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.
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.
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.
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.
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.
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.
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.
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.