Economic and Product Design Considerations in Machining

Posted on Posted in Blog, Mechanical Engineering, Mid Level
Economic and Product Design Considerations in Machining
  1. Machinability
  2. Tolerances and Surface Finish
  3. Selection of Cutting Conditions
  4. Product Design Considerations in Machining
Machinability

Relative ease with which a material (usually a metal) can be machined using appropriate tooling and cutting conditions.

  • Depends not only on work material
    • Type of machining operation, tooling, and cutting conditions are also important factors
Machinability Criteria in Production
  • Tool life – longer tool life for the given work material means better machinability
  • Forces and power – lower forces and power mean better machinability
  • Surface finish – better finish means better machinability
  • Ease of chip disposal – easier chip disposal means better machinability
Machinability Testing

Most tests involve comparison of work materials

  • Performance of a test material is measured relative to a base material
  • Relative performance is expressed as a machinability rating (MR)
    • MR of base material = 1.00 (100%)
    • MR of test material > 1.00 (100%) means better machinability
Machinability Tests
  • Tool life (most common test)
  • Tool wear
  • Cutting force
  • Power required in the operation
  • Cutting temperature
  • Material removal rate under standard test conditions

Product Design

Mechanical Properties and Machinability
  • Hardness
    • High hardness means abrasive wear increases so tool life is reduced
  • Strength
    • High strength means higher cutting forces, specific energy, and cutting temperature
  • Ductility
    • High ductility means tearing of metal to form chip, causing chip disposal problems and poor finish
Tolerances and Surface Finish
  • Tolerances
    • Machining provides high accuracy relative to most other shape-making processes
    • Closer tolerances usually mean higher costs
  • Surface roughness in machining is determined by:
    1. Geometric factors of the operation
    2. Work material factors
    3. Vibration and machine tool factors
Geometric Factors
  • Machining parameters that determine surface geometry:
    • Type of machining operation, e.g., milling vs. turning
    • Tool geometry, especially nose radius
    • Feed
  • Surface geometry that results from only these factors is the “ideal” or “theoretical” surface roughness
Geometric Factors Affecting Surface Finish
Product Design
Effect of (a) nose radius, (b) feed, and (c) ECEA
Ideal Surface Roughness

Product Design

Work Material Factors
  • Built-up edge effects
  • Damage to surface caused by chip
  • Tearing of surface when machining ductile materials
  • Cracks in surface when machining brittle materials
  • Friction between tool flank and new work surface
Effect of Work Material Factors

Multiply theoretical surface roughness by the ratio of actual to theoretical roughness for the given cutting speed to obtain estimate of actual surface roughness.

Product Design

Vibration and Machine Tool Factors

Related to machine tool, tooling, and setup:

  • Chatter (vibration) in machine tool or cutting tool
  • Deflections of fixtures
  • Backlash in feed mechanism

If chatter can be eliminated, then surface roughness is determined by geometric and work material factors

How To Avoid Chatter
  • Add stiffness and/or damping to setup
  • Operate at speeds that avoid cyclical forces with frequencies close to natural frequency of machine tool system
  • Reduce feeds and depths to reduce forces
  • Change cutter design to reduce forces
  • Use a cutting fluid
Selection of Cutting Conditions
  • One of the tasks in process planning
  • For each operation, decisions must be made about machine tool, cutting tool(s), and cutting conditions
    • Cutting conditions: depth of cut, feed, speed, and cutting fluid
  • These decisions must give due consideration to workpart machinability, part geometry, surface finish, and so forth
Selecting Depth of Cut
  • Depth of cut is often predetermined by workpiece geometry and operation sequence
    • In roughing, depth is made as large as possible to maximize material removal rate, subject to limitations of horsepower, machine tool and setup rigidity, and strength of cutting tool
    • In finishing, depth is set to achieve final part dimensions and surface finish
Determining Feed
  • Select feed first, speed second
  • Determining feed rate depends on:
    • Tooling – harder tool materials require lower feeds
    • Roughing or finishing?
      • In roughing, limits on feed are imposed by forces, setup rigidity, and maybe horsepower
      • In finishing, select feed to achieve desired finish
Optimizing Cutting Speed
  • Select cutting speed to achieve a balance between high metal removal rate and suitably long tool life
  • Mathematical formulas available to determine optimal speed
    • Two alternative objectives in these formulas:
      • Maximum production rate
      • Minimum unit cost
Maximum Production Rate
  • Maximizing production rate is equivalent to minimizing cutting time per unit
  • In turning, total production cycle time for one part consists of:
    1. Part handling time per part = Th
    2. Machining time per part = Tm
    3. Tool change time per part = Tt/np

