Metal-Cutting Technology Section 8

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

Physics of Metal Cutting Unit 27

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

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

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.

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

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)

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

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

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

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

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

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

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 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

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.

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

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

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

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

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

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

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