ADVANCED MANUFACTURING TECHNOLOGY (UNIT-2) BY Asst. Prof. CP SAINI

UNIT II

Introduction, Polymers, Polymerization, Addition of Polymers, Plastics, Types of plastics, Properties of Plastics, Processing of Thermoplastic Plastics, Injection Moulding, Extrusion Process, Sheet forming processes, Processing of Thermosetting Plastics, Compression Moulding, Transfer Moulding, Casting of Plastics, Machining of plastics, other processing methods of plastics

Introduction, casting, thread chasing, Thread Rolling, Die Threading and Tapping, Thread Milling, Thread Measurement and Inspection

2 INTRODUCTION OF PLASTICS AND POLYMERS

Plastics belong to the family of organic materials. Organic materials are those materials, which are derived directly from carbon. They consist of carbon    chemically combined with hydrogen, oxygen and other non-metallic substances, and their structures, in most cases, are fairly complex. The large and diverse organic group includes the natural materials: wood, coal, petroleum, natural rubber, animal fibers and food, which have biological origins. Synthetics include the large group of solvents, adhesives, synthetic fiber- s, rubbers, plastics, explosives, lubricants, dyes, soaps and cutting oils etc. Which have no biological origins? Of them, plastics and synthetic rubbers are termed as “polymers”.

2.1 POLYMERS

The term “polymer” is derived from the two Greek words: poly, meaning “many“, and meros meaning “parts“ or “units”. Thus polymers are composed of a large number of repeating units (small molecules) called monomers are joined together end-to-end in a polymerization reaction. A polymer is, therefore, made up of thousands of monomers joined tog -ether to form a large molecule of colloidal dimension, called macromolecule. The unique characteristic of a polymer is that each molecule is either a ling chain or a network of repeating u -nits all covalently bonded together. Polymers are molecular materials and are generally noncrystalline solids at ordinary temperature, but pass through a viscous stage in course of their formation when, shaping is readily carried out.

The most common polymers are those made from compounds of carbon, but polymers can also be made form inorganic chemicals such as silicates and silicones. The naturally occurring polymers include: protein, cellulose, resins, starch, shellac and lignin. They are commonly found in leather, fur, wool, cotton, silk, rubber, rope, wood and many others. There are also synthetic polymers such as polyethylene, polystyrene, nylon, terylene, dacron etc., termed under plastics, fibers and elastomers. Their properties are superior to those of the naturally occurring counter- parts. Our concern, here, is therefore, with synthetic polymers, also called plastics (again from Greek plastics, derived from plassein: to form, to mould) or resins.

2.2 POLYMERIZATION

The process of linking together of monomers, that is, of obtaining macro -molecules is called “polymerization”. It can be achieved by one of the two processing techniques:

(a) Addition Polymerization. In addition or chain polymerization under condition of temperature and pressure and in the Presence of a catalyst called an initiator, the polymer is produced by adding a second monomer to the first, then a third monomer to this dimmer, and a fourth to the trimmer, and so on until the long polymer chain is terminated. Polyethylene is produced by the addition polymerization of the addition polymerization of ethylene monomers. This linear polymer can also be converted to a branched polymer by removing a side group and replacing it with a chain. It many such branches are formed, a network structure results.

“Co-polymerization” is the addition polymerization of two or more different monomers. Many monomers will not polymerize with themselves, but will copolymerize with other compounds.

(b) Condensation polymerization. In this process, two or more reacting compounds may be involved and there is a repetitive elimination of smaller molecules, to form a by-product for example, in the case of phenol formaldehyde (bakelite), the compounds are: formaldehyde and phenol. Metacresol acts as a catalyst and the by- product is water. The structure of the ‘mer’ is more complex. Also, there is the growth perpendicular to the direction of chain. This is called ‘cross-linking’.

Size of a Polymer. The polymer chemist can control the average length of the molecules by terminating the reaction. Thus, the molecular weight (the average weight, in grams, of 6.02×10²³ molecules) or degree of polymerization, D.P., (the number of mers in the average molecule) can be controlled. For example, the length of molecules may range from some 700 repeat units in low-density polyethylene to 1,70,000 repeat units in ultrahigh molecule are weight polyethylene.

2.3 ADDITIONS TO POLYMERS

The properties of polymers can be further modified by the addition of agents, which are basically of two types. Those that enter the molecular structure are usually called “additives”, whereas those that form a clearly defined second phase are called “fillers”.

1. Plasticizers. Plasticizers are liquids of high boiling point and low molecular weight, which are added to improve the plastic behavior of the polymer. The board role of a plasticizer is to separate the macro- molecules of the polymer, that is, making deformation easier. They are essentially oily in nature. Organic solvents, resins and even water are used as plasticizers.
2. Fillers. Filler is used to economize on the quantity of polymer required and / or to vary the properties to some extent, for example, mechanical strength, electrical resistance etc. Filler, whose function is to increase mechanical strength, is termed “reinforcing filler”. A filler is commonly fibrous in natures and is chemically inert with respect to the polymer with Which it is to be used? Common fillers are wood flour, cellulose, cotton flock, and paper (for improving mechanical strength); mica and asbestos (for heat resistance); talc (for acid resistance).Wood flour is general purpose filler. It improves mould ability, lowers the cost with fairly improved strength of the plastics. Mica also imparts excellent electrical properties to plastics and results in low moisture absorption. The commonly used “reinforcing filler agents” with plastics are: fibers/filaments of glass, aramid, graphite or boron. Reinforcing by metal and glass fibers make plastics strong, flexible and light materials such as used in bullet proof vests. Cotton fibres improve toughness. Carbon fibers are used for high performance installations such as aircrafts etc. requiring high strength and stiffness.
3. Catalysts. These are usually added to promote faster and more complete polymerization and as such they are also called ‘accelerators’ and ‘hardeners’ e.g., ester is used as a catalyst for urea formaldehyde.
4. Initiators. As the name indicates, the initiators are used to initiate the reaction, that is, to allow polymerization to begin. They stabilize the ends of the reaction sites of the molecular chains. H₂O₂ is a common initiator
5. Dyes and pigments. These are added, in many cases, to impart a desired colour to the material. For example, titanium dioxide is an excellent white pigment; iron oxides give yellow, brown or red colour; carbon black is not only a pigment but also a UV light absorbent. Finely divided calcium carbonate dilutes (extends) the colour and is used in large quantities as low – cost filler.
6. Lubricants. Lubricants are added to the polymers for the following purposes: to reduce friction during processing, to prevent parts from sticking to mould walls, to prevent polymer films from sticking to each other and to impart an elegant finish to the final product. Commonly used lubricants include: oils, soaps and waxes.
7. Flame-retardants. Most Plastics will ignite at sufficiently high temperatures. The non-inflammability of the plastics can be enhanced either by producing them from less inflammable raw materials or by adding “flame retardants”. The common flame retardants are: compounds of chlorine, bromine and phosphorous.
8. Solvents. Solvents are useful for dissolving certain fillers or plasticizers and help in manufacturing by allowing processing in the fluid state. For example, alcohol is added in cellulose nitrate plastics to dissolve camphor. However, subsequently, the solvents must be removed by evaporation.
9. Stabilizers and anti-oxidants are added to retard the degradation or polymers due to heat, light and oxidation.
10. Elastomers are added to plastics to enhance their elastic properties.

