Manufacturing & Turning

Production or manufacturing

Production or manufacturing of any object is a value addition process by which raw material of low utility and value due to its irregular size, shape and finish is converted into a high utility and valued product with definite size, shape and finish imparting some desired function ability.

Machining is an essential process of semi-finishing and often finishing by which jobs of desired shape and dimensions are produced by removing extra material from the preformed blanks in the form of chips with the help of cutting tools moved past the work surfaces in machine tools.

Machined parts can be classified as rotational or non-rotational. A rotational work part has a cylindrical or disk-like shape. The characteristics operation that produces this geometry is one in which a cutting tool removes material from rotating work part. Examples include turning and boring. Drilling is closely related except that an internal cylindrical shape is created and the tool rotates in most drilling operations.

A non-rotational (also called prismatic) work part is block-like or plate like. This geometry is achieved by linear motions of the work part, combined with either rotating or linear tool motion. Operations in this category include milling, shaping, planning and sawing etc.

(a)

(b)

(c)

Fig. 2.1 (a) Rotational (b) & (c) Non-rotational

Each machining operation produces characteristics geometry due to two factors

The relative motions between the tool and the workpart and

The shape of the cutting tool. These operations may be classified as generating and forming.

In generating, the geometry of the workpart is determined by the feed trajectory of the cutting tool. Examples of generating the work shape in machining include straight turning, taper turning, contour turning, peripheral milling and profile milling.

In forming, the shape of the part is created by the geometry of the cutting tool. In effect, the cutting edge of the tool has the reverse of the shape to be performed on the part surface. Form turning, drilling, and broaching are examples of the case.

Forming and generating are sometimes combined in one operation, such as in thread cutting on a lathe and slot milling.

(a) Straight turning (b) Taper turning (c) Contour turning

(d) Plain milling (e) Profile turning

Fig. 2.2 Generating shape in machining

(a)

(b)

(c)

Fig. 2.3 Forming to create shape in machining (a) form turning (b) broaching and (b) drilling

(a) (b)

Combination of forming and generating to create shape (a) slot milling and (b) thread cutting

TURNING & RELATED OPERATIONS

Turning is a machining process to produce parts round in shape by a single point tool on lathes. The tool is fed either linearly in the direction parallel or perpendicular to the axis of rotation of the workpiece, or along a specified path to produce complex rotational shapes.

The primary motion of cutting in turning is the rotation of the workpiece (wood, metal, plastic even stone), and the secondary motion of cutting is the feed motion.

It can be done manually, in a traditional form of lathe, which frequently requires

continuous supervision by the operator, or by using a computer controlled and automated lathe. This type of machine tool is referred to as having computer numerical control, better known as CNC and is commonly used with many other types of machine tool besides the lathe.

Turning can be either on the outside of the cylinder or on the inside (also known as boring) to produce tubular components to various geometries.

Cutting parameters of turning

(a) Cutting speed, V (m/min)-Job, tangential. Cutting speed in turning V in m/min is related to the rotational speed of the workpiece by the equation: Where, D is the diameter of the workpiece, mm; N is the rotational speed of the workpiece, rev/min (rpm).

V m/min

1000

NDV

(b) Feed rate, f (mm/rev)- Tool, along the job axis. Feed in turning is generally expressed in mm /rev (millimetres per revolution).

(c) Depth of cut, d (mm)-Tool, radial. The turning operation reduces the diameter of the workpiece from the initial diameter D1 to the final diameter D2. The change in diameter is actually two times depth of cut, d. So, the depth of cut, d = (D1 D2)/2

Cutting speed, Feed rate and Depth of cut also

denoted by Vc , So and t.

These values are dependent on each other and tool

material, required surface finish----etc.

Material removal rate (MRR): The volumetric rate of material removal (so-called material

removal rate, MRR) is defined by, MRR = Vfd

When using this equation, care must be exercised to assure that the units for V are consistent with those for f and d.

The time for a single pass of turning:

The time t for a single pass is given by,

Where, L= length of the job, mm Lo= over travel of the tool beyond the length of the job to help in the setting of the tool, mm f= feed rate, mm/rev N= rotational speed of the work piece, rpm The value of Lo depends upon the operators choice but usual values could be 2 to 3 mm on either side.

The number of Roughing and finishing pass: The roughing passes Pr is given by, Where, A= total machining allowance, mm Af= finishing machining allowance, mm dr= depth of cut in roughing pass, mm The value calculated from the equation is to be rounded to the next integer. Similarly the finishing passes, Where, df = depth of cut in finishing, mm

min Nf

LLt o

d

AAP

r

fr

d

AP

f

ff

Power requirement for turning operation: The power required in the spindle for turning operation depends upon the cutting speed, depth of cut feed rate and the work piece material hardness and machinability. The power required depends upon the cutting force F which is a function of feed rate f and depth of cut d. However, for the sake of gross estimation it can be safely assumed that, Cutting force, F=K * d * f Where K is a constant depending on the work material.

