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Manufacturing Processes > Machining processes > Turning and Boring

 

Turning and Boring

 

Process description

The removal of material by chip processes using sequenced or simultaneous machining operations on cut to length bar or coiled bar stock. The stock can be automatically or manually fed into the machine.

Materials

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

Turning and Boring

Process variations

  • Manually operated machines include: bench lathes (can machine non-standard shape parts) and turret lathes (limited to standard stock material).
  • Automatic machines: fully or semi-automated. Follow operations activated by mechanisms on the machine.
  • Automatic bar machines: used mainly for the production of screws and similar parts. Single spindle, multiple spindle and Swiss-types are available.
  • CNC machines: movement and control of tool, headstock and saddle are performed by a computer program via stepper motors.
  • Machining centers: fully automated, integrated turning, boring, drilling and milling machines capable of performing a wide range of operations.
  • Extensive range of cutting tool geometries and materials available.

Economic considerations

  • Production rates ranging 1–60/h for manual machining, 10 to 1000/h for automatic machining.
  • Lead times vary from short to moderate.
  • Material utilization is poor to moderate depending on specific operation (10–60 per cent scrap generated typically). Large quantities of chips generated which can be recycled.
  • Flexibility is low to moderate for automatic machines: changeover and setup times can be many hours. Manual machines very flexible.
  • Economical quantities are 1000+ for automatic machines. Production volumes of 100 000+ are common. Manual and CNC machining commonly used for small production runs, but can also be economic for one-offs.
  • Tooling costs are moderate to high for automatic machines, low for manual.
  • Equipment costs are high for automatic/CNC machines. Moderate for manual machining.
  • Direct labor costs are high for manual machining, low to moderate for automatic/CNC machining.
  • Finishing costs are low. Only cleaning and deburring required.

Typical applications

  • Any component with rotational symmetrical elements requiring close tolerances
  • Non-standard shapes requiring secondary operations
  • Shafts
  • Screws and fasteners
  • Transmission components
  • Engine parts

Design aspects

  • Complexity limited to elements with rotational symmetry.
  • Little opportunity for part consolidation.
  • Can perform many different operations in a logical sequence on the same machine.
  • Potential for linking with CAD very high.
  • Machining operations should be reduced to a minimum (for simplicity and lower cycle time).
  • Fillet corners and chamfer edges where possible to increase tool life.
  • Holes should be drilled with a standard drill point at the bottom for economy.
  • Required number of full threads should always be specified.
  • Leading threads on both male and female work should be chamfered to assure efficient assembly.
  • Auxiliary operations made possible by special attachments, for example, drilling and milling perpendicular to the length of the work.
  • Some special machines allow larger pieces but then operations restricted.
  • Sizes rangingØ0.5mm–Ø2m+ for manual and CNC machining. Automatic machines usually have a capacity of less than 160mm.

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.
  • Multiple setups can be a source of variability.
  • Selection of appropriate cutting tool, coolant/lubricant, feed rate, depth of cut 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 to excellent.
  • Surface roughness values ranging 0.05–25 µm Ra are obtainable.
  • Process capability charts showing the achievable dimensional tolerances for turning/boring (using conventional and diamond tipped cutting tools) are provided . Note, the tolerances on these charts are greatly influenced by the machinability index for the material used and the part geometry.

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