Holding Down Nested Plywood Parts

      Vacuum alone may not hold parts in place well enough, but there are many ways to get around the problem. Here's an assortment of tips and techniques. July 24, 2005

Question
I am implementing a new Weeke BHC550NB. When we try to machine nested nursing station studs 3"x36" from 1" poplar plywood, the stud moves. The vacuum gauge reads within manufacturer's specifications, and I am using new tooling, and also using 3/4" MDF with sealer skimmed off for spoil board that meets manufacturer's specification’s as well. Does anyone have any ideas?

Forum Responses
(CNC Forum)
From contributor J:
I have a problem routing wire holes and toe kicks in poplar studs for walls. Regardless of what the vacuum gauge says, you can not buy too large of a vacuum pump. My gauge is on 28 and parts still move, and you need a high flow rate also.



From the original questioner:
On the panel saw, that is how we were/are doing it, but the studs also have wire chases that need to be drilled, and the drilling time is what we are trying to eliminate in addition to other uses for the new CNC router.


From contributor S:
Depending on the software, make sure you sequence the studs to be cut first. Use two passes if necessary. Plywood can be tricky to cut on a flat table CNC because it tends to bow, no matter how powerful your vacuum is. You can also insert a drill sequence to drill a hole in the part. Then program an optional stop in the program so the table will come out. Run a screw in the part and resume the program. Tabbing is a good way of combating this too, but if the material is bowed at that point, the router will cut thru the tab and leave it loose.


From contributor N:
To the original questioner: Are the studs precut to 3" x 36"? If so, you can make a vacuum jig with a pocket. The stud sets in the pocket which prevents any movement. The jig and the stud are both held down with vacuum.

If you are cutting them out of a large sheet on the router, the ideas already mentioned are pretty good. We would not quite cut all the way through on these parts. I have a small router table setup with a bearing guided bit. It’s an easy job for the operator to separate the parts while the next job is running.



From contributor T:
To the original questioner: In addition to multiple passes, make sure that your lead in and lead outs are on the long side of the part. This reduces the load on the part that causes slippage.


From contributor M:
With 1" plywood, you will want to take two passes. When I do studs, I always conventional cut. That puts less horizontal pressure on the finished part. Cut all the wire chases first and all the way through. For the perimeter, place your start point so that it is attached to the larger sheet until it is ultimately through cut and removed from the sheet.

Order your cutting so that you are not cutting a piece that has already been removed by other operations. Also, set all the parts first to a depth of .03" to .06" first, then return and cut through in the same order. This is referred to as a return onion skin. Also, if your panel is warped, make sure the edges of the panel curl down toward the bleeder. Also, make sure you have a sufficient length of Z ramp for your lead in. This will keep the bit from getting hot and dulling quickly, which will push your pars around.



From contributor C:
I've done similar parts by using a spoil board. I first run the program on two empty spoil boards 2 mm deep. This gives a pattern to hand drill two screw holes per part. I put index marks on two edges so everything gets loaded same way everytime. Lay spoil board face down on stack of plywood, screw it down, load it, and while the machine cuts, screw down the second piece. Work on one while machine works on the other. The depth of cut stays at the same 2 mm into the spoil board, so when the screws are removed you have a finished part.



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