Material being cut K (N/mm2) Steel, 100-150 BHN 1200 Steel, 151-200 BHN 1600 Steel, 201-300 BHN 2400 Steel, 301-400 BHN 3000

Cast Iron 900 Brass 1250

Aluminium 700 Then power, P=F * V Combining the above two equations, power P= K * d * f * V

Basic Turning Operations

Facing: The tool is fed radially into the rotating work on one end to create a flat surface on the end.

Cutoff: The tool is fed radially into the rotating work at some location along its length to cut off the end of the part. This operation is sometimes referred to as parting.

Form turning: In this operation, sometimes called forming, the tool has a shape that is imparted to the work by plunging the tool radially into the work.

Grooving: In this operation, a Groove on workpiece is produced. Here, Shape of tool is similar to the shape of groove. Carried out using Grooving Tool (A form tool ). Also called Form Turning.

Contour turning: Instead of feeding the tool along a straight line parallel to the axis of rotation as in turning, the tool follows a contour that is other than straight, thus creating a contoured form in the turned part

Taper turning: Instead of feed the tool parallel to the axis of rotation of the work, the tool is fed at an angle, thus creating a taper cylinder or conical shape

Chamfering: The cutting edge of the tool is used to cut an angle on the corner of the cylinder, forming what is called a chamfer.

Drilling: Drilling can be performed on a lathe by feeding the drill into the rotating work along its axis. Reaming can be performed in a similar way.

Internal grooving: : In this operation, an internal groove on workpiece is produced.

Boring: A single point is fed linearly. Parallel to the axis of rotation, on the inside diameter of an existing hole in the part.

Knurling: This is not a machining operation because it does not involve cutting of material. Instead, it is a metal forming operation used to produce a regular cross hatched pattern in the work surface.

Threading: A pointed tool is fed linearly across the outside surface of the rotating workpart in a direction parallel to the axis of rotation at a large effective feed rate, thus creating threads in the cylinder.

Various types of tools used in turning related operations

Lathe machine and its various parts

A lathe is a machine tool that rotates the workpiece against a tool whose position it controls. The different parts of the lathe machine are described below: Bed: The bed is the base or foundation of the parts of the lathe. It supports all major components. The main feature of the bed is the ways, which are formed on the beds upper surface and run the full length of the bed. The ways keep the tailstock and the carriage, which slide on them, It provides precise guidance to carriage assembly and tailstock. Headstock The headstock contains the headstock spindle and the mechanism for driving it.

Spindle-The spindle is the part of the lathe that rotates. Various work holding attachments such as three jaw chucks, collets, and centers can be held in the spindle. The spindle is driven by an electric motor through a system of belt drives and gear trains. Spindle rotational speed is controlled by varying the geometry of the drive train.

Tailstock The tailstock can be used to support the end of the work piece with a center, or to hold tools for drilling, reaming, threading, or cutting tapers. It can be adjusted in position along the ways to accommodate different length work pieces. The tailstock barrel/quill can be fed along the axis of rotation with the tailstock hand wheel. Carriage- The carriage controls and supports the cutting tool. It consists of:

a saddle that slides along the ways; an apron that controls the feed mechanisms; a cross slide mounted to the saddle that controls transverse motion of the tool

(toward or away from the operator);

a tool compound rest mounted to the cross slide that adjusts to permit angular tool movement. This is the component that holds the tool post.

a tool post that holds the cutting tools. There are a number of different lathe designs, and some of the most popular are discussed here.

Feed Rod - Feed rod is driven by the spindle through a train of gears. The ratio of feed rod speed to spindle speed can be varied by using change gears to produce various rates of feed. The feed rod transmits power to the apron to drive the longitudinal feed and cross-feed mechanisms. The rotating feed rod drives gears in the apron; these gears in turn drive the longitudinal feed and

cross-feed mechanisms through friction clutches. The feed rod will move the apron and cutting tool slowly forward. This is largely used for most of the turning operations. Lead Screw-The lead screw is used for thread cutting. It has accurately cut Acme threads along its length. The lead screw is driven by the spindle through a gear train. Therefore, the rotation of the lead screw bears a direct relation to the rotation of the spindle. When the half-nuts are engaged, the longitudinal movement of the carriage is controlled directly by the spindle rotation. Consequently, the cutting tool is moved a definite distance along the work for each revolution that the spindle makes. The lead screw will cause the apron and cutting tool to advance quickly. This is used for cutting threads, and for moving the tool quickly.

