Engineering Manufacturing 2 Cutting mechanics and machining (3)

which region is the primary shear zone

it is the region where shear deformation takes place due to the tool digging into the workpiece

which region is the secondar shear zone

region where sheared material moves over rake face to form chip

things to note about secondary shear zone

-part of the chip adheres to rake face due to high pressure exerted by the chip on the tool and highly reactive chip surface

-there is sliding friction

which region is the tertiary zone

contact area between flank face and freshly generated workpiece surface

things to note about tertiary zone

-contact forces are cause by the elastic springback of the freshly machined surface

-contact between tool and workpiece is less intimate due to lower contact forces

-workpiece surface is highly reactive as it has not started to oxidise, therefore highly reactive and thus subject to intense interactions with tool flank face

what is the primary source of heat generation in metal cutting processes?

the deformation in the primary shear zone is mostly plastic, resulting substantial shear strain and converted into heat energy

where is the highest temperature recorded?

where the chip separates from the rake face

rule of thumb of heat distribution

75-80% chip, 10-20% tool, 10-15% workpiece

how to increase the heat distribution taken by chip and why?

increase relative velocity between tool and workpiece, as less time for heat to migrate to tool from chip and reduced chip adhesion due to softening of materials in response to higher temperatures

5 chip types

Continuous

Continuous with built up edge

segmented

serrated

discontinuous

continuous chips, why they form, and how they can be promoted

Form when material is sufficiently ductile, uniform microstructure and deformation does not cause embrittlement. Can be promoted by low tool-chip friction on the rake face

Why do BUE continuous chips form, and any negatives of it

due to a strong bond between chip and tool, workpiece material can accumulate on the rake face, creating a new built up edge for the tool. When the BUE breaks off, segments are embedded in the fresh surface, deteriorating tool quality

Why do segmented chips form

when material is sufficiently ductile, but with periodic variation in thickness, caused by stick-slip oscillation and fracture. Only happens in certain cutting speed ranges

Why do serrated chips form

when material is not ductile enough or microstructure is not uniform. Typical for high cutting speeds due to extremely high shear rates or materials that exhibit low thermal conductivity

Why do discontinuous chips form

When cutting brittle material with uneven microstructure., there is a periodic rupture in the shear zone, caused by an oscillatory compressive stress on the shear zone, originating at the tool edge and propagating toward the free surface