Metal Machining Flashcards
Machining
material removal by a sharp cutting tool
Abrasive processes
material removal by hard or abrasive particles
Chip formation
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.
Single-Point Tools
One dominant cutting edge. Point is usually rounded to form a nose radius. Turning uses single point tools.
Multiple Cutting Edge Tools
More than one cutting edge. Motion relative to work achieved by rotating. Drilling and milling use rotating multiple cutting edge tools.
Major independent variables
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.
Continuous chip
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).
Continuous chip with build up edge
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.
Serrated (segmented) chip
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).
Discontinuous chip
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.
Chip Breaker
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.
98% of the energy in machining…
…is converted into heat.
temperatures at the tool-chip…
…are very high.
The remaining energy (~2%)…
…is retained as elastic energy in the chip.
High cutting temperatures…
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.
Cutting Fluids
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.
Oils – mineral, animal
vegetable or synthetic; typically used for low speed operations where temperature rise is not significant.
Emulsions (soluble oils)
mixture of oil and water and additives; generally used for high speed operations with significant temperature rises.
Semi-Synthetics
chemical emulsions containing little mineral oils diluted in water and additives that reduce the size of oil particles making them more effective.
Synthetics
chemicals with additives which are diluted in water; containing no oil.
Mist
– supplied to inaccessible areas similar as with an aerosol can; effective with water-based fluids.
High-pressure systems
– refrigerated coolant systems provide cutting fluid via specially designed nozzles; powerful jet can also act as chip breaker.
Cutting tool system
– high pressure cutting fluid is applied through narrow passages in the tool or the workholder.
Near-dry and Dry Machining
- very little or no cutting fluid used
Flooding
- work area is flooded with cutting fluid to wash away and chips and keep the area cool.
Toughness
ability to avoid fracture failure.
Hot hardness
ability to retain hardness at high temperatures.
Wear resistance
hardness is the most important property to resist abrasive wear.
Thermal shock resistance
to withstand the rapid temperature cycling encountered in interrupted cutting.
Chemical stability
inertness of the material to avoid any tool-chip diffusion that could contribute to tool wear.
Cutting Tool – Materials
High-Speed Steel Cemented Carbides Coated Carbides Cermets Ceramics Synthetic Diamonds Cubic Boron Nitride
Holding options
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.
Fracture failure
cutting force becomes excessive and/or dynamic, leading to brittle fracture.
Temperature failure
cutting temperature is too high for the tool material.
Gradual wear
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)
Crater wear
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.
Flank wear
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).
Break-in period
sharp cutting edge wears rapidly at beginning of use.
Steady state wear
wear at a fairly uniform rate.
Failure rate
higher cutting temperatures; efficiency of machining is reduced.
Alternative Tool Life Criteria
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.
