Hey Guys

Im having issues holding down plastic sheets on a nesting table. The equipment being used is a Stratos pro 5x12 with 3 rotary vane pumps, 9 hp each. Im trying to hold down a 4'x10' sheet. so with all pumps on and the open spoilboard covered I get 16 HG on my vacuum guage versus 21" if I was cutting Mel sheet goods. Lots of leakage around the edges because the sheets arent flat, all the edges are convex and the vacuum cant pull them down. wondering if any of you guys have experience with plastics and if theres anything I can do to make this work..

Regards
Nathan

It's an airflow issue.

Rotary vanes pull decent vacuum but not a lot of flow.

Can you tape the edges down?

Also, use a down spiral bit as it will tend to push the material down onto the spoilboard.

Pretty basic response here that I can't imagine you didn't consider but I'll offer it up anyway. Can you flip the sheet over so it cups down at the edges?

BH Davis

BH, some plastic sheet is wavy not cupped like ply. Horrible stuff.

Yeah. I've found that as well but with the very thin stuff.....especially polyethylene. But I've found PVC sheet stock to be very flat. I'm experimenting with clear acrylic right now and that is also good and flat....at least in the max. 12" x 12" pieces I'm using.

PVC is I think one of the most commonly cut of the plastics. If that is what he's using and it isn't flat he needs to find a different brand.

BH Davis

So I tried taping the edges and that seems to work. I do need to get some downcut oflute bits, Hopefully those will give me a better edge then the endmill I used. It is a 1/8" polyethlene product. Seems to work for now, Just sucks that I have to tape the edges..

Thanks to both of you for your help.

Nathan

I was thinking about this a bit more this morning and remembered something from years ago. I actually thought about trying to build something like this and even got hold of some parts. But the parts sat in box for years and I eventually got rid of them.

There are CNC routers that instead of using vacuum hold down for sheet stock use pressure rollers that run parallel to the gantry. There are typically two full table width rollers, one in front and one behind the gantry. They press down on the sheet stock as the gantry (or moving table) moves back and forth.

This makes me wonder if some sort of outer end pressure foot might be constructed to press down on the edge of the polyethylene sheets as the gantry (or again, moving table) moves back and forth. Perhaps a wood plate with a slippery material (carpet?) attached to the bottom. The vac hold down system would keep the sheet stock in place while the pressure foot keeps the edge down in the general region where the cutting is taking place.

This of course would require time and effort to develop for your particular CNC. So if you had perhaps 10 to 20 sheets to do it most likely wouldn't be worth the time and effort. However if there are hundreds of sheets to be cut it could be worthwhile to investigate the concept.

BH Davis