Note. Above, excepting fillers, all other materials used fall under the category of “Additives”.

2.4 PLASTICS

Polymer can be divided in to three broad divisions: plastics, fibers and elastomers (polymer of high elasticity, for example rubber). Synthetic resins are usually referred to as plastics. Plastic derive their name from the fact that in a certain phase of their manufacture, they are present in a plastic stage (that is, acquire plasticity), which makes it possible to impart any desired shape to the product. Plastics fall in to a category known chemically as high polymers.

Thus, “plastics” is a term applied to composition consisting of a mixture of high molecular compounds (synthetic polymers) is fillers, plasticizers, stains and pigments, lubricating and other substances. Some of the plastics can contain nothing but resin (for instance, polyethylene, polystyrene).

2.4.1 TYPES OF PLASTICS: Plastics are classified on the broad basis of whether heat causes them to set (thermosetting) or causes them to soften and melt (thermoplastic).

2.4.1.1 Thermosetting plastics: These plastic undergo a no. of chemical changes on heating and cure to infusible and practically insoluble articles, the chemical change is not reversible Thermosetting plastics do not soften on reheating and can not be worked. They rather become harder to completion of any leftover polymerization reaction. The commonest thermosetting plastics are: alkyds, peroxides, melamines, polyesters, phenolics and urea.

2.4.1.2 Thermoplastic Plastics: These plastics soften under heat, harden on cooling, and can be re softened under heat. Thus, they retain their fusibility, solubility and capability of being repeatedly shaped. The mechanical properties of these plastics are rather sensitive to temperature and to sunlight and exposure to temperature may cause thermal degradation. Common thermoplastics plastics are: acrylics, poly tetra fluoroethylene (PTFE), polyvinyl chlorides (PVC), nylons, polyethylene, polystyrene, etc.

Thermosetting plastics are cross-linked polymers and thermo-plastics are linear and branched-linear polymers. The method of processing a plastic is determined largely by whether a plastic is thermosetting or thermoplastics.

2.4.2 PROPERTIES OF PLASTICS: There is great variety of physico-chemical and mechanical properties and the ease with which they can be made into various articles have found plastics their wide application in the engineering and other industries.

1. Their comparatively low density substantial mechanical strength and high anti friction properties have enable plastics to be efficiently used as substitute for metals, for example, non-ferrous metals and alloy bronze, lead, tin, babbit etc., for making bearing.
2. With certain special properties (silent operation, corrosion resistance etc), plastics can sometimes replace ferrous metals.
3. From the production point of view, their main advantage is their relatively low melting points and their ability to flow into a mould.
4. Simple processing to obtain machine parts. Generally there is only one production operation required to convert the chemically manufactured plastic into a finished article.
5. In mass production, plastics substituted for ferrous metals allow the production costs to be reduced by a factor of 1.5 to 3.5 and for non-ferrous metals by a factor of 5 to 20.
6. Good damping capacity and good surface finished of the product.
7. The high heat and electric insulation of plastics permits them to be applied in the radio and electrical engineering industries as and substitutes for porcelain, ebonite, shellac, mica, natural rubber, etc.
8. Their good chemical  stability , when subjected to the action of solvents and certain oxidizing agents, water resistance, gas and steam proof properties, enable plastics to be used as valuable engineering materials in the automo9bile and tractor, ship building and other industries

2.4.3 DISADVANTAGES

1.  Comparatively higher costs of materials.
2. Inability of most plastics to withstand even moderately high temperatures.

The factors which have determined the rapid growth of polymer materials in the recent past are:

1. Ready availability of the basic raw chemical materials in large quantities and, in general, at the low cost.
2. The large no. of available starting materials for their production provide us with an almost continuous spectrum of composition and structure, and hence of mechanical, optical, electrical and thermal properties of the resulting polymers.
3. The engineer now has at his disposal many well developed processes and machines to convert (as they from the factory and that he can choose according to the specification of the ultimate product) into useful goods.

2.4.4 PROCESSING OF THERMOPLASTIC PLASTICS

Thermoplastic can be processed to their final shape by moulding and extrusion processes. However, extruding is often used as an intermediate processes, for example, vacuum forming or machining. 

2.4.4.1 EXTRUSION PROCESS:

The extrusion process, in many cases, produces material in an intermediate form for subsequent reprocessing to its final component form. The process is the same as for metals, that is, the expulsion of material through a die of the required cross-section .The earliest extrusion machines were of the ram type. The cylinder of the machine is filled with prepared plastic and extruded through a die under the pressure of the ram. The advantages of this machine are: simplicity in operation and a controlled pressure which can be virtually as high as required. If the polymer can be plasticized by pressure, then the ram extruder is advantageous in view of its simplicity. But for plastics which require heat, the separate pre-processing may be regarded as a drawback. Another major drawback of this type of machine is reciprocating action of the ram which is time wasting since the ram must be withdrawn after its power stroke and a new dolly of material inserted in the container. Also, with many materials the die orifice must be cleaned between each working stroke.

                         

                                Fig. 2.1 Screw Extruding Machine                                                                                         

Now a days, the ram machine is mainly used for “wet extrusion” that is for extruding plastics which have been softened by the addition of solvents. Although useful in homogenizing materials which contain hard inclusions, wet extrusion has the disadvantage of producing a component from which the solvent has to be remolds.

For the extrusion of the plastics, single-screw machine has completely replaced the ram type machine. There are two basic types of screw extrudes: the melt extruder and the plasticizing extruder. In the former, the material is delivered to the extruder already melted and thus the function of the extruder is merely to push the material to the die and through the orifice. In the plasticizing extruder the material is in the form of granules or particle and so the extrude has to compress and work it until it melts before delivering it, under pressure, to the die orifice.

Complex shapes with constant cross sections can be extruded with relatively inexpensive tooling. The extruded product can be coiled or cut into desired lengths.

2.4.4.2 SHEET –FORMING PROCESS:

Many plastic articles are formed from sheet. The process resembles those for metals, but requires very low forces. Even atmospheric pressure may be sufficient. In “Drape Forming”, the sheet is heated to a moderate temperature. It is then clamped at the edges and stretch – formed over a die. One of the encountered is that the portion of sheet first touching the die will be chilled and remain thicker than the rest. This is overcome or minimized by blowing hot air between the sheet and the die. 

Fig. 2.2(a) Principle of Vacuum Forming Process

Vacuum forming is a process, in which a heated plastic is changed to a desired shape by causing it to flow against the mould surface by reducing the pressure between one side of the sheet and mould surface. The process consist of clamping the heat heated plastic sheet over a mould in such a way that the air b/w the sheet and mould can be evacuated, this vacuum, of increasing intensity, draws the sheet against the surface of the mould, where it cools and solidifies. The solidifies will take place earliest in those resigns which touch the mould first. This will cause differential cooling and, as a result of non uniform temp. Distribution, there will be a marked change in thickness along any given section of the component.

