machine tool design lab manual

Department of Mechanical Engineering,RTU

EXPERIMENT NO.1

Object: - Study of functional requirements of machine tools.

Introduction: - Any machine should satisfy the following requirements.

1. High productivity

2. Ability to provide the required accuracy of shape and size and also necessary surface

finish

3. Simplicity of design

4. Safety and convenience of control

5. Good appearance

6. Low cost of manufacturing and operation

1. Productivity: - Productivity of a metal cutting machine tool is given by the expression

Q= (1/tc+tn0).n

tc = machine time

tn0 = non-productivity time that include job handling time.

a. Cutting down machining time: - This is possible if high cutting speeds and feed rates

are available on the machine tool in accordance with the latest development in cutting

tool material and design.

b. Machining with more than one tool simultaneously: - This principle employed in

multiple-spindle lathes, drilling machine etc.

c. Improving the reliability of the machine tool to avoid break down and adopt proper

maintenance policy to prevent unscheduled stoppages and delays.

2. Accuracy: - The accuracy of a machine tool depends upon its geometrical and kinematic

accuracy and its ability to retain this accuracy during operation. Accordingly the ability of a

machine tool to consistency machine parts with a specified accuracy with in permissible

tolerance limits can be improved by the following method.

a. Improving the geometrical accuracy of the machine tool: This is mainly determined by

the accuracy of guideways, power screw etc.

b. Improving the kinematic accuracy of the machine tool: This is determined the

relationship between velocities of two or more forming motion and it depends upon the length

of kinematic accuracy of machine tool can be improved.

c. Increasing the static and dynamic stiffness of machine tool structure. The greater in the static

stiffness of the machine tool structure the smaller will be its deformation due to cutting forces

and will be the accuracy of machining.

d. Providing accurate devices for measuring distance of travel.

e. Arranging the machine tools units in such a manner that the thermal deformation during the

machining operation result in the least possible change in the relative position between the tool

and the workpiece.

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Department of Mechanical Engineering, RTU

3. Simplicity of design: - Simplicity of design of machine tool determines the ease of its

manufacture and operation. The design of machine tool can be simplified by using standard

parts and sub assembly as far as possible. The complexity of design of a machine tool depends

to a large extend upon the degree of its university. Thus a general purpose machine tool is a

rule more complex than a special purpose machine tool design doing similar operation.

4. Safety and convenience of control: - A machine tool cannot be deemed fit for use

unless it machine tools the requirement of safety and convenience of operation.

a. Shielding the rotating and moving parts of the machine tool with hoods.

b. Protecting the worker from chips, abrasive dust and coolant by means of screws shield etc.

c. Providing reliable clamping for the tool and workpiece.

d. Providing reliable earthing of the machine, providing device for safe handling of heavy

workpiece.

5. Appearance:- Good appearance of the machine tool influence the mood of the worker

favourably and thus facilities better operations it is generally conceded that a machine tool that

is simple in design and safe in operation and also good in appearance although factors, such as

external finish colour.

Nowadays, painting of machines in different colours according to the production purpose is

becoming popular.

6. Cost of manufacturing and operation: - The cost of manufacturing a machine tool

is determined by the complexity of its design. Therefore factors that help in simplifying the

machine tool design also contribute towards lowering its manufacturing cost.

The cost can also be brought down by reducing the amount of metal required in manufacturing

the machine tool. This is achieved by using stronger materials and more precise design

calculation pertaining to the strength and rigidly of parts to keep the safety margins as low as

possible.

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EXPERIMENT NO.2

Object: - Study of working and auxiliary motion of machine tool.

Introduction: - Obtaining the required shape on the workpiece, it is necessary that the

cutting edge of the cutting tool should move in a particular manner with respect to the

workpiece the relative movement between the workpiece and cutting edge can be obtained

either by the motion of the workpiece the cutting tool or by a combination of the motion of the

workpiece and cutting tool.

These motion which are essential are working to impart the required shape to the workpiece

are known as working motion. Working motions are further classified into two categories:

1. Drive motion or primary cutting motion

2. Feed motion

Working motion in machine tools generally of two types:

1. Rotary

2. Translatory

Fig: lathe fig: drilling

Fig: shaping fig: grinding

1 .For lathes and boring machines

Drive motion: Rotary motion of workpiece

Feed motion: Translatory motion of cutting tool in the axial or radial direction

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2 .For drilling machines

Drive motion: Rotary motion of workpiece

Feed motion: Translatory motion of drill

3 .For milling machines

Drive motion: Rotary motion of the cutter

Drive motion: Translatory motion of workpiece

4 .For shaping, planning and slotting machines

Drive motion: Reciprocating motion of cutting tool

Feed motion: Intermitted translatory motion of the workpiece

5 .For grinding machines

Drive motion: Rotary motion of grinding wheel

Feed motion: Rotary as well as translatory of the workpiece

Besides the working motion a machine tool also has provision for auxiliary motions. In

machine tool, the working motions are powered by sources of energy. The auxiliary motion

may be carried out manually or may also be power operated depending upon the degree of

automation of the machine tool. In general purpose machine tools, most of the auxiliary

motions are executed manually.

Parameters defining working motions of a machine tool

The working motions of the machine tool are numerically defined by their velocity, the velocity

of the primary cutting motion or drive motion is known as cutting speed while the velocity of

feed motion is known feed.

The cutting speed is denoted by ‘v’ and measured in the units m/min. Feed is denoted by‘s’

and measured in the following units.

1. mm/rev. in machine tool with rotary drive motion e.g. lathes, boring machine etc.

2. mm/tooth, in machine tool using multiple-tooth cutters e.g. milling machines.

3. mm/stroke, in machine tools with reciprocating drive motion e.g. shaping and planning

machine.

4. mm/min, in machine tools which have a separate power source for feed machines.

In machine tools with rotary primary cutting motion, the cutting speed is determined by the

relationship

v = 𝜋𝑑𝑛

1000 m/min

d= diameter of workpiece or cutter

n= revolution per minute (rpm) of the workpiece or cutter

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In machine tools with reciprocating primary cutting motion, the cutting speed is determined by

u = 𝐿

1000Tc m/min

L= length of stroke mm

Tc = time of cutting stroke min

If the time of the idle stroke in minutes is denoted by Ti, the number of strokes per minute can

be determined as

n = 1

Tc +Ti

Generally, the time of idle stroke Ti, is less than the time of cutting stroke, if the ratio Tc/Ti is

denoted by K, the expression for number of strokes per minute may be written as

n = 1

𝑇𝑐(1+𝑇𝑖

𝑇𝑐) =

𝐾

𝑇𝑐(1+𝐾)

Now combining equations the relationship between cutting speed and number of strokes per

minute may be written as follows

v = 𝑛𝐿(𝐾+1)

1000𝐾

The feed per revolution and feed per stroke are related to the feed per minute by the relationship

Sm = s.n

Where, Sm = feed per minute

s = feed per revolution

n = number of revolution

The feed per tooth in multiple tooth cutter is related to the feed per revolutions as follows:

S = Sz.z

Where, S = feed per revolution

Sz = feed per tooth of cutter

z = number of tooth on the cutter

The matching time of any operation can be determined from the following basic expression

Tm = 𝑙

Sm min

Where, Tm = matching time, min

l = length of machined surface, mm

Sm = feed per minute

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Experiment No.-3

Objective: Design criterion for machine tool structure, Static & dynamic stiffness.

