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We had a 40mm thick MDF bed on our old ShopBot (96x48"), built up over the years of glueing on and skimming off "spoilboards" all of MDF, and we removed it after about 5 years because it had too many holes through it. Well, when we loosened the screws, that board curled up by about 30mm! Imagine if you try to pull a 40mm thick MDF, curled by 30mm, flat . . . . . .

Plywood might be a slightly better choice, but we havn't tried it because here it isn't available in big sizes. The reason I say this is because we did have some problems with the bolt heads tearing out the bottom of the MDF. Decent plywood might be better for this. The typical plywood we have here would be a disaster because it splits with the slightest provocation. The drawing mentions pouring epoxy over the bolt head to soak into the MDF and reinforce the area - this has cured our problem.

Gerald,
Have you ever tried thin Cyanoacrylate adhesive for strengthening the board holes? It is very thin and would soak right into the pores of the mdf. Lots of people use it when they want to thread a hole in mdf to strengthen the threads. Just a thought.:)

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Socketed cap screws, for the support board, have heads that are too high, too hard and too small a shoulder. You shouldn't use them in wood without washers to increase the bearing area, which will make them even taller. Too hard?......well, you are probably going to hit them with a cutter one day......

About the socket head bolts,... whats the maximum depth I can drill into a 3/4" thick board? In your plans you have a 30mm thick board? There are low profile socket head bolts. But they for sure need washers seeing that their shoulders are even smaller. With the washer the overall height of the bolt/ washer will be about the same as a regular socket head bolt. What bolts are usually used?

. . . . . .
For hardware to hold down the spoilboard, I actually use a large #14 Phillips head pan machine screw. (although plans call for different- use what I had in stock as usually and is Low profile enough. )

My spoilboard cutting profile in Mach shows a 3/8" depth for the countersink.

Unless you use a furniture style hex head with a low profile truss head, you may not find a hex head cap that will work as well as you hope for.
. . . . .

I was actually thinking about using 3/8" - 16 countersunk socket head. It has a large shoulder and I can sink it a good amount. As of now I have about .25 from bolt face to spoilboard surface and about .25" of spoilboard left. If this is not good then I'll just go down to .25" bolts instead. I'm going to post new pics soon.

Charles, consider that you will probably have to re-tighten those screws after a few months. If you glue a spoilboard over the heads, you won't have future access from the top side. The drawings show resin poured over the carriage bolt heads - that is also intended to firm up the wood in that area so that there is less settling/relaxing of the wood over time.

wait wait wait,... glue a spoilboard over the heads?!?!? Why would I do that? First,... how long does one usually keep a spoildboard for?!?!? 5 years seems like aloooooong time. I would think that any wood would curl up after abusing it for 5 years. I was thinking that i would use 3/8" countersunk hex head screws with some locktite to keep them tight. I wouldn't mind changing out my spoilboard everyonce in while if I do in fact tear it up. I would like to be able to remove the board.

Let's take that discussion to Fixing the spoilboard to the support board

I have to order my spoil boards soon.
The supplier can only supply 16 & 18mm from his warehouse.
Am I on the right track if I glue it together like this? :

Attachment 4719

Glueing it up like that would be fine.

Thanks Gerald.
If I counter bore the second sheet as per drawing then I should have the clamping efect as well. I was just looking at my sketch again!:)

A related question:

Is it necessary to seal or paint the bottom of the support board before installation? For my purposes, I have no intention of using vacuum clamping.

No need if your not going to do any of the vacuum. I spent 2 years with a bare bottom! :D spoil board that is.

my 2 cents has anyone thought of using elevator bolts i have used them on several projects where pullthrew might be a problem you can see specks here
http://www.grainger.com/Grainger/Elevator-Bolt-2XJ19

Bill, that looks just perfect! . . . . . for those that can get them in their country.