tool geometry and tool life
DESCRIPTION
tool geometryTRANSCRIPT
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