Vacuum Hold-Down with Rough Materials

      Advice on how to achieve a strong working vacuum when holding down rough sheet stock on a CNC router table. April 29, 2006

I have had my CNC for approximately a year now and I haven't had any problems with moving parts - until I signed a job that requires cutting 1" rough plywood. I have never had problems with 3/4" plywood so I assumed there wouldn't be much of problem with the 1". But it seems there is. The material has such a rough finish that my vacuum table won't hold it down. The CNC I have is an Omnitech Selexx mate, 4 x 8 table, 9 horsepower pump. I have been using a 1/2 down spirial chipbreaker to skin it down to 1 mm (anything less and the skin breaks) and 3/8 compression to get rid of the skin. This actually works about 80% of the time and 80% is being generous. Does anyone have any suggestions?

Forum Responses
(CNC Forum)
From contributor A:
On the 3/8" tool, use the shorter up/shear. Based on the part size, assuming its small, other firms put the material thru a sander to separate the wood. We would also suggest a true "ruffer" on the first cut and a "smoffer" on the second. The smoffer is a compression tool that looks like a ruffer, but will provide for a great edge finish.

From contributor B:
Your spoilboard is everything. First - make sure you have fresh knives on your fly cutter. Second - rpm and feed speed are important (3" or 4" cutter under 14,000 rpm at 30 mpm). Third - edgeband your spoilboard. Finally, make sure there is a good seal on spoilboard with foam gasket. Also use good quality MDF not LDF.

From contributor C:
Check the filter on your vacuum pump as well. You may be experiencing a lower level of vacuum if it is partially clogged with dust.

From contributor F:
Make sure you machine both sides of the spoilboard to start. I have seen vacuum issues because people have only machined the top side. You want to open up the underside to maximize air flow. The rough texture may be your main issue. Besides the cutter blades, rpm and feedrates mentioned above, also use a conservative stepover. If your head is just slightly out of tram, a 2", 3" or 4" flycutter can leave steps and/or cups in the surface, depending on which way it is out of tram.

From contributor D:
I know this sounds like a simple fix, but I have had great success simply taping the edge of the material/spoilboard to close the gap that the rough surface creates.

From contributor E:
Use a nice fresh spoilboard with a little water sprayed on it. You will have a strong hold.

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