Metal Machining

Question 1

Machining

Answer 1

material removal by a sharp cutting tool

Question 2

Abrasive processes

Answer 2

material removal by hard or abrasive particles

Question 3

Chip formation

Answer 3

Cutting action involves shear deformation of work material to form a chip. As the chip is removed the new surface is exposed.
Chips are produced by shearing which takes place in a shear zone along a well defined plane referred to as the shear plane at an angle (shear angle).
Below the shear plane the workpiece remains undeformed; above it the chip that is already formed moves up the rake face of the tool.

Question 4

Single-Point Tools

Answer 4

One dominant cutting edge. Point is usually rounded to form a nose radius. Turning uses single point tools.

Question 5

Multiple Cutting Edge Tools

Answer 5

More than one cutting edge. Motion relative to work achieved by rotating. Drilling and milling use rotating multiple cutting edge tools.

Question 6

Major independent variables

Answer 6

 Tool material, coating and condition.
 Tool shape, surface finish and sharpness.
 Cutting parameters, such as speed, feed and depth of cut.
 Characteristics of the machine tool - stiffness & damping.
 Workholding, fixturing etc.
 Use of cutting fluid.

Question 7

Continuous chip

Answer 7

 Narrow, straight and primary shear zone.
 Ductile materials at high speed or high rake angles.
 Small feeds and depth of cut.
 Bad for automation (use chip breakers).

Question 8

Continuous chip with build up edge

Answer 8

 Friction between tool and chip tend to cause portions of material to adhere to the rake face.
 Formation is cyclical, becomes unstable and breaks off.
 Ductile materials at low/medium cutting speeds.

Question 9

Serrated (segmented) chip

Answer 9

 Low thermal conductivity materials and strength that
decreases sharply with temperature.
 Cyclical chip formation of alternating high shear strain followed by low shear strain (sawtooth like appearance).

Question 10

Discontinuous chip

Answer 10

 Low ductility materials and/or negative rake angle.
 Brittle materials as they do not have high shear strains.
 Very low or very high cutting speeds.
 Workpiece with inclusions or impurities.

Question 11

Chip Breaker

Answer 11

 Long chips are often generated machining ductile materials.
 Cause hazard to operator, bad for workpiece finish and interfere with automatic operations.
 Groove-type chip breaker designed into the cutting tool.
 Obstruction-type chip breaker designed as an additional device on the rake face.

Question 12

98% of the energy in machining…

Answer 12

…is converted into heat.

Question 13

temperatures at the tool-chip…

Answer 13

…are very high.

Question 14

The remaining energy (~2%)…

Answer 14

…is retained as elastic energy in the chip.

Question 15

High cutting temperatures…

Answer 15

 Reduce tool life.
 Produce hot chips that pose safety hazards to the machine operator.
 Can cause inaccuracies in part dimensions due to thermal expansion of work material.

Question 16

Cutting Fluids

Answer 16

 Reduce friction and wear, thus improving tool life and surface finish.
 Cool the cutting zone, thus improving tool life and reducing the temperature and thermal distortion of the workpiece.
 Reduce forces and energy consumption.
 Flush away the chips from the cutting zone.

Question 17

Oils – mineral, animal

Answer 17

vegetable or synthetic; typically used for low speed operations where temperature rise is not significant.

Question 18

Emulsions (soluble oils)

Answer 18

mixture of oil and water and additives; generally used for high speed operations with significant temperature rises.

Question 19

Semi-Synthetics

Answer 19

chemical emulsions containing little mineral oils diluted in water and additives that reduce the size of oil particles making them more effective.

Question 20

Synthetics

Answer 20

chemicals with additives which are diluted in water; containing no oil.

Question 21

Mist

Answer 21

– supplied to inaccessible areas similar as with an aerosol can; effective with water-based fluids.

Question 22

High-pressure systems

Answer 22

– refrigerated coolant systems provide cutting fluid via specially designed nozzles; powerful jet can also act as chip breaker.

Question 23

Cutting tool system

Answer 23

– high pressure cutting fluid is applied through narrow passages in the tool or the workholder.

Question 24

Near-dry and Dry Machining

Answer 24

• very little or no cutting fluid used

Question 25

Flooding

Answer 25

• work area is flooded with cutting fluid to wash away and chips and keep the area cool.

Question 26

Toughness

Answer 26

ability to avoid fracture failure.

Question 27

Hot hardness

Answer 27

ability to retain hardness at high temperatures.

Question 28

Wear resistance

Answer 28

hardness is the most important property to resist abrasive wear.

Question 29

Thermal shock resistance

Answer 29

to withstand the rapid temperature cycling encountered in interrupted cutting.

Question 30

Chemical stability

Answer 30

inertness of the material to avoid any tool-chip diffusion that could contribute to tool wear.

Question 31

Cutting Tool – Materials

Answer 31

 High-Speed Steel
 Cemented Carbides
 Coated Carbides
 Cermets
 Ceramics
 Synthetic Diamonds 
 Cubic Boron Nitride

Question 32

Holding options

Answer 32

 Solid tool (typically HSS) is ground from a solid shank.
 Insert is brazed to a tool shank (tool steel for strength and toughness).
 Mechanically clamped inserts with multiple cutting edges can be easily unclamped and indexed when worn out.

Question 33

Fracture failure

Answer 33

cutting force becomes excessive and/or dynamic, leading to brittle fracture.

Question 34

Temperature failure

Answer 34

cutting temperature is too high for the tool material.

Question 35

Gradual wear

Answer 35

gradual wearing of the cutting tool causes loss of tool shape.
 Crater wear – on top on rake face
 Flank wear – on flank (side of tool)

Question 36

Crater wear

Answer 36

 Concave section on the rake face.
 Formed by the action of the chip sliding against the surface.
 Measured either by its depth or its area.

Question 37

Flank wear

Answer 37

 Occurs on the flank or relief face.
 Formed by rubbing between the new work surface and the flank face adjacent to the cutting edge.
 Measured by the width of the wear band (land).

Question 38

Break-in period

Answer 38

sharp cutting edge wears rapidly at beginning of use.

Question 39

Steady state wear

Answer 39

wear at a fairly uniform rate.

Question 40

Failure rate

Answer 40

higher cutting temperatures; efficiency of machining is reduced.

Question 41

Alternative Tool Life Criteria

Answer 41

 Complete failure of cutting edge.
 Visual inspection of flank wear (or crater wear) by the machine operator.
 Changes in sound emitted from operation.
 Chips become ribbon-like, stringy, and difficult to dispose of.
 Degradation of surface finish.
 Increased power requirements.
 Workpiece count decreases.