Machining :- Is the process of cold working the metal into different shapes by using different type

of machine tools.

Machinability is defined as the ease of removing metal while maintaining dimension and

developing a satisfactory surface finish.

Working Principle: The lathe is a machine tool which holds the work piece between two rigid and

strong supports called centers or in a chuck or face plate which revolves. The cutting tool is rigidly

held and supported in a tool post which is fed against the revolving work. The normal cutting

operations are performed with the cutting tool fed either parallel or at right angles to the axis of

the work. The cutting tool may also be fed at an angle relative to the axis of work for machining

tapers and angles.

1) Speed lathes

2) Engine lathes

3) Bench lathes

4) Tool room lathes

5) Capstan and turret lathes

6) Special purpose lathes

• Drilling: to produce a hole (Fig. i), which may be followed by

boring to improve its dimensional accuracy and surface finish.

• Parting: also called cutting off, to cut a piece from the end of

a part, as is done in the production of slugs or blanks for

additional processing into discrete products (Fig. j).

• Threading: to produce external or internal threads (Fig. k).

• Knurling: to produce a regularly shaped roughness on

cylindrical surfaces, as in making knobs and handles

(Fig. l).

• Turning: to produce straight, conical, curved, or grooved

workpieces (Figs.a through d), such as shafts, spindles, and

pins.

• Facing: to produce a flat surface at the end of the part and

perpendicular to its axis (Fig.e), useful for parts that are

assembled with other components. Face grooving produces

grooves for applications such as O-ring seats (Fig.f).

• Cutting with form tools: (Fig. g) to produce various ax

symmetric shapes for functional or aesthetic purposes.

• Boring: to enlarge a hole or cylindrical cavity made by a

previous process or to produce circular internal grooves

(Fig. h).

Taper turning methods.

• Setting over the tailstock centre.

• Swivelling the compound rest

• Using a Taper turning attachment

• Manipulating the transverse and longitudinal feeds of the slide tool

simultaneously.

• Using a broad nose form tool.

Setting over the tailstock centre

Swivelling the compound rest

Using a Taper turning attachment

Using a broad nose form tool

1) Speed lath :- This lathe is simplest of all the lathes and is easily understandable to everyone. As

the name indicates vary high speed and generation This lath are general employed for wood

working ,c centering position and pinning .

2) Engine lathe It is the most commonly used lathe and is for high importance in lathe family. The

name engine is kept as in the past steam engine are used for running robust in construction.

Engine lathe have additional mechanism to produce a wide range of speed. Speed both in anti

clockwise and clockwise direction can be generated very effectively.

3) Bench lathe This is a very small lathe and is generally mounted over a bench. This contains all

the parts of engine lathe but difference arises in size.

4) Tool room lathe:- It has similar feature of engine lathe but it is more accurately built and a

wide range of speed range for very low to very high speeds up to 2500rpm can be generated.

5) Capstan and Turret lathe :- Turret lathe is a machine generally larger than a capstan lathe, but

using similar tools. The main difference between them is that the turret saddle has longitudinal

movement on the guides of the bad. These are particularly suitable for mass production of

identical parts in minimum time.

6) Automatic lathes :- These lathes are so designed that the tool are automatically fed to the work

and withdrawn after all operation.

7) Special purpose lathes :- These lathes are primarily designed for carrying out a particular

operation with utmost efficiency .

Lathe Accessories• Chucks

• Face Plate

• Angle plate

• Driving Plate

• Lathe carriers or dogs

• Lathe Mandrels

• Rests

• Jigs and Fixture

Face Plate

Angle Plate

Driving Plate

Lathe Carriers or Dogs

Lathe Mandrels

Rests

• THE TOOL SIGNATURE OR NOMENCLATURE

FOR A SINGLE-POINT CUTTING TOOL IS A

SEQUENCE OF ALPHA AND NUMERIC

CHARACTERS REPRESENTING THE VARIOUS

ANGLES, SIGNIFICANT DIMENSIONS, SPECIAL

FEATURES, AND NOSE RADIUS SIZE

• The seven elements that comprise the signature of a single point cutting tool are always stated in the following order:

• Back rake angle (0°)

• Side rake angle (7°)

• End relief angle (6°)

• Side relief angle (8°)

• End cutting edge angle (15°)

• Side cutting edge angle (16°) and

• Nose radius (0.8 mm)

• It is usual to omit the symbols for degrees and mm, simply listing the numerical value of each component in single point cutting tool:

• A typical tool signature is 0-7-6-8-15-16-0.8

• 1: Side Cutting Edge Angle:

• The angle between side cutting edge and the side of the tool shank is called side cutting edge angle. It is often referred to as the lead angle.

• 2: End Cutting Edge Angle:

• The angle between the end cutting edge and a line perpendicular to the shank of the tool shank is called end cutting edge angle.

• 3: Side Relief Angle:

• The angle between the portion of the side flank immediately below the side cutting edge and a line perpendicular to the base of the tool.

