YILDIZ TECHNICAL UNIVERSITY
FACULTY OF MECHANICAL ENGINEERING
DEPARTMENT OF MECHANICAL ENGINEERING
MAK4462 MACHINE TOOLS
Alperen Koyuncu
16065213
Group:1
LECTURER: Orhan Çakır
CONTENTS
1. Motions In Machine Tool
2. Driving Mechanism
2.1. Stepped Mechanism
a. Belt Type
b. Gear Type
2.2. Stepless Mechanism
a. Mechanical Drive
b. Hydraulic Drive
c. Electrical Drive
REFERENCES
1Assoc.Prof. Orhan ÇAKIR,Assist.Prof. Mihrigül Ekşi ALTAN .Lectures Notes of Machine
Tools
2. PH Joshi - Machine Tools Handbook-McGraw-Hill Osborne Media
MOTIONS IN A MACHINE TOOL
To obtain a machined part by a machine tool, coordinated motions must be imparted to its
working members. These motions are either working (cutting and feed) motions, which
removes the chips from theworkpiece or auxiliary motions that are required to prepare for
machining and ensure the successive machining of several surfaces of one workpiece or a
similar surface of different workpieces.
Driving Units in Machine Tools
There are two main types of motions that are effective;
1-Rotational motion
2-Lineer (forward-backward motions)
The extreme spindle speeds of a machine tool main gearbox nmax and nmin can be
determined by;
In machine tools, there are two types of primary motion: rotary and linear. In some machine
tools, this motion is a combination of rotating and reciprocating motions. The most effective
factor in rotary movements is the revolution speed (rpm).
In a machine tool, the revolution speeds (rpm) need to provide the optimum cutting
speedaccording to the characteristics of the workpiece. Linear motions are mostly used for
the feed movement. And feed movement may be continuous (lathes, milling machine, drilling
machine) or intermittent (shapers, planers). The speed adjustment in the linearly moving
machine parts is possible by adjusting the revolution speeds of the rotary motion that drive
these parts
ROTARY DRIVES
For rotary cutting, it is necessary to compute revolutions per minute (R.P.M.) of the machine
spindle, using the following equation:
V = π DN/100
V = Linear (cutting) speed (m/min);
D = Workpiece diameter (mm);
N = Revolutions per minute (R.P.M.) of machine spindle
Cutting Speed Variation
A few rotary the speeds (N ) suffice for special purpose machines that are used for a few
operations, on workpieces of the same size and material. But general purpose machines, used
for a range of workpiece sizes and materials, call for a much wider variation in rotary speeds
(N ). The variation can be of two kinds: stepped or stepless. Stepped variation provides a
limited number of fixed rotary speeds. Stepless variation can provide infinite number of
speeds, within the maximum and minimum values. Stepless variation is necessary when there
is a wide variation in the hardness of the workpiece material. Infinitely variable, stepless
drives are generally unsuitable for low-speed, high-torque applications
1.STEPPED DRIVING MECHANISM
As stepped driving mechanisms;
• Belt type mechanisms
• Gear type mechanisms
Stepped driving mechanisms are either two-axis main mechanisms or their combinations.
1.1 BELT TYPE MECHANISM
A belt type mechanism consist of a belt and pulley. It ispossible to change the location of the
belt according to the selected revolution speed.
Belting: The belting system 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:
1.2. GEAR TYPE MECHANISM
A wide variety of gearboxes utilize sliding gears or frictionor 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. 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 iscapable of
transmitting higher torque and is small in radial dimensions. Among the disadvantages of
these gearboxesis 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.
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 gearboxes as shown in
FIG
2.STEPLESS DRIVING MECHANISM
Stepless 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 stepless 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 Stepless Drives: Infinitely variable speed (stepless) 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 stepless drives are 4 types:
•Friction Stepless Drive
•Kopp Variator
•Toroidal and Reeves Mechanisms
•Positive Infinitely Variable Drive
Friction Stepless Drive: The disk-type friction stepless 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 D.
Because n1d = n2D
Hence n2=n1*d/D.
The diameter ratio d/D can be varied in infinitely small steps by the axial displacement of the
friction roller. If the friction force between the friction roller and the disk is F,
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.
Kopp Variator: The drive balls (4) mounted on inclinable axes (3) run in contact with
identical, effective radii r1 = r2, and drive cones (1 and 2) are fi xed on coaxial input and
output shafts. When the axes of the drive balls (3) are parallel to the drive shaft axes, the input
and output speeds are the same. When they are tilted, r1 and r2 change, which leads to the
increase or decrease of the speed. Using Kopp mechanism, a speed range of 9:1, effi ciency of
higher than 80% and 0.25–12 hp capacity are obtainable.
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
Electrical Stepless Speed Drive
Figureshows 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.
Hydraulic Stepless 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 deliversthe 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 pumpeccentricity 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
