1. Metal-Cutting Technology
Section 8

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

3. Physics of Metal Cutting
Unit 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

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

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
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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

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)

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

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, narrow-faced, and knife-edge

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

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

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

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

15. Chip Types
Machining operations performed on lathes, milling machines, or similar machine tools produce ships of three basic types
Discontinuous (segmented) chip
Continuous chip
Continuous chip with built-up edge
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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
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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

18. Type 2 – Continuous Chip
Continuous ribbon produced when flow of metal next to tool face not retarded by built-up 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

19. Direction of Crystal Elongation
Tool
As Cutting action progresses, metal ahead of tool is compressed with resultant deformation (elongation) of crystal structure.
Plane of Shear
Shear
Angle
Shear Zone

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

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

22. Type 3 - Continuous Chip with Built-Up Edge
Low-carbon machine steel and high-carbon alloyed steels
Low cutting speed with high-speed steel cutting tool
Without use of cutting fluids
Poor surface finish
Tool
chip
Built-up Edge
Finished Surface of Work

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