PowerPoint to accompany
Technology of Machine Tools
6th Edition
Krar • Gill • Smid
Metal-Cutting
Technology
Section 8
Copyright © The McGraw-Hill Companies, Inc.
Permission required for reproduction or display.
27-2
Metal Cutting Technology
• Metals used in products must be machined
efficiently to be economical
• Cutting metals efficiently requires
– Knowledge of metal to be cut
– How cutting tool material and its shape will
perform under various machining conditions
• Many new cutting-tool materials introduced
in last few decades
– Improved machine construction, higher cutting
speeds and increased productivity
PowerPoint to accompany
Technology of Machine Tools
6th Edition
Krar • Gill • Smid
Physics of Metal
Cutting
Unit 27
Copyright © The McGraw-Hill Companies, Inc.
Permission required for reproduction or display.
27-4
Objectives
• Define the various terms that apply to
metal cutting
• Explain the flow patterns of metal as it is
cut
• Recognize the three types of chips
produced from various metals
27-5
How Metal Is Cut
• Have used tools without understanding how
metal is cut
• Prior thought held that metal ahead of cutting
tool split (like ax splits wood)
• Since WWII, research conducted
– Found metal compressed and flows up face of
cutting tool
– Led to new cutting tools, speeds and feeds,
cutting-tool angles and clearances and cutting
fluids
27-6
Metal-Cutting Terminology
• Built-up edge
– Layer of compressed metal which adheres to and
piles up on face of cutting
tool edge
• Chip-tool interface
– Portion of face of cutting
tool on which chip slides
as cut from metal
Copyright © The McGraw-Hill Companies, Inc.
Permission required for reproduction or display.
27-7
Metal-Cutting Terminology
• Crystal elongation
– distortion of crystal structure of work material
occurring during
machining operation
• Deformed zone
– Area in which work
material deformed during
cutting
27-8
Metal-Cutting Terminology
• Plastic deformation
– Deformation of work material occurring in shear
zone during cutting action
• Plastic flow
– Flow of metal that occurs on shear plane (extends
from cutting-tool edge to corner between chip and
work surface)
27-9
Metal-Cutting Terminology
• Rupture
– Tear that occurs when brittle materials are cut
and chip breaks away from work surface
• Shear angle or plane
– Angle of area of material where plastic
deformation occurs
• Shear zone
– Area where plastic deformation of metal occurs
– Along plane from cutting edge of tool to
original work surface
27-10
Plastic Flow of Metal
• Study flat punches on ductile material
–
–
–
–
Stress pattern
Direction of material flow
Distortion created in metal
Used blocks of photoelastic materials
• Polarized light used to observe stress lines
– Saw series of colored bands – isochromatics
• Tested three punch types: flat, narrowfaced, and knife-edge
27-11
Flat Punch
• Flat punch forced into block of photoelastic
material
– Lines of constant maximum shear stress appear
– Isochromatics (shape of stress lines)
• Appear as family of curves almost passing through
corners of flat punch
• Greatest concentration occurs at each corner of punch
• Larger circular stress lines appear farther away from
punch
• Spacing relatively wide
27-12
Narrow-Faced Punch
• Narrow-faced punch forced into block of
photoelastic material
– Stress lines concentrated
• Punch corners
• Where punch meets top surface of work
– Isochromatics spaced closer than with flat
punch
27-13
Knife-Edge Punch
• Knife-edge punch forced into block of
photoelastic material
– Isochromatics becomes series of circles tangent
to the two faces of punch
– Flow of material occurs upward from point
toward free area along faces of punch
27-14
When Cutting Tool Engages
Workpiece
• Internal stresses are created
• Compression occurs in work material because
of forces exerted by cutting tool
• Concentration of stresses causes chip to shear
from material and flow along chip-tool
interface
– Since most metals ductile to some degree, plastic
flow occurs
• Determines type of chip produced
27-15
Chip Types
• Machining operations performed on lathes,
milling machines, or similar machine tools
produce ships of three basic types
Discontinuous
(segmented) chip
Copyright © The McGraw-Hill Companies, Inc.
Permission required for reproduction or display.
Continuous
chip
Continuous chip
with built-up edge
27-16
Type 1 - Discontinuous
(Segmented) Chip
• Produced when brittle metals are cut
• Point of cutting tool contacts metal
some compression occurs and chip
begins to flow
• More cutting action produces more
stress, metal compresses until
rupture, and chip separates from
unmachined portion
• Poor surface created
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
27-17
Production of Type 1
Discontinuous Chip
• Conditions that favor the production
– Brittle work material
– Small rake angle on the cutting tool
– Large chip thickness (coarse feed)
– Low cutting speed
– Excessive machine chatter
27-18
Type 2 – Continuous Chip
• Continuous ribbon produced when flow of
metal next to tool face not retarded by builtup edge or friction
• Ideal for efficient cutting action
• Results in better surface finishes
• Plastic flow as deformed metal slides on
great number of crystallographic slip planes
– No fractures or ruptures occur due to ductile
nature
27-19
Direction of Crystal Elongation
Tool
As Cutting action progresses,
metal ahead of tool is compressed
with resultant deformation
(elongation) of crystal structure.
Shear
Angle
Plane of Shear
Shear Zone
27-20
Conditions Favorable to
Producing Type 2 Chip
•
•
•
•
•
•
Ductile work material
Small chip thickness (relatively fine feeds)
Sharp cutting-tool edge
Large rake angle on cutting tool
High cutting speeds
Cutting tool and work kept cool using
cutting fluids
27-21
Conditions Favorable to
Producing Type 2 Chip
• Minimum resistance to chip flow
– High polish on cutting-tool face
– Use of cutting fluids
– Use of cutting-tool materials which have low
coefficient of friction
• Cemented carbides
– Free-machining materials
• Those alloyed with lead, phosphor, and sulphur
27-22
Type 3 - Continuous Chip
with Built-Up Edge
• Low-carbon machine steel and high-carbon
alloyed steels
Tool
• Low cutting speed with
high-speed steel cutting tool
chip
• Without use of cutting fluids
• Poor surface finish
Built-up
Edge
Finished Surface of Work
27-23
Type 3 – Continuous Chip with
Built-Up Edge
• Small particles of metal adhere to edge of tool
– Build-up increases until becomes unstable and
breaks off
– Portions stick to both chip and workpiece
– Buildup and breakdown occur rapidly during
cutting action
• Shortens cutting-tool life
– Fragments of build-up edge abrade tool flank
– Cratering effect caused short distance back from
cutting edge where chip contacts tool face