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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 centered Drill ground off-center |
| 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 center 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 center |
| 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 & 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 center |
| 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 center Sharp point on tool (slight nose radius required) |
| Chatter | Too much nose radius on tool Tool not mounted solidly Material not supported properly |