Problems Holding Down Thin Sheet Stock

Suggestions for vacuum hold-down solutions when cutting small parts out of thin stock on the CNC router. October 1, 2010

I'm cutting small parts out of 1mm birch plywood sheets 24" x 48". About half way through the cutting, the vacuum is leaking through the cuts to the point where the material is lifting off the table. I'm using a 2mm upcut tool because I have to plunge into the material around the tabs that keep the parts connected to the sheet. Does anyone have experience with this that they would be willing to share? Double sided tape?

Forum Responses
(CNC Forum)
From contributor U:
Cutting thin material is always a problem and small parts as you are experiencing are the most common problem. There are different ways to overcome this. When you are cutting thin material as you are that suffers from bleed through (vacuum sucking through the material). This can make it extremely difficult to hold in place.

If you have created a custom pod/vacuum spoilboard for this part, then you should be concentrating all your vacuum to this part and with its density, you will experience bleed through. The best option I think that will help you is to place a dummy board on top of your part and allow the vacuum to suck through your part to this dummy board. You can then suck the dummy board down and this in turn will help hold your material in place and keep it from rising on the edges breaking the vacuum seal. I would recommend using a non-porous material for this to help seal up the vacuum. With the thickness of your part, you will not experience vacuum loss through the edges that will cause any problems.
If you have any questions, feel free to contact me.

From contributor E:
The low-tech solution to this is to use another piece of spoilboard on top of your existing one to accept screws around the edges of your parts. If you are starting with a square piece of material, 4 drawer-slide screws in the corners will probably hold down the main material. The tabs, if placed judiciously, will hold the parts. I would try a down-cut bit to minimize uplift, and use a ramping lead-in, no lead-out. If you are cutting the same pattern repeatedly, you might want to place some additional screws toward the center of the sheet, in-between parts.

From contributor M:

If it's an option, try using a down spiral bit instead of the up.

From contributor R:
I keep a grouping of acetate pieces or vinyl of different sizes on hand and just toss the pieces over the cut lines. It works perfect.

From contributor L:
If your material thickness allows, onion skin all the parts then flip the whole sheet and run it through the wide belt sander to release the parts.

From contributor S:
You could leave tabs and it you cut with a .25 bit you can use a trim router to release them.

From contributor G:
I know everybody likes to use bleeder board and resurface for each job as the norm. I dedicate spoilboards as the norm and use bleederboards/resurface only on occasion. Investing $38.00 and about 40 minutes takes away any anxiety of parts moving or coming off the table. It's always been worth it to me as my runs are usually days long. I store the spoilboad / fixture for the next run or throw it away.

Create vacuum tracks 3/8" inside of your part, drill holes through to your table and lay weather-stripping inside your cut to seal everything. I can hold 50 plus parts on a 5' x 10' table using a 10 hp Dekker vacuum pump. Example: 1/2" birch plywood, single pass, no tabs or onion skin, 49 pieces on a sheet, approx size 6" x 12". Cycle time about 6 1/2 minutes. 3/8" compression bit. We cut over 6000 pieces and lost maybe five parts. In all the hour I invested in the spoilboard was well worth it.

From the original questioner:
Thanks for all the great ideas. It looks like I'm going with a dedicated fixture board and ZGrabber. We'll see how that works.

From contributor Y:
You do not say how small the small parts are that you are attempting to cut on the CNC Router. A "dedicated fixture" with gasketing will get you through without any need for tabs or turning the sheet over for the old "onion skin method". You should be able to get finished parts in a single pass without dropping a single part.