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Manufacturing Processes > Machining processes > Broaching

 

Broaching

 

Process description

The removal of material by chip processes using a multiple-point cutting tool, which is pushed or pulled across the workpiece surface. With successively deeper cuts, the desired profile is gradually generated in a single pass.

Materials

All metals (mostly free machining).

Broaching

Process variations

  • Horizontal, vertical or rotary broaching machines with push and/or pull capability.
  • Broaching tools can be single or combination types, internal or external, performing either roughing or finishing operations.
  • Some indexable insert broaches are available for surface broaching and titanium nitride coatings are also used to increase tool life.

Economic considerations

  • Production rates up to 400/h.
  • To improve production rates, many parts can be machined at once, called stacking. Stacking is best suited to internal features.
  • Automation possible to improve production rates.
  • Lead times moderate.
  • Material utilization poor. Large quantities of chips are generated, which can be recycled.
  • Flexibility high. Little dedicated tooling and setup times are generally short.
  • Accurate re-grinding of the broaching tool required on large production runs, which uses expensive fixtures and grinding machines.
  • Production volumes usually very high, 10 000–100 000.
  • Tooling costs high. Broaching tools are very expensive due to their complexity and the economics of this process must be carefully studied on this basis.
  • Equipment costs low to moderate.
  • Direct labor costs low to moderate. Some skilled labor may be required.
  • Finishing costs low. Some deburring may be required.

Typical applications

  • Many regular or irregular, internal or external profiles
  • Turbine blade root forms
  • Connecting rod ends
  • Rifling on gun barrels
  • Flat surfaces
  • Key seats and slots
  • Splines, both straight and helical
  • Gear teeth

Design aspects

  • Complexity is limited by nature of process, i.e. straight, curved and complex profiles, slots and flat surfaces along length of workpiece.
  • Part design should allow for sufficient clamping area and clearance for broaching tool.
  • A hole is initially required for internal broaching for broaching tool access. This can be achieved by either punching, boring or drilling the blank.
  • Ideally, between 0.5 and 6mm should be removed by the broaching tool on any one surface.
  • More than one surface can be cut simultaneously.
  • Workpiece must be strong enough to withstand the pressure of continuous cutting action of broach.
  • Large surfaces, blind holes and sharp corners should be avoided.
  • Chamfers are preferred to radiused corners.
  • Minimum stroke =25 mm.
  • Maximum stroke =3m.

Quality issues

  • Machinability of the material to be processed is an important issue with regards to: surface roughness, surface integrity, tool life, cutting forces and power requirements. Machinability is expressed in terms of a ‘machinability index’ for the material.
  • For materials with high surface hardness, the first tooth on the broach should cut beneath this layer to improve tool life.
  • Soft or non-uniform materials may tear during machining.
  • Adequate clearance should be provided for to prevent rubbing and chipping of the broaching tool on return strokes.
  • Broaching tools may require chip breakers for very ductile materials to efficiently remove swarf from cutting area.
  • Selection of appropriate cutting tool material, coolant/lubricant, depth of cut per tooth and cutting speed with respect to material to be machined is important.
  • Coolant also helps flush swarf from cutting area.
  • Surface detail is excellent.
  • Surface roughness values ranging 0.4–6.3 µm Ra are obtainable.
  • A process capability chart showing the achievable dimensional tolerances is provided . Note, the tolerances on this chart are greatly influenced by the machinability index for the material used and geometry complexity.

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