                            Fig. 2.2(b) Vaccuum Forming Process

2.4.5 PROCESSING OF THERMOSETTING PLASTICS

Compression molding and transfer molding are the most common methods of processing thermosetting plastics. Although, suitable for thermoplastics also, the main application of these methods is to thermo sets.

 2.4.5.1 COMPRESSION MOULDING:

Fig. 2.3 Compression Moulding

Compression moulding is the equivalent of closed-die forging. In this process, a premature quantity of plastic in the form of plastics or briquettes is placed in heated mould and compressed at suitable pressure and temperature. Hydraulic pressure usually employed to provide the pressure (which may range from 20 to 30 MPa or even higher up to 80 MPa in same case) for compressing the plastic compound. Other equipment, such as friction and presses, can also be used. The object of compression moulding is to bring the plastic to virtually molten state. Thus the process is, effectively, forming from the liquid state, the material being healed in the mould until the curing stage is over when polymerization is complete. The process is rather slow with the phenolics and urea resin, but some of the newer resins have shorter curing time and this has improved the production rates appreciably.
When the plastic is completely trapped between the male and female die, it is called as” positive mould”. Cluster tolerances can be held if a small flash is allowed to extrude, usually along the male die perimeter in “semi positive moulds”. More plastics is lost in “flash moulds”, similar to those used in impression-die forging.

Typical product application are: disches, handles, container caps, fitting,. Electrical and electronic components, washing machine agitators and housing etc. 

2.4.5.2 TRANSFER MOULDING:

 

                                  Fig.  2.4 Transfer Moulding

Transfer moulding is a modification of compression moulding in which the moulding is first placed in separate chamber (transfer pot), from which it pushed through an orifice into the mould cavity as the mould closes. The material to be moulded is often preheated by radio frequency methods and, where it is desired to improve toughness and strength, reinforcing fillers may be used.

The process has got following advantages:

1. There is little pressure inside the mould cavity until it is completely filled, at which stage the full liquid pressure is transmitted.

2. The plastic acquires uniform temperature and properties in the transfer pot prior to transfer .The plastic is further heated by sheering through the orifice, viscosity is reduced, and the plastic fills the intricate mould cavities.

3. It scores over normal compression moulding in that presses can be used, since; heating of plastic is affected, not by press itself, but by a simple heating jacket round the transfer chamber.

2.4.6 CASTINGS OF PLASTICS:

Casting of plastics in moulds finds application when making parts of plastic material with a binder but no filler.  It is also used to obtain various kinds of casts thermosetting plastics, for example, cast carbolite, as well as certain cast thermoplastic material, such as organic glass, polystyrene and others.

The method is simple and cheap since no expensive tooling or equipment is required, and no pressure needs to be applied to fabricate the part.  There are many variations of the casting method for plastics:-

(a)    By using flexible moulds, very intricate shapes can be fabricated. The mould is peeled off afterwards.

(b)   Using plate glass moulds produces thick plastic sheets.

(c)   Using moving stainless steel belts, which contain and cool the resin, produces thinner plastic sheets.

(d)   Hollow shapes can be obtained by centrifugal casting of the molten plastic material.

(e)    Potting:   In this method, the plastic material is cast around an electrical component, which gets embedded in the plastic material.  This is achieved by pouring the molten plastic material in a housing or case, which is an integral part of the component and in which the component is repositioned before pouring the plastic.

(f)    Encapsulation:   Here, the component is covered with a layer of cooled and solidified plastic.

(g)   Both potting and encapsulation are very important to the electrical and electronics industry.  The plastic material serves as a dielectric.

(h)   Foam moulding / casting: In this method a foaming agent is mixed with the plastic resin.  The mixture is placed in a mould and heated.  The foaming agent makes the material to expand (even up to 50 times the original size) to take up the shape of the mould.  The amount of expansion can be controlled through temperature and time. Both rigid and flexible foamed plastics can be obtained from thermo-plastics and the thermo-setting plastics.  Rigid construction is used for structural purposes and flexible for cushioning.  Product application include: Shaped packaging materials for cameras, appliances and electronics etc. insulating blocks, food containers and Styrofoam cups.

2.4.7 MACHINING OF PLASTICS:

Plastic can be machined, but in most cases, machining of plastics is not required.  Moulding and forming methods can obtain acceptable surface quality and dimensional accuracy.  However, there are certain plastics like PTFE (Polytetra fluoroethylene) which are sintered products and are not mould able by usual techniques, as they do not melt. For such “thermo stable plastics” machining is a viable alternative to moulding.

The machining of plastics (by operation such as turning, drilling and milling) has special features due primarily to the structure of the material.  It also depends upon the binder upon the binder and the filler and the method of moulding the component. For example, the machining of thermosetting plastics allows optimum cutting variables and tool geometry to be employed because these do not soften on heating, whereas thermoplastic resins soften under heat. The permissible maximum temperature in the cutting zone is 1600C for thermo-setting resins and only 600 C to 1000C for thermoplastics.

Special features of the machining of plastics are:-

•  The tendency of certain plastics to splitting.
• High elasticity (40 times as much as that of steels). Therefore, they must be carefully supported, to avoid their deflection during machining.
• Non-homogeneous structure of the material, with components of different hardness. This results in poor surface finish after machining.
• Plastics have a strong abrading action on cutting tools.
• Their low thermal conductivity results in poor heat dissipation from the cutting zone and in over-heating of the cutting edges.
• The intense dust formation, especially for thermosetting plastics, makes it necessary to use special dust -removing devices.
• The hygro scopicity of plastics excludes the use of liquid cutting fluids. Compressed air is commonly used for cooling.
• Reinforced plastics are very difficult to machine.

Plastics can be machined with H.S.S. and cemented -carbide tools.  In machining a plastic material with a filler of glass, quartz or mica type, a satisfactory tool life can be obtained only with carbide -tipped tools.  Only diamond tools are suitable for turning high-strength plastics of this type.  The strength of cast parts of laminate plastics is 40 to 50 percent less than that of the parts made by compression moulding.   Therefore, higher cutting speeds and feeds can be used in their machining than for strong thermo-setting plastics.  The main trouble in turning laminated plastics is the peeling of the surface layer.

The cutting variables are also influenced by the life of cutting tool which is subject to abrasive wear in machining most engineering plastics.  Dulling of the cutting tool leads to a poor surface finish and to breaking out of the material at the points the cutting tools enters and leaves the cut. This makes it necessary to use more keenly sharpened cutting tools for plastics.  The need for sharp cutting edges follows from the high elasticity of plastics.

The selection of cutting variables is also influenced by the low heat conductivity of the plastics, since, in machining the tool may be within a closed volume (as in drilling) with no cooling facilities. This may lead to charring of the machined surface.

The cutting tool angles for machining plastics are made somewhat different than those of tools for ferrous and non-ferrous metals. The rake angles are positive and relatively larger.  Because of the visco elastic behavior of thermoplastics, some of the local elastic deformation is regained when the load is off.  Therefore tools must be made with large relief angles (200 to 300).

Abrasive machining of plastics has many advantages over machining with metal cutting tools. These include the absence of splitting and crack formation, and the better surface finish that can be obtained.

