Hold-Down Problems with Small, Thin, Porous Parts

      CNC pros suggest a range of solutions for holding down small parts during machining. August 3, 2009

Question
We are having trouble with a particular part on our CNC. We are trying to hold down a small (approx. 4"x8"x1/8" thick) MDF panel for edge routing on all four edges (therefore we cannot use a fence on the spoil board). The part must remain fixed (critical dimensions).

We have tried pinning the panel down, but this is a pain since you have to pull the part off the pins and then pull the pins out; not to mention the resulting holes in the part. Double sided tape helps, but is only good for a single part. I thought of trying the silicone tape that Lee Valley sells, but wanted to see if any of you had better suggestions. Thanks in advance for your help.

Forum Responses
(CNC Forum)
From contributor M:
Why don't you use a vacuum fixture using gasketing? We make lots of them for all kinds of applications. 4" x 8" is huge compared to what some are doing.



From the original questioner:
A few more details: we run gasketed Melamine spoilboards (with holes drilled for the vacuum) on our other CNC's, but on this one we process numerous door and front components and therefore use gasketed non-melamine (with no holes) so we don't have to constantly swap out spoil boards. Does that make sense?


From contributor K:
Can't you use a bigger piece and do the detail edging first? Then cut the part loose from the bigger piece. That opens up a whole list of options for holding it in place.


From contributor B:
If you are not making an aggressive cut, you may not be able to hold it down via vacuum but that is a small foot print.

There are three other ways to hold down.

1. Mechanical fastening: if your parts have holes in the body, you can make a fixture that can be mechanically fastened down.

2. Using two operations. Hold down with clamps and router two sides. Turn and then do the two other sides.

There are many variations like not cutting all the way through or all the way around. Many times when a person is machining small parts, the part will have to be machined a slower way.



From contributor M:
I'm not real clear on the process here. Are these parts loose (sized previously), or are you cutting them in a nest from a larger sheet? Is the spoilboard MDF, LDF, or something else? Cutting one at a time, or multiples?


From the original questioner:
Thanks for everyone's help so far. I'll try to answer all of your questions. Our spoilboard is LDF, and we tape the edges of it. The parts are not nested (they are pre-cut 1/4" total (1/8" per edge) over the final dimensions. We run them two at time (side by side with adjacent heads - not stacked on top of each other).

We can't have larger-than-pin-sized holes in the panel, as we are vinly-wrapping the panels after machining. The panels must fit very precisely into a recess on a door, which is the reason we are sizing them on the CNC. There is no edge profile or anything, just cut to final size with radiused corners to match the routed recess in the door. We would like to avoid the use of clamps, as we are routing other parts on the same board, plus we are routing different sized panels constantly.



From contributor P:
We make an aggregate tool that is referred to as a "floating head". Typically this unit is used to compensate for inconsistent material but we have had customers that use this head to provide additional "hold down" force on smaller work pieces such as what you have described. With this head, you will be able to machine all the way around the part without having to change anything halfway through the machining cycle.


From contributor J:
First, are you covering the unused portion of your spoilboard? Use some thin melamine or laminate, this will help concentrate the vacuum in the area where your small parts are located. Also, might you try gasketing the matrix table under the area around where your parts are located to further focus the vacuum on your parts? LDF may be too porous, pulling air through border areas around the parts. Try standard MDF instead.

Do you surface both sides of the spoilboard? If you have a multi-zone vacuum table, activate only the vacuum zone under your parts. Nest the parts out of a larger sheet of material and use tabs to keep the parts connected to the sheet. Maybe a couple 1/32" thick tabs 1/4 " to 3/8" wide would help. The tabs are easily cut with a sharp utility knife or chisel.



From contributor B:
There are a couple of things that are in play here. You have a very thin porous board and you have a very small area. There is a high likelihood that there is vacuum is going to be sucked through the board. Can you put the vinyl on the board before you router it? I would also use a down cut bit so the bit is not pulling your part up. I would also use a slower speed so you do not have as much torsional force trying to twist your part.

With that said, there are a few tricks also but they come with time expense and that would be something on the outside that keeps the part from spinning (it could be a border that that outlines the uncut part. It could also be some pins that keep it from spinning). You can cut all the way through on most of the part but leave three tabs. These tabs can be manually trimmed after the part is routered. This will of course take more time.

One other thing that you may consider are small powerful magnets. The main drawback though would be that steel or another magnetic material would need to be imbedded in your fixture. This also is just good for holding the part down and does not help the part being twisted torsionally. This sounds silly but it can greatly help assist the vacuum in holding the part down.

My best solution would be to laminate first if this is possible and to use a down-cut bit if your arenít already using it. If this does not work, then you will have to employ some method so the part wonít twist out of its fixture. I hope I am not just throwing things at you but these are methods that I have used in the past in similar situations.



From the original questioner:
Contributor P - I'm hoping for a more economical fix (business is slow), but I'm still not clear on the floating head. How could the router spindle get around another head on such a small part?

Contributor J/B - I'll try blocking off the remainder of the spoil board for increased suction, and will try MDF as well. We cannot vinyl the part first, as the panel/door assembly is vinyled as one unit (but I do agree that we're probably losing suction through the part - maybe I could cover it with a temporary covering)?

As to leaving the material attached with tabs - that would probably help, but it would increase our waste, right, as we're only cutting 1/4" oversized currently. We don't currently surface the bottom of the spoil board, but I will try that as well. Also, we are using a down cut bit. Thanks for all of your help; you've given me some good ideas to try.



From contributor J:
It's very important to surface the both sides of the spoilboard. I suggest that you consider cutting the parts from a nested sheet using the tabs as you'll have very little waste.


From contributor R:
Even on a full vacuum table, you can build fixtures to hold parts like this. I have built many fixtures like this. A plywood panel with a standoff (to raise the part) with a large hole in the middle of the standoff. The ply panel "concentrates" the vacuum through the hole under the part providing a large amount of holding power. Feel free to contact me if you'd like advice on how to create a fast and accurate fixture.


From contributor T:
If you donít want to build a dedicated fixture or gasket off the part, the easiest fix is to give the backside of you part a very light spay with 3M spray adhesive. The slight tack in conjunction with the vacuum should be enough to hold the part down. We use this method often to hold down small solid wood, melamine, and MDF parts. Be sure to apply a very light coat otherwise you will destroy your spoil board and or your part trying to separate the two.

I might also add that you should try using 45 lb. ĺĒ thick MDF for your spoilboard with a gasket between your grid table and the MDF. 35 lb. LDF allows too much vacuum loss around the uncovered areas and edges of the spoil board. MDF densities greater than 45 lb. and 55 lb HDF are too dense and do not allow enough flow thru. You have to fly cut both sides of the MDF prior to using.



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