Introduction:- Machine tool consists of machine tool structure, bed, column, housings. These are the base of machine tool on which the guideways, spindle, carriage, etc. are mounted. These elements must able to withstand at higher permissible load. These elements are discussed in detail in the following section. Objectives is to understand

Functions of machine tool structure and the design criteria for selection of material for

sideways,

The design of bed,

The design of column, and

The design of housing.

FUNCTIONS OF MACHINE TOOL STRUCTURE:

Machine tool structure consists of bed, base, columns, box type housings, overarms, carriages,

tables etc.

The structures are divided into three categories according to their functions :

Category 1 An element, upon which various subassemblies are mounted, falls under this

category. Example: bed and base.

Category 2 Elements consist of box type housings in which individual parts are assembled fall

under this category. Example: Speed box housing, spindle head, etc.

Category 3 Elements consist of parts that are used for supporting and moving the workpiece

and cutting tool fall under this category. Example: Table, carriage, knee, tailstock etc.

Machine tool structure must satisfy the following requirement :

(a) The initial geometrical accuracy of the structure should be maintained for the whole life of

the machine tool.

(b) All mating surfaces of the structure should be machined with a high degree of accuracy to

provide the desired geometrical accuracy.

(c) The shape and size of structure should not only provide safe operation and maintenance of

the machine tool but also ensure that working stresses and deformation should not exceed

specific limits.

(d) The selection of material and high static and dynamic stiffness are the fundamental

requirement to fulfill above-mentioned requirement.

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DESIGN CRITERIA FOR MACHINE TOOL STRUCTURE:

The simple machine tool bed with two-side wall is represented as a simply supported beam. Figure 3.1 depicts a simply supported beam. Point load F acts at its center. The maximum normal stress acting on the beam is given by

Figure 3.1: Simply Supported Beam

Where Bmax = Maximum bending moment = FL/4 ,

In = Moment of inertia of the beam section about the neutral axis=bh3/12

On substituting these values in Eq. σnmax changes to

The permissible normal stress under tension for the beam material is given by

Or minimum volume of material (Vmin) required to make sure that beam has sufficient strength

is given by

The maximum deflection of simply supported beam is given by the following expression :

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Where E is young modulus of beam material.

If the deflection of the beam dper is not to exceed a permissible value, then

Where Vmin = minimum volume of metal required to make sure that deflection of the beam

under load does not exceed the permissible value.

The condition of optimum design is given by

Hence Eq. Indicates that for every structure, there exists an optimum ratio l/h and the ratio l/h

depends upon:

a) Operation constraint i.e. dper.

b) The material of the structure i.e. E and σper.

Materials for Machine Tool Structure

The commonly used material for machine tool structures are cast iron and steel. Earlier cast

iron structures were widely used but due to advances in welding technology, welded steels are

widely used now days.

The selection of material for machine tool structure depends upon following factors:

Material properties

a) Cast iron has higher damping properties than steel. Welded steel also shows good

damping properties.

b) Cast iron has better sliding properties.

c) Steel has higher strength under static and dynamic loading.

d) The unit rigidity of steel under tensile, torsional and bending loads is higher than cast

iron.

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Manufacturing Problems

Welded structures of steel have much thinner wall thickness as compared to cast structure.

Walls of different thickness can be welded more easily than casting it. Machining allowances

for cast structures are generally greater than for weld steel structures. Machining allowance is

necessary in casting to remove defects such as inclusions, scales, etc. Welded structure can be

easily repaired as compared to cast structure.

Economy

The selection of material for structure will also depend upon its cost. The weight of steel is

lesser and but actual metal consumption is higher than that of cast iron. Hence in such cases

the cost increases. Holes are obtained with the help of core in the casting structure but holes

are made in welded steel structure by machining. These will not only increase the material cost

but also increases labour cost. Cost of patterns, welding fixtures, and cost of machining are

considered while selecting material for structure.

On considering above factors, the cast iron and steel may be used for following application:

a) Cast iron should be used for complex structure subjected to normal loading which are

to be produced in large number.

b) Steel should be used for simple and heavy loaded structures which are to be produced

in small number.

c) Combined welded steel and cast iron should be used where steel structure is

economically suitable. Example: Cast bearing housings that are welded into the feed

box.

Machine Dynamics: The machining and machine dynamics within the machine system should be well understood, optimized and controlled, because they have the following direct effects:

They may degrade machining accuracy and the machined surface texture and integrity.

They may lead to chatter and unstable cutting conditions.

They may cause accelerated tool wear and breakage.

They may result in accelerated machine tool wear and damage to the machine and part.

They may create unpleasant noises and sounds on the shopfloor because of the chatter

and vibrations.

Loop Stiffness within the Machine-tool-work piece System: Stiffness: stiffness normally can be defined as the capability of the structure to resist

deformation or to hold a position under the applied loads. Static stiffness in machine tools

refers to the performance of structures under the static or quasi-static loads. Static loads in

machine tools normally come from gravity and cutting force etc. apart from the static loads,

machine tools are subjected to constantly changing dynamic forces and the machine tool

structure will deform according to the amplitude and frequency of the dynamic excitation loads,

which is termed dynamic stiffness

Machine-tool-workpiece Loop Concept

From the machining point of view, the main function of a machine tool is to accurately and

repeatedly control the contact point between the cutting tool and the uncut material - the

‘machining interface’. Figure 3.2shows a typical machine tool-work piece loop. The machine-

tool-work piece loop is a sophisticated system which includes the cutting tool, the tool holder,

the slideways and stages used to move the tool and/or the workpiece, the spindle holding the

workpiece or the tool, the chuck/collet, and fixtures, etc. If the machine tool is being taken as

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a dynamic loop, the internal and external vibrations, and machining processes should be also

integrated into this loop as shown in Figure 3.3.

Stiffness can normally be defined as the capability of the structure to resist deformation or hold

position under the applied loads. Whilst the stiffness of individual components such as spindle

and slideway is important, it is the loop stiffness in the machine-tool system that determines

machining performance and dimensional and forming accuracy of the surface being machined,

i.e., the relative position between the workpiece and the cutting tool directly contributes to the

precision of a machine tool and correspondingly leads to the machining errors.

Fig. 3.2 A typical machine tool loop

Fig.3.3 The machine-tool-workpiece loop taking account of machining processes and

dynamic effects

Static Loop Stiffness

Static loop stiffness in machine tools refers to the performance of the whole machine-tool loop

under the static or quasi-static loads which normally come from gravity and cutting forces in

machine tools.

A simplified analogous approach to obtaining the static loop stiffness is to regard the machine

tool individual elements as a number of springs connected to each other in series or in parallel,

so that the static loop stiffness can be derived based on the stiffness of each individual element:

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Typically, a well-designed machine-tool-workpiece system may have a static loop stiffness of

around 50N/µm; a figure of 500 N/µm is well desired for heavy cutting machine tools in

particular. While a loop stiffness of about 10N/µm seems not rigid enough, it is quite common

in precision machines. Static loop stiffness can be predicted at the early design stage by

analytical or numerical methods and thus design optimization and improvement are essential;

also, a continuous process because of the increasing demands from the various applications.

Dynamic Loop Stiffness and Deformation

Apart from the static loads, machine tools are subjected to constantly changing dynamic forces

and the machine tool structure will deform according to the amplitude and frequency of the

dynamic excitation loads, which is termed dynamic stiffness. Dynamic stiffness of the system

can be measured using an excitation load with a frequency equal to the damped natural

frequency of the structure.