In grinding, the contact between the wheel and the surface being ground, should be as short as possible, to avoid burns.  Organic glass is commonly ground with coated abrasive, applying an ample amount of water as a coolant. If possible, however, grinding should be replaced by polishing with a felt, broadcloth or flannel wheel charged with lapping paste, the process is known as “Buffing”. The buffing wheels are of diameter 250 mm, 40 to 60 mm wide and of speed 2000 rev/min.  Medium and fine lapping pastes are used as the buffing compound for plastics. Laminate fabric base, asbestos-fiber and glass -fiber laminate can be cut with abrasive wheels (SiC) of grain size 24 to 46 and with a 5% emulsion as the cooling fluid.

2.4.8 OTHER PROCESSING METHODS FOR PLASTICS

1. Calendering:   It is an intermediate process where the extruded plastics sections are reduced to sheet which may or may not, then be formed to final shape by vacuum forming.  It is clear that the calendering process can be used for thermoplastics and not for thermosetting plastics.

Calendering is some ways similar to rolling process in that the material is compressed between rolls and emerges as sheet (Fig 2.5a).  However, there are differences. There is appreciable thickening after the material has reached minimum thickness at the roll gap and the pre-calendered material is not in sheet form, but of indefinite shape. The method of producing vide sheet and foil is illustrated in Fig (2.5b).  The thermoplastic melt is fed to a multi roll calendar. 

                                       Fig 2.5 Calendering Process

The first roll gap serves as a feeder, the second as a metering device, and the third roll gap sets the gauge of the gradually cooling plastic which is then wound, with about 25% stretching onto a drum.

Calendering is a high -production rate (typically 100 m/min) process, mostly for flexible PVC, for example, upholstery, rainwear, shower curtains, tapes, etc. and rigid PVC, for example, trays, credit cards, lamination.  PVC is also calendered into the well known transparent film widely used for packaging.

2. Rotational Moulding:   In this process, also called “rotomoulding”, large relatively thin -walled hollow (open or closed) parts are made. A measured quantity of Polymer powder is placed in a thin-walled metal would. The mould is closed and is rotated about two mutually perpendicular axes as it is heated.  This causes the powder of sinter against the mould walls, building up the wall thickness of the component.  At the end of the heating and sintering operations, the mould is cooled while it is still rotating.  Applying cold water and air to the outside of the rotating mould does cooling. The rotation is then stopped and the component is removed. To increase production rates, three moulds at the end of three arms joined together to the central spindle (just like centrifuge casting) are used, with one mould for each stage of the process, that is, load-unload, heat and cool positions.

The process is simple as no pressure is employed and the part is free of moulded in stresses. The technique is extensively used for the production of toys in P.V.C. such as boats, horses etc.  Large containers of Polythene (or up to 20,000 liter capacity) and large components like laminated petrol tanks for motor care are made from polythene (outer shell) and nylon (inner shell). Other products include: Trash Cans, Boat hulls, buckets, housings and carrying cases etc.

3. Blow Moulding:   In this process, a hot extruded tube of plastic, called a parison, is placed between the two part open moulds (Fig 2.6 a).

 

3D View of Blow Moulding

Fig 2.6 Blow Moulding

The two halves of the mould move towards each other so that the mould closes over the tube. The tube gets pinched off and welded at the bottom by the closing 

Moulds (Fig. 2.6 b). The tube is then expended by internal pressure, usually by hot air, which forces the tube against the walls of the mould. (Fig2.c). The component is cooled and the mould opens to release the component (Fig. 2d). Typical products applications are: Plastic beverage bottles and hollow containers.Fig. 2.6

4. Reaction Injection Moulding (RIM) :   The method differs from the conventional  injection moulding process in the sense that  it is not molten polymer which is injected into a  mould, but a mixture of two or more monomers  (reactants ) are forced  into  a mould cavity.  Chemical reaction takes place between the constituents of the mixture giving off heat to form a plastic polymer, which solidifies producing a thermo set component.  The major product applications include: Automotive bumpers, and fenders, thermal insulation for refrigerators and freezers and stiffness for structural components.

5. Solid State Forming:   The term is a misnomer because the temperature of the polymer is just (100 to 200C) below the melting point of the polymer. The main operations involved are: Sheet metal techniques such as stretching, bending and deep drawing. Many food-packaging tubs and containers are fabricated from Polypropylene.  Forging is also used mainly for producing gears.

6. Cold Forming:   All the cold working methods used for metals can be used for polymers. Filaments and fibers are produced by “Cold drawing”, that is, continuous stretch drawing.  Conventional rolling can also be used for producing fibers.   In “Cold pressing” or “Cold moulding”, the raw thermosetting material (or mixed plastic compounds) are put in the mould and cured in an oven.  Pressure applied by the press range from 14 MPa to 84 MPa. The moulds are made of abrasion resistant tool steel.  The process is quite economical and the process cycle is relative short. However, the surface quality and dimensional accuracy of the part is not very good.

7. Thermoplastic Stamping: Thermoplastic Stamping or matched-die forming is a method in which thermoplastic polymer sheet at melt temperature is worked between mating dies.  The cycle time is greatly reduced and the spring back is minimum.

8. Spinning: The extrusion process can be modified to produce filaments, fibers and yarns. The molten thermoplastic polymer   is extruded through a die containing holes.  For obtaining strands, the dies can be rotated to produce twists and wraps.”

2.5 THREAD MANUFACTURING

2.5.1 INTRODUCTION

Threads are prime importance to the engineering. These are used as fasteners, to transmit power and motion and for adjustment. The subject of thread manufacture has assumed a great significance because of the ever-increasing demand for high precision fastening devices and power transmission devices. At present, the threads are manufactured by the following processes:

1.      Casting

2.      Thread chasing

3.      Thread rolling

4.      Die thread and tapping.

5.      Thread milling

6.      Thread grinding

2.5.2 CASTING

The accuracy and finish of threads made by casting will depend upon the method of casting. Threads made by sand casting are rough and are not used much, except sometimes in vises and rough machinery. Threads made by die casting and permanent mould casting are very accurate and high finish, if properly made. However, these can be made only of low melting point non-ferrous metals and, therefore, are not fit for repeated use, being not hard and durable. Lost wax method can produce highly accurate threads of good finish. But the method is costly and difficult. Sewing machine vending machines, typewriter parts and toys may have their threads cast in place by die casting and permanent mould casting. Such parts are rarely taken apart, so, the method is very satisfactory. The drawbacks of sand casting can overcome by using shell molding method. Due to the inherent drawbacks of casting methods of thread production.

2.5.3 THREAD CHASING

The method of cutting threads with a single point tool on a center lathe and with a multipoint tool on a turret lathe is called “thread chasing”. Thread chasing is a form cutting operation, with the form tool corresponding to the profile of desired thread space.

