Tool life Flashcards
modes of tool failure
- fracture failure: cutting forces of dynamic vibrations break the tool
- temperature failure: cutting temperature too high for the material
- gradual wear
the first two are unexpected events that can give high cost, they must be avoided;
the third can be modeled and predicted
phenomena causing tool wear
- mechanical: abrasion of the material on the rake face
- thermal: cutting temperature too high
- chemical: related to thermal effects
mechanisms of wear formation
- abrasion by high hardness particles
- diffusion: atoms passage between part and tool
- oxidation: oxygen forms oxides
- adhesion: built up edge
- plastic deformation
- fatigue: thermal or mechanical cycles
factors influencing tool wear
- tool characteristics
- temperature of the cutting zone, related to cutting speed
- cutting parameters
- cooling and lubrication of the cutting zone
- presence of thermal cycles
- chemical affinity between tool and workpiece
locations where tool wear occurs
- crater wear, on top of the rake face
- it weakens the tool
- it is quantified by crater depth KT and position of the deepest point of the crater KM (wrt the original tool) - flank wear
- it affects dimensional accuracy and surface finish
- it is quantified by VB average or VBmax height of the wear zone
flank wear as a function of cutting time
- break-period: rapid initial wear
- steady-state region: uniform wear rate
- failure region: accelerating wear rate
flank wear as a function of cutting speed
- slope increases in any zone as speed increases
- failure region starts before
rake wear as a function of cutting time
linear relation
rake wear as a function of cutting speed
slope increases as speed increses
definition of failure criteria
- a maximum wear value is defined
- from this value the tool life is estimated using the graph
Classical Taylor’s law
The relation between Tool life T and cutting speed vc can be approximated by a line with negative slope in the log-log graph
vc * T^n = C
where n and C are parameters depending on workpiece material, feed and cutting depth, to be identified with exeriments
Generalized Taylor’s law
vc * T^n * f^m * ap^q = C*
with 4 parameters to be identified
more accurate, but the classical law is more used because it’s simpler and gives good results
tool life as a function of cutting speed: real graph
- log(T) - log(vc) graph
- there is an initial zone where tool life decreases for increasing speed, due to built up edge at low speeds
- minimum point
- region where tool life increases with increasing cutting speed
- maximum point
- approximated linear zone, where taylor’s law is valid.
- for the same tool life, there may be two possible speeds; the highest is the best to be chosen
cutting tool materials: main requirements
- toughness
- wear resistance
- hardness at high temperature
- heat conductivity
- low termal expansion coefficient
- chemical inertia
- limited cost
cutting tool materials
with constant tool life, in order of increasing cutting speed (and cost):
- carbon steels
- high speed steels
- cast alloys
- carbides (coated or uncoated)
- ceramics
- ceramic metals
- CBN: cubic boron nitride
- diamond
note that very hard materials at high temperature (diamond) have low strength and toughness, while steels have the opposite behaviour. the desired behaviour actually is not achievable.
Ceramics and coated carbides balance these characteristics