Following Equations provide a rough approximation of dynamic stiffness kdyn and deformation

xdyn:

Where F is the dynamic load applied to the machine tool, kstatic is the static stiffness of the

machine tool, and Q is the amplification factor which can be calculated from:

Where M and c is the mass and damping:

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Therefore,

In order to accurately predict and calculate dynamic loop stiffness or the behaviour of a whole

machine-tool system, a dynamic model including all elements in the machine-tool loop needs

to be developed. The finite element method has been widely used to establish the machine tool

dynamics model and provide the solution with reasonable accuracy.

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Experiment No: 4

Objective: - Function & important requirements of spindle unit.

Functions: The spindle unit of machine tool performs the following functions:.

1. Centering the workpiece, e.g., in lathes, turrets, boring machines, etc., or the cutting

tool as in drilling and milling machines, 2. Clamping the workpiece or cutting tool, as the case may be, such that the workpiece or

cutting tool is reliably held in position during the machining operations, and

3. Imparting rotary motion (in lathes) or rotary cum translatory motion (drilling machines)

to the cutting tool or workpiece.

The operational capabilities of a machine tool in terms of productivity as well as accuracy and

finish of machined parts largely depends upon the extent to which these functions are

qualitatively satisfied.

Important design requirements of spindle units are listed below:

The spindle should rotate with a high degree of accuracy. The accuracy of rotation is

determined by the radial and axial runout of the spindle nose, and these must not exceed

certain permissible values which are specified depending upon the required machining

accuracy. The rotational accuracy is influenced maximum by the stiffness and accuracy

of spindle bearings, particularly the one located at the front end.

The spindle unit must have a high static stiffness. The stiffness of the unit is made up

of the stiffness of the spindle unit proper and the spindle bearings. Machining accuracy

is influenced by bending axial as well as torsional stiffness.

The spindle unit must have high dynamic stiffness and damping. Poor dynamic

stability of the spindle unit adversely affects the dynamic behaviour of the machine

tool as a whole, resulting in poor surface finish and loss of productivity due to

restriction on the limiting unreformed width of cut.

The mating surfaces that are liable to wear restrict the life of the spindle unit. These

surfaces, such as journals, quills(in drilling machine) etc., must be hardened to improve

their wear resistance. The spindle bearing must also be selected or designed to retain

the initial accuracy during the service life of the machine tool.

The deformation of the spindle due to heat transmitted to it by the bearings, cutting

tool, workpiece, etc. should not be large as this has an advers effect on the machining

accuracy.

The spindle unit must have a fixture which provides quick and reliable cantering and

clamping of the cutting tool or workpiece. The centering is achieved by means of an

external or internal taper at the front end of the spindle.

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Table: Spindle Ends

DESIGN OF SPINDLE

Figure 4.1 shows schematic diagram of spindle. A spindle represents a shaft with

(a) length ‘a’ which is acted upon by driving force F2, and

(b) Cantilever of length ‘m’ acted upon by external force F1.

(c) The total deflection of spindle nose consists of deflection d1 of the spindle axis due to bending

forces F1 and F2 and deflection d2 of the spindle axis due to compliance of the spindle supports. When the spindle has tapered hole in which a center or cutting tool is mounted, the

total deflection of the center or cutting tool consists of deflections d1, d 2 and d3 of the center or cutting tool due to compliance of the tapered joint.

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d max

≤ d

per

F2 F1

a m

Figure 4.1: Principle of Working of Spindle

Deflection of Spindle Axis due to Bending To calculate the deflection of the spindle nose due to bending, one must establish a proper design

diagram. The following guidelines may be used in this regard.

(a) If the spindle is supported on a single anti-friction bearing at each end, it may be

represented as a simply supported beam, and

(b) If the spindle is supported in a sleeve bearing, the supported journal is analyzed as a beam on an elastic foundation; for the purpose of the design diagram the sleeve bearing

is replaced by a simple hinged support and a reactive moment Mr acting at the middle

of the sleeve bearing. The reactive moment is given as :

Mr = C . M Where M = bending moment at the support, and

C = constant = 0 for small loads and 0.3 to 0.35 for heavy load.

F2 F1

k b M

(a) Mr

F2 F1

(b) Mr

(c) d1

Figure 4.2 : Effect of Various Force on Spindle

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Figure 4.2 (a) shows schematic diagram of spindle. Figure 4.2 (b) depicts the design diagram of the

spindle and figure 4.2 (c) illustrates deflected axis of the spindle. Consider the spindle shown in Figure 4.2 (a). By replacing the rear ball bearing by a hinge and the

front sleeve bearing by a hinge and reactive moment Mr, the spindle can be reduced to the design

diagram as shown in Figure 4.2 (b). The deflection at the free end of the beam (spindle nose) can be determined by Macaulay’s method and is found out to be

Where E is Young’s modulus of the spindle material.

Ia is average moment of inertia of the spindle section. The

deflection of the beam is shown in Figure 4.2 (c).

Materials of Spindles

The blank for a machine tool spindle may be:

1. Rolled stock in the case of spindles having diameter < 150 mm, and

2. Casting in case of spindles having diameter >150 mm.

In machine tool spindle design the critical design parameter is not strength but stiffness.

If we compare the mechanical properties of various steels, we found that modulus of elasticity

is more or less equal, although the strength of alloy steels can be considerably greater than that

of mild steel. Since stiffness is primarily determined by the modulus of elasticity of the

material, it may be concluded that no particular benefits accrues from using costly alloyed

steels for making spindles.

Recommendations for selecting spindle materials:

1. For normal accuracy spindles, plain carban steels C45 and C59, hardened and tempered

to RC =30.

2. For above normal accuracy spindles- low alloy steel 40Cr1Mn60Si27Ni25 induction

hardened to Rc= 50-56.

3. For spindles of precision machine tools, particularly those with

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Experiment No. 5

Objective: Importance of machine tool compliance with respect to machine tool accuracy.

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Consider a uniform shaft being machined between centres on a lathe (Fig. 5.1). Let KA be the

stiffness of centre A and Kg that of centre B. Due to radial component Py of the cutting force,

centre A will be displaced by a distance

YA = PA /KA

And centre B by

YB= PB/KB

Fig. 5.1 Schematic diagram of a simple turning operation

Here PA and PB are the forces of reaction at ends A and B, respectively. They can be determined

from the following equations of static equilibrium:

1. Moment of Forces about Point B = 0, i.e.,

PAl= Py (l— x)

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Substituting the values of yA and yB from Eqs. And yields

If it is assumed that KA/KB = α, Eq.

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EXPERIMENT NO.6

Object – Study of different mechanism used for transforming rotary motion into translatory

motion. (Application and sketching of Slider-crank mechanism, Cam mechanism, Rack &

pinion mechanism, Nut & screw mechanism.)

1. Cam mechanism

2. Rack and pinion mechanism

3. Nut and screw mechanism

Theory – These elementary transmissions are employed in feed mechanism of most of the

machine tools and also in the drives of machine tools have a reciprocating primary cutting

motion.

The Important elementary transmissions that are used in machine tools for transforming rotary

motion into translatory motion are:

1. Slider crank mechanism –

Fig: Slider crank mechanism

The machine consists of a crank, connecting rod and slider. The forward and reverse stroke

each take place during a revolution of crank therefore the need speed of forward and reverse

speed in slider crank mechanism since metal removal occur during one stroke. It is desirable

from the point of view of productivity to have a higher speed of the other stroke. Due to this

property of slider crank mechanism is used only in an appreciable increase of productivity e.g.

in the driving of primary cutting motion of gear shaping machine the length of stroke may be

change by adjusting the crank radius and is equal to

L = 2R, where R is the crank radius

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2. Crank and Rocker mechanism –

Figure. Crank and Rocker mechanism

The crank and rocker mechanism consist of a rotating crank which makes the rocker arm

oscillate by means of a block sliding along the groove in the rocker arm the clockwise rotation.

