Cutting Tools

Cutting Tools:

1. One of most important components in machining process

2. Performance will determine efficiency of operation

3. Two basic types (excluding abrasives)

     - Single point and multi point

4. Must have rake and clearance angles ground or formed on them

Cutting-Tool Materials:

1. Lathe toolbits generally made of five materials

     - High-speed steel

     - Cast alloys (such as stellite)

     - Cemented carbides

     - Ceramics

     - Cermets

2. More exotic finding wide use

     - Borazon and polycrystalline diamond

Lathe Toolbit Properties

1. Hard

2. Wear-resistant

3. Capable of maintaining a red hardness during machining operation

     Red hardness: ability of cutting tool to maintain sharp cutting edge even when turns red because of high heat during cutting

4. Able to withstand shock during cutting

5. Shaped so edge can penetrate work

High-Speed Steel Toolbits

1. May contain combinations of tungsten, chromium, vanadium, molybdenum, cobalt

2. Can take heavy cuts, withstand shock and maintain sharp cutting edge under red heat

3. Generally two types (general purpose)

     - Molybdenum-base (Group M)

     - Tungsten-base (Group T)

4. Cobalt added if more red hardness desired

Cemented-Carbide Toolbits

1. Capable of cutting speeds 3 to 4 times high-speed steel toolbits

2. Low toughness but high hardness and excellent red-hardness

3. Consist of tungsten carbide sintered in cobalt matrix

4. Straight tungsten used to machine cast iron and nonferrous materials (crater easily)

5. Different grades for different work

Cemented-Carbide Applications

1. Used extensively in manufacture of metal-cutting tools

     - Extreme hardness and good wear-resistance

2. First used in machining operations as lathe cutting tools

3. Majority are single-point cutting tools used on lathes and milling machines

Types of Carbide Lathe Cutting Tools

1. Brazed-tip type

     - Cemented-carbide tips brazed to steel shanks

     - Wide variety of styles and sizes.

2. Indexable insert type

     - Throwaway inserts

     - Wide variety of shapes: triangular, square, diamond, and round

     - Triangular: has three cutting edges

     - Inserts held mechanically in special holder

Grades of Cemented Carbides

1. Two main groups of carbides

     - Straight tungsten carbide

          - Contains only tungsten carbide and cobalt

          - Strongest and most wear-resistant

          - Used for machining cast iron and nonmetals

     - Crater-resistant 

           - Contain titanium carbide and tantalum carbide in addition to tungsten carbide and cobalt

          - Used for machining most steels

Coated Carbide Toolbits

1. Made by depositing thin layer of wear-resistant titanium nitride, titanium carbide or aluminum oxide on cutting edge of tool

     - Fused layer increases lubricity, improves cutting edge wear resistance by 200%-500%

     - Lowers breakage resistance up to 20%

     - Provides longer life and increased cutting speeds

2. Titanium-coated offer wear resistance at low speeds, ceramic coated for higher speeds

Ceramic Toolbits

1. Permit higher cutting speeds, increased tool life and better surface finish than carbide

     - Weaker than carbide used in shock-free or low-shock situation

2. Ceramic

     - Heat-resistant material produced without metallic bonding agent such as cobalt

     - Aluminum oxide most popular additive

     - Titanium oxide or Titanium carbide can be added

Diamond Toolbits

1. Used mainly to machine nonferrous metals and abrasive nonmetallics

2. Single-crystal natural diamonds

     - High-wear but low shock-resistant factors

3. Polycrystalline diamonds

     - Tiny manufactured diamonds fused together and bonded to suitable carbide substrate

Carbide Cutting Tools

1. First used in Germany during WW II as substitute for diamonds

2. Various types of cemented (sintered) carbides developed to suit different materials and machining operations

     - Good wear resistance

     - Operate at speeds ranging 150 to 1200 sf/min

3. Can machine metals at speeds that cause cutting edge to become red hot without loosing harness

Cutting-Tool Nomenclature

Cutting edge: leading edge of that does cutting

Face: surface against which chip bears as it is separated from work

Nose: Tip of cutting tool formed by junction of cutting edge and front face

Base: Bottom surface of tool shank

Flank: surface of tool adjacent to and below cutting edge

Shank: body of toolbit or part held in toolholder

Nose radius: radius to which nose is ground

Size of radius will affect finish

Rough turning: small nose radius (.015in)

Finish cuts: larger radius (.060 to .125 in.)

Point: end of tool that has been ground for cutting purposes

Lathe Toolbit Angles and Clearances

Lathe Cutting-tool Angles

Positive Rake Angle

1. Considered best for efficient removal of metal

     - Creates large shear angle at shear zone

     - Reduces friction and heat

     - Allows chip to flow freely along chip-tool interface

2. Generally used for continuous cuts on ductile materials not too hard or abrasive

Factors When Choosing Type and Rake Angle for Cutting Tool

1. Hardness of metal to be cut

2. Type of cutting operation

     - Continuous or interrupted

3. Material and shape of cutting tool

4. Strength of cutting edge

Shape of Chip

1. Altered in number of ways to improve cutting action and reduce amount of power required

2. Continuous straight ribbon chip can be changed to continuous curled ribbon

     - Changing angle of the keeness

     - Included angle produced by grinding side rake

     - Grinding chip breaker behind cutting edge of toolbit

Factors Affecting the Life of a Cutting Tool

1. Type of material being cut

2. Microstructure of material

3. Hardness of material

4. Type of surface on metal (smooth or scaly)

5. Material of cutting tool

6. Profile of cutting tool

7. Type of machining operation being performed

8. Speed, feed, and depth of cut

Turning:

Assume cutting machine steel:  If rake and relief clearanceangles correct and proper speed and feed used, a continuouschip should be formed.

