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Cutting Tool Materials

Cutting Tool Materials. Questions p.333. List the properties of the ideal cutting tool material. What is the main advantage of carbon steel cutting tools ? Explain the term red hardness . What makes up high speed steel ? What are tungsten carbides ?

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Cutting Tool Materials

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  1. Cutting Tool Materials

  2. Questions p.333 • List the properties of the ideal cutting tool material. • What is the main advantage of carbon steel cutting tools ? • Explain the term red hardness . • What makes up high speed steel ? • What are tungsten carbides ? • Explain how tungsten carbides are produced . • List the advantages of tungsten carbides as cutting tools. • How are they graded ? • List 4 uses for tungsten carbides. • Give the uses and properties for the following cutting tool materials (i) cast alloys (ii) ceramics (iii) cubic boron nitride (iv) diamond .

  3. Cutting tool materials • Selection of cutting tool materials is very important • What properties should cutting tools have • Hardness at elevated temperatures • Toughness so that impact forces on the tool can be taken • Wear resistance • Chemical stability

  4. Cutting Tool Construction

  5. Types of tool materials • Carbon steel • High speed steel (HSS) • Cemented Carbides • Cast alloys • Ceramics • Cubic boron nitride (CBN) • Diamond

  6. Carbon Steel • Oldest of tool materials • Used for drills taps,broaches ,reamers • Inexpensive ,easily shaped ,sharpened • No sufficient hardness and wear resistance • Limited to low cutting speed operation

  7. Temperature effects

  8. High speed steel • Retains its hardness at high temperature • Red hardness…. • Relatively good wear resistance

  9. Tungsten Carbide • Composite material consisting of tungsten-carbide particles bonded together • Alternate name is cemented carbides • Manufactured with powder metallurgy techniques p335 Fig. 2 • Small particles are pressed & sintered to desired shape • Amount of cobalt present affects properties of carbide tools • As cobalt content increases – the tougher the tool

  10. Making tungsten carbides

  11. Step 1 Tungsten & carbon mixed then heated to give tungsten carbide

  12. Mix tungsten carbide powder with binder Usually cobolt Step 2

  13. Step 3 Pressing to shape

  14. Step 4 Sintered

  15. Cast alloys • Commonly known as stellite tools • Composition ranges – 38% - 53 % cobalt 30%- 33% chromium 10%-20%tungsten • Good wear resistance ( higher hardness) • Less tough than high-speed steels and sensitive to impact forces • Less suitable than high-speed steels for interrupted cutting operations • Continuous roughing cuts – relatively high g=feeds & speeds • Finishing cuts are at lower feed and depth of cut

  16. Inserts

  17. Inserts • Individual cutting tool with severed cutting points • Clamped on tool shanks with locking mechanisms • Inserts also brazed to the tools • Clamping is preferred method for securing an insert • Carbide Inserts available in various shapes-Square, Triangle, Diamond and round • Strength depends on the shape • Inserts honed, chamfered or produced with negative land to improve edge strength

  18. Insert Attachment Fig : Methods of attaching inserts to toolholders : (a) Clamping and (b) Wing lockpins. (c) Examples of inserts attached to toolholders with threadless lockpins, which are secured with side screws.

  19. Ceramics • Used as grinding wheels. • as cutting tool inserts. These are used in a similar way to cemented carbide inserts. • they can withstand extremely high machining temperatures. • They also have a high resistance to abrasion.

  20. Ceramics • Ceramic cutting tools can he used to machine ‘difficult’ materials at really high cutting speeds — sometimes over 2000 m/min. Compare this with the cutting speed for carbon steel cutting tools — 6 m/min. • Ceramic cutting tools are very brittle. • They can be used only on machines which are extremely rigid and free of vibration.

  21. Cubic boron Nitride ( CBN ) : • Made by bonding ( 0.5-1.0 mm ( 0.02-0.04-in) • Layer of poly crystalline cubic boron nitride to a carbide substrate by sintering under pressure • While carbide provides shock resistance CBN layer provides high resistance and cutting edge strength • Cubic boron nitride tools are made in small sizes without substrate Fig : (a) Construction of a polycrystalline cubic boron nitride or a diamond layer on a tungsten-carbide insert. (b) Inserts with polycrystalline cubic boron nitride tips (top row) and solid polycrystalline CBN inserts (bottom row).

  22. Diamond : • Hardest known substance • Low friction, high wear resistance • Ability to maintain sharp cutting edge • Single crystal diamond of various carats used for special applications • Machining copper—front precision optical mirrors for ( SDI) • Diamond is brittle , tool shape & sharpened is important • Low rake angle used for string cutting edge

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