The forward cutting stroke takes place during the clockwise rotation of the crank through angle

‘𝛼’ and the reverse stroke during rotation of the crank through angle ‘𝛽’ since “𝛼 > 𝛽” and the

crank rotation with uniform speed. The ideal stroke completes transfer than the cutting stroke.

The length of stroke can be varied by adjusting the crank radius with a decrease in crank radius.

The ratio of angle 𝛼 𝛽⁄ decrease and the speed of cutting and reverse stroke tend to become

equal preferred in machine tool with large stroke (up to 1000 mm) where it can be effectively

employed e.g. in drive of the primary cutting motion of shaping and slotting machine.

Length of stoke can be calculated,

L = 2(𝐿

𝑒) R mm

L = length of rocker

e = offset distance

R = radius of crank

3. CAM Mechanism –

Figure. Cam follower and link mechanism

The cam mechanism consists of a cam and a follower the cam mechanism provides the desired

translatory motion is a suitable profile is selected. The profile may be provided.

a. On the periphery of a disc-disc type mechanism.

b. On the face of a disc-face type cam mechanism.

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c. On a cylindrical surface-drum type cam mechanism.

The main advantage of cam mechanical is that the velocity of the operative element is

independent of the design of driving mechanism and is controlled by the cam profile.

In a disc type cam if the radius change from R1 to R2 along an spiral while the cam rotate

through angle 𝛼, the velocity of the follower can be determined from the expression.

v = 𝑅2−𝑅1

𝛼. 360.

𝑛

1000 m/min

n = rpm of the cam

R1,R2 = radius mm

In face or drum type cam mechanism the speed of the follower depends upon the steepness of

the grove consider for instance. The profile development of drum cam segment a deplict the

steep rise of follower corresponding to the rapid advanced segment deplict the slow rise

corresponding to the steep full corresponding to the rapid withdraw of cutting tool.

v = ℎ

𝑏 .

𝜋.𝐷.𝑛

1000 m/min

h = rise during the working stroke

b = length of the working stroke

D = diameter of the drum in mm

s = rpm of drum

4 .Nut and Screw transmission –

Figure Schematic diagram of anti-friction nut and screw transmission

A nut and screw mechanism is schematically depicted the screw and nut have a trapezoidal

thread. The direction movement can reverse by reversing rotation of the screw. The nut and

screw transmission is compact but has a high load carrying case capacity its other advantage

are simplicity case of manufacturing the possibility of achieving slow and uniform movement

of the operating member.

The speed of operating member can be found from relationship

Sm = t.k.n mm/min

t = pitch of thread

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k = number of thread

n = rpm of the screw

5. Rack and pinion transmission

Figure: rack and pinion transmission

When the rotating gear meshes with a stationary rack, the centre of the gear moves in straight

line on the other hand if the gear axis is stationary then the rack executes translatory motion.

The direction of motion can be reversed by reversing the rotation of the pinion.

Sm = 𝜋.z.n mm/min

Sm = feed per minute of the operative member

m = module of the pinion

z = number of teeth of the pinion

n = rpm of the pinion

Rack and pinion transmission is the simplest and cheapest among all types of transmission used

in reversible driven. It also has high efficiency and provides a large transmission ratio which

makes it possible to use it in the feed as well as main drive mode.

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EXPERIMENT NO.7

Objective: Discuss various device for intermittent motion and draw the schematic diagram

for various application. (Application and sketching of Ratchet gear mechanism, Geneva

mechanism, Reversing mechanism, Differential mechanism, Norton mechanism, Mender’s

mechanism.)

1. Ratchet gear mechanisms

2. Geneva mechanism

3. Reversing mechanism

4. Differential mechanism

Introduction –

Devices for intermittent motion –

In some machine tools, it is required that the relative position between the cutting tool and

workpiece should change periodically.

a .Machine tools with a reciprocating primary cutting motion e.g. shaping machine in which

the workpiece must be intermittently upon completion of one full stroke of the cutting tool.

b .machine tools with reciprocating feed motion.

1. Ratchet gear mechanism –

The Ratchet gear mechanism is generally consists of a pawl mounted on an oscillating pin.

During each oscillation in the anticlockwise direction, the pawl turns the ratchet wheel through

a particular angle. During the clockwise oscillating in the opposite direction, the pawl simply

slides over the ratchet teeth and the latter remain stationary. The ratchet wheel is linked to the

machine tool table through a nut and screw transmission. Therefore the periodic rotation of the

ratchet wheel is transformed into the intermittent translator motion of the table for a particular

nut and screw pair of some constant transmission ratio. The feed of the table during each

oscillation depends upon the swing of the oscillating pawl. The rotation of the ratchet wheel in

one stroke of the pawl should not exceed 45’. The ratchet gear mechanism is most suitable in

case when the periodic displacement must be completed in a short time.

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2. Geneva mechanism -

Fig: Geneva mechanism

Geneva mechanism consists of a driving disc which rotates continuously and a wheel a wheel

with four radial slots. The arc on the driving disc and wheel provide a locking effect against

rotation of the slotted wheel e.g. position of the wheel cannot rotate. As the disc continuous to

rotate, point A of the disc comes out of contact with the arc and immediately thereafter pin ‘p’

mounted at the end of the driving arm enters the radial slot.

The wheel now begins to rotate when it has turned an angle 90° the pin comes out of the radial

slot and immediately thereafter point ‘B’ comes in contact with the next arc of the wheel

preventing its further rotation. In the Geneva mechanism the angle of rotation of the wheel

cannot varied.

Application –

(i) Mainly used in torrents.

(ii) Single spindle automatic machine for indexing cutting tools.

(iii) Multi spindle automatic machines for indexing spindle through a constant angle.

3. Reversing mechanism –

These mechanisms are used for changing the direction of motion of the operative member.

Reversing is accomplished generally through spur and helical gears. A few reversing

arrangements using spur and helical gear. In this arrangement the gear on the driving shaft are

mounted rigidly. While the idle gear and gears on the driven shaft are mounted freely. The jaw

clutch is mounted on a key, rotation may be transmitted to the driven shaft either through gear

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(A/B), (B/C) or through D/E depending upon whether the jaw clutch is shifted to the left to

mesh with gear C or to the right to mesh with gear E.

In the second arrangement, the gears on the driving shaft are again rigidly mounted and the idle

gear is free. On the driven shaft a double cluster gear is mounted on a spline. By sliding the

cluster gear transmission to the driven shaft may again be achieved either through gear (A/B),

(B/C) or through gear pair D/E.

In the third arrangement gear A on the driving shaft and gear D on the driven shaft are both

rigidly mounted. A quadrant with constantly meshing gear B and C can be swivelling about the

axis of the driven shaft. By swivelling the quadrant with the help of a lever transmission to the

driven shaft may be achieved through (A/C), (C/D) or through (A/B) (B/C)(C/D).

4. Differential mechanism –

Differential mechanisms are used for summing two motions in machine tools in which

operative member gets input from two separate kinematics trains. They are generally employed

in thread and gear cutting machines where the machined surface is obtained as a result of the

summation of two or more forming motions.

A simple differential mechanism using spur or helical gears is shown. The mechanism is

essentially a planetary gear mechanism consisting of sun gear A, planetary gear B and arm C.

The planetary gear is mounted on the arm which can rotate about axis of gear A. suppose gear

A makes nA and arm C, nC revolutions per minute in the clockwise direction. The relative

motion between the elements of the mechanism will remain unaffected if the whole mechanism

is rotated in the anticlockwise direction with nC revolution per minute.