where np = number of pieces cut in one tool life

  • Total time per unit product for operation:

Product Design

Cycle time Tc is a function of cutting speed

Actually, Tm and np are functions of cutting speed.

Cycle Time vs. Cutting Speed

Product Design

Minimizing Cost per Unit

In turning, total production cycle cost for one part consists of:

  • Cost of part handling time = CoTh , where Co= cost rate for operator and machine
  • Cost of machining time = CoTm
  • Cost of tool change time = CoTt/np
  • Tooling cost = Ct/np, where Ct= cost per cutting edge

Total cost per unit product for operation:

Product Design

Again, unit cost is a function of cutting speed, just as Tc is a function of v

Product Design

Comments on Machining Economics
  • As C and n increase in Taylor tool life equation, optimum cutting speed increases
    • Cemented carbides and ceramic tools should be used at speeds significantly higher than for HSS
  • vmax is always greater than vmin
    • The reason: Ct/np term in unit cost equation pushes optimum speed to left in the plot of Cc v
  • As tool change time Tt and/or tooling cost Ct increase, cutting speed should be reduced
    • Tools should not be changed too often if either tool cost or tool change time is high
    • Disposable inserts have an advantage over regrindable tools because tool change time is lower
Product Design Guidelines
  • Design parts that need no machining
    • Use net shape processes such as precision casting, closed die forging, or plastic molding
  • If not possible, then minimize amount of machining required
    • Use near net shape processes such as impression die forging
  • Reasons why machining may be required:
    • Close tolerances
    • Good surface finish
    • Special geometric features:
      • Threads
      • Accurate holes
      • Accurate cylindrical sections
      • Flat and/or straight surfaces

Tolerances should be specified to satisfy functional requirements, but process capabilities should also be considered

  • Very close tolerances add cost but may not add value to part
  • As tolerances become tighter, costs generally increase due to additional processing, fixturing, inspection, sortation, rework, and scrap
Relationship between relative cost and tolerance

Product Design

Tolerances as a function of component size for various manufacturing processes

Product Design

Relative production time as a function of surface finish produced by various manufacturing methods

Product Design

 

  • Surface finish should be specified to meet functional and/or aesthetic requirements
    • However, better surface finish generally increases processing cost by requiring additional operations such as grinding or lapping
  • Machined features such as sharp corners, edges, and points should be avoided
    • They are difficult to machine
    • Sharp internal corners require pointed cutting tools that tend to break during machining
    • Sharp corners and edges tend to create burrs and are dangerous to handle
  • Machined parts should be designed so they can be produced from standard stock sizes
  • Example: rotational parts with outside diameters equal to standard bar stock diameter
  • Select materials with good machinability rating of a material
    • As a rough guide, allowable cutting speed and production rate correlates with machinability rating of a material
      • Thus, parts made of materials with low machinability take longer and cost more to produce
    • Avoid undercuts
    • Additional setups, operations, and often special tooling are required

Product Design

  • Design parts with features that can be produced in a minimum number of setups
  • Two parts with similar hole features:
    • (a) holes that must be machined from two sides (two setups), and
    • (b) holes that can all be machined from one side

Product Design

  • Machined parts should be designed with features that can be achieved with standard cutting tools
    • Avoid unusual hole sizes, threads, and features requiring special form tools
    • Design parts so that the number of individual cutting tools needed is minimized
Reference

Fundamentals of Modern Manufacturing: Materials, Processes, and Systems Third (3rdEdition By Mikell P. Groover

 

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