Handy hints and tips for turning plastics

• Thermal expansion is up to 10 times greater with plastics than metals
• Plastics lose heat more slowly than metals, so avoid localised overheating.
• Softening (and melting) temperatures of plastics are much lower than metals.
• Plastics are much more elastic than metals.
• Ground peripheries and polished top surfaces generally reduce material buildup and improve the surface finish.
• A fine grained carbide tool is generally best for turning operations.Use adequate chip clearance to prevent clogging.
• Adequately support the material to restrict deflection away from the cutting tool.

Because of these differences, you may wish to experiment with fixtures, tool materials, angles, speeds and feed rates to obtain optimum results.

Coolants are generally not required for most machining operations, however for optimum surface finishes and close tolerances, non-aromatic, water soluble coolants are recommended. Spray mists and pressurised air are both very effective means of cooling the cutting interface. General purpose petroleum based cutting fluids although suitable for many metals and plastics, may contribute to stress cracking of some plastics.

TURNING TROUBLESHOOTING
PROBLEM: Spiral marks
Suggested Cause:
1. Tool rubs during its retreat
2. Burr on point of tool

TURNING TROUBLESHOOTING
PROBLEM: Concave or convex surfaces
Suggested Cause:
1. Point angle too great
2. Tool not perpendicular to spindle
3. Tool deflecting (use negative rake)
4. Feed too heavy
5. Tool mounted above or below centre

TURNING TROUBLESHOOTING
PROBLEM: Nibs or burrs at cut-off point
Suggested Cause:
1. Point angle not great enough
2. Tool dull or not sharp
3. Feed too fast

TURNING TROUBLESHOOTING
PROBLEM:Melted surface
Suggested Cause:
1. Tool dull or heel rubbing
2. Insufficient side clearance
3. Feed rate too slow
4. Spindle speed too fast

TURNING TROUBLESHOOTING
PROBLEM: Rough finish
Suggested Cause:
1. Feed too fast
2. Incorrect clearance angles
3. Sharp point on tool (slight nose radius required)
4. Tool not mounted on centre

TURNING TROUBLESHOOTING
PROBLEM:Cracking or chipping of corners
Suggested Cause:
1. Too much positive rake on tool
2. Tool not eased into cut (tool suddenly hits work)
3. Dull tool
4. Tool mounted below centre
5. Sharp point on tool (slight nose radius required)

TURNING TROUBLESHOOTING
PROBLEM:Chatter
Suggested Cause:
1. Too much nose radius on tool
2. Tool not mounted solidly
3. Material not supported properly
4. Width of cut too wide

See more hints at the pages below