• 4: End Relief Angle:

• The angle between the end flank and the line perpendicular to the base of the tool is called end relief angle.

• 5: Back Rake Angle:

• The angle between the face of the tool and line perpendicular to the base of the tool measures on perpendicular plane through the side cutting edge. It is the angle which measures the slope of the face of the tool from the nose, towards the rack. If the slope is downward the nose it is negative back rake.

• 6: Side Rake Angle:

• The angle between the face of the tool and a line parallel to the base of the tool measured on plane perpendicular to the base and the side edge. It is the angle that measure the slope of the tool face from the cutting edge, if the slope is towards the cutting edge it is negative side rake angle and if the slope is away from the cutting edge, it is positive side rake angle. If there is no slope the side rake angle is zero.

•

Terminology of single point cutting tool

•

Shank – It is main body of tool. The shank used to grippesd in tool holder.

• Flank – The surface or surface below the adjacent of the cutting edge is called flank of the tool.

• Face – It is top surface of the tool along which the chips slides.

• Base – It is actually a bearing surface of the tool when it is held in tool holder or clamped directly in a

tool post.

• Heel – It is the intersection of the flank & base of the tool. It is curved portion at the bottom of the tool.

• Nose – It is the point where side cutting edge & base cutting edge intersect.

• Cutting edge – It is the edge on face of the tool which removes the material from workpiece. The cutting

edges are side cutting edge (major cutting edge) & end cutting edge ( minor cutting edge)

• Tool angles-Tool angles have great importance. The tool with proper angle, reduce breaking of tool, cut

metal more efficiently, generate less heat.

• Noise radius –It provide long life & good surface finish sharp point on nose is highly stressed, & leaves

grooves in the path of cut.Longer nose radius produce chatter.

Three Modes of Tool Failure

• Fracture failure

– Cutting force becomes excessive and/or

dynamic, leading to brittle fracture

• Temperature failure

– Cutting temperature is too high for the tool

material

• Gradual wear

– Gradual wearing of the cutting tool

Preferred Mode of Tool Failure:

Gradual Wear

• Fracture and temperature failures are

premature failures

• Gradual wear is preferred because it leads

to the longest possible use of the tool

• Gradual wear occurs at two locations on a

tool:

– Crater wear – occurs on top rake face

– Flank wear – occurs on flank (side of tool)

Taylor Tool Life Equation

This relationship is credited to F. W. Taylor

where v = cutting speed; T = tool life; and n and C are parameters that

depend on feed, depth of cut, work material, tooling material, and the

tool life criterion used

• n is the slope of the plot

• C is the intercept on the speed axis

CvTn=

Tool Life Criteria in Production

1. Complete failure of cutting edge

2. Visual inspection of flank wear (or crater wear) by the machine operator

3. Fingernail test across cutting edge

4. Changes in sound emitted from operation

5. Degradation of surface finish

6. Increased power

7. Cumulative cutting time

Tool Materials

• Tool failure modes identify the important

properties that a tool material should

possess:

– Toughness - to avoid fracture failure

– Hot hardness - ability to retain hardness at

high temperatures

– Wear resistance - hardness is the most

important property to resist abrasive wear

High Speed Steel (HSS)

Highly alloyed tool steel capable of maintaining hardness at elevated temperatures better than high carbon and low alloy steels

• One of the most important cutting tool materials

• Especially suited to applications involving complicated tool geometries, such as drills, taps, milling cutters, and broaches

Cemented Carbides

Class of hard tool material based on

tungsten carbide (WC) using powder

metallurgy techniques with cobalt (Co) as

the binder

• Two basic types:

1. Non-steel cutting grades - only WC-Co

2. Steel cutting grades – TiC (Titanium carbide)

and TaC (Tantalum carbide)added to WC-Co

Steel Cutting Carbide Grades

• Used for low carbon, stainless, and other

alloy steels

– For these grades, TiC and/or TaC are

substituted for some of the WC

– This composition increases crater wear

resistance for steel cutting, but adversely

affects flank wear resistance for non-steel

cutting applications

Cubic Boron Nitride

• Next to diamond, cubic boron nitride

(cBN) is hardest material known

• Fabrication into cutting tool inserts same

as SPD: coatings on WC-Co inserts

• Applications: machining steel and

nickel-based alloys

• SPD and cBN tools are expensive

Lathe Operation Calculation

• Cutting Speed :- it is the peripheral speed of

the work piece past the cutting tool

S= Cutting /Peripheral speed, m/min

D= Diameter of the job,mm

N= Job or spindle speed rpm

S= πDN/1000 m/min

Feed (f)

f=L/(NxTm)

L= Length of cut, mm

N= rpm

Tm = Machining / cutting time , min

Depth of cut (d)

d=(Di – Df)/2 mm

Di = initial diameter of the work piece in mm

Df = final diameter of work Piece

Material Removing Rate (MRR)

Volume of Material remove in one revolution V = ∏ Di x d x f mm3

For N revolution = VN mm3/ min

RAKE ANGLE- TYPES AND THEIR SIGNIFICANCE

• Positive rake :

• Positive rake or increased rake angle reduces compression, the forces , and the friction, yielding a thinner ,less deformed and cooler chip

• But increased rake angle reduces the strength of the tool section and heat conduction capacity

• Some areas of cutting where positive rake may prove more effective are , when cutting tough, alloyed materials that tend to work harden, such as certain stainless steel, when cutting soft or gummy material or when low rigidity of work piece, tooling , machine tool, or fixture allows chatter to occur.

