Spoilboard flatness woes

[johnm]

Folks -

We've had the 'bot up and running for a couple of months now, but are having trouble with table flatness.

Our spoilboard setup is a sheet of 3/4 maple cabinet plywood as a base, with a sheet of 1/2 MDF on top. Doc Palumbo has told us that MDF will creep quite a bit with changes in humidity, and he must be right - I have the MDF screwed down (up) from underneath with brass screws, but it bows and doesn't sit flat. I've thrown a few more screws in it, but have some serious bows - 1/8" or so out of true over ~4' - frankly I am kind of amazed that it would be that sensitive to humidity. I haven't had any trouble with MDF movement in other applications - heck - that's why I *USE* it in those other applications. We already had everything set up when we got Doc's warning...

Bill has suggested tru-pan, but it isn't available locally.

What else are y'all using, or what would you suggest? We are doing some Z-sensitive work (never use those 3 words together, before SB...)so it does matter, and I am getting tired of killing time surfacing the table.

Thanks in advance!

John Moorhead

[paco]

If you want to keep using MDF, get thicker board (1" or more; I have some 2" list here). I personnaly get into wraping problem when it's about 3/8" and less. You can glue it along with screwing it... Extira would be more stable but you still need to use it thick (it's list up to 1-1/4" max). Or look at more exotic material like PVC or high quality plywoods... how about a 2" HDU spoilboard?!... no, please, don't do that!

[gerald_d]

In which direction does your table bow/sag? If it is along the y-direction, I would look carefully at that notch you left in the table for the indexer....

[billp]

John,
Two options; Use birch plywood as your top sheet as well (which is how I went). OR drive down to PALS in Richmond, as they carry Trupan,in ALL sizes, BUT Trupan is the same as MDF in the sense that you HAVE to totally seal it, and keep it sealed...Even with the plywood you need to seal it with shellac or water based polyurethane... With the amount of rain you have already had this Winter (80+ inches?)I'm not surprised...

[Brady Watson]

John,
I use MDX (Medex) and it is fairly stable...much better than MDF. It has plastic in it that helps stablize it.

Instead of screws, glue the top sheet down to the base plywood. This way you don't get any buckling, have no screws to worry about hitting and can surface it many times, right down to the base ply. When you get down that far...just surface and glue down another sheet.

You may also want to try Extirra as a spoilboard. It is very stable, although it is hard on tools.

-Brady

[gerald_d]

"a sheet of 3/4 maple cabinet plywood as a base, with a sheet of 1/2 MDF on top.......screwed down (up) from underneath with brass screws, but it bows and doesn't sit flat. I've thrown a few more screws in it, but have some serious bows - 1/8" or so out of true over ~4'"

Surely you have screws at much closer centers than 4', therefore I deduce from your description that the whole system is bowing - the spoilboard, baseboard and crossmembers? Good solid crossmembers are the starting point to prevent bowing over a large area.

[johnm]

Folks -

Thanks for all of your remarks - The ply under the spoilboard is flat - the table isn't low due to x members sagging or the ply - looking at the edge of the ply/spoilboard, I could see that the MDF was "wavy" and had some gaps between it and the ply. I only had one area about 1'x2 1/2" that was bowed up, so that's where I threw some extra screws.

I had thought about gluing the whole mess together, but really didn't want risk hitting the screws I've mounted the plywood with when it comes to surfacing for a new spoilboard - but I can replace the screws with shorter screws or brass ones.

I guess my main surprise in all of this is just how much, apparently, MDF moves around. That sure hasn't been my experience in using it in other applications.

I'm out of MDF, so I might end up using another sheet of plywood. My existing MDF spoilboard is now about 3/8"+ and I can see how that could contribute to instability.

Thanks again for all of your remarks.

John Moorhead

[gerald_d]

"I had thought about gluing the whole mess together, but really didn't want risk hitting the screws I've mounted the plywood with when it comes to surfacing for a new spoilboard"

If you have a glued spoilboard, there is no reason to remove it totally before adding a new spoilboard. With a glued spoilboard, you can work it down a couple of times until it is only about 0.2" thick. Then glue a new one on top of that.

See this old thread.

[hippo]

Umm... This is that old thread.

[gerald_d]

Oops, broken link - here (http://www.talkshopbot.com/forum/messages/7/4186.html) it is.

[kerrazy]

also, Screw from the top down, you are inevitably pushing dust and debris in between the two sheets when screwing up. I predrill countersunk pilot holes every 8-10 inches from the surface then screw to the plywood with drywall screws while standing on the table. This works real well, Then a quick surfacing and bob's you uncle.
Dale

[ed_conley]

Here is something we've tried, and seems to be working when we've replaced the spoilboard.

First, use regular MDF, not lightweight MDF or Trupan, both of which are more subject to water absorption. LWMDF and Trupan will do well for suction, but they suck up the water just as well. So stick with regular MDF.

Second, use a roofing silicon caulk on your plenum with 23 gauge nails on the perimeter, nailed at an angle. Be prepared to sharpen your surfacing bit more often, but you will be suprised to learn that the 23 gauged nails don't do much damage to the bit. We get about 12 to 15 surfacings before we sharpen our bit and we could probably get a few more out of the bit. Regarding the silcon caulk, we put some daps on the rectangular tops of the plenum to secure it better in the middle. Seems to help buckling and warping of the MDF as it gets thinner. Be sure to keep your vaccum on or clamps on edge until the silcon is cured.

Finally, surface your spoilboard once a week. If you do it often, you really don't have to take off much, maybe 0.025" at a time. We've tried a number of different materials but find that MDF, for the cost, works best. Using this approach, I've used my spoil board until its less than 1/16th" thick.

Give it a try and let me know if you find any better solutions or ideas.

Ed.