2.5.3.1 Thread chasing on a center lathe. The first step in cutting threads on a lathe is to get an accurately shaped and mounted tool. The form and setting of the tool is checked with the help of a thread template or center gauge, Fig. The job is either mounted between centers or held in a chuck (for external threads) and held in a chuck for internal threads. When mounting the tool in the tool post, it must be ensured that the top of the job, Fig. After this, the second step is to establish a specific relationship between the longitudinal movement of the tool parallel to the axis of rotation, and the rotation, of the job. This will determine the pitch or lead of the thread. This is achieved with the help of lead screw and a split nut. The two halves of the split nut are fastened to the carriage. When the nut is closed on to the lead screw, it acts as a complete nut, and the carriage starts moving as the lead screw rotates. The lead screw is geared to the spindle and the proper speed ratio between the two is sit by means of a gear-change box. Therefore, as the lead screw rotates, the carriage will move a predetermined distance (depending upon the pitch of lead of the thread) per revolution of the job. The third requirement an exact predetermined time, for taking successive cuts, so that the tool enters the helical groove of the cut previously produced; otherwise the tool may remove some of the desired thread.

Fig 2.7 Center Gauge

This is achieved with the help of a ‘thread dial’, which is mounted on the carriage and is driven by the lead screw through a worm gear. The face of the thread dial is graduated into a even number of full and half divisions, Fig. Whenever the lead screw rotates and the split nut is not engaged, the thread dial rotates. The split nut must be engaged when a particular line on the dial face coincides with the zero line. For cutting even number of threads, the split nut should be engaged when any line on the dial coincides with zero line, and for cutting odd-number threads, when and homebred line coincides with zero line.

To start cutting a thread, the tool is fed inward until it first scratches the surface of the job. The graduated dial on the cross-slide is noted or set to zero. The split nut is then engaged and the tool moves over the desired job length. At the end of tool travel, it is quickly withdrawn by means of cross slide. The spot nut is disengaged and the carriage is returned to the starting portion, for the next cut. 

Fig 2.8 Setting of Cutting Tool

These successive cuts are continued until the thread reaches its desired depth (checked on the dial of cross-slide). The depth of first cut is usually 0. 25 to 0.40 mm. This is gradually decreased for the successive cuts until for the final finishing cut; it is usually 0.027 to 0.075 mm. The tool can be fed inward either radially or at an angle of 29 by swiveling the compound rest, Fig. 

Fig. 2.9 Feeding the tool into the job 

The drawback of the first method is that the absence of side and back rake will not proper cutting except on brass and cast iron. In the second method, the cutting mainly takes place on one face of the tool and some side rake can be provided. Also, the chip will curl more easily. For cutting square, acme and worm threads, the first method is used. For cutting L.H. threads, the tool is moved from left to right and for cutting right hand threads; it is moved from right to left. Thread cutting on a lathe is a slow process, but it is the only process of producing square threads, as other methods develop interference on the helix.

2.5.3.2 Thread chasing on a turret lathe. The main drawback of cutting threads on a center lathe is that the operation cannot be done at higher cutting speeds, since the permissible speed is limited by the quickness with which the operator can withdraw the cutting tool from the job at the end of a cut. This drawback is overcome in turret lathe, where thread-chasing attachment is used to cut the thread. The attachment has no thread dial, which enables the operation of the machine even by a semi-skilled worker. A simple thread chasing attachment for a turret lathe is illustrated in Fig. From the headstock of the machine, power is given to a short lead screw, known as the leader, by means of change gears. The fed nut and the tool slide are carried on a shaft, which can be engaged or disengaged to the leader by means of a hand lever. The major advantage of the arrangement is that the fed nut can be engaged to the leader at any portion of the work rotation. 

Fig. 2.10 Thread Chasing Attachment

2.5.4 THREAD ROLLING

Thread rolling is a cold working process in which a blank of diameter approximately equal to the pitch diameter of the required thread, is rolled between hardened steel rolling dies having the negative counter of the thread to be produced. As the thread shaped ridges on the dies penetrate the blank material, material is displaced from the bottom of the thread and forces radially out to form the thread crests. These are three types of the thread rolling machine:

1.      Reciprocating, flat die machines.

2.      Cylindrical die machines.

3.      Rotary planetary machines, having a rotary die and one or more stationary concave-die segments.

In the reciprocating, flat die machine, one die is stationary and the other reciprocating. The part to be threaded is rolled between the dies, as the moving die reciprocating in reference to the stationary die. The stroke of the reciprocating die is will depend upon the diameter of the thread being produced, since during one stroke, the blank makes are completely revolution and the thread is completely formed. This is highly versatile machine, since at the same time threading and knurling can be done on a part of right and left hand threads can be rolled, by assembling two or three sets of flat dies. This method is mainly used for the manufacture of commercial bolts and nuts.

In cylindrical die machine, the part is to be threaded is rolled between rotating cylindrical dies. The machine can have two rounded dies located diametrically opposite each other, or three rounds dies usually spaced. This machine is slower than the reciprocating flat die machine and is more suitable for large sized precision threads and for short run production. This machine operates with the following motions:

1.      Positive rotation of the both the dies (in a two die machine) in the same direction.

2.      Radial motion of one of the dies for its rapid approach, infeed and retraction. This method has the main application of the threads on taps.

Fig. 2.11 Thread Rolling

In rotary planetary machine, the job is rolled between a central die that rotates continuously about a fixed axis, and one or more concave-shaped die segment located adjacent to the periphery of the rotating die. This being a continuous process is the fastest method of thread rolling.

2.5.4.1 Advantages of thread rolling

1.      It is the fastest method of producing a thread, with production rate more than 2000 piece per minute.

2.      Being a chipless forming process (no material wastage), there is lot of material saving (about 16 to 27 %).

3.      During thread rolling, the material is strained plastically and is work-hardened, and is, therefore, stronger against both tension and fatigue, especially the latter.

Increase in tensile strength is form 10 to 20% and that in fatigue strength is from 10 to 75%

4.      The grain fibers remain continuous and follow the contous of the threaded surface. Due to his, the threads are less easily sheared off than machined threads.

5.      The surface of rolled thread is harder than a cut thread, so wear resistance increases.

6.      Surface finish is better as controlled by the rolls.

7.      Dimensional accuracy is better, as very little wear occurs on the rolls as it would on a cutting tool.

2.5.5 DIE THREADING AND TAPPING

2.5.5.1 Die Threading. Die threading is a method of cutting external threads on cylindrical or tapered surfaces by the use of the solid or self-opening dies. The main advantage of die threading is that it can be performed along with other operations on turret lathes and on automatics (in the case of self opening dies).

2.2.5.2 Solid Dies. In principal, a solid consists of a hardened, threaded nut with several longitudinal grooves cut away and shaped to provide cutting edges to the remaining portions of the thread. To facilitate their use from either end, entry chamfers are provided at both the ends. To cut the threads, the die is screwed on the bar upon which the threads are to be cut. To move the die along the bar, it is held in stock. This is rotated manually. To cut a smoother thread and to prolong the life of the die, a suitable lubricant is used. The solid type dies are used rarely, because they do not have any adjustment for wear. The solid adjustment can be adjust for size and wear over a small amount by means of a screw, these dies are made of carbon or high speed tool steel and can be used on a turret lathes with suitable holders.