• Negative rake:

• To provide greater strength at the cutting edge and better heat conductivity , zero or negative rake angles employed on carbide, ceramic, polycrystalline diamond, and poly crystalline cubic boron nitride cutting tools.

• These materials tend to be brittle but their ability to hold their superior hardness at high temperature results in their selection for high speed and continuous machining operation.

• Negative rakes increases tool forces but it is necessary to provide added support to the cutting edge. This is particularly important in making intermittent cuts and in absorbing the impact during the initial engagement of the tool and work.

• Negative rakes are recommended on tool which does not possess good toughness ( low transverse rupture strength)

• Negative rake causes high compression, tool forces and friction , resulting in highly deformed , hot chip.

The rake angle for a tool depends on the following factors

1. Type of the material being cut: A harder material like cast iron may be machined by

smaller rake angle than that required by soft material like mild steel or aluminum.

2. Type of tool material. Tool material like cemented carbide permits turning at high speed. At

high speeds rake angle has little influence on cutting pressure. Under such condition the rake

angle can minimum or even negative rake angle is provided to increase the tool strength.

3. Depth of cut: In rough turning, high depth of cut is given to remove the maximum amount of

the material. This means that the tool has to withstand severe cutting pressure. So the rake angle

should be decreased to increase the lip angle that provides the strength of the cutting edge.

4. Rigidity of tool holder and machine: an importantly supported tool on old or worn out

machine cannot take up high cutting pressure. So while machining under above condition, the

tool used should have larger rake angle.

• The tool is perfectly shaped and there is no contact along the clearance face

• The shear surface is a plane extending upward form the cutting edge.

• The cutting edge is a straight line , extending perpendicular to the direction of motion and generate a plane surface as the work moves past it.

• The chip does not flow to either side

• The depth of cut is constant

• Width of tool is grater than that of work piece

• The work moves relative to the tool with uniform velocity

• A continuous chip is produce with no built up edge

• Plane strain condition exist I,e.the width of the chip remains equal to the width of the work piece

• Chip is assumed to shear continuously across plane on which shear stress reaches the value of shear flow stress.

• The chip is formed by shear along a single plane inclined at an angle ø.

• The resultant force F is resolved in components FN and FF that are normal to the tool

face the former and along the tool face the latter.

• It is also resolved to FSN and FS that are normal to and along the shear plane

respectively.

• it can also be resolved into components Fc, the cutting force, and Ft the feed or thrust

force.

• the rake angle γ, the shear angle ø and the mean angle of friction between chip and

tool ρ are shown.

• Friction μ = tan ρ =FF /FN

Assuming that the cutting action is continuous we can develop a continuous model of cutting

conditions.

Orthogonal Cutting - assumes that the cutting edge of the tool is set in a position that is

perpendicular to the direction of relative work or tool motion. This allows us to deal with forces

that act only in one plane

We can obtain orthogonal cutting by turning a thin walled tube, and setting the lath bit cutting

edge perpendicular to the tube axis.

Next, we can begin to consider cutting forces, chip thicknesses, etc.

First, consider the physical geometry of cutting

Next, we assume that we are also measuring two perpendicular cutting forces that are horizontal,

and perpendicular. This then allows us to examine specific forces involved with the cutting. The

cutting forces in the figure below (Fc and Ft) are measured using a tool force dynamometer

mounted on the lathe.

Force Calculations

Thread Manufacturing• Screw threads are cut with the lathe for accuracy and for versatility. Both inch and

metric screw threads can be cut using the lathe.

• Classification of thread • External thread

• Internal thread

• According to direction of thread • Right handed thread

• Left handed thread

• According to number of start

• Single Start thread

• Multi Start thread

• In cutting threads on a lathe, the pitch of the

thread or number of threads per inch

obtained is determined by the speed ratio of

the headstock spindle and the lead screw

which drives the carriage.

• The carriage is connected to the lead screw of

the lathe for threading operations by engaging

the half nut on the carriage apron

In cutting threads on a lathe, the pitch of the thread or number of threads per inch

obtained is determined by the speed ratio of the headstock spindle and the lead

screw which drives the carriage

Back rake angle (0°)

Side rake angle (7°)

End relief angle (6°)

Side relief angle (8°)

End cutting edge angle (15°)

Side cutting edge angle (16°) and

Nose radius (0.8 mm)

A typical tool signature is 0-7-6-8-15-16-0.8