Chapter 17 Cutting Tool Technology

Three modes of failure

1. fracture

   1. cutting force excessive & dynamic → brittle fracture

2. temperature

   1. temp too hi

3. gradual wear

   1. preferred mode, only non premature failure

   2. two locations:

      1. crate (rake face)

      2. flank: flank (side)

Taylor tool life equation

vT^n = C

v = cutting speed

T = tool life

n and C parameters based on feed, depth of cut, work mtl, tool mtl, tool life criterion

n is slope of plot

Tool mtls

• toughness: avoid fracture failure

• hot hardness: ability to retain hardness @ hi temp

  • ceramics > carbides > hi speed steel > C steel

• wear resistance: hardness most important to resist abrasive wear

Hi speed steel HSS

highly alloyed steel capable of maintaining hardness @ elevated temp better than high carbon / low alloy steels

• drills, taps, milling cutters, broaches

types of HSS:

• tungsten type T grade

• molybdenum type M grade

other ingredients:

• chromium & vanadium

• carbon

• cobalt

Cemented carbides

class of hard tool mtl based on tungsten carbide (WC) using powder metallurgy with cobalt (Co)

types:

• non steel cutting grades (WC-Co)

• steel cutting grades (TiC and TaC added to WC-Co)

properties:

• high compressive strength, low to mod tensile

• high hardness and hot hardness

• wear resistance

• hi thrmal conductivity

• high elastic mod

• toughness < hi speed steel

Cermets

combos of TiC, TiN, titanium carbonitride w/ nickel & molybdenum as binders

applications:

• hi speed finishing & semi finishing of steels, stainless steels, and cast irons

• high speeds, low feeds than steel cutting cemented carbide grades

• better finish achieved

Coated carbides

cemented carbide insert coated in one or more layers of TiC, TiN, or Al2O3

• thickness: 2.5 - 13 micro meters

applications:

• cast irons and steels in turning and milling operations

Tool geometry

• single point:

  • turning, boring, shaping and planing

• multiple:

  • drilling, reaming, tapping, milling, broaching, sawing

More on single point tools

• solid shank (HSS)

• brazed cemented carbide insert

• mechanically clamped insert

  • cemented carbides, ceramics, other hard tool mtls

Chip breaker purpose

1. eliminating long chips

2. controlling chip flow

3. reducing vibration and heat generated

Twist drill

most common cutting tool for hole making

HSS

• flutes must provide sufficient clearance to allow chips to move from bottom of hole during cutting

Cutting fluids

problems addressed:

• heat generation @ shear & friction zones

• friction @ tool chip and tool work interfaces

• washes away chip

• reduces temp

• improved dimensional stability

class:

• coolant: reduce effects of heat, water based

• lubricant: reduce tool chip & tool work friction, oil based

Mechanical properties and machinability

• hardness: hi hardness → abrasive wear increases → tool life reduced

• strength: hi strength → hi cutting forces, specific energy, cutting temp

• ductility: hi ductility → tearing of metal to form chip → chip disposal problems → poor finish