Machining PVC Foam with a CNC Router

      Advice on feed rates, cutter choice, and RPMs when cutting sheet PVC foam on CNC equipment. March 21, 2006

I got an interesting job today. This is my first time machining a large sign out of 5mm Foamalux, produced by Brett Martin company. I followed the feeds and speeds on their website to the letter and after a few tools had broken and I had melted the cut material back into the groove, I slowed down the feed VIS: 3/16 upcut 2 flute solid carbide, 18000 rpm feed reduced to 1000mm per minute, and used compressed air to blow on the tool the whole time. I eventually got the job done.

1. What tools should I have used?
2. Do I need air for cooling or was this just an incorrect tool choice?
3. I have an opportunity to do a lot of this including machining ABS. What should I be doing to get a) a faster cut (the above took ages), and b) a clean finish?

Forum Responses
(CNC Forum)
From contributor P:
Take a look at the Onsrud cutter site. They do some specialized cutters for foam cutting and publish recommended feeds and speeds (at least in the printed catalogue). Haven't done Foamalux myself.

From contributor G:
I have never used the brand of material you mentioned (Foamalux - expanded PVC?), but just looking at your material thickness, the feed speeds and cutter diameter, I can see why you are having a challenge here. First, make sure you are using a solid carbide, or carbide tipped cutter that has never touched wood. Your diameter of the cutter for that thickness of material should be 3/8". You may get away with 1/4. Use a single flute and not 2 flutes (you need more clearance). Did you say you were moving at 100mm/min? I calculate that to be around 40 inches per minute, which is about 10X too slow. I run 300-450 IPM, which is about 7000-8000mm per minute (18000rpm).

The ABS will cut like butter. My two favorite cutters are Onsrud (3/8") 52-638, which I use for a great finish on acrylics and such, and Onsrud (1/4") 61-082 o-flute. For what it's worth, I've been cutting plastics since the late 80's and a day doesn't pass when I don't learn something new about how to cut plastic a little better. I hope the next endeavor goes better!

From contributor E:
As mentioned above, use a single "O" flute tool for this application. Better edge finish and longer lasting tool. With Courmatt's "O" flute, we have a 7 grind back relief, which provides for a superior edge. A cold air gun can be very beneficial for this application and other cutting applications. It blows approximately 100 degrees of cold air, which helps with the tool life and will blow the chips away from the cut. Here in the desert, we use this on our CNC control, so they do not overheat.

From the original questioner:
Thanks a heap. I got a sheet of ABS today and used 3/8 5000mm per minute and 2mm deep cuts at 18000 rpm. It cut beautifully and made chips like you would get from a steel plate being machined dry with a carbide rose cutter, great for getting rid of the heat. I still want to rig up a cold air jet next to the cutter to assist cooling.

I reckon I could go a lot faster, though, and will keep trying until I scrap a job. I am using Onsrud cutters, but it is a bit of a mission, as I have to fly them in to Australia FEDEX, as they do not appear to be stocked here, nor Courmatt. Pity. Maybe I should set up an agency here. For cutting 25mm thick acrylic, recommendation please.

From contributor G:
Depending on the size of your parts and hold down capability, I would use multi passes on this one. 6mm per pass at about 5000-6000mm/min. Use a 3/8" diameter or greater with an upspiral if possible. Leave the part oversized by about 1-1.5 mm and then clean it up with a single pass for a nice finish. Also, how thick is the ABS you're cutting? You should be able to go deeper than 2mm while cutting it.

From contributor E:
You don't have to fly them from the US, just New Zealand. The dealer is Icam. On the 25mm depending on the size of the parts, you can use the CP-500-AZ or the CO-500-AZ. Both will provide you with great results. CP for larger parts CO for smaller parts.

From the original questioner:
I have a very good Becker vacuum pump and hold down. If I am nervous, I add duct tape to the 4 sides and that makes a hell of a difference. As far as the 3/8 is concerned, most of the cutters I currently have (will be buying the others as recommended) are short and can only cut about 25mm deep without fouling up the flutes. I have some very long ball cutters, but am hesitant to use these, at they are for some specialized wood carving I am trying to do. The ABS was only 5mm sheet and cuts very easily and leaves a good edge. No complaints from the customer there.

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