The transmission ratio of the mechanism may be written as

nA – nC/nB – nC =- zB/zA

Where zA and zB are the number of teeth of gear A and B, respectively. The above expression

may be written as follows.

nB = nC(1+zA/zB) – nA(zA/zB)

Differential mechanisms are using a double cluster planetary gear. The mechanism consists of

gear A, cluster gear block B-B’ mounted on arm C and gear D. If nA, nB, nC are the rpm’s of

gear A, arm C and gear D, respectively then the transmission ratio of the kinematic train

between gear A and D may be expressed as

nD – nC/nA – nC = zA/zB . zB’/zD

29


The mechanism consist of bevel gears A and D and planetary level gears B and C. Planetary

gear can be rotated about the common axes of gear A and D.

1. By means of a ring gear – this differential is used in automobiles.

2. By means of a T- shaped shaft – this differential is used in machine tools.

3. if gear A,B and D make nA, nB, nD revolutions per minute, respectively, then the transmission

ratio of the kinematic train between gear A and D can be written as

nA – nB/nD – nB = - zA/zB . zB/zD

Where zA, zB, zD are the number of teeth of gears A, B and D respectively.

30


EXPERIMENT NO.8

Objective: Aim of speed & feed rate regulation, stepped regulation of speed.

Speed and Feed Rate Regulation:

Various Laws of Steeped Regulation:

31


Table: Diameter range for different rpm values in an A.P.

32


The rpm values constitutes a G.P.

Table: Diameter range fordifferent rpm values in G.P.

33


The rpm values constitutes a H.P.

Table shown below:

34


Table: Diameter range fordifferent rpm values in H.P.

35


EXPERIMENT NO.9

Object: Which Speed Series are used in machine tool gear box. (G.P. series is used in steeped

regulation of speed.)

Gear boxes

-Ap & Gp for steeping speeds of gears.

-Structural Formula & Structural diagrams.

Gear boxes

Machine tool characterized by their large number of spindle speeds and feeds of cape with the

requirements of machine parts of different materials and dimension 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. Surfaces finish required and economical consideration.

Speed Range for different Machine Tools

Machine Range

Numerically Controlled lathes 250

Boring 100

Milling 50

Drilling 10

Surface Finish 4

Stepping of Speed According to Arithmetic Progression (AP)

Let 𝑛1,𝑛2,……..,𝑛𝑛 be arranged according to arithmetic progression.

Then 𝑛1-𝑛2 = 𝑛3-𝑛2 = Constant

The saw tooth diagram in such a case is show in fig. Accordingly, for an economical cutting

Speed 𝑣0, the lowest speed 𝑣1 is not constant, it decrees with increasing dia. Therefore, the

arithmetic progression does not permit economical machine at large diameter ranges. The main

disadvantage of such an arrangement is that the percentage drop from step to step decrees as

the speed increase. Thus the speed are not evenly distribution 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 gear box with a sliding key when the number of

shaft is only two.

36


Fig. 8.1 Speed stepping according to A.P.

Stepping of Speed According to Geometric Progression (GP)

As show in Figure 8.2, the percentage drop from one step to the other is constant, and the

absolute loss of economically expedient cutting speed ∆v is constant all over the whole diagram

range. The relative loss of cutting speed ∆𝑉𝑚𝑖𝑛/𝑉0 is also constant Geometric progression.

Therefore, allow machining to take place between limits 𝑉0 and 𝑉𝑢 independent of the WP

diagram, where 𝑉0 is the economical cutting speed and 𝑉𝑢 is the allowable minimum cutting

speed. Now suppose that 𝑛1,𝑛2,……..,𝑛𝑧 are the spindle speeds. According to the geometric

progression

𝑛2

𝑛1=

𝑛2

𝑛1= ∅

Where Ø is the progression ratio. The spindle speed can be expressed in term of the minimal

speed n1 and progression ratio Ø

n1 n2 n3 n4 nz

n1 n1 Ø n1 Ø2 n1 Ø

3 n1 Øz-1

37


Fig.8.2 Speed stepping according to G.P.

Hence, the maximum spindle speed nz is given by

𝑛𝑧 = 𝑛1∅𝑧−1

Where z is the number of spindle speed, therefore

∅ = √𝑛2

𝑛1

𝑧−1

= √𝑅𝑛𝑧−1

𝑧 =log 𝑅𝑛

log ∅+ 1

ISO Standard values of progression ratio Ø

(1.06, 1.12, 1.26, 1.4, 1.6, 1.78, 2.0)

Justify ensuring with reason

1. Transmission ratio imax =2, imax =1/4, ig = imax/ imin=8

2. Minimum total shaft size

The torque transmitted by a shaft is given by

𝑇 ∝1

𝑁

From the strength consideration: (𝑑1

𝑑2⁄ ) = (

𝑁2𝑁1

⁄ )1/3

3. For last radial dimensions of gear box imax* imin = 1

4. No of gears on last shaft should be minimum.

5. No of gears on any shaft should be limited to 3

38


EXPERIMENT NO. 10

Object: Design Procedure of machine tool gear box design. (Design a four / six speed Gear

Box.)

Gear Box

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 nmax and nmin can be

determined by

𝑛𝑚𝑎𝑥 =1000 𝑉𝑚𝑎𝑥

𝜋𝑑𝑚𝑖𝑛

𝑛𝑚𝑖𝑛 =1000𝑉𝑚𝑖𝑛

𝜋𝑑𝑚𝑎𝑥

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

𝑅𝑛 =𝑛𝑚𝑎𝑥

𝑛𝑚𝑖𝑛=

𝑉𝑚𝑎𝑥

𝑉𝑚𝑖𝑛 .

𝑑𝑚𝑎𝑥

𝑑𝑚𝑖𝑛= 𝑅𝑣𝑅𝑑

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.

Design procedure for gear box

1. Determine the maximum and minimum speed of the output shaft. Then calculate the

number of steps or speeds reduction stages for this range. This depends on the

39


application as well as space optimization. Higher reduction stages require more space

because of more number of gears and shafts requirements.

2. Select types of speed reduction or gear box based on the power transmission

requirements, gear ratio, and position of axis space available for speed reducer.

Also make sure that for low gear ratio requires single speed reduction. Select worm

gear for silent operation and level gear for interesting axis.

3. Determine the progression ratio which is ratio maximum speed and minimum speed of

output shaft of the gear box the nearest progression ratio should be a standard one and

it taken either from R20 or R40 series.

4. Draw the structural diagram and kinematic arrangement indicating various arrangement

possibilities during speed reduction or increment.

5. Select materials for gears so that gear should sustain the operating condition and

operating load. Normally cast iron is chosen for housing and cast steel or other all can

be selected as per the load requirements.

6. Note down the maximum power output in horse power (H.P) or transmission power

and revolution per minute of shaft i.e. rpm of each shaft.

7. Determine the centre distance between the driven and driver shaft based on the surface

compressive stress.

8. Determine the module of gear by beam strength as well as fix the number of teeth

required.

9. Calculate the diameter of the shaft by torque requirements and bending moment

consideration.

10. Calculate the key size, shape or type of transmission key for each gears.

11. Select appropriate fit and tolerance for matting parts like shaft and gear.

12. Select bearing types or the loading and operating conditions. Also make sure to include

consideration of maximum speed and expected life of gear and gear box.

13. Make the shaft stepped or provide collar to prevent axial displacement of bearing and

gear.

