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Plastics Troubleshooting

When machining plastic stock shapes, remember…

  • Thermal expansion is up to 10 times greater with plastics than metals
  • Plastics lose heat more slowly than metals, so avoid localized overheating
  • Softening (and melting) temperatures of plastics are much lower than metals
  • Plastics are much more elastic than metals

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

Drilling
 
Problem Experienced Common Cause
Tapered drill hole Incorrectly sharpened drill - cutting edges not symmetrical
Drill speed in relation to feed not correct
Insufficient clearance
Feed too heavy

Burnt or melted surface Wrong type drill
Drill speed in relation to feed not correct
Incorrectly sharpened drill
Feed too light
Worn flutes on drill
Check tool clearance

Chipping of surfaces Feed too heavy
Clearance too great
Too much rake
Material not clamped properly — movement on material.

Chatter Too much clearance
Feed too light
Drill over-hang too great
Too much rake
Check RPM

Feed marks or spiral lines on inside diameter Material not square
Feed too heavy
Drill not centred
Drill ground off-centre

Oversize holes Insufficient clearance
Feed rate too heavy
Point angle too great

Undersize holes Dull drill or worn flutes
Too much clearance
Point angle too small

Holes not concentric Feed rate too heavy
Spindle speed too slow
Drill enters next piece too far
Cut-off tool leaves nib, which deflects drill
Drill speed too heavy at start
Drill not mounted on centre
Drill not sharpened correctly

Rapid dulling of drill bit Feed too light
Spindle speed too fast
Insufficient lubrication from coolant
Check support under work piece
 
Saw Cutting
Melted surface Blunt saw blade
Insufficient side clearance

Rough finish Feed too heavy
Saw blade improperly sharpened
Cutting edge not honed
Saw blade wrong sided

Spiral marks Saw blade rubs against material during its retreat
Burr on point of tool

Concave or convex surfaces Point angle too great
Tool not perpendicular to spindle
Tool deflecting (use negative rake)
Feed too fast
Tool mounted above or below centre

Nibs or burrs at cut-off point Point angle not great enough
Saw blade blunt or not honed
Feed too fast

Burrs on outside diameter No chamfer before cut-off
Blunt saw blade
 
Turning and Boring
Melted surface Tool blunt or heel rubbing
Insufficient side clearance
Feed rate too slow
Spindle speed too fast

Rough finish Feed too heavy
Incorrect clearance angles
Sharp point on tool (slight nose radius required)
Tool not mounted on centre

Burrs at edge of cut No chamfer provided at sharp corners
Blunt tool
Insufficient side clearance
Lead angle not provided on tool
(tool should ease out of cut gradually, not suddenly)

Cracking or chipping of corners Too much positive rake on tool
Tool not eased into cut (tool suddenly hits work)
Blunt tool
Tool mounted below centre
Sharp point on tool (slight nose radius required)

Chatter Too much nose radius on tool
Tool not mounted solidly
Material not supported properly