Vacuum Pod Tips

      Techniques for achieving good hold-down for machining small parts on CNC equipment. December 6, 2006

I am looking for advice on holding small parts. The image is of a pod I am trying to use to hold a small oval part about 3 1/2" x 1 5/8". I screw the pod to the flat table on my CNC machine and hook a smaller vacuum pump to the fitting on the side. I have used this setup for parts as small as 6" around with success, but not with anything this small. Any advice would be appreciated.

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Forum Responses
(CNC Forum)
From contributor R:
It's pretty low tech, but I usually put a strip of double faced carpet tape down on the spoilboard and it's all good.

From contributor T:
Double sided tape would be my first choice, as well. If that doesn't hold it to your satisfaction, you could glue/screw the fixture base to a piece of MDF, or whatever, as a backing. Then screw the whole assembly to the spoil board.

From contributor M:
With parts this small, you may be better off leaving a thin onion skin around the perimeter of the part that can be sanded off when you're done. In this manner, you can nest a bunch of them together and not worry about pods.

From contributor F:
We just cut 2000 circles made of 3/4" thick plywood with a 2" diameter. I just made a fixture out of scrap melamine with a 1/8" wide channel for gasketing and a 1" diameter pocket with a through-hole for vacuum. Worked great using onion skin followed by through-cut at a programmed rate of 275ipm. Making the fixture, which held a quantity of 87 finished pieces, took about 45 minutes to machine and insert all the gasketing, but it saved a guy from trying to route away or clean up the onionskin by hand.

From the original questioner:
Thanks for the replies. A groove needs to be routed around the outside of the part, so the onion skin will not work. The part also needs to be elevated to provide clearance for the grooving cutter. I think I'm stuck with the pod setup, but am hoping for some advice regarding vacuum to hold the parts secure.

From contributor K:
I have had fair success with the following procedure on particularly hard to hold small parts. I use a fixture exactly like the one you pictured, and add several screws driven up through the bottom of the jig just enough for the points to protrude into the part. When placing the blank on the fixture, a sharp hit with the heel of your hand will "set" the tips into the blank, helping the vacuum to hold the parts. Then, I adjust the depth of cut and feed rates to the maximum without causing the part to slip.

From the original questioner:
Excellent idea! Thanks!

From contributor E:
I have found that for some small, hard to hold parts, it helps to seal the bottom of the part with something like shellac or water-base poly; this will help seal the vacuum. If you moisten the gasket a little bit when you put the part on, that will help also.

From contributor M:
That little tab off your 80 grit PSA sanding disks will adhere real nice to the perimeter of the pod shown for some extra grab. We also, when making the smaller pods, go with a 1/4" full round seal; seems to have less side to side flex.

From contributor D:
What type of vacuum pump are you using?

From the original questioner:
I've tapped off of my flat table's vacuum while covering the rest of the table. I've also used a separate small 3/4 hp pump that was kicking around the shop. What I'm finding is that even though the gasket appears to fully compress, there is still a small amount of side to side movement when the perimeter is being cut.

There will be a hole in the center of the part, so I'm thinking of going with some sort of mechanical fastener through the hole, through a spacer to elevate the part above the spoil board and give up on the vacuum.

From contributor M:
I see a lot of extra room around the seal on your pod. Try the full round as I mentioned earlier, or even two rows.

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From the original questioner:
What is the diameter of the seal you have in those pods? What are the pods made of?

From contributor M:
We use 1/4" neoprene, and the material is 3/4" Corian type stuff we had laying around. Corian works well - just don't drop them or they will break at the thin edges.

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