Polycarbonate Feeds & Speeds?

[kr_fitz]

I am to cut a 48" round disk out of polycarbonate for someone. It has a 2" hole in the center. Thickness is 1/4". I have three 1/4" bits; a single straight flute, a double straight flute and a single flute upcut. Hold down will be with clamps and/or drywall screws. Any suggestions as to bit selection, number of passes, and feeds and speeds?

[Westcoast Sign Guy (Unregistered Guest)]

With my Belin spiral single flute upcut bits I go 15,000 RPM's at 2.5" per sec. with a 1/4" bit I'll do 2 passes for acrylic. They say it's the same for poly-carb. I'm about to cut some 3/16" this weekend for the first time

[paco]

As from what I can remember of my tests, polycarbonate cut with even better chips extraction than acrylics... I've tested wood straight edges (2) carbide tipped router bit (1/8" and 1/4" CED); O-flute would be a better choice for any plastic I believe. I'll suggest a finishing pass with a light allowance (0.01" to 0.03") to get the smoothest finish possible... maybe a slower finishing pass.

Make a small test first...

From most router bits supplier, with a 1/4" CED tool, one could cut 1/4" thick plastics material with a single pass (100% stepdown); 2 pass will make this only safer... again, a quick test will say.

Tell us how it go...

[kr_fitz]

Obviously I only get one shot at this, so it has to be right. The only test cuts possible are at the corners, but that might be enough.

If I do a light finish cut as you suggest, Paco, I am wondering about holding down with two DW screws in the center (plus camps on the outside; move as necessary), make my rough and finish cuts on the outside first, then clamp from 2 or 3 edges and cut out the center hole LAST. ???

So, one says single flute spiral upcut and the other says double straight. 2.5" ips seems pretty fast, are you sure?

[richards]

You might want to visit http://www.plasticrouting.com/van.asp for some good articles about cutting plastics with recommended speeds.

[paco]

Kevin,

make the "roughting pass" not through the thickness, then make the "finishing pass" machining both the finishing allowance and the remaning skin of the material thickness (say 0.063")... "always" cut interior vectors first. You may consider some tabs (removing 'em with a flush trim bit) if the material does'nt have a protecting mask/paper which could help with the hold down "issue".

O-flutes will/should do a better job on plastic. One or two flutes depends on desire finish; from what router bit supplier say/advertise... I've always choose single flute in O-flutes type because of the large flute gullet (less body-strenght). As my test proove to me; wood router carbide tipped straight edges have done a "decent job" (http://www.cooptel.qc.ca/~usinum/acryl_5.jpg) (clear part)... this is in two pass with a 1/8" CED tool (wood router bit carbide tipped) without finishing pass... sorry, no feed/speed specs. to remember of...

2.5"/sec. seem only fast (to me) with a single pass (in 1/4" thick poly with 1/4" CED tool)... but not with two pass (0.125" stepdown). I'd suggest like Mike; make some reading at the link he provided.

[bleeth]

Kevin: Double sided tape could help you immensely

Dave

[kr_fitz]

WSG, Paco, Mike and Dave; thanks for all your suggestions. For some reason I am feeling particularly stressed about this project, but you all have helped quite a bit. Went to the site Mike suggested and looks like I have the right bit in the single flute upcut (it is an "O" flute). Will use double sided tape in addition to the clamps and scews (thanks Dave). Will run two roughing passes at 2.5 ips and a single finish pass a little slower (thanks WSG and Paco). Will also take a deep breath first and pray the whole time.

[paco]

Yes, me too; since I have my Bot, I've become somewhat a prayer too... on some projects at least. I think the test could relief you some of your stress...

[UK Andy (Unregistered Guest)]

Not on the cutting subject but might be useful to know;

Acrylic edges will flame polish nicely with the required hydrogen based flame equipment.

For poly I was told wet and dry paper with meths as a lubricant would get a nice polished edge.

On my last polycarb job I used hot air instead of a flame and it does the same job.

I was using a leister triac hot air welder so it was pretty hot but any heat gun with a narrow nozzle will do the trick.

When I cut poly I cut 6mm in one pass using a double flute up cut bit at 200mm p/m at 18,000 rpm with good results.

Hope this info is useful

[kr_fitz]

Project completed successfully! Thanks for all your help! I really like the double-faced tape idea. That stuff holds like you wouldn't believe. Would not have needed anything else. I wish I had known/thought of it when I was trying to hold a 6" foam block down when machining the Velociraptor last year. Next time...

Thanks again,
Kevin