Lathe Specification

Distance between centers- Maximum length of the job Swing over the cross slide- maximum diameter that can be rotated on the lathe

Example: 300 - 1500 Lathe

Maximum Diameter of Workpiece that can be machined= SWING (= 300 mm) Maximum Length of Workpiece that can be held between Centers (=1500 mm)

Length of

Workpiece

Diameter of

Workpiece

Workholding Devices for Lathes

Lathe Chucks

Lathe Chucks are adjustable mechanical vises that hold the work piece and transfer rotation motion from the drive motor to the work piece

Lathe Chucks come in two basic types

Three-jaw self-centering chucks-Used to center round or hexagonal stock

Four-jaw independent chucks-Each jaw moves independently to accommodate various work piece shapes

Lathe Collets

Collet consists of tubular bushing with longitudinal slits. Collets are used to grasp and hold barstock/round stock of standard sizes. A collet of exact diameter is required

to match any bar stock diameter. Its a most accurate holding method for round stock.

Run out less than 0.0005 inch. Stock should be no more than 0.002 inch larger or 0.005 smaller than the collet. Typically used for drill-rod, cold-rolled, extruded, or previously machined stock.

Lathe Centers

A lathe center hold the end of the work piece, providing support to preventing the work piece from deflecting during machining. Lather centers can be mounted in the spindle hole, or in the tailstock quill.

Lathe centers fall into two categories:

Dead Center: solid steel tip that work piece spins against Live Center: centers contact point is mounted on bearings and allowed to spin with

work piece

Mandrels

A work piece which cannot be held between centers because its axis has been drilled or bored, and which is not suitable for holding in a chuck, is usually machined on a mandrel. A mandrel is a tapered axle pressed into the bore of the workpiece to support it between centers.

A solid machine mandrel is generally made from hardened steel and ground to a slight taper of from 0.0005 to 0.0006 inch per inch. It has very accurately countersunk centers at each end for mounting between centers.

Cutting Tools for Lathes

Tools consists of cutting surface and support

Cutting surfaces can be of same material as support or a separate insert Supports materials must be rigid and strong enough to prevent tool deflection

during cutting Cutting materials are typically carbides, carbide coatings, ceramics, or high carbon

steels Inserts are used to decrease cost in that the insert is disposed of, and the support

reused.

Tool insert Solid tool

Taper turning:

Taper: Defined as a uniform increase or decrease in diameter of a piece of work measured

along its length.

Taper Turning: To produce a conical surface by gradual reduction in diameter from a

cylindrical work-piece.

Methods:

With a broad nose form tool

By Swiveling the compound rest

By Setting off a tail stock centre

By Combining longitudinal & cross feed

By Taper turning attachment

Amount of taper (K): Ratio of difference in diameter of taper to its length, K = (D- d) / L From, triangle ABP and AEF,

2. By Swivelling the Compound Rest

This method employs the principle of turning taper by rotating the work piece on the lathe axis and feeding the tool at an angle to the axis of rotation of the work piece.

The tool mounted on the compound rest is attached to a circular base, graduated in degree, which may be swivelled and clamped at any desired angle.

Once the compound rest is set at the desired half taper angle, rotation of the compound slide screw will cause the tool to be fed at that angle and generate a corresponding taper.

This method is limited to turning a short taper owing to the limited movement of the cross slide. The compound rest may be swivelled at 45on either side of the lathe axis enabling it to turn a steep taper.

The movement of the tool in this method is being purely controlled by hand, thus giving a low production capacity and poor surface finish.

The setting of the compound rest is done by swivelling the rest at half taper angle, if this is already known. If the diameter of the small and large end and Length of taper are known, the half taper angle can be calculated from the equation, Tan = (D-d) / 2L.

3. By Setting off the Tailstock

The principle of turning taper by this method is to shift the axis of rotation of the work piece, at an angle to the lathe axis, and feeding the tool parallel to the lathe axis.

The angle at which the axis of rotation of the work piece is shifted is equal to half the angle of the taper.

This is obtained by sliding body of tail stock towards or away from operator by a set over screw

The amount of set over being limited, this method is suitable for turning small taper on long jobs.

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The main disadvantage of this method is that the live and dead centres are not equally stressed and the wear is not uniform. Moreover, the lathe carrier being set at an angle, the angular velocity of the work is not constant.

Required amount of set over can be calculated as follows: Amount of off-set = (D-d)L/2l

Where D= larger diameter, d= smaller diameter L= length of work, l= length of taper.

4. By Combining Feeds

Taper turning by combining feeds is a more specialized method of turning taper. In certain lathes both longitudinal and cross feeds may be engaged simultaneously

causing the tool to follow a diagonal path. This is the resultant of the magnitudes of the two feeds. The direction of the

resultant may be changed by varying the rate of feeds by change gears provided inside the apron.

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5. By a Taper turning attachment

The principle of turning taper by a taper attachment is to guide the tool in a straight path set at an angle to the axis of rotation of the work piece.

