Getting Initial Vacuum

      CNC operator with a spoilboard installed has trouble getting the machine to suck plywood down when first placed for cutting. Pros offer advice. September 24, 2006

Question
I made a spoilboard and it works pretty good. I'm getting about 20.5 in/hg. I have taped the edges, sealed the toolpaths, etc. My only problem is getting it to suck the plywood down initially. Some sheets suck down without putting any weight on it, some you have to have one or two people standing on to establish a good seal. I have thought of getting an engine hoist and using a 4x8 sheet of metal to put weight on some of the stubborn plywood. Am I missing something?

Forum Responses
(CNC Forum)
From contributor T:
You might try using cheap masking tape to quickly tape all edges of the plywood to your spoilboard. You just need a seal around the edges and then it will suck down. We ended up going with a higher grade of plywood that is flatter.



From contributor O:
Having gone through this with many clients, the choices are to increase the quality of your plywood or go with roller holddowns on the machine.


From contributor D:
At the risk of repeating the obvious, always place the warped sheet so it is frowning (the long ends are on the table and the center is bowed up).

Then as was already said, cheap tape all the two sides with the vac on - it should pull down. Depending on the size of your parts, it might even stay.

No question that roller holddowns would help a ton if you are running a lot of this stuff. When the sheet has a twist or a double warp, it gets harder. Sometimes it smiles the short way and frowns the long - these sheets are just about impossible to machine with vacuum alone. Even if you suck them down, they tend to pop up as you cut the parts. A couple of brads through the part into the table goes a long way to helping if you only have a couple of parts/sheets to run. If you are running the same parts over and over, gasketed spoilboards will solve some of the challenge also.



From contributor G:
Weather stripping will work well. Use it as a gasket around all your cuts. It's the same stuff you would use to keep the cold air leaks out of your house. It has adhesive on one side so it is easy to install. An alternative option is to get a big piece of flexible plastic sheeting. Place your plywood on your spoilboard and then drape the plastic sheeting over the top. Your vacuum pump will suck out all the air and the plastic sheeting will pull your warped board down. Remove the sheeting and your ply should hold and you can be off and cutting. If it doesn't hold, try the weatherstripping. Your next option would be to invest in better plywood.


From contributor L:
Try All-Star adhesive products in RI for gasket material - lots of sizes and very helpful. If your main business is running plywood, a roller system is the best solution by far.


From the original questioner:
I should have been more specific. I'm already using a dedicated spoilboard. My parts stay on the table, it's just getting the initial suck that I'm having problems with sometimes. We also upgraded to Akume plywood, which if you are not familiar with, is a flat veneered plywood. I will try the masking tape and the plastic sheet. Where would I find rollers? The machine is a Phoenix rg612, made by Magnum, who went out of business. The machine is funky; I can't even find a serial number on it. I don't know if it was a demo or a prototype. We can't trace back the original purchaser of the machine; Centroid doesn't even know who bought it from the control serial number. Some of the G and M codes were way different. Three technicians and a lot of time on my part later and we finally got it running. It is a fixed gantry machine, if that makes any difference on the roller idea.


From contributor C:
I used akume plywood once - I found it to be a cheap import with a crappy core and will not use it again.


From contributor A:
When I have problems with plywood bowing, I use a sub routine in the program, which calls a tool that has a blank shank instead of a router bit. Then it moves to the middle of my board and moves down in the Z-axis until it presses the board down so that the vacuum can grab the plywood. This only takes a few seconds and it saves time trying to seal or press it down manually.


From contributor V:
What type of vacuum are you using? What horsepower? What size is the hose from the pump to the machine? Maybe you are just lacking the initial volume of vacuum needed to get the material to suck down. Try adding an empty tank in between vac pump and machine - this should help with the volume issue. The bigger the tank, the better. Large vac hoses help also.


From the original questioner:
The vacuum is a 7.5kw, which I think is 9 or 10hp. I have 2" PVC going to my zone valves, and the hoses going to the bottom of the plenum are probably 1 or 1/1/2". It does have a tank made out of square tube which is about 4x4x20'' long.


From contributor L:
With that size vacuum, you are lucky to get plywood to hold at all!

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