Workpiece materials, tool materials and cutting fluids Flashcards
Material characteristics affecting machinability
- hardness
- ductility
- strain hardening
- abrasiveness
- thermal conductivity
steel:
- hardness
- carbon content
- crystal structure
- alloys
Tool materials
- HSS
- wolframcarbide
- ceramic
- Cubic Boron Nitride
- diamond
growing hardness and decreasing toughness
Characteristics of cutting tool materials
- hardness (harder than workpiece)
- toughness (variation of chip thickness)
- heat resistance (cutting temperature)
- low friction (wear and surface quality)
- low chemical reactivity (air and workpiece)
Composition of HSS
Increasing Mo: toughness
Increasing W: wear resistance
Increasing Co: Both
Types: conventional and powder
What is a typical coating for HSS tools?
Top Layer to lowest:
- TiN
- Al2O3
- TiC
- WC
Ceramics
• Manufacturing similar to WC: powder pressed and sintered
• Mostly:
- Al2O3 (white)
- Si3N4 (dark)
- Mixed ceramics Al2O3 with 20 - 30% TiC or TiN
• High cutting speeds
• Brittle: Do not stand vibration or impacts
• Typically for turning of cast iron and hard steels
PCBN (Polycrystalline Cubic Boron Nitride)
- Man-made material, not found in nature
- Extremely hard, also in high temperatures, inert to iron
- Binder: WC, Al, TiAl, TiC, TiN
- Applications: hard steels, hard cast materials
PCD (Polycrystalline Diamond)
- Synthethic diamond, which consists of randomly oriented diamond grains, which are bound together with WC binder
- Types are separated according to diamond crystal size (fine 2 µm, medium 10 µm, rough 25µm)
- cutting of very abrasive materials
- not for iron
- typically aluminium, plastic, magnesium, copper
Purpose of cutting fluids
- Tool and workpiece cooling
- Friction reduction
- Chip evacuation
- Corrosion protection
Characteristics of cutting fluids
- high heat capacity
- high thermal conductivity
- good pressure resistance
- non-foaming
- stable ph
- corrosion protective
Types of cutting fluids
Emulsions: - oil content 10-20% - milk-like - solvable to water cooling of water and lubrication of oil
Oils:
- mineral, plant or animal
- good lubrication and corrosion protection
- poor cooling
When do cooling lubricants have the best lubrication?
Better lubrication when
- Small cutting speed
- Poor machinability
- Difficult cutting method
When do cooling lubricants have the best cooling?
Better cooling when
- Small cutting speed
- Poor machinability
- Difficult cutting method
- BUE problems
Controlling of cutting fluid
- Concentration using refractometer
- pH value with litmus paper or pH meter
- Bacteria content
- Electrical conductivity
Degradation of cutting fluid
- Dirt from tools and workpieces - Reaction with air and metal
- Leak oil
- Bacteria
- Concentration change due to water evaporation
