Machinability and Chip Formation
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Transcript Machinability and Chip Formation
Chip Formation
Machinability
relative difficulty of a machine
operation with regard to tool
life, surface finish, and power
consumption
generally softer materials easier to machine
Metal does NOT split off
ahead of cutter as in wood
Metal is sheared off at
SHEAR PLANE
metal is forced ahead of tool
Surface finish affected by:
speeds
feeds
depth of cut
tool shape
use of cutting fluid
rigidity of the setup
Optimum chip for operator
safety is figure “9”
Chip breaker
curls chip to break it off
keep from tangling in machine
safer
3 basic types of chip
formation
1. Continuous chip
Continuous chip
characteristics
better surface finish
soft or medium hardness materials
that are ductile
low coefficient of friction - pass
across top of well polished tool
chips curl or are straight and stringy
chip breaker used to break the chip
2. Discontinuous chip
(segmented)
Discontinuous chip
characteristics
materials that fracture easily (cast
iron)
fails or breaks after only a small
amount of deformation
no chip breaker required
chips are cleaned up easily
3. Continuous chip (with
built up edge on tool)
Continuous chip (built of
edge) characteristics
soft materials - high coef of friction
stick to top of entering edge of tool
caused by heat and pressure of
cutting action
material temporarily welds to tip of
cutting tool then releases
rougher surface finish
tool life shortened
Solutions for built up edge
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no single solution
change tool geometry
use chip breaker
cutting fluids
best combination of speeds and feeds
Cutting tool geometry
positive rake
neutral rake
negative rake
Positive rake tools
Positive rake tool
characteristics
freer cutting at low speeds
pos rake tools, cutting fluids, and
higher speeds decrease tendency for
built up edge - however,
large pos rake = continuous chip
Negative rake tools
Negative rake tool
characteristics
surface disrupted more
require more power
stronger and have longer working life
than positive
low cutting speeds = poor surface
finish
high cutting speeds = good surface
finish
Most carbide tools have
negative rake because:
indexable insert can be turned over
withstands more cutting pressure
higher cutting speeds used