UNIT-1
MACHINE
TOOL DRIVES
• Broadly Classification of transmission of rotary motion
• Stepped Speed Drives in Machine Tools & Belting, Pick-Off
Gears , Gear boxes
• AP &GP for steeping speeds of gears
• Structural formula & structural diagrams
• Feed gear boxes
• Steeples Speed Drives in Machine Tools.
MACHINE
TOOL DRIVES
To obtain a machined part by a machine tool, coordinated motions
must be imparted to its working members. These motions are either primary
(cutting and feed) movements, which removes the chips from the WP or auxiliary
motions that are required to prepare for machining and ensure the successive machining of
several surfaces of one WP or a similar surface of different WPs. Principal
motions may be either rotating or straight reciprocating. In some machine
tools, this motion is a combination of rotating and reciprocating motions. Feed
movement may be continuous (lathes, milling machine, drilling machine) or
intermittent (shapers, planers). As shown instepped motions are obtained using
belting or gearing. Stepless speeds are achieved by mechanical, hydraulic, and
electrical methods.
Classification
of transmission of rotary motion.
STEPPED
SPEED DRIVES:
Belting:
The belting system, shown in Figure 2. is used to produce four
running rotational speeds n1, n2, n3,and n4. It is cheap and absorbs
vibrations. It has the limitation of the low-speed
changing, slip, and the need for more space. Based on the driver
speed n1, the following speeds can be obtained in a decreasing order:
Belting transmission.This type is commonly used for grinding and bench-type drilling
machines.
Pick-Off Gears
Pick-off gears are used for machine tools of mass and batch
production (automatic and semiautomatic machines, special-purpose machines, and
so on) when the changeover from job to job is comparatively rare. Pick-off
gears may be used in speed or feed gearboxes. As the change of speed is
achieved by setting gears A and B on the adjacent shafts. As the center distance is constant,
correct gear meshing occurs if the sum of teeth of gears A and B is constant.
Gearboxes
Machine tools are characterized by their large number of spindle
speeds and feeds to cope with the requirements of machining parts of different
materials and dimensions using different types of cutting tool materials and
geometries. The cutting speed is determined on the bases of the cutting ability
of the tool used, surface finish required, and economical considerations. A
wide variety of gearboxes utilize sliding gears or friction or jaw coupling.
The selection of a particular mechanism depends on the purpose of the machine
tool, the frequency of
Speed change, and the duration of the working movement. The
advantage of a sliding gear transmission is that it is capable of transmitting
higher torque and is small in radial dimensions.
Among the disadvantages of these gearboxes is the impossibility of
changing speeds during running. Clutch-type gearboxes require small axial
displacement needed for speed changing, less engagement force compared with
sliding gear mechanisms, and therefore can employ helical gears. The extreme
spindle speeds of a machine tool main gearbox n max and n min can be
Determined by
where Vmax = maximum
cutting speed (m/min) used for machining the most soft and machinable material
with a cutting tool of the best cutting property
Vmin = minimum cutting speed
(m/min) used for machining the hardest material using a cutting tool of the
lowest cutting property or the necessary speed for thread cutting dmax, dmin
= maximum and minimum diameters (mm) of WP to be machined
The speed range Rn becomes where
Rv = cutting speed range
Rd = diameter range
In case of machine tools having rectilinear main motion (planers
and shapers), the speed range Rn is dependent only on Rv. For other machine
tools, Rn is a function of Rv and Rd, large cutting speeds and diameter ranges
are required. Generally, when selecting a machine tool, the speed range Rn is
increased by 25% for future developments in the cutting tool materials.
Speed Range for Different Machine Tools
Machine Range
Numerically controlled lathes 250
Boring 100
Milling 50
Drilling 10
Surface grinding 4
Stepping of Speeds According to Arithmetic Progression:
Let n1, n2 , … , nz be arranged according to arithmetic
progression. Then
n1 – n2 = n3 – n2 = constant
The saw tooth diagram in such a case is shown in Figure 4.
Accordingly, for an economical
Cutting speed v0, the lowest speed vl is not constant; it
decreases with increasing diameter. Therefore, the arithmetic progression does
not permit economical machining at large diameter
Ranges The main disadvantage of such an arrangement is that the
percentage drop from step to step δn decreases as the speed increases. Thus the
speeds are not evenly distributed and more concentrated and closely stepped, in
the small diameter range than in the large one.
Stepping speeds according
to arithmetic progression are used in Norton gearboxes or gearboxes with a sliding
key when the number of shafts is only two.Speed stepping according to
arithmetic progression.
