# Cutting polycarbonate - jumping away from bit



## idontcare (Sep 10, 2011)

Hello,

I am attempting to use my router/table to make smooth, accurate cuts in 3/8" polycarbonate plastic. I got a 1/4" 2 flute full carbide up spiral bit - Diablo DR75102. Whenever I set the fence to take off say 1/16 or 1/8" off the edge it works fine and cuts well. I learned I have to go slow but otherwise it works as anticipated.

Whenever I try to make a complete cut (pass the full width of the bit through the material) the material violently jumps back. What am I doing wrong? I have tried going very slow, very fast, slowing the bit down a little (25k rpm router, speed control got it down to maybe 3/4 or 1/2 of full)... nothing seems to help. Is it just the wrong bit for the job (and if so, what bit is recommended for this?) or am I just missing something? I had a wood 4x4 handy - of course I couldn't cut completely through because its only a 1" long bit, but I had no problem making a deep slot in the middle of the 4x4.


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## Phil P (Jul 25, 2010)

idontcare said:


> Whenever I set the fence to take off say 1/16 or 1/8" off the edge it works fine and cuts well. I learned I have to go slow but otherwise it works as anticipated.
> 
> Whenever I try to make a complete cut (pass the full width of the bit through the material) the material violently jumps back. What am I doing wrong?


Hi

I think you have already answered your own question in many ways. Polycarbonate is quite a flexible plastic which has a tendency to compress under cutting and when the cutting pressure is reduced its' polymer memory causes it to spring back. For that reason it's often better to band saw or jigsaw to the approximate shape (i.e. only slightly oversize) then trim to finish with the router. Pretty much the same approach is required with lower grades of HDPE/MDPE, softer nylons and polyurethanes. In terms of feeds and speeds softer plastics are often best worked with lower spindle speeds (typically 12,000 to 18,000 rpm), but with a fairly high feed speed. That approach generates a chip as opposed to dust and reduces "churning" of waste materials in the cut (which makes for heat build-up and reduces surface quality in some materials due to "weld back" of waste). Dust extraction for the router also helps improve the quality of the cut and keeps the tool/material cooler. In the CNC world the technique employed when cutting plastics like polycarbonate is to make a rough cut of the material 0.5 to 1mm mm or so oversize then to do a finishing pass at the finished size. The spring back at the finish cut is much reduced and the problem of kick back disappears. For table routing the same effect could be had by attaching a layer of Formica laminate to the fence, taking a pass then removing the laminate for the finish pass, although this assumes that you aren't attempting to mill a slot exactly the same width as your cutter (not advisable BTW) 

For further reading I'd suggest taking a look at the Onsrud Cutters website (they specialise in plastics tooling for routers) and in particular at a section with expert papers on the subject. The most helpful for you might be the Van Niser article on machining polycarbonates

Hope this helps

Regards

Phil


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## Quillman (Aug 16, 2010)

Full thickness cuts require an offset fence to account for the waste, like a jointer.
Do not continue to do this. You might break the sample, the cutter or the work can get loose and go its own way.


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## idontcare (Sep 10, 2011)

Yep, I found that out about the offset fence during my experimenting. The kickback happens at the instant it hits the bit though before the stock has a chance to reach the far side of the fence. Only way around that is to use the router right-side-up with a straight edge against the base.

I'm just an average Joe in his garage at home. I have a hand held circular saw that cuts the polycarbonate well with a plywood blade but it's a royal pain getting everything square. I wanted to just use the router to get square cuts and not have to do further sanding to get a glue ready edge. Ideally I'd have a table saw but accurate saws aren't cheap.

So if I understood you guys right, cutting polycarbonate the full width of a bit is just not do-able? Would a single flute or straight bit be less aggressive and do what I want or am I just out of luck doing what I want to do with a router?


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## jschaben (Jun 21, 2009)

idontcare said:


> Yep, I found that out about the offset fence during my experimenting. The kickback happens at the instant it hits the bit though before the stock has a chance to reach the far side of the fence. Only way around that is to use the router right-side-up with a straight edge against the base.
> 
> I'm just an average Joe in his garage at home. I have a hand held circular saw that cuts the polycarbonate well with a plywood blade but it's a royal pain getting everything square. I wanted to just use the router to get square cuts and not have to do further sanding to get a glue ready edge. Ideally I'd have a table saw but accurate saws aren't cheap.
> 
> So if I understood you guys right, cutting polycarbonate the full width of a bit is just not do-able? Would a single flute or straight bit be less aggressive and do what I want or am I just out of luck doing what I want to do with a router?


