TOOL GEOMETRY

AND TOOL LIFE

T

The mechanics of chip formation:

Single point cutting tool

Single point cutting tool

Shank: It is the main body of the tool

Flank: The surface below and adjacent to cutting

edge is called flank of the tool.

Face: The surface on which chip flows is called face

Nose : It is the point where side cutting edge and

end cutting edge intersect.

Designation of cutting tools: The two systems widely used designate tool shape are,

1) American standard association system (ASA)

2) Orthogonal rake system(ORS)

ASA system:

Side cutting edge angle(SCEA): It is the angle between

side cutting edge and side of the tool shank.

End cutting edge angle(ECSA): It is the angle between end

cutting edge and a line normal to tool shank.

Side relief angle(SRA): It is the angle between portion of

the flank immediately below side cutting edge and a line

normal to base of the tool

End relief angle(ERA): It is the angle between portion of

flank immediately below the end cutting edge and a line

normal to base of the tool.

Back rake angle(BRA): It is the angle between face of

tool and a line parallel to base of the tool. this angle is

positive if side cutting edge slope downwards from the

point towards shank and negative if the slope of side

cutting edge is reverse.

Side rake angle(SRA): It is the angle between tool face

and a line parallel to base of the tool. This angle gives

the slope of the face of the tool from the cutting edge.

side rake angle is negative if slope is towards cutting

edge and positive if slope is away from cutting edge.

Importance of tool angles

Side cutting edge angle: It is the angle which prevents

the interference as the tool enters the work material.

The tip of tool is protected at the start of the cut as it

enables the tool to contact the work first behind the tip.

This angle affects tool life and surface finish. This angle

can vary from 0 to 90.

The side cutting edge at increased value of SCEA will

have more of its length in action for given depth of cut

and the edge lasts longer also,

The chip produced will be thinner and wider which will

distribute the cutting and heat produced over more of

the cutting edge. BUT

larger this angle, greater the component of force

tending to separate the work and tool. This promotes

chatter.

The general value of SCEA vary from 15 to 30

End cutting edge angle : The ECEA provides a

clearance to trailing end of cutting edge to prevent

rubbing between machined surface and trailing(non

cutting) part of cutting edge.

Only small angle is sufficient for this purpose, generally

it varies from 8 to 15

Too large an ECEA takes away material that supports

the point and conducts away heat.

Side relief angle(SRA), End relief angle(ERA):

These angles are provided so that the flank of the tool clears

the work piece surface and there is no rubbing action

between two.

These angles varies from 5 to 15. these angles are

necessary to give strength to the cutting edge when

machining hard and strong material.

Too large relief angles weaken the cutting edge as there is

less mass to absorb and conduct the heat away from cutting

edge.

Back and side rake angles:

Rake angle is small for cutting hard material and

large for cutting soft ductile material. an exception is

brass which is machined with negative rake angle to

prevent the tool from digging into the material.

For carbide cutting tool negative rake angle is used

as carbide is brittle lacks shock resistance and will

fail if positive rake angle is used.

Conditions for using positive rake angle When machining low strength ferrous and non ferrous

materials and work hardening materials

When using low power machines

When machining long shafts of small diameters

When cutting at low speeds

Conditions for using negative rake angle When machining high strength alloys

When using high power machines

When cutting at high speeds

Tool Life Criteria in Production

1. Complete failure of cutting edge 2. Visual inspection of flank wear (or crater wear)

by the machine operator3. Fingernail test across cutting edge4. Changes in sound emitted from operation5. Chips become ribbony, stringy, and difficult to

dispose of6. Degradation of surface finish7. Increased power8. Workpiece count9. Cumulative cutting time

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 faceFlank wear – occurs on flank (side of tool)

Diagram of worn cutting tool, showing the principal locations and types of wear that occur

Figure:-

(a) Crater wear, and

(b) flank wear on a cemented carbide tool, as seen through a toolmaker's microscope

Tool wear as a function of cutting time Flank wear (FW) is used here as the measure of tool wear

Crater wear follows a similar growth curve

Taylor Tool Life Equation

Taylor Tool Life Equation

CvT n 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

‑ Effect of cutting speed on tool flank wear (FW) for three cutting speeds, using a tool

life criterion of 0.50 mm flanckwear

Figure :- Natural log‑log plot of cutting speed vs tool life

Variables affecting tool life Process variables- cutting speed, feed

and depth of cut which ultimately leads to increase in cutting temperature

Tool material Tool geometry Workpiece material, its hardness and

microstructure . Surface condition of the workpiece Cutting fluid