Stepping of Speeds According to Geometric Progression
As shown in Figure, the percentage drop from one step to the other
is constant, and the absolute loss of economically expedient cutting speed Δv
is constant all over the whole diameter range. The relative loss of cutting
speed Δvmax/v0 is also constant. Geometric progression, therefore, allows
machining to take place between limits v0 and vu independent of the WP
diameter, where v0 is the economical cutting speed and vu is the
allowable minimum cutting speed. Now suppose that n1, n2, n3, … ,nz are the
spindle speeds. According to the geometric progression,
Where φ is the progression ratio. The spindle speeds can be
expressed in terms of the minimal speed n1 and progression ratio φ.
Hence, the maximum spindle speed nz is given by
Where z is the number of spindle speeds, therefore, From which
Speed stepping according to geometric progression. ISO Standard
values of progression ratios φ (1.06, 1.12, 1.26, 1.4, 1.6, 1.78,2.0 )
Structural formulae & Structural Diagrams:
Suppose a speed on one shaft yields two speed values on the next
shaft. The noo of speed steps of the particular transmission group is p=2.if
the transmission is through gears, the transmission ratios that provide the two
new speed values must lie in the following range:
GUIDELINES FOR SELECTING BEST STRUCTURAL FORMULA:
1. Transmission ratio i max=2, i min=1/4, ig= i max/ i min=8.
2. Minimum total shaft size:
The torque transmitted by a shaft is given by
T α 1/N;
From the strength consideration : (d1/d2)= (N2/N1)1/3
3. For least radial dimensions of gear box i max* i min=1.
4. No of gears on last shaft should be minimum.
5. No of gears on any shaft should be limited to three.
Feed Gearboxes:
Feed gearboxes are designed to provide the feed rates required for
the machining
operation. The values of feed rates are determined by the specifi
ed surface fi nish, tool life, and the rate of material removal.
The classification of feed gearboxes according to the type of
mechanism used to change the rate of feed is as follows:
1. Feed gearboxes with pick-off gears. Used in
batch-production machine tools with infrequent Change over from job to job,
such as automatic, semiautomatic, single-purpose, and special purpose machine
tools. These gearboxes are simple in design and are similar to those used for speed
changing
2. Feed gearboxes with sliding gears. These
gearboxes are widely used in general-purpose machine tools, transmit high
torques, and operate at high speeds. Figure shows a typical gearbox
That provides four
different ratios. Accordingly, gears Z2, Z4, Z6, and Z8 are keyed to the drive shaft
and mesh, respectively, with gears Z1, Z3, Z5, and Z7, which are mounted freely
on the driven key shaft. The sliding key engages any gear on the driven shaft.
The engaged gear transmits the motion to the driven shaft while the rest of the
gears remain idle.
The main drawbacks of such feed boxes are the power loss and wear
occurring due to the rotation of idle gears and insufficient rigidity of the
sliding key shaft. Feed boxes with sliding gears are used in small- and
medium-size drilling machines and turret lathes.
3. Norton gearboxes. These gearboxes provide an
arithmetic series of feed steps that is suitable for cutting threads and so are
widely used in engine lathe feed .
STEPLESS SPEED DRIVES:
Step less speed drives may be mechanical, hydraulic, or electric.
The selection of the suitable drive depends on the purpose of the machine tool,
power requirements, speed range ratio, mechanical characteristics of the
machining operation, and cost of the variable speed unit.
In most step-less drives, the torque transmission is not positive.
Their operation involves friction and slip losses. However, they are more
compact, less expensive, and quieter in operation than the stepped speed
control elements
Mechanical Step-less Drives:
Infinitely variable speed (step-less) drives provide output speeds,
forming infinitely
variable ratios to the input ones. Such units are used for main as
well as feed drives to provide the most suitable speed or feed for each job,
thereby reducing the machining time. They also enable machining to be achieved
at a constant cutting speed, which leads to an increased tool life and ensures
uniform surface finish.
Mechanical step less drives are 4 types:
• Friction Step less Drive
• Kopp Variator
• Toroidal and Reeves Mechanisms
• Positive Infinitely Variable Drive
Friction Step-less Drive
The disk-type friction step-less mechanism. Accordingly, the drive
shaft rotates at
a constant speed n1 as well as the friction roller of diameter d.
The output speed of the driven shaft rotates at a variable speed n2 that
depends on the instantaneous diameter.
The diameter ratio d/D can be varied in infi nitely small steps by
the axial displacement of the friction roller. If the friction force between
the friction roller and the disk is F,
F = input torque (T1) = output torque (T2)
input radius (d/2) output radius (D/2)
If the power, contact pressure, transmission force, and effi
ciency are constant, the output torque T2 is inversely proportional to the
speed of the output shaft n2.
