manufacturing engineering 2 tool wear and cooling strategies

3 types of tool failure

mechanical breakage, dulling of cutting edges, gradual wear

3 types of gradual tool wear

mechanical wear (abrasion and adhesion), thermo-mechanical wear (diffusion), chemical wear (oxidation)

what is abrasion and how can we reduce it

caused by microscopic variations on the bottom surface of the chips and the freshly machined workpiece surface constantly rubbing against the tool

can be reduced by increasing cutting temp, therefore softening cutting material

what is adhesion

micro-welding of workpiece material from underside of chip to tool, promoted by high pressure between chip and workpiece and elevated cutting temp. When joint is fractured as result of chip motion, bits of tool are torn out

what is diffusion

when atoms from tool diffuse to chip due to high temps and mutual solubility

what is oxidation

surfaces form an oxide film due to exposure to environment, usually occurring when using cutting fluids in high cutting velocities.

how does increasing temperature affect tool wear

abrasion wear always present but becomes worse due to softening of tool substrate

Adhesion wear lowers due to weakening bond strength between constituents

diffusion and oxidation require high temps to occur

2 geometric patterns of tool wear

crater wear and flank wear

what is crater wear caused by

caused by diffusion

how does crater wear affect everything

increases rake angle and reduces coefficient of friction, making cutting easier. However, it weakens tool cutting edge and increases probability of abrupt failure

what is flank wear caused by

rough abrasion in the tertiary shear zone

what is flank wear shown as

flank wear land, where the cutting tool is flattened at the flank face

how does flank wear affect everything

it increases friction between workpiece and tools, requiring higher cutting force and higher temps

makes machined surface much rougher

decreases clearance angle

how can we reduce overall tool wear

lower temperatures as much as possible

different methods of cooling

flood cooling, minimal quantity lubrication

what is flood cooling

dumping lots of fluid near the cutting zone to cool, lubricate and flush away chips. supplied at 5->50l/min at up to 100 bar. Coolants are typically 90-95% water and rest is additives to enhance cooling, lubrication and prevent corrosion and foaming

bad flood cooling

high cost, health problems, long term impacts on env

What is MQL

provision of little lubricants, usually oil or synthetic esters, carried to the cutting zone in a stream of compressed air

good-bad MQL

+reduce friction

+much more env friendly

-cooling less strong due to tiny quantities of liquid supplied and low heat capacity of oil( but some have alcohol mixed in to enhance cooling quality)