14. Provide suitable clearance between gear and walls of the housing of gear box and based

on this considerations design the casing/housing of gear box.

40


15. Complete the design of casing in drawing by providing fires if necessary to have

increased heat transfer by convection and conduction. Put inspection hole/man hole as

well as drain hole to drain lubricating oil. Also provide oil level indicator to have proper

amount of oil during operation, if not out, this will lead to failure of gear and shaft due

to over heating or due to friction failure.

16. Draw neat a clean working drawing in suitable software like auto cad, pro engineer etc.

indicating required details during manufacturing or assembly.

17. One can also perform finite element analysis of the complete gear box after it

completely designed.

41


EXPERIMENTS NO. 11

Objective: Study of Lathe Bed. (Design of Lathe bed. (Including Torque analysis of lathe

bed, bending of lathe bed, designing for torsional rigidity, use of reinforcing stiffener in

Lathe bed))

Figure. Lathe

Explanation of the standard components of most lathes:

Bed: Usually made of cast iron. Provides a heavy rigid frame on which all the main components

are mounted.

Ways: Inner and outer guide rails that are precision machined parallel to assure accuracy of

movement.

Headstock: mounted in a fixed position on the inner ways, usually at the left end. Using a

chuck, it rotates the work.

Gearbox: inside the headstock, providing multiple speeds with a geometric ratio by moving

levers.

42


Spindle: Hole through the headstock to which bar stock can be fed, which allows shafts that

are up to 2 times the length between lathe centers to be worked on one end at a time.

Chuck: 3-jaw (self centering) or 4-jaw (independent) to clamp part being machined. Chuck

allows the mounting of difficult workpieces that are not round, square or triangular.

Tailstock: Fits on the inner ways of the bed and can slide towards any position the headstock

to fit the length of the work piece. An optional taper turning attachment would be mounted to

it.

Tailstock Quill: Has a Morse taper to hold a lathe center, drill bit or other tool.

Carriage: Moves on the outer ways. Used for mounting and moving most the cutting tools.

Cross Slide: Mounted on the traverse slide of the carriage, and uses a handwheel to feed tools

into the work piece.

Tool Post: To mount tool holders in which the cutting bits are clamped.

Compound Rest: Mounted to the cross slide, it pivots around the tool post.

Apron: Attached to the front of the carriage, it has the mechanism and controls for moving the

carriage and cross slide.

Feed Rod: Has a keyway, with two reversing pinion gears, either of which can be meshed with

the mating bevel gear to forward or reverse the carriage using a clutch.

Lead Screw: For cutting threads.

Split Nut: When closed around the lead screw, the carriage is driven along by direct drive

without using a clutch.

Quick Change Gearbox: Controls the movement of the carriage using levers.

Steady Rest: Clamped to the lathe ways, it uses adjustable fingers to contact the workpiece

and align it. Can be used in place of tailstock or in the middle to support long or unstable parts

being machined.

Follow Rest: Bolted to the lathe carriage, it uses adjustable fingers to bear against the work

piece opposite the cutting tool to prevent deflection.

Table below showing commonly used bed section and wall arrangement and their

applications.

43


Wall Arrangement Applications

(a) (b)

1.) Beds on logs or sheas

a.) without stiffening, diagonal wall, used in

lathe, turrets, etc.

b.) without stiffening diagonal wall has 30-

40% height stiffness than arrangement (a);

used in multiple tool and height production

lathes.

(c) (d)

c.) with stiffening wall and provision of

chip disposal through opening in rear wall,

used in large sized lathes & turret with

stiffing wall, also used in large-sized laths.

(d) With stiffening wall also used in large

size lathe and turret

2.) Covered top closed profile bed, used in

plan milling, clothing & boring machines.

3.) Open top closed profile bed, used when

the bed is also require, commonly employed

in grinding machine.

44


EXPERIMENT NO. 12

Object: Free body diagram of following machines. (Analysis of force under headstock, tail

stock and saddle.)

(1) Lathe

(2) Drilling

(3) Shaping

(4) Milling

Introduction

Lathe

The lathe is a machine tool used principally for shaping articles of metal (and sometimes wood

or other materials) by causing the workpiece to be held and rotated by the lathe while a tool bit

is advanced into the work causing the cutting action. The basic lathe that was designed to cut

cylindrical metal stock has been developed further to produce screw threads, tapered work,

Drilled holes, knurled surfaces, and crankshafts. The typical lathe provides a variety of rotating

speeds and a means to manually and automatically move the cutting tool into the workpiece.

Machinists and maintenance shop personnel must be thoroughly familiar with the lathe and its

operations to accomplish the repair and fabrication of needed parts.

Types of lathe

Lathes can be divided into three types for easy identification: engine lathes, turret lathes, and

special purpose lathes. Small lathes can be bench mounted, are lightweight, and can be

transported in wheeled vehicles easily. The larger lathes are floor mounted and may require

special transportation if they must be moved. Field and maintenance shops generally use a lathe

that can be adapted to many operations and that is not too large to be moved from one work

site to another. The engine lathe is ideally suited for this purpose. A trained operator can

accomplish more machining jobs with the engine lathe than with any other machine tool. Turret

lathes and special purpose lathes are usually used in production or job shops for mass

production or specialized parts. While basic engine lathes are usually used for any type of lathe

work. Further reference to lathes in this chapter will be about the various engine lathes.

45


Figure. FBD of Lathe

Drilling Machine

A drilling machine comes in many shapes and sizes, from small hand-held power drills to bench

mounted and finally floor-mounted models. They can perform operations other than drilling,

such as counter sinking; counter boring, reaming, and tapping large or small holes. Because

the drilling machines can perform all of these operations, this chapter will also cover the types

of drill bits, took, and shop formulas for setting up each operation. Safety plays a critical part

in any operation involving power equipment. This chapter will cover procedures for servicing,

maintaining, and setting up the work, proper methods of selecting tools, and work holding

devices to get the job done safely without causing damage to the equipment, yourself, or

someone nearby. A drilling machine, called a drill press, is used to cut holes into or through

metal, wood, or other materials. Drilling machines use a drilling tool that has cutting edges at

its point. This cutting tool is held in the drill press by a chuck or Morse taper and is rotated and

fed into the work at variable speeds. Drilling machines may be used to perform other

operations. They can perform countersinking, boring, counter-boring, spot facing, reaming, and

tapping.

Drill press operators must know how to set up the work, set speed and feed, and provide for

coolant to get an acceptable finished product. The size or capacity of the drilling machine is

usually determined by the largest piece of stock that can be center-drilled. For instance, a 15-

inch drilling machine cans center-drill a 30-inch-diameter piece of stock. Other ways to

determine the size of the drill press are by the largest hole that can be drilled, the distance

between the spindle and column, and the vertical distance between the worktable and spindle.

All drilling machines have the following construction characteristics: a spindle, sleeve or quill,

column, head, worktable, and base.

1. The spindle holds the drill or cutting tools and revolves in a fixed position in a sleeve. In

most drilling machines, the spindle is vertical and the work is supported on a horizontal table.

2. The sleeve or quill assembly does not revolve but may slide in its bearing in a direction

parallel to its axis. When the sleeve carrying the spindle with a cutting tool is lowered, the

cutting tool is fed into the work: and when it is moved upward, the cutting tool is withdrawn

from the work. Feed pressure applied to the sleeve by hand or power causes the revolving drill

to cut its way into the work a few thousandths of an inch per revolution.

3. The column of most drill presses is circular and built rugged and solid. The column supports

the head and the sleeve or quill assembly.

