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24 - ECONOMIC AND PRODUCT DESIGN CONSIDERATIONS IN


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Machinability
Machinability can be defined as the relative ease with which a material can be machined using an appropriate cutting tool under appropriate cutting conditions
Criteria for good machinability
Criteria that indicate good machinability are: (1) ease of chip disposal, (2) high value of R a, (3) long tool life, (4) low cutting forces.
Testing machinability
The most important method of testing machinability is tool life.
Machinability rating
Machinability rating greater than 1.0 means that the work material is easier to machine than the base metal.
Criteria for machinability
The machinability assesement criteria include (1) tool wear and tool life, (2) forces and power, (3) surface finish, and (4) ease of chip disposal
Properties of machinable material
The properties for good machinability of material include hardness, strength, and thermal diffusivity.
Surface finish increases cost
Costs tend to increase when better surface finish is required because additional operations such as grinding, lapping, or similar finishing processes must be included in the manufacturing sequence.
Factors that affect surface finish
The factors that affect surface finish are (1) geometric factors such as type of operation, feed, and tool shape (nose radius in particular); (2) work material factors such as built-up edge effects, and tearing of the work surface when machining ductile materials, which factors are affected by cutting speed; and (3) vibration and machine tool factors such as setup and workpart rigidity, and backlash in the feed mechanism.
Parameters for surface finish in turning
The ideal surface roughness is determined by the following geometric parameters of the machining operation: (1) tool nose radius and (2) feed. In some cases, the end cutting edge and end cutting edge angle of the single-point tool affects the feed mark pattern on the work surface.
Vibration elimination in machining
Steps to reduce vibration in machining include (1) increase stiffness or damping in the setup; (2) operate at speeds away from the natural frequency of the machine tool system; (3) reduce forces in machining by changing feed or depth and cutter design (e.g., reduced rake angle), and (4) change the cutter design to reduce forces
Cutting speed increase in ductile materials
When cutting ductile material, the increase of cutting speed increases surface finish.
Feed selection in machining
The factors are (1) type of tooling (e.g., a cemented carbide tool should be used at a lower feed than a high-speed steel tool), (2) whether the operation is roughing or finishing (e.g., higher feeds are used in roughing operations), (3) cutting forces limitations that would require lower feeds, and (4) surface roughness requirements.
Cost of machining operation
The unit cost in a machining operation is the sum of four cost terms: (1) part load/unload cost, (2) cost of time the tool is actually cutting the work, and (3) cost of the time to change the tool, (4) the cost of the tool itself (purchasing the tool and grinding it, if applicable).
Which cutting speed is always lower for a given machining operation?
Cutting speed for minimum cost is always lower because of the fourth term in the unit cost equation, which deals with the actual cost of the cutting edge. This term tends to push the U-shaped function toward a lower value in the case of cutting speed for minimum cost.