Screwing Down a Spoilboard

Here's a detailed discussion of whether, and how, to screw a spoilboard down to a phenolic CNC table when vacuum isn't doing the job. November 27, 2012

I have a Rover 30 with a phenolic vacuum grid. I would like to screw down the spoilboard, as some have suggested. How do you accomplish this in a phenolic top? I do a lot of drilling, so any fasteners would have to be in the outside .5" inches of the grid. I have been looking for some 4mm or 5mm brass expansion anchors, but can't find any other than wholesale from India. Any suggestions?

Forum Responses
(CNC Forum)
From contributor D:
Plastic screws work, and free your mind from the stress of hitting metal fasteners with your expensive bits.

From the original questioner:
How would the plastic screw hold in the phenolic top?

From contributor R:
Either drill and tap, or put in a threaded insert.

From contributor P:
Why are you screwing down the spoilboard? What is the application? I've done it several ways and found that usually the spoilboard did not have to be screwed down.

From contributor J:
I have made some jigs and had reason to use nylon screws near my CNC. Got me a good size bag of flush 1/4 - 20s from Alliance.

From the original questioner:
I am cutting 3/4 melamine. I am having issues with the spoil board warping and not holding down for surfacing when the board reaches about 1/2". Also, when it starts to warp upward at the edges, sometimes chips get under the edges of the board and cause leaks. We have to stop, lift up, and clean under the board (a result of only using 4X8 on a 5X12 plenum). I am going to try drilling and tapping. If that does not work, does anyone have a source for threaded inserts that would work?

From contributor M:
Have you inspected the phenolic table to see if inserts for this purpose aren't already there? Typically they had plastic screws screwed into them.

From contributor P:
From my experience, the size of the table doesn't matter if you use a phenolic table and you use the gasket method. This should prevent the spoilboard from ever coming up without screwing it down. They have programs that you can run on your phenolic for gasket seating. It takes a little time to run the grid, but in the long run you can use any size spoilboard in the future, and then plug off the holes that are not in use. Trust me on this one. It's a lot easier - I've tried so many other methods and you can't go wrong with the gasket method. Then you don't take the chance of going into a screw. I used to surface down to a 1/4" on my spoilboard, and never lost suction but one time, and that was because I didn't have the gasket all the way under the board's edge. You can also run multiple parts in different areas on the table. The versatility is endless with this system.

From the original questioner:
Contributor M, yes, I have. There are some in the phenolic. They are very large and only the outer edges of the 2 - 5X6 zones. Not well placed for a 4X8 board. They are also in the field of work of drilling.

Contributor P, that is the method I am currently using. It is possible that you are using a much more powerful pump. I use a single 10hp Becker. With changes in humidity come the natural warp of the MDF spoil-board. This brings the two problems I mentioned earlier.

1. First the spoil-board begins to slightly lift at the edges. This sometimes lets dust get under the spoil-board and onto the gasket. This happens between sheets when the vac is turned off.

2. It will finally get so I can't get the spoil-board to pull flat on the phenolic for surfacing.

Sometimes with 2 people you can hold down each of the 4 corners and it will catch the vac. I don't really like to make it a habit of climbing up on a powered up CNC, even with all the safeties in place.

From contributor P:
We typically ran all of our parts during the day and never turned the vacuum off at all, unless really necessary. I see your point and I know it's hard to get it to suck down when it warps like that, but also using a gasket system you can never be sure that your spoilboard is still at 0 when you turn off the vacuum and back on. You know yourself that that gasket gives more in some places than in others, so if you stretch it more in one place, it's going to set that point lower than another place. But it sounds like you're on the right track with that spoilboard.

From contributor N:
I will have that happen throughout the day and I just flip the spoilboard. Starts out good and flat, and then after a couple hours, the corners want to curl up. Much easier to pull the center down than the edges. I drilled and taped for 1/4 x 20 screw also, and we used nylon flat head screws, but that became a real hassle, so now no screws.

From contributor M:
The existing ones work pretty good with most of my other customers. Be advised that the intent is for you to drill and counter bore the spoil so that the screw sits sub-flush to the surface of the spoil. This way you would never drill into the screw over the effective life of the spoil.

From the original questioner:
We do a lot of through boring, which goes about .25" into the spoil-board. Has anyone drilled and tapped into phenolic? Has this held up well? I would use plastic screws.

From contributor M:
Phenolic taps relatively freely, however I would recommend tapping about less effective thread instead of the standard percent thread. For instance, a 1/4"-20 tap typically calls for a #7 or .201 drill. In phenolic I would use a #3 or .213 drill. This will still allow for 65~70% effective thread but make for much easier tapping. You could also always purchase the inserts and just interpolate counterbores to press the inserts into.

From the original questioner:
Thanks for the advice. Do you have a source for inserts? I was looking for some brass expandable ones. I could not find them for sale anywhere on the web. I found plenty of companies in India that make them. None for sale. Kind of strange.

From contributor T:
Good old McMaster Carr has all the inserts you want in stock.

From contributor S:
We've got our spoilboard screwed down with machine screws and just keep the heads down about 3/16 or so from the surface of the board. You should be able to do all the drilling you want if you set your bits and drills carefully and pay attention with your programming. Never had a problem hitting a screw on the spoilboard on two different routers in the last 15 years. Just be careful in setting your bits and in programming.

From contributor I:
I found the nylon screws to be a pain, as they are slotted and have a tendency to draw through the holes in the spoil board, and are easily cross threaded against the brass inserts. I just go with metal screws and countersink them to 3/8", but I don't do much through boring. This would only be a problem when the spoil board gets thin. I also put some silicone on top of the bolt heads.