I used 1/4” MDF for my first spoilboard and mounted it to my 3/4” baltic birtch base with roofing screws (because I had those material around). That worked fine for a few months but the spoilboard gradually developed a warp in the middle - not ideal. I can’t use thicker material for the SB because I’m using the CNC to cut guitar bodies and I barely have enough Z-axis travel for the 2” bit to clear the work piece as it is. I’m thinking of mounting a new spoilboard with either blue tape/SG or 3m 77 adhesive. Are any of these tried and tested methods or are there better ideas?
Larry,
Did you paint the spoil board? I used a 3/8 MDF for a Pen plotting surface and it has stayed flat for a couple years. I used water-based poly-acrylic, painted onto both sides. I did one side with one coat, let it dry, and the did the second side. After that dried, I put another coat on both sides.
Another thing to consider: if you don’t cut too deep in the center, you can put a screw there to keep the center down.
If I was doing a jigged-pattern frequently, I.E. the same part many times. I might build the jig directly on the 1/4 MDF and just remove the whole spoil board when I’m not making that part.
Mike
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How bad is the warp? Could you surface your spoil board to get rid of it? Bonus is it would make it thinner too 
Also yeah I’d totally try sinking a screw down the center of it, just put it a little deeper than normal to be safe.. if it gets you up and going super fast, then it’s worth a try at least!
ETA we need @Michael_Melancon to respond next for a Triple Mike Post lol
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Obligatory Mike post for the fans 
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The MDF SB is not painted.
Though I’ve only made a few, I cut a variety of guitar parts: several different bodies, necks, pickguards, etc. And each part is rarely the same unless I’m prototyping.
I’ve though about countersunk screw, or several, in the middle but when I press down on the bow in the middle, I can see that it is still not flat across all of the board. That’s why I’m leaning towards a adhesive or tape/SG.
The warp is 1-2mm or so - enough to ruin a parting cut on a pickguard with V-bit. For the guitar bodies, it’s no problem.
The current SB needs to be replaced in any case, so I’m just thinking ahead to the next SB and hoping I can prevent the warp. Countersinking a screw, at least with the screws I’m using, in a 1/4” board leaves very little room for error, even though I try no to cut deeper than ~0.2-0.5 into the SB.
I still plan to screw the SB to the base in the corners and mid-way on the long edges, but taping or adhesive in the middle section seems like a reasonable idea.
For it to warp from humidity, it had to get longer.
Can’t push it back flat without allowing the edges to move.
The tape I use that I believe I saw recommended on this site is very thin but also very tenacious. I’ve use 3M77 but that may leave a huge mess when you remove. 3/4" Baltic Birch is what I have and as you know way overkill but I love it.
Would it make sense to perhaps cut a shallow pocket right in the BB perhaps a 5-6 mm (1/4") and then use 3/4 MDF? I assume raising your Y Rails on Xmin and Xmax is not an option?
Impressive making your own guitars BTW!!
For thicker spoil boards, just mount the rails on top of the same spoil board material. I generally cut a couple strips of it to go under mine. Then the center board is replaceable. If your top isn’t too chowdered up, you can flip it over. Then the bow will be held down by the edges.
I have considered placing 3/4” BB strips under the Y rails to let me use 1/2” spoil boards (giving me 1/4” more Z-clearance) but I already have just enough clearance for cutting guitar bodies (~2” work piece) and, other than the SB warping, 1/4” MDF is cheaper and less wasteful than 1/2” MDF.
I do flip the SB after one side gets mangled up.
I might try fabricating some sort of screw-down clamps along the edges of the SB, along with adhesive in just the center of the SB to keep it in pace yet provide for expansion.
It would cost a bit of money up front, but you can get composite brad nails to hold down the center of the spoilboard. They are essentially plastic, so they don’t damage the end mills.
The major drawback is that you need to also purchase a special nailer, as a standard pneumatic brad nailer doesn’t work very well with them (nails shatter due to excessive force).
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