A taper turning attachment consists essentially of a bracket or frame which is attached to the rear end of the lathe bed and supports a guide plate pivoted at the centre.

The plate having graduations in degrees may be swivelled on either side of the zero graduation and is set at the desired angle with the lathe axis.

When the taper turning attachment is used, the cross slide is first made free from the lead screw by removing the binder screw.

The rear end of the cross slide is then tightened with the guide block by means of a bolt.

When the longitudinal feed is engaged, the tool mounted on the cross slide will follow the angular path, as the guide block will slide on the guide plate set at an angle to the lathe axis.

The required depth of cut is given by the compound slide which is placed at right angles to the lathe axis. The guide plate must be set at half taper angle and the taper on the work must be converted in degrees.

The maximum angle through which the guide plate may be swivelled is100 to12on either side of the centre line. If the Large diameter (D), Small diameter (d), and the taper length (L) are specified, the angle of swivelling the guide plate can be determined from equation. Tan = (D-d) / 2L.

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Mathematical problem:

Q 1. Estimate the actual machining time required for the component (C 40 steel) shown in

the figure. The available spindle speed are 70, 110,176, 280, 440, 700, 1100, 1760, 2800

etc. use a roughing speed of 30 m/min and finishing speed of 60 m/min. the feed for

roughing is 0.24 mm/rev while that for finishing is 0.10 mm/rev. the maximum depth of

cut for roughing is 2 mm. finish allowance may be taken as 0.75 mm. Blank to be used for

machining is 50 mm in diameter.

Solution:

Stock to be removed= (50-42)/2=4 mm

Finish allowance=0.75 mm

Roughing pass

Roughing stock available= 4-0.75=3.25 mm

Since maximum depth of cut to be taken is 2 mm, there are two roughing passes.

Given cutting speed V= 30 m/min

Average diameter =(50+43.5)/2= 46.75 mm

Spindle speed

The nearest rpm available from the list is 176 rpm as 280 is very high compared to 204.26

as calculated.

Machining time for one pass=

Finishing pass

Given cutting speed V= 60 m/min

Average diameter =(43.5+42)/2= 42.75 mm

Spindle speed

The nearest rpm available from the list is 440 rpm

120 mm

42 mm

rpm 204.26rpm46.75

301000N

min 2.89min17624.0

2120

rpm 75.446rpm42.75

601000N

40 mm

75 mm

1 2


Total machining time

Q 2. In following figure a component to be machined from a stock of CRS C40 steel, 40 mm

in diameter and 75 mm long is shown. Calculate the machining times required for

completing the part with the following cutting conditions:

(a) HSS tool, Cutting speed 30m/min, feed rate 0.3 mm/rev and maximum depth of cut

2mm

(b) Tungsten carbide tool, Cutting speed 145m/min, feed rate 0.38 mm/rev and maximum

depth of cut 2mm

Solution: The machining to be carried out in two stages as pockets marked in figure as 1

and 2.

HSS tool: For pocket 1-

Average diameter= (40+32)/2=36mm

Spindle speed, N=


Total machining time for pocket 1

For pocket 2-


Spindle speed, N=



Total time for HSS tool=1.94+1.17=3.11min

min 77.2min44010.0

2120

min55.8min)77.289.22(

32 mm

75 mm

40 mm

22 mm

rpm 265rpm 36

301000

min 97.0min26530.0

275

min94.1min)97.02(

min 39.0min35430.0

240

min17.1min)39.03(

rpm 35467.533rpm 27

301000

Tungsten carbide tool: For pocket 1-


Spindle speed, N=



For pocket 2-


Spindle speed, N=



Total time for HSS tool=0.316+0.195=0.511min

Q 3. Calculate the power required for roughing and finishing passes in Q 1.

Solution:

For roughing pass:

Given feed rate, f=0.24 mm/rev

Depth of cut, d=2 mm

Actual cutting speed, V=

The value of K =1600 N/mm2

Required power, P=

For finishing pass:



Actual cutting speed, V=


Required power, P=

rpm 1282rpm 36

1451000

min 158.0min128238.0

275

min316.0min)158.02(

min 065.0min171038.0

240

min195.0min)065.03(

rpm 171044.1709rpm 27

4511000

m/min 25.85 1000

46.75176

kW 0.331 W331W60

20.2425.581600

kW 0.118 W2.181W60

0.175.01.951600

m/min 1.95 1000

42.75440

Q 4. Calculate the power required for roughing and finishing passes in Q 2.

Solution:

For HSS Tool:



Actual cutting speed, V=30 m/min


Required power, P=

For Tungsten Carbide Tool:



Actual cutting speed, V= 145 m/min


Required power, P=

kW 0.480 W480W60

20.30031600

kW 938.2 W67.2938W60

20.381451600

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Manufacturing & Turning