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Manufacturing Processes > Machining processes > Milling

 

Milling

 

Process description

The removal of material by chip processes using multiple-point cutting tools of various shapes to generate flat surfaces or profiles on a workpiece of regular or irregular section.

Materials

All metals (mostly free machining) and some plastics and ceramics.

Milling

Process variations

  • Horizontal milling: axis of cutter rotation is parallel to surface of workpiece. Includes slab milling, form milling, slotting, gang milling and slitting. Can be either up-cut or down-cut milling.
  • Vertical milling: axis of cutter rotation is perpendicular to surface of workpiece. Includes face milling, slotting, dovetail and woodruff milling.
  • CNC machines: movement and control of tool, headstock and bed are performed by a computer program via stepper motors.
  • Extensive range of cutting tool geometries and tool materials available.

Economic considerations

  • Production rates ranging 1–100/h.
  • Lead times vary from short to moderate. Reduced by CNC.
  • Material utilization is poor. Large quantities of chips generated.
  • Recycling of waste material is possible but difficult.
  • Flexibility is high. Little dedicated tooling.
  • Production volumes are usually low. Can be used for one-offs.
  • Tooling costs are moderate to high depending on degree of automation (tool carousels, mechanized tool loading, automatic fixturing, etc.).
  • Equipment costs are moderate to high.
  • Direct labor costs are moderate to high. Skilled labor required.
  • Finishing costs are low. Cleaning and deburring required.

Typical applications

  • Any standard or non-standard shapes requiring secondary operations
  • Aircraft wing spars
  • Engine blocks
  • Pump components
  • Machine components
  • Gears

Design aspects

  • Complexity limited by cutter profiles and workpiece orientation.
  • Potential for linking with CAD very high.
  • Chamfered edges preferred to radii.
  • Standard sizes and shapes for milling cutter used wherever possible.
  • Auxiliary operations made possible by special attachments, for example gear cutting using an indexing head.
  • Minimum section less than 1 mm, but see below.
  • Minimum size limited by ability to clamp workpiece to milling machine bed, typically 1.5m², but length 5m have been milled on special machines.

Quality issues

  • Machinability of the material to be processed is an important issue with regard to: surface roughness, surface integrity, tool life, cutting forces and power requirements. Machinability is expressed in terms of a ‘machinability index’* for the material.
  • Rigidity of milling cutter, workpiece and milling machine is important in preventing deflections during machining.
  • Selection of appropriate cutting tool, coolant/lubricant, depth of cut, feed rate and cutting speed with respect to material to be machined is important.
  • Coolant also helps flush swarf from cutting area.
  • Regular inspection of cutting tool condition and material specification is important for minimum variability.
  • Surface detail is good.
  • Surface roughness values ranging 0.2–25 µm Ra are obtainable.
  • A process capability chart showing the achievable dimensional tolerances for milling and a chart for positional tolerance capability of CNC milling centers are provided. Note, the tolerances on the milling process capability chart are greatly influenced by the machinability index for the material used.

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