46


4. The head of the drill press is composed of the sleeve, spindle, electric motor, and feed

mechanism. The head is bolted to the column.

5. The worktable is supported on an arm mounted to the column. The worktable can be adjusted

vertically to accommodate different heights of work. or it may be swung completely out of the

way. It may be tilted up to 90° in either direction, to allow for long pieces to be end or angled

drilled.

6. The base of the drilling machine supports the entire machine and when bolted to the floor,

provides for vibration-free operation and best machining accuracy.

7. The top of the base is similar to a worktable and maybe equipped with T-slots for mounting

work too large for the table.

47


FBD of Drilling Machine

Shaping Machine

The main functions of shaping machines are to produce flat surfaces in different planes. The

cutting motion provided by the linear forward motion of the reciprocating tool and the

intermittent feed motion provided by the slow transverse motion of the job along with the bed

result in producing a flat surface by gradual removal of excess material layer by layer in the

form of chips. The vertical infeed is given either by descending the tool holder or raising the

48


bed or both. Straight grooves of various curved sections are also made in shaping machines by

using specific form tools. The single point straight or form tool is clamped in the vertical slide

which is mounted at the front face of the reciprocating ram whereas the workpiece is directly

or indirectly through a vice is mounted on the bed.

Fig. Shaping Machine

Milling Machine

Milling is the process of machining flat, curved, or irregular surfaces by feeding the workpiece

against a rotating cutter containing a number of cutting edges. The milling machine consists

basically of a motor driven spindle, which mounts and revolves the milling cutter, and a

reciprocating adjustable worktable, which mounts and feeds the workpiece. Milling machines

are basically classified as vertical or horizontal. These machines are also classified as knee-

type, ram-type, manufacturing or bed type, and planer-type. Most milling machines have self-

contained electric drive motors, coolant systems, variable spindle speeds, and power-operated

table feeds. Free body diagram of the milling machine shown in figure below

49


Fig. Milling Machine

50


EXPERIMENT NO.13

Object: Application of slide ways profiles and their combinations. (Design of Guide ways /

Slide ways.)

FUNCTIONS OF GUIDEWAYS

The Guideway is one of the important elements of machine tool. The main function of the guideway

is to make sure that the cutting tool or machine tool operative element moves along predetermined

path. The machine tool operative element carries workpiece along with it. The motion is generally

circular for boring mills, vertical lathe, etc. while it is straight line for lathe, drilling, boring machines,

etc.

Requirements of guideways are:

(a) Guideway should have high rigidity.

(b) The surface of guideways must have greater accuracy and surface finish.

(c) Guideways should have high accuracy of travel. It is possible only when the deviation of the actual

path of travel of the operative element from the predetermined normal path is minimum.

(d) Guideways should be durable. The durability depends upon the ability of guideways to retain the

initial accuracy of manufacturing and travel.

(e) The frictional forces acting on the guideway surface must be low to avoid wear.

(f) There should be minimum possible variation of coefficient of friction.

(g) Guideways should have good damping properties.

Guideways can be classified as:

(a) Guideways with sliding friction

(b) Guideways with rolling friction

Guideways with Sliding Friction

The friction between the sliding surfaces is called as guideways with sliding friction. These

guideways are also called as slideways. The slideways are further classified according to the

lubrication at the interface of contacting surfaces. The friction between the sliding surfaces

may be dry, semi-liquid, and liquid. When the lubrication is absent in between contacting

surfaces, it is called as dry friction. Dry friction is rarely occurred in machine tools.

When two bodies slide with respect to each other having lubrication between them, the sliding

body tends to rise or float due to hydrodynamic action of the lubricant film. The principle of

slider is shown in Figure 12.1.

51


Figure12.1 Principle of a Slider

The hydrodynamic force,

𝐹ℎ = 𝐶 ∗ 𝑣𝑠 (1)

Where C is constant and depends upon wedge angle θ, the geometry of sliding surfaces,

viscosity of the lubricant and parameter of lubricant film.

vs is sliding velocity.

W is weight of the sliding body.

The resultant normal force acting on sliding body,

R = Fh

– W

From Eq. (1), it is clear that the hydrodynamic force increases with increase in sliding velocity.

The sliding body rests on the stationary body when hydrodynamic force is less than the weight

of the sliding body. Here, there are semi-liquid type friction conditions and under these

conditions the two bodies are partially separated by the lubricant film and partially have metal

to metal contact. The resultant normal force on sliding body starts to act upwards and the body

floats as hydrodynamic force is greater than the sliding weight of the body. The sliding surfaces

are completely separated by the lubricant film and liquid friction occurs at their interface. The

slideways in which the sliding surfaces are separated by the permanent lubricant layer are

known as hydrodynamic slideways. This permanent lubrication layer is due to hydrodynamic

action. A permanent lubricant layer between the sliding surfaces can be obtained by pumping

the liquid into the interface under pressure at low sliding speed. The sliding body is lifted by

this permanent lubricant layer. Such slideways are called as hydrostatic slideways.

52


Guideways with Rolling Friction

These are also called as anti friction ways. The anti friction slideways may be classified

according to the shape of the rolling element as:

(a) Roller type anti friction ways using cylindrical rollers.

(b) Ball type anti friction ways using spherical balls.

DESIGN OF SLIDEWAYS

Slideways are designed for wear resistance and stiffness.

1. Design of Slideways for Wear Resistance

The wear resistance of slideways is mainly dependent upon maximum pressure acting on the

mating surfaces. This condition may be given as

pmax ≤ pmp (2)

Where

pm = maximum pressure acting on the mating surface, and

pmp = permissible value of the maximum pressure.

It is seen during the subsequent analysis that slideway designed for maximum pressure is quite

complicated. Sometimes, this design is replaced by a simple procedure based upon the average

pressure acting on the mating surfaces. The condition is that:

Pa ≤ Pap (3)

Where Pa = average pressure acting on the mating surface, and

Pap = permissible value of the average pressure.

Hence from Eqs. (2) and (3), the design of slideways for wear resistance requires that.

(a) pm and pa to be known,

(b) pmp and pap to be known, and

(c) The values of pmp and pap are given for different operating conditions of slideways on the

basis of experience. For determining pm and pa, the first and foremost task is to determine the

forces acting on the mating surfaces.

Forces acting on the mating surfaces in combination of V and flat slideways.

The combination of V and flat slideways is commonly used in lathe machines. The schematic

diagram of slideways and the forces acting on the system for the case of orthogonal cutting are

illustrated in Figure 12.2.

53


Figure 2 Forces Acting on Combination of V and Flat Sideways

The forces acting on V and flat slideways are :

(a) Cutting force component Fz (in the direction of the velocity vector) and Fy (radial),

(b) Weight of carriage W, and

(c) Unknown forces F1, F2 and F3 acting on the mating surfaces.

The unknown forces are calculated from following equilibrium conditions:

Sum of components of forces acting along Y-axis = 0

Substituting value of F3 in Eq., we get

54


If the apex angle of the V is 90o, and assume that present angle γ may change to γ = 90 – λ, the solution

of simultaneous algebraic Eqs. gives :

Substituting the values of F1 and F2 in Eq., we get,

Above eq. represents the forces acting on the mating surfaces in combination of two flat sideways.

The schematic diagram of the slideways and the forces acting on the system under orthogonal cutting

conditions are shown in Figure 12.3.

Fig.12.3 Forces Acting on Combination of Two Flat Slideways

The forces acting on combination of two flat slideways are :

(a) Cutting force components, i.e. axial Fx, radial Fy, and Fz in the direction of velocity vector.

