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

• 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