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Clear plastic speeds and feeds for vacuum & double-sided tape workholding

Corn

Aluminum
Joined
Nov 3, 2019
Dear All,

Every now and then need to make lens from plastic and mostly used vacuum table. That however is not good for very small lenses. So I tried first time double sided tape and oh my, that was awesome. However I’d like to hear your experiences & suggestions with speeds and feeds for that purpose? In this case I was very conservative to not move the part, but it ended up some chip welding back to the plastic. I had 6mm three teeth YG1 alupower and 6000 rpm 400mm/min (15.7 inch/min) feed and airblast chip evacuation. Lens material was SAN (Styrene acrylonitrile).

Thank you in advance
 
I use double sided 3m tape and superglue with kicker on the outside on MDF or PVC base. Run the cutter flat out with 1% of cutter diameter feed per tooth (6000×3×.06mm feed=1080mm/min) I leave .05mm of stock and stop .25mm short of breaking through on the Z depth. Depth of cut depends on material thickness. Final cut is full depth and no stock at half the feed rate, sometimes I stop .05mm short on depth and scrape the edge with a scalpel blade. Be careful of the cutter hitting the tape as it can build up on the cutter and lift or shift the workpiece. If machining polycarbonate I use neat water as coolant otherwise air blast.

I only do bigger workpieces on vacuum using same method for maximum hold down during roughing.
 
Try slowing your spindle down, your chip load is .00087" which is wayyyy too small. Nearly all plastics want big chip loads and slow speeds, and the sharpest tools possible. At thst feed try dropping your spindle to 1000 rpm or less, not joking. I run a 2 flute mill at 3000 rpm and up to 120 ipm in polyethylene.

I like the idea about not cutting all the way through the stock to give you more surface to hold the part in place. How fast you can feed depends on how strong your tape is and if you have outside corners, which like to lift or chip. Also I find I can machine plastic about twice as fast using coolant instead of air.
 
Here's picture of that lens. Flange is only 31x13mm so bet no vacuum would have worked. Also I left only 0.1mm stock so it might have punctured and end up in disaster. Didn't expect the tape to work so good. You can see those scratches on that flange due to sloppy removal of the welded chips. Flange will not be visible so it doesn't matter. Chips only welded to the side of the cut, not on bottom, that's why the bottom surface is relatively smooth. 1000rpm with that feed sounds awfully low, do you think it would still work and not push the part off? This was just one off, so next time I'll try much smaller speed.
lens.jpeg
 
All you can do is try, but it all depends on the details you are working with. Knowing how to machine metal will only get in the way of learning how to machine plastic, they are very different.
 
Double-sided tape has long been a part of the plastic fabrication toolbox, from well before my start in that business in the mid 70's. (that is 1970's, not 1870's...) Many parts passed through many hands running routers with parts taped to router patterns and templates, where there was no other workholding opportunity. It has always been very effective when required. Vacuum is also good when the part is large enough, but that is a pretty fuzzy line to decipher sometimes, with all the variables included.

The SAN material, while better than single-polymer styrene, is nevertheless a low-melting point material, with a deflection temp around 185F. As others note, less RPM (less heat) and more cooling at the cutter is a good prescription, which will also help in longer-term part survival from less thermal stress associated with machining. Both vacuum chip evacuation and airblast will help to avoid chip melting.
 








 
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