Fig. 2.12 Solid Threading die

2.2.5.3 Self-opening die heads. The major drawback of the solid type dies is that they must be unscrewed from the work piece by reversing the machine spindle, to disengage the die from the work. Due to this, these dies are not suitable for use on high speed production machines, for use on high speed production machines, e.g. turret lathes and automatics. This drawback is overcome by using self-opening die heads. When the required length is thread is cut, the die open automatically. At the end of the turret slide travel, the front portion of the dire head continuous to move forward by a small amount until the chasers in the die head move outward in the body, under the action of a scroll or a cam. This action clears the chasers from the cut thread and enables the die head to be withdrawn without reversing the machine spindle. The die head, while cutting threads may advance its own guidance once it screws itself along the work, until the die trip opens. However, for better accuracy, there is increasing use of lead screw guides.

Depending upon the type of chaser, there are three types of die heads

1.      Radial

2.      Tangent

3.      Circular

Radial chasers can be more rigidly, supported than other types. These are difficult to resharpen and their life is short. Tangential chasers give a long life, because the length of the teeth makes possible a large number of regrinds on the cutting face. Due to this they are very suitable for heavy duty work and large batch production. Circular chasers also have a long working life since these can be resharpened a number of times. All the die-heads can either be stationary or revolving.

When used on automatics, the feed motion of the die-head is controlled by the cam rise, which can be designed according. At the end of the return stroke, the dies are closed automatically when the closing handle strikes a rod. Die-hands are available for cutting threads from 6.35 mm to 114 diameter and chasers are available for any thread form.

2.2.5.4 Thread tapping. Taps are the tools for cutting internal threads. A tap is similar to a threaded bolt, with one to four flutes cut parallel to its axis. The flutes perform three functions:

1.      Provide cutting edges.

2.      Conduct the cutting fluid to the cutting region, and

3.      Act as channels to carry away the chips formed by the cutting action.

Fig. 2.13 Self opening die head

The flutes can be straight, spiral, helical or spiral pointed. Taps with straight flutes are most commonly used, since it is easier to cut and sharpen these flutes. Tapping can be dome manually or on drilling machines, tapping machines, turret lathes and automatics. A hole of diameter slightly larger than the minor diameter of the thread to be cut must already exist, for thread tapping. Drilling can make the hole, boring or casting. The two main types of taps are: solid taps and collapsing taps.

Solid taps. Solid taps are of one-piece construction. These taps are usually worked manually but can also be used on machine tools, such as lathes, drill presses and special tapping machines. Taps are made of high carbon or high-speed steel. The shank of the taps is kept plain and the end is squared. To operate the tap by hand (Hand taps), it is held at the squared end with the help of a “tap wrench”, which is used to screw the tap into the hole. To cut any particular size, hand taps are available in sets of three: taper, plug and bottoming. The three taps are identical in size and length, but differ in the amount of chamfer at the bottom end. The taper plug has about 8 to 10 threads chamber at the bottom end, the plug tap has 2 to 3 threads chamfered, whereas, a bottoming tap has no taper threads at its bottom end. The tapered are cut to the full depth gradually, so less effort is required. If a hole is open at both ends, then, after the taper tap, plug is used for finishing the treads as deep into the hole as its shape will permit. Lastly, the bottoming tap is used to finish the entire thread portion. So, the three taps should be used in the order mentioned above. The bottoming tap is the only tap, which would nearly reach the bottom of a blind hole. The three taps are shown in Fig. 2.14. 

Fig. 2.14 Solid Thread Taps

 While threading a combined rotary and axial motion is given to the tap. When using a solid tap on a drill press, a special tapping attachment is used. This makes the tap to rotate slowly as it is fed downward into the job. At the end of tapping, when the spindle is raised, the tap automatically starts rotating in the reverse direction at a higher speed to back the tap out of the hole in a shorter time. On screw machine or turret lathe, a special holder is used for the tap, in which a pin prevents the tap from rotating while it is fed into the job. At the end of travel, the tap pulls the pin so that it is free to rotate with the work. The machine spindle is then reversed in motion and the pin again stops the tap from rotating while it is being backed out of the hole.

(b) Collapsing taps. For better results, a tap (or a die) should not be backed off the thread it has just produced, because, during backing off, they catch tiny chips which can do damage to the product. So for good finish and to speed up openings, collapsing taps are used, which collapse inward automatically when the thread is completed. This makes it possible to withdraw the tap from the hole without reversing the machine spindle.

Nomenclature: Refer to Fig. 2.15 

Fig. 2.15

1.      Axis. It is the longitudinal centre line through the tap.

2.      Body. The body of a tap is the thread and fluted part of the tap.

3.      Thread. It is the cutting tooth of the tap which produces the thread in a hole.

4.  Angle of thread. It is the angle included between the sides of the thread, measured in the axial plane.

5.      Crest. It is the top surface joining the two sides of a thread.

6.      Root. It is the bottom surface joining the sides of two adjacent threads.

7.      Base of thread. It is the bottom section of a thread; the greatest  section

section between the two adjacent roots.

8.     Depth of thread. The depth of the thread profile is the distance between the top of crest and the base or root of thread measured perpendicular to the axis of the tap.

9.    Side of thread. It is the surface of the thread which connects the crest with the root.

10.  Land. It is the threaded web between flutes.

11.  Cutting face. It is the front part of the threaded section of the land.

12.  Hook. It is the curved undercut of the cutting face of the land.

13.  Heel. It is the back part of the threaded section of the land.

14. Chamfer. The tapered outside diameter at the front end of the threaded section.

15.  Point diameter. It is the outside diameter at the front end of the chamfered portion.

16. Flute. It is the groove providing for the cutting facts of the teeth, chip passage and cutting fluid.

17.  Helix. It is the curve of an ordinary screw thread.

18. Helix angle. It is the angle made by the helix of the thread at the pitch diameter with a plane perpendicular to the axis.

19.  Shank. It is the part of the tap behind the threaded and fluted section of the tap. The tap held or located and driven by the shank.

20.  Square. It is the squared end of the tap.

21. Radial rake angle. It is the angle formed in a diametric plane between the face and a radial line from the cutting edge at the crest of the thread from.

22.  Chamfer angle. It is the angle formed by the tapered outside diameter at the front end with the top axis.

23.  Web. The central portion of the tap situated between the roots of the flutes and extending along the fluted section of the tap. Its thickness increases from the front and towards the shank end of the flutes.

24. Back taper. The reduction in diameter of the tap body of the threaded portion from the front end towards the shank end.

25.  External Centre. It is the cone-shaped end of the tap. It is provided only for manufacturing purposes and only for small taps and usually at the thread end.  

26.  Internal Centre. A small drilled and countersunk hole at the end of the tap, necessary for manufacturing purposes.

27.  Thread Relief. It is the radial clearance providing a gradual decline in the major, pitch, and minor diameters of the lands, back of the cutting face.

2.5.5.5 DESIGN FEATURES OF A TAP

A tap is essentially a screw that has been fluted to form cutting edges. The cutting end of the tap has a relieved chamfer, which forms the cutting edges and permits it to enter the untapped hole. The design features are illustrated below:

1.      Chamfer Diameter. The chamfer diameter, of the chamfer at the front end of the tap is made smaller than the minor diameter  of the thread as given below:

   d= Minor diameter of thread – 0.10 to 0.15 mm for diameter upto 18 mm

     = Minor diameter of thread – 0.20 to 0.25 mm for diameter from 20 to 39 mm

    = Minor diameter of thread – 0.30 to 0.35 mm for diameter from 42 to 52 mm.

