I’ve finally got my base sheet of MDF attached to my CNC. I want to put another piece of 3/4 inch MDF on top so I can add T-Tracks and Dog Holes. What’s the best way to attach the two together? Gluing seems best but I’m not sure how long a spoil board will last and that would mean replacing both sheets.
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I bolt the first sheet in permanently to the frame and then use screws to secure the second temporary sheet on top of that. Countersink the screws 1/2" down on 3/4" MDF and replace after it is worn down 3/8". After the spoil board is flat .03" is usually sufficient to renew it at least for me. When making through cuts on a work piece I always reference the spoil board not the top of the work piece to help keep the spoil board clean. Gluing is an option but then you have to machine off the entire top sheet.
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I have one sheet attached to the machine, T-Tracks attached to that sheet, and glued MDF between the T-Tracks.
I replace the MDF when it reaches the level of the T-Tracks. You can macine it down to the level you require with your router when it is time to do so.
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Everyone works differently, but I went with Jay Bate’s approach and it’s working well for me. Let us know what you go with.
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I was in the same situation, and used 3M Hi-Strength 90 spray adhesive (contact cement) to glue the sheets together. This is a bit tricky, especially if you have a large spoilboard, because you only have one chance to get the two sheets exactly aligned. To accomplish that, before using the cement, I lay one sheet (call it “A”) on top of the other (“B”). After carefully aligning A and B, I drilled four 3/8" holes, one through each corner of the aligned sandwich. I then cut four 3" long pieces of 3/8" diam. dowel, and using a grinding wheel, partially sharpened one end of each piece to allow it to easily slide into a hole. I then removed sheet A and inserted the unsharpened ends of the dowels into the holes in sheet B (so that the pointed ends of the dowels stood up). I then sprayed two or three coats of the contact cement on one side of each of A and B (the sides that touched in the sandwich) per the 3M instructions. When ready, I had a helper (my wife) help me flip A over and temporarily rest it on the pointed dowels already inserted in B (i.e. slightly ajar) with the two sprayed sides facing each other. On the count of three, we lifted A slightly, aligned its four holes with the pointed dowels in B, and let it drop. Bingo! The two sheets joined each other perfectly aligned. We would then clamp around the edges of the sandwich so that there was no space between the two sheets around the perimeter, and then remove the dowels.
Actually a lot simpler than it sounds from this long-winded description. A bit of work to make the holes and cut the dowel pieces but it worked like a charm, giving a very strong, flat lamination.
I glued my second layer,then surfaced it then installed my t tracts ,i used some heavy stuff to put on top of the sheets
I built the bed for my 8x4 by screwing a base MDF sheet to the frame, then laying another sheet on top.
I used the machine to drill and counter-sink a grid of holes and fastened the top sheet to the bottom with nylon screws and nuts from McMaster-Carr.
Screws: part# 92942A765
Nuts: part# 94812A700
No problem, no failure, no issues when I go too deep thru a cut and dig into the spoilboard (which i NEVER EVER DO 
)
rick
For the record my machine does not have a welded base nor an AVID aluminum base, it has a steel bolted base.
For most builds I used a double MDF top contact cemented together bolted to the frame with countersunk 1/2" bolts and then I surface the top of that double thick top.
AFTER that I then add the replaceable 3/4" MDF beater board(sometimes 3/8" or 1/2") and screw that in, sometimes even just sticky double tape it.
This way I don’t need surface every time I replace my beater board, this beater board follows bmy resurfaced top perfectly most every time. It has been very rare over the years where the MDF beater board is not consistent through its thickness or would not lay flat enough to follow the flatness of my surfaced double thick MDF table top to where I had to resurface my beater board.
Ever since my first Joe’s build over 20 years ago I started using this double cemented MDF top instead of a single sheet. Initially it was to help stop the shaking, vibration, etc with unistrut bases. I found in subsequent builds that the added weight of a double MDF top helped even my welded bases. Depending on machine size that one sheet of MDF could add 150lbs of even weight. Some guys just try to weight underneath the table with shelves and things and that is fine, but adding an easy no space taking weight by using a double table top adds even more mass. In one build I used a triple top(the machine I have now has a triple top, with beater on top)and truly I dont’ see any diminishing returns with that third sheet as part of the top. For me that loss in height is meaningless. In my shop with my work it seems the heaver I can make that table and base as a whole(as long as the legs, bracing and over all structure can handle it)no matter where the weight is, the legs, the frame, the table top, it just makes my set up more stiff during cutting. Me not having to resurface as much by using a double or triple top is just another benefit of it.
Well I decided to just glue the two sheet together. Used a couple of quarts of glue and about 60 screws. Jay’s solution was good for his choice of T-Tracks but they are too expensive for me. My T-Tracks won’t be here until the 14th 
and this way I can continue with the dog holes and the vertical spoil board. I need to 3D print a holder for the Auto-Z tool. With my spoil board offset one crosspiece I need to set 0,0 on a different cross member. Thanks for everybody’s help with this.