I've cut discs and holes in polycarbonate with this bit:
Solid Carbide Spiral Plastic 'O' Flute -ToolsToday.com- Industrial Quality Solid Carbide Bits
I use the 1/4" up cut and haven't had any trouble except if I stop the feed to rearrange the power cord. Then I end up with the bit welded into the cut. With a regular spiral bit I come up with a big wad of plastic on the end of the bit. :fie:


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## bobj3 (Jan 17, 2006)

Hi

I will 2nd John's post I do the same all the time.. 

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jschaben said:


> I've cut discs and holes in polycarbonate with this bit:
> Solid Carbide Spiral Plastic 'O' Flute -ToolsToday.com- Industrial Quality Solid Carbide Bits
> I use the 1/4" up cut and haven't had any trouble except if I stop the feed to rearrange the power cord. Then I end up with the bit welded into the cut. With a regular spiral bit I come up with a big wad of plastic on the end of the bit. :fie:


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## Phil P (Jul 25, 2010)

jschaben said:


> I use the 1/4" up cut and haven't had any trouble except if I stop the feed to rearrange the power cord. Then I end up with the bit welded into the cut. With a regular spiral bit I come up with a big wad of plastic on the end of the bit. :fie:


Hi John

That's the "weld-back" I referred to. The plastics trade people use dust extraction and sometimes spritz the cutter with water to avoid this. As you've found out a purpose-made plastics cutter works far better. I rarely try to do an internal cut without first drilling or hogging away the waste - I've had 13 stitches and 22 lacerations in my left thumb to remind me why! (although that was 1in thick MDPE)

Regards

Phil


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## Quillman (Aug 16, 2010)

Full thickness cuts in AC and PC not a problem. Templets, adjustable fences, many ways to do this.


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## BigJimAK (Mar 13, 2009)

An untested idea but you might try sandwiching the poly between two pieces of wood (couble stick tape) prior to cutting, cutting throught all 3 pieces at once. This may well hold the poly rigid enough for the cut.

May not be feasible, depending on the amount to cut. Just an idea.


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## idontcare (Sep 10, 2011)

Phil - I came about the Onsurd bits during my research. They have a dizzying variety, I may contact them to see what is recommended for this application.

Quillman - Oooo, looks s3xy! Any chance you know what bit that was done with?

jschaben thankyou for the link. Seems those bits fit the ticket, per the description they are intended for this exact purpose.

Jim - interesting concept. I suspect double sided take won't provide a firm enough hold, they would have to be clamped together though. 

All in all it seems it certainly is do-able but I just have the wrong bit for the job.


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## Quillman (Aug 16, 2010)

No special cutters. straight, on-shear, solid carbide, brazed carbide on steel.
Cuttings are fixture and technique sensitive, very few cutter design sensitivities. I cut the stuff for a living.


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## idontcare (Sep 10, 2011)

Interesting looking bits, nothing like I've seen before.

I can't post links because I don't have 10 posts yet, but if you google "diablo dr 75-102" you'll find it easily. Double flute up spiral bit. 

Granted my technique isn't perfect but I've tried all I can with it on the table. I was about to get some different bits but now your making me doubt whether it is the bit or really just me. Perhaps I'll have better luck using the router right-side-up by hand. I can put a cheap sacrificial material underneath it so the PC is sandwiched between it and the router.


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## WBLHANDYMAN (May 18, 2021)

Quillman said:


> Full thickness cuts require an offset fence to account for the waste, like a jointer.
> Do not continue to do this. You might break the sample, the cutter or the work can get loose and go its own way.


 I can sharpen a bit for polycarbonate that will cut into polycarbonate with no kickback. I have cut into 3/8 polycarbonate free hand With a bit I sharpened.


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## John Smith_ (Jan 10, 2020)

welcome to the forum, WBL.
even though this is a ten year old thread, the content is still a little relevant.
photos of your process would be good to see. (not just a vague description of what you did).


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## old55 (Aug 11, 2013)

Welcome to the forum @WBLHANDYMAN


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