   

2.    Chamfer Length and Chamfer Angle. It shows the material removal in tapping threads. The cross-hatched area represents the part of the thread groove removed in the first revolution of the tap.

Fig. 2.16 Tap Chamfer Element

The uncut chip thickness (measured perpendicular to the tap axis for simplicity) removed by each land is,

                            t = h/zf

But

                            f =  L_ch/p;              L_ch= chamfer length

  

                            t = ph/z.L_ch = p.tanф/z

It is clear that,

                        L_ch = h/tanф  or  = h/kz

where

                        h  = depth of thread

                        ф = angle of chamfer of the tap

                        p = pitch of the thread being tapped.

and                  k =t/p is a characteristic of the construction of a tap.

Its values are:       k = 0.012 to 0.02 for nut taps

                                = 0.03 to 0.04 for die taps

                                = 0.06 to 0.10 for hand and machine taps. 

The chamfer angle is given as,

                              tan ф= (d_o-d_b)/2 L_ch         therefore  L_ch = (d_o-d_b ).cotф/2

where

                              d_o= major diameter of the tap thread

                              d_b = diameter of blank hole for tapping.

3. Flutes: Most taps have straight flutes, but special taps have helical flutes. Changing the hand of the helical flutes on the tap can change the direction of the chip flow. Left-handed flutes will drive the chips forward, ahead of the tap. Left-handed flutes will drive the chips forward, ahead of the tap, and, so, are used for tapping through holes. For tapping blind holes, right-handed flutes are used for which the chip flow will be towards the shank. With straight fluted taps, the chips can be made to flow forward, ahead of the tap, by grinding a spired point on the cutting face of each land at the chamfered end.

The number of flutes may vary from 2 to 8, the higher number is used for larger diameter taps.

Larger the number of flutes, better will be the quality of the tapped thread. However, the cut chips will be thinner, the specific cutting force and the torque will be higher.

 Table: 2.1 Number of Flutes

Type of Tap	Number of flutes                              Major diameter, mm 2 to 6        8 to 14        16 to 24        27 to 36          39 to 52
Hand, nut and machinetaps : For metric and inch threads For pipe threads Master taps	3                   3               3 or 4                4                 4 to 6  -                 3 or 4               6                   6                    6 3                     4                   6                  6                 6 to 8

Tap geometry :

i) Rake angle. of  the sizing and chamfer part is given below , depending upon the type of material to be tapped.

                         α  = 15^o,for steel with σ_t <600 MPa.

                           = 10^o , for steel with σ_t 600 to 900 MPa

                           = 5^o ,for steel with σ_t >900 MPa

                           = 5^o for Grey C.I.

                           = 0^o for  Bronze

                           = 20^o to 30^ofor Aluminium and its alloys.

ii) Relief angle. Relief is provided only on the chamfer length. It is obtained by relieving the thread only on the crests along the length of the chamfer. Its recommended values are :

                          γ        = 8^o to 10^o for machine taps

                                     = 6^o to 8^o for hand taps

                                    = 8^o to 12^o for nut and machine taps

                                    = 3^o to 4^o for die calibrating taps

                                     = 4^o to 8^o for taps for light alloys.

The relieving over the chamfer length will be given as,

   

                       K =  пd_o.tanγ  /z

There is usually no relief on the sizing section and at the flank of the thread. Relieving reduces the friction between the tap and the surface of the hole.

iii) Back-Taper : Axial back taper is provided on the tap from the front end towards the shank end to to avoid rubbing of the tap with the surface of the hole so as to reduce friction. It is taken as :

                      = 0.05 mm to 0.10mm/100 mm for ground taps

                     = 0.08 mm to 0.12 mm/100 mm for unground taps in which threads                           are formed by rolling.

                       = 0.20 mm for tapping especially tough , high strength materials,                               such as heat resistant and stainless steels and alloys and tough                                 row-carbon steels etc.

iv) Chamfer Angle. The leading edges of a tap are chamfered to help in starting the tap. Smaller the chamfer angle, longer will be the chamfer length. This will result in thinner uncut chips, resulting in increase in cutting force, eventhrough longer chamfer length provides better guiding to the tap and the quality of the thread improves. 

Table: 2.2 Chamfer Angles

Taps in a set	Type of Tap	ф ,degrees
1.                 2.                 3.	Nut           Taper          Rougher          Bottoming          Finisher           Taper          Rougher          Second         Intermediate           Bottom           Finisher	2                    7                    7                   20                   20                                      5                    5                   10                   10                   20                   20

Cutting speeds. The cutting speeds for machine taps are :

Table: 2.3 Cutting Speeds

Work material	Lubricant	Tapping speed             m/min
Aluminium Bakelite Brass Cast Iron Steel :       Mild       Medium alloy       Stainless Zinc die-cast	Kerosene and hard oil Air blast Soluble or light base oil Dry or Soluble oil Soluble or sulphur based oil Sulphur-base oil Sulphur-base oil Soluble oil	30                24                42                 24                18                12                 6                24

Materials. Taps are usually made of carbon tool steel or H.S.S.

2.5.6 THREAD MILLING:

In thread milling, the threads are cut by a revolving from milling cutter conforming to the shape of the thread to be produced. Both external and internal threads can be cut by this method. Thread milling has got the following characteristic:

1)      This is a fast thread cutting method for producing threads usually of too large a diameter for die heads.

2)      The threads produced are more accurate than those cut by dies, but less accurate than produced by grinding.

3)      Threads running upto a shoulder on the workpiece can be cut without any difficulty.

4)      Worms and lead screws, which are too large to be cut with a single point tool, can be milled.

5)      This method is desirable, when the pitch of the thread is too coarse to be cut with a die.

6)      The method is more efficient than cutting thread on a lathe; especially when the job is long or when large amounts of metal are to be removed.

For thread milling either single or multiple cutters may be used. A single-form cutter has a single, annular row of teeth, lying in one plane. While thread cutting with a single cutter, it is tilted through an angle equal to the helix angle of the thread to avoid interference while cutting. To start milling the threads, the cutter is fed radially inward equal to the depth of the thread , while the job is stationary , being held between centers of the machine. The job is then rotated slowly and the cutter, while rotating, is also traversed longitudinally parallel to the axis of the job, or  vice versa , by means of a lead screw. This operation is stopped when the thread is completed. This method of thread milling is used for cutting coarse (large-pitch or multiple-pitch) threads. The threading can be completed in a single cut or roughing and finishing cuts may be used.

The method of cutting threads with single-thread or single-rib milling cutters is chiefly employed to cut long threads (chiefly of square and trapezoidal profiles) on various lead screws and worms. Usually, the threads are cut rough by milling and then chasing with a single-point tool or a formed grinding wheel finishes these.

Fig. 2.17 Thread milling with Multiple Thread Cutter; a) External Threads; b) Taper

Multiple cutter is used when the thread to be cut is not too long and it is desired to cut the threads in one revolution of the work. The width of the cutter has to be slightly more than the length of the thread. The cutter is set parallel to the axis of the job and is fed radially inward equal to the depth of the thread while the job is stationary. The job is equal to depth of the thread while the job is stationary. The job is then rotated slowly, with the cutter moving axially a distance equal to the lead of the threads plus a small over travel to complete the thread in one pass.

