[mooreaa]

Hey Guys,

I've been milling a number of flat plastics (polycarbonate) recently and running into fixturing challenges as well as some feeds and speeds issues relating to swarf and was hoping I could get some feed back.

So the setup I am using consists of a sacrificial sheet on the bottom, layered with my main sheet on top. I use a strip of aluminium on the edges to clamp the sheet down.


Flat Plate Milling | Flickr - Photo Sharing!

This works pretty well with the 6mm sheets, but when I'm doing the 3mm sheets, I definitely see it flexing in the middle. The bowing is really noticeable during peck drilling as the cutting tool lifts up the plastic as the tool lifts.

Since I'm cutting a whole part out of the center, I also get a nub at the end when the part disconnects. It's easy to finish off but I'm wondering if anyone has any suggestions on how to do this kind of work easier.


Flat Plate Milling | Flickr - Photo Sharing!

As for the cut, I was getting a bit of swarf built up on the tool (enough to marr the surface of the plastic). I keep hearing that you can cut though this stuff fast, but I've been running at 200mm/min feed rate doing a 1.6mm DOC using a 1/8" endmill. 2200-3000rpm.

I'm thinking about trying something like this:

Solid Carbide Spiral Plastic 'O' Flute -ToolsToday.com- Industrial Quality Solid Carbide Bits

Wanted to get some opinions.

[beanbag]

double stick tape

[mooreaa]

Hmm interesting that you brought this up. We did use this method before when milling PCBs on a smaller mill.

I actually tried this with Scotch double sided tape on the polycarbs. Covered the bottom side with many strips and adhered it to the bottom plate. Not sure if it was the tape that didn't seem to hold the material down of if the bottom plate was flexing with it but I was still getting some float in the center. It could also have to do with the cover material. The Scotch tape didn't stick too well to that plastic covering.

Just googled various tapes and wow there are MANY different types.

What kind of tape should I be using? Any suggestions for suppliers?

[davebaldwin]

I have used carpet tape with great success. One problem with tape though is the cutter gets gummed up so can interfere with getting a clean cut. I have started using hot melt glue as an alternative but this is no good for plastic because of the heat and solvents needed to break the bond and clean the glue off.

Not tried it but recently I have been playing around with some neodymium magnets and they are strong. Use some of these in the centre of your shape to keep it flat.

Dave.

[beanbag]

I don't have the model number of my double stick tape, but it is definitely thicker than scotch double stick tape. I would guess 4 mil or so. What I would do is first cut a very shallow pass into your backing plate so you know where the cutter is actually going to go. Then place tape around that line, making sure that the cutter won't actually hit the tape. Finally, be sure to barely not cut thru on your second to last pass. This ensures that the last pass is a light cut with minimal forces. In general, I like to avoid using tabs.

[odonatas]

I had to cut some accurate slots in thin plastic. Made a vacuum table by plugging one end of a piece of rectangular tubing, put a hose barb in the other end and drilled a bunch of holes and surfaced one of the sides. I couldn't get two sided tape to work consistently.

[mvtop]

I too use the double sided tape for plastics, but leave 1 to 2 thou. left and then use a box cutter to trim out the part. i don't like runnin the cutter thru the tape.
as another mentioned, make an outline of where you'll cut then tape close the edge so you can cut thru without running into the sticky.
i use this method a lot with cutting aluminum.

[Steve Seebold]

I haven't cut any plastics on my Tormach yet, but when I had my FADAL, I cut lots of it. I would use a cutter with a right hand cut, left hand spiral.

That cutter wouldn't try to lift the part off the table. It actually tried to push the part down.

At the time, the smallest cutter you could get was .250 diameter. They were made by Quinco.

I never used flood coolant on plastic. I always used spray mist. And your cutter has to be dead sharp. As soon as the cutting edge starts to break down, your cutter will start to melt the plastic, then you've got a big mess.

[Steve Seebold]

Hey Guys,

I've been milling a number of flat plastics (polycarbonate) recently and running into fixturing challenges as well as some feeds and speeds issues relating to swarf and was hoping I could get some feed back.

So the setup I am using consists of a sacrificial sheet on the bottom, layered with my main sheet on top. I use a strip of aluminium on the edges to clamp the sheet down.


Flat Plate Milling | Flickr - Photo Sharing!

This works pretty well with the 6mm sheets, but when I'm doing the 3mm sheets, I definitely see it flexing in the middle. The bowing is really noticeable during peck drilling as the cutting tool lifts up the plastic as the tool lifts.

Since I'm cutting a whole part out of the center, I also get a nub at the end when the part disconnects. It's easy to finish off but I'm wondering if anyone has any suggestions on how to do this kind of work easier.


Flat Plate Milling | Flickr - Photo Sharing!

As for the cut, I was getting a bit of swarf built up on the tool (enough to marr the surface of the plastic). I keep hearing that you can cut though this stuff fast, but I've been running at 200mm/min feed rate doing a 1.6mm DOC using a 1/8" endmill. 2200-3000rpm.

I'm thinking about trying something like this:

Solid Carbide Spiral Plastic 'O' Flute -ToolsToday.com- Industrial Quality Solid Carbide Bits

Wanted to get some opinions.

Watch the video about the Onsrud cutters, and pay attention to what he says. He knows what he's talking about.

You might pay a little more for good cutters, but good tools make cheap parts.

[Gerry Sweetland]

I have cut thin sheet aluminum profiles by drilling and taping into the sacrificial plate in the waste areas between parts and if available (ie. a hole that needs to be in the part) in the part itself then using small screws to attach the work pc. to the sacrificial plate below.

I plugged your numbers into GWizard and came up with RPM = 5140 and feed rate = 327 mm/min.

Gerry

[Steve Seebold]

You might also consider making your sacrificial sheet out of either thicker plastic or 1/2 inch aluminum.

Remember, good tools make cheap parts.

[shred]

I think the down-cut bits are probably a good way to go. I've used home made hex cutters a time or two to avoid the lift-up problem as well.

[rowbare]

A vacuum fixture might help a lot. If you are cutting the same shapes all the time, you could make fixtures for each shape with the vacuum grooves just inside of the cut lines.

bob

[Steve Seebold]

A vacuum fixture with the reverse, push down cutters would be ideal.

Vacuum fixtures are easy to make and they are reusable.

[twocik]

A light spray of WD40 or coolant would definitely help, but it can get messy. A vacuum table would be best, but the double sided tape also works. I might have to try the DIY vacuum table....

I normally throw a few screws along the outskirts of the plastic into a piece of 3/4" wood that I vise down. You can clamp down pretty hard on the wood without the plastic flexing and you'll have more machining room without the worry of hitting a fixture. I'd bump up your feed to at least 10.5 IPM, 5140 rpms, using a .125" 2 FL carbide cutter is a good place to start. Unfortunately cutters that small your rpms are limited (use very sharp bits). This is based on my results, others might have a different opinion. Good luck