(b) Weight of carriage, W.

(c) Unknown forces F1, F2 and F3 acting on the mating surfaces.

(d) Frictional forces μF1, μF2, μF3, where μ is the coefficient of friction between the sliding surfaces.

55


The unknown forces F1, F2 and F3 are calculated from following equilibrium conditions :

From above eq.

On substituting the value of F3 in Eq.

56


Slideway

Profile and

Combinatio

n for Bads

Sketch Application

Open V +

Open V

Planning

Machines

Closed V +

Closed V

Precision lathes

and turret lathes

Open flat +

Open V

Surface-grinding

machines

Closed flat +

Closed V

Genral-purpose

lathes & heavy

duty boring

machine

For vertical columns

Closed flat +

Closed flat

Most Commonly

used for all types

of vertical

columne

Closed flat +

Closed flat

Knee types

milling machine

small vertical

drilling machine

and traverses of

radial drilling

machine

57


Closed flat +

heavy-closed

dovetail

Same us above

For cross slides and compound rests

Closed

devotail

Cross slides &

compound rests

Closed flat +

Closed flat

Cross slides of

heavy duty

machine tools

For Rotary Blades

Flat

Surface-grinding

machine and

small hobbing

machine

W

Precission gear-

hobby machine

58


Shapes of slideways

(a) (b)

(c)

(d) (e)

Slideways profiles:

(a) Flat; (b) Symmetrical V; (c) Asymmetrical V; (d) dovetail

(e) Cylindrical

59


EXPERIMENT NO. 14

Object: Draw a neat schematic diagram of herring bone gear and explain

Introduction

A herringbone gear, a specific type of double helical gear, is a special type of gear that is a side

to side (not face to face) combination of two helical gears of opposite hands. From the top,

each helical groove of this gear looks like the letter V, and many together form a herring bone

pattern (resembling the bones of a fish such as a herring). Unlike helical gears, herringbone

gears do not produce an additional axial load.

Like helical gears, they have the advantage of transferring power smoothly because more than

two teeth will be in mesh at any moment in time. Their advantage over the helical gears is that

the side-thrust of one half is balanced by that of the other half. This means that herringbone

gears can be used in torque gearboxes without requiring a substantial thrust bearing. Because

of this herringbone gears were an important step in the introduction of the steam turbine to

marine propulsion.

Precision herringbone gears are more difficult to manufacture than equivalent spur or helical

gears and consequently are more expensive. They are used in heavy machinery.

Where the oppositely angled teeth meet in the middle of a herringbone gear, the alignment may

be such that tooth tip meets tooth tip, or the alignment may be staggered, so that tooth tip meets

tooth trough. The latter alignment is the unique defining characteristic of a Wuest type

herringbone gear, named after its inventor.

https://en.wikipedia.org/wiki/Gear

https://en.wikipedia.org/wiki/Herringbone_pattern

https://en.wikipedia.org/wiki/Herringbone_pattern

https://en.wikipedia.org/wiki/Fish_anatomy#Vertebrae

https://en.wikipedia.org/wiki/Herring

https://en.wikipedia.org/wiki/Helical_gear

https://en.wikipedia.org/wiki/Thrust_bearing

https://en.wikipedia.org/wiki/Steam_turbine

https://en.wikipedia.org/wiki/Spur_gear

https://en.wikipedia.org/w/index.php?title=Wuest_type_herringbone_gear&action=edit&redlink=1

https://en.wikipedia.org/w/index.php?title=Wuest_type_herringbone_gear&action=edit&redlink=1

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Benefits

Since a herringbone gear is non-linear in the teeth the gears won't slip out from grabbing one

another if the axle or another force moves the gears up and down. This is also a benefit with

machinery that needs very straight movement, because a herringbone gear is designed to 'self

center' and is much less likely to skip a tooth or fall out of place. With some gears sets that use

herringbone gears; an axle can be lost and the gear will stay in place, a herringbone planetary

gear system.

Manufacture

A disadvantage of the herringbone gear is that it cannot be cut by simple gear

hobbing machines, as the cutter would run into the other half of the gear. Solutions to this have

included assembling small gears by stacking two helical gears together, cutting the gears with

a central groove to provide clearance, and (particularly in the early days) by casting the gears

to an accurate pattern and without further machining. With the older method of fabrication,

herringbone gears had a central channel separating the two oppositely-angled courses of teeth.

This was necessary to permit the shaving tool to run out of the groove. The development of the

Sykes gear shaper made it possible to have continuous teeth with no central gap. Sunderland,

also in England, also produced a herringbone cutting machine. The Sykes uses cylindrical

guides and round cutters; the Sunderland uses straight guides and rack-type cutters. The W. E.

Sykes Co. dissolved in 1983–84. Since then it has been common practice to obtain an older

machine and rebuild it if necessary to create this unique type of gear. Recently, the Bourn and

Koch Company has developed a CNC-controlled derivation of the W. E. Sykes design called

the HDS1600-300. This machine, like the Sykes gear shaper, has the ability to generate a true

apex without the need for a clearance groove cut around the gear. This allows the gears to be

used in positive displacement pumping applications, as well as power transmission. Helical

gears with low weight, accuracy and strength may be 3D printed.

The herring bone gear is essentially a pair of helical gear in which the helix angel is oppositely

direct.

In a gear transmission, the rpm of the drives shapes is determined as

𝑛2 = 𝑛1 .𝑧1

𝑧2

Where 𝑛1=rpm of the driven shaft

𝑛2=rpm of the driving shaft

𝑧1=no. of teeth of the drawing gear

𝑧2=no. of teeth of the driven gear

The ratio 𝑧1/ 𝑧2 is known as the transmission ratio of the gear driven and is constant for a

particular gear pair.

https://en.wikipedia.org/wiki/Gear_hob

https://en.wikipedia.org/wiki/Gear_hob

https://en.wikipedia.org/wiki/Gear_shaper

https://en.wikipedia.org/wiki/3D_printing

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EXPERIMENT NO.15

Object: Description of stick-slip and sliding friction in machine tool design.

Stick-slip Friction

Stick-slip can be described as surface alternate between sticking to each other and sliding over

each other with a corresponding change in the force of friction coefficient between two surfaces

is larger than the reduction of the friction to the kinetic friction can cause a sudden jump in the

velocity of the movements. The attached picture shows symbolically an example of stick-slip.

V is the drive system, R is the elasticity in the system and M is the load i.e. lying on floor and

is being pushed horizontally. When the drive is started, the spring R is loaded and its pushing

force against load M increases until the static friction coefficient between load M and floor is

not able to hold the load anymore. The load start sliding and the friction coefficient decreases

from its static value to its dynamic value. At this moment, the spring can give more power and

accelerate M.

During M’s movements, the force of the spring decreases, until it is insufficient to the overcome

the dynamic friction. From this point M de-accelerate to a stop. The drive system however,

continues and the spring is loaded again etc.

Fig.- Stick-Slip Phenomenon

Sliding (motion) Friction

Sliding is a type of friction motion between two surfaces in contact. This can be constructed to

rolling friction. Both types of motion may occur in bearing.

Friction may damage or wear the surface in contact. However, it can be reduced by lubrication.

The science and technology of friction, lubrication, and wear is known as tribology.

Sliding may occur between two objects of arbitrary shape, whereas rolling friction is the

friction force associated with the rotational movement of a somewhat dislike or other circular

object along the surface.

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In engg sliding friction occur in numerous types of sliding components such as journal bearing,

cams, linkage, and pistons in cylinders. Static friction is the friction required to move two

surfaces that are not in relative motion.