2.5.7 THREAD GRINDING

Thread grinding is used to produce very accurate threads. It is also employed to cut threads on hardened materials for which the other methods of thread cutting are not possible. The method is also useful for materials too soft to get a good surface finish by other methods. Thread grinding is used to cut threads on: taps, micrometer screws, lead screws, thread gauges and milling cutters.

The principle of thread grinding is similar is principle to thread milling. The grinding wheels can be single ribbed or multi-ribbed, which are shaped (conforming to the thread profile) by special diamond dressers. In the case of single-ribbed wheel, the wheel turns against rotation of the job. In addition to this rotary motion, a relative axial motion between the wheel and the job is provided with the help of a precision lead screw. The wheel is tilted an angle equal to the helix angle of the thread, to the axis of the job. This method is known as ‘Traverse Thread Grinding, and is used to produce long and coarse pitch threads. Also, the pressure on the work and hence the heat generated during grinding is not excessive, resulting in a more accurate thread.

Fig. 2.18 Thread Grinding

A multi-ribbed wheel, which is slightly longer (one or two threads) than the work, is used to cut the entire threads in one revolution of the work. The wheel is fed into the work to the required depth and moves axially a distance equal to the pitch of the thread while the work revolves through one revolution. The cutter is set parallel to the axis of the job. This method is known as ‘plunge cut grinding’. This method is employed when production is more important than accuracy. The principle of these two methods is shown in Fig. A thread grinding machine is similar to centre type cylindrical grinding machine with an arrangement for precise movement of the machine table and provision for tilting the grinding wheel at the helix angle of the thread.

2.6 THREAD MEASUREMENT AND INSPECTION

 The elements to be checked for a thread are: major diameter, pitch diameter, pitch and helix angle.

2.6.1 Major Diameter. The major diameter of the screw or the minor diameter of a nut can be checked by a plain snap and plug gauges respectively. They can also be measured with micrometer and vernier calipers. To measure the major diameter of a screw with a micrometer, the anvils should be of sufficient diameter so as to span two threads. To eliminate the effect of errors between the micrometer screw and the anvil faces, it is always better to first check the instrument on a cylindrical standard of about the same diameter as the screw.

2.6.2 Minor Diameter. Minor diameter of a screw can be measured with a screw thread micrometer Caliper. This instrument is similar to the ordinary micrometer, but instead of usual flat measuring faces, it has specially designed anvil and spindle inserts.

Fig. 2.19 Screw Thread Micrometer

Thread pitch. To check the minor diameter of a screw, two V-shaped inserts are used, so that their sharp apexes contact the roots of the screw thread. 

To check the pitch diameter, inserts of a type that contact the sides of the screw thread near the pitch diameter are employed. For this, a truncated thread form is used on the inserts.

2.6.3 Pitch Diameter. One of the most accurate methods for checking the pitch diameter is the ‘three-wire method’. The method consists in placing three shall diameter cylinders  (three wires of equal and precise diameter) in the thread grooves at opposite sides of a screw and measuring the distance W over the outer surfaces of the wires with an ordinary micrometer caliper having flat measuring faces. Three wires are required to prevent misalignment of the measuring faces on the micrometer caliper. The pitch or effective diameter is calculated from the value W in the following manner:

Fig. 2.20

It is clear that, W=P+2*d/2

Where                                              P=pitch or effective diameter

And                                                 d= wire size

Now                                                           AC=AD-CD=d/2cosec a/2- P/4cot a/2

Where                                             a=thread angle

And                                                 p=pitch of threads`

                      

After simplification, it can be seen that,

                                                               

W=p+d (1+coseca/2)-p/4cota/2

In case of  I.S.O. metric threads, a=60^o

W=p+3d-0.866p

P=w-3d+0.866p

Here, the pitch diameter lies 0.3248p inside the crest of the thread that is,

                                             P=D-0.6496p

D=Outside diameter

        D=w-3d+1.5156p

                                          

Thus, if the wire diameter d, the thread pitch p and w are known, the pitch diameter of the screw may easily be computed for the above relations.

2.6.4 Wire Size- Wire of any diameter can be used to measure the pitch diameter, provided it makes contact on the true flank of the thread and provided the thread angle is correct. A wire of best size is the one that makes contact with the flanks of the tread at the pitch diameter. The effected diameter calculated with the help of any wire touching the true flanks of the thread will differ from the obtained by using wire of best size if there is any error in the angle or form of the thread. In the case of best size wire, the point B Fig (b) at which the wire touches the flank of the thread lies on the pitch line, that is, BC lies on the pitch line and that AB is perpendicular to the flank position of the thread. If there is a possibility of the thread angle being incorrect, the wire of best size should be used to determine effective diameter, since such wire will be independent of any error in the thread angle.

2.6.5 Pitch. The pitch of the thread is usually measured with “Screw pitch gauge”. Screw pitch gauges, fig are sets of flat steel blades which are notched on one edge according to various thread pitches represented by the gauge.  The blades are pivoted at the end of a holder. To use it, the blade with the required thread pitch is applied to the thread being checked at the radial plane. If the pitch is correct, the gauge will fit tightly at the thread profile and no light will pass between the gauge and the thread profile.

To estimate the values of the screw pitch error, screw pitch comparators are available. A comparator comprises a frame with two or three rods ending in ball shaped contacts. The rods are linked to a measuring tool, for example, a dial indicator and the ball shaped contacts are inserted into the thread grooves to be checked. If the comparator has three contacts it will align along the thread axis. A two contact comparator checks the thread pitch in a direction perpendicular to the helix angle. The scale of the dial indicator will indicate the accumulated pitch error by the number of pitches. This instrument must be set up with gauge blocks to the nominal size of the length of the measurement. Pitch measuring machines are also available to inspect a screw for pitch. The machine consists of a bed with centers at each end (just like a centre lathe) for supporting the screw. Alternate means are also available for holding nuts and sleeves. A head carrying a stylus shaped to fit in the vee of the thread is moved along the bed with the help of an accurate micrometer. The head is provided with an indicator, which shows when the stylus is in its lowest position in the groove that is, bedded home centrally in the groove of the thread. When the head is moved along the bed, the stylus seats successively in each of the threads over the length being examined. The pitch is determined by analyzing the micrometer ending.

2.6.6 Angle of thread. The screw pitch gauge can also give an indication about the correctness of the thread angle. If the angle is incorrect, light will be seen between the gauge and the thread of the profile. The thread angle of the screws is measured on an optical instrument, the ‘toolmakers microscope’. Checking of an internal thread is very difficult since molded copies of the thread profile must be made.

Optical projection methods are very convenient for inspecting the form and angle of a thread. The screw is held between centers provided in the apparatus and tilted to the helix angle so as to get a clear profile of the thread. When a beam of light is thrown on the thread, the magnified image of the thread is projected on to a screen or onto some part of the apparatus and compared with a master template.

The angle of thread can also be measured from the projected image with the help of a shadow protractor provided with the apparatus. The blade of the projector is set to each side of the thread and the angle with the vertical is measured to get the total angle of the thread.