Nomenclature of an End Mill

Operating Conditions

1. Three operating variables influence metal-removal rate and tool life

     - Cutting speed

     - Feed rate

     - Depth of cut

General Operating Condition Rules

1. Proper cutting speed most critical factor to consider establishing optimum conditions

     - Too slow:  Fewer parts produced, built-up edge

     - Too fast: Tool breaks down quickly

2. Optimum cutting speed should balance metal-removal rate and cutting-tool life

3. Choose heaviest depth of cut and feed rate possible

Blending

1. Five types of powders

     - Tungsten carbide, titanium carbide, cobalt, tantalum carbide, niobium carbide

2. One or combination blended in different proportions depending on grade desired

3. Powder mixed in alcohol (24 to 190 h)

4. Alcohol drained off

5. Paraffin added to simplify pressing operation

Compaction

1. Must be molded to shape and size

2. Five different methods to compact powder

     - Extrusion process

     - Hot press

     - Isostatic press

     - Ingot press

     - Pill press

3. Green (pressed) compacts soft, must be presintered to dissolve paraffin

Presintering

1. Green compacts heated to about 1500º F in furnace under protective atmosphere of hydrogen

2. Carbide blanks have consistency of chalk

3. May be machined to required shape

     - 40% oversize to allow for shrinkage that occurs during final sintering

Sintering

1. Last step in process

2. Converts presintered machine blanks into cemented carbide

3. Carried out in either hydrogen atmosphere or vacuum

     - Temperatures between 2550º and 2730º F

4. Binder (cobalt) unites and cements carbide powders into dense structure of extremely hard carbide crystals

Cutting Speeds and Feeds

1. Important factors that influence speeds, feeds, and depth of cut

     - Type and hardness of work material

     - Grade and shape of cutting tool

     - Rigidity of cutting tool

     - Rigidity of work and machine

     - Power rating of machine

Milling

1. Face milling

     - Ring-type distributor recommended to flood cutter completely

     - Keeps each tooth of cutter immersed in cutting fluid at all times

2. Slab milling

     - Fluid directing to both sides of cutter by fan-shaped nozzles ¾ width of cutter.

Cutting tool materials

1. Selection of cutting tool materials is very important

2. 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

Types of tool materials

1. Carbon  steel

2. High speed steel (HSS)

3. Cemented Carbides

4. Cast  alloys

5. Ceramics

6. Cubic boron nitride (CBN)

7. Diamond

Carbon Steel

1. Oldest of tool materials

2. Used for drills taps,broaches ,reamers

3. Inexpensive ,easily shaped ,sharpened

4. No sufficient hardness and wear resistance

5. Limited to low cutting speed operation

High speed steel

1. Retains  its  hardness  at  high  temperature

2. Red  hardness….

3. Relatively  good wear resistance

Tungsten Carbide

1. Composite material consisting of tungsten-carbide particles bonded together 

2. Alternate name is cemented carbides

3. Manufactured with powder metallurgy techniques  p335  Fig. 2

4. Small  particles  are pressed & sintered to desired shape 

5. Amount of cobalt present affects properties of carbide tools

6. As cobalt content increases – the  tougher  the  tool

Making tungsten carbides

Tungsten  &  carbon  mixed  then  heated  to  give  tungsten  carbide

Mix  tungsten  carbide  powder  with  binder

Usually  cobolt

                   Pressing  to  shape

                   Sintered

Cast alloys

1. Commonly known as stellite tools

2. Composition ranges –  38% - 53 % cobalt

                   30%- 33% chromium

                   10%-20%tungsten

3. Good wear resistance ( higher hardness)

4. Less tough than high-speed steels and sensitive to impact forces

5. Less suitable than high-speed steels for interrupted cutting operations

6. Continuous roughing cuts – relatively high g=feeds & speeds

7. Finishing cuts are at lower feed and depth of cut

Inserts

1. Individual cutting tool with severed cutting points

2. Clamped on tool shanks with locking mechanisms

3. Inserts also brazed to the tools 

4. Clamping is preferred method for securing an insert

5. Carbide Inserts available in various shapes-Square, Triangle, Diamond and round

6. Strength depends on the shape 

7. Inserts honed, chamfered or produced with negative land to improve edge strength

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.

Ceramics

1. Used as grinding wheels. 

2. As cutting tool inserts. These are used in a similar way to cemented carbide inserts.

3. They can withstand extremely high machining temperatures. 

4. They also have a high resistance to abrasion.

5. Ceramic cutting tools can he used to machine ‘difficult’ materials at really high cutting speeds — sometimes over 2000 m/min. 6. Compare this with the cutting speed for carbon steel cutting tools — 6 m/min. 

7. Ceramic cutting tools are very brittle. 

8. They can be used only on machines which are extremely rigid and free of vibration.

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).

Diamond:

1. Hardest known substance 

2. Low friction, high wear resistance

3. Ability to maintain sharp cutting edge

4. Single crystal diamond of various carats used for special applications

5. Machining copper—front precision optical mirrors for ( SDI)

6. Diamond is brittle , tool shape & sharpened is important

7. Low rake angle used for string cutting edge.