T2 α T1n1 /n 2
Due to the small contact area, a certain amount of slip occurs,
which makes this arrangement suitable for transmitting small torques and is
limited to reduction ratios not more than 1:4.
Toroidal and Reeves Mechanisms
the principle of toroidal stepless speed transmission. Figure
shows the Reeves variable speed
transmission, which consists of a pair of pulleys connected by a
V-shaped belt; each pulley is made up of two conical disks. These disks slide
equally and simultaneously along the shaft and rotate with it. To adjust the
diameter of the pulley, the two disks on the shaft are made to approach each
other so that the diameter is increased or decreased. The ratio of the driving
diameter to the driven one can be easily changed and, therefore,
any desired speed can be obtained without stopping the machine. Drives of this
type are available with up to 8:1 speed range and 10 hp capacity.
Positive Infinitely Variable Drive
Positive torque transmission arrangement that consists of two
chain wheels, each of which consists of a pair of cones that are movable along the
shafts in the axial direction. The teeth of the chain wheels are connected by a
special chain. By rotating the screw, the levers get moved thus changing the
location of the chain pulleys, and hence the speed of rotation provides a speed
ratio of up to 6 and is available with power rating up to 50 hp.
The use of infinite variable speed units in machine tool drives
and feed units is limited by their higher cost and lower efficiency or speed
range. Positive Infinitely Variable Drive
Electrical Stepless Speed Drive
The Leonard set, which consists of an induction motor that drives
the direct current generator and an exciter (E). The dc generator provides
the armature current for the dc motor, and the exciter provides the field
current; both are necessary for the dc motors that drive the machine tool .Leonard
set (electrical step-less speed drive).
The speed control of the dc motor takes place by adjusting both
the armature and the field voltages by means of the variable resistances A and
F, respectively. By varying the resistance A, the terminal voltage of the dc generator and hence the rotor
voltage of the dc motor can be adjusted between zero and a maximum value. The
Leonard set has a limited efficiency: it is large, expensive, and noisy.
Nowadays, dc motors and thyrestors that permit direct supply to the dc motors
from alternating current (ac) mains are available and, therefore, the Leonard
set can be completely eliminated. Thyrestor feed drives can be regulated such
that the system offers infinitely variable speed control.
Hydraulic Step-less Speed Drive
The speeds of machine tools can be hydraulically regulated by
controlling the oil
discharge circulated in a hydraulic system consisting of a pump
and hydraulic motor, both of the vane type, as shown in fig This is achieved by
changing either the eccentricity of the pump ep or the eccentricity of
the hydraulic motor em or both. The vane pump running approximately at a
constant speed delivers the pressurized oil to the vane type hydraulic motor,
which is coupled to the machine tool spindle.
To change the direction of rotation of the hydraulic motor, the
reversal of the pump
eccentricity is preferred. Speed control in hydraulic circuits can
be accomplished by throttling the quantity of fluid flowing into or out of the
hydro cylinders or hydro motor. The advantages of the hydraulic systems are as
follows:
1. Has a wide range of speed variation
2. Changes in the magnitude and direction of speed can be easily
performed
3. Provides smooth and quiet operation
4. Ensures self-lubrication
5. Has automatic protection against overloads
Hydraulic step-less speed drive.
The major drawback to a hydraulic system is that the operation of
the hydraulic drive
becomes unstable at low speeds. Additionally, the oil viscosity
varies with temperature and may cause fluctuations in feed and speed rates.
GROUP AND INDIVIDUAL DRIVE IN MACHINE TOOLS
1. Where there are
compact groups of constant-speed machines which are to be run continuously or
simultaneously.
2. Where there are
compact groups of machines which, because of their diversity of load, may be
driven by a single motor of much smaller rating than the combined capacities of
the motors required for individual drives.
3. Where groups of
constant-speed machines with heavy peak-load demands might require individual
motors of a size much in excess of the average running load.
4. Where the motors
required for individual drive are small.
5. Where, in changing
over an existing installation, the old system of line-shafting may be used as
already installed.
The initial cost will
usually be less with individual drive under the following conditions:
1. Where the machines
are isolated and line-shafting is impracticable.
2. Where the roof
construction will not safely support line-shafting and the floor structure does
not admit of hanging the line-shafting beneath.
3. Where the speed of
the machines must be independently variable.
4. Where it is
necessary to move the machines from one location to another frequently.
In addition to the
foregoing considerations it should be remembered that group drive with open
belting presents some personal hazard. Furthermore, where good appearance and
working conditions are of great importance, the individual drive is preferable
because of the absence of belts and line-shafting.
