[MechanoMan]

I have a full-size CNC router that has moderate MDF bed issues.

It's made out of 2x layers of 3/4" MDF. One layer is the base, bolted down with nylon bolts in case a milling error sends the bit down that far.

The second piece of MDF is sacrificial. I attached it with fiberglass-composite Raptor nails which won't damage a milling bit. I didn't glue it because I need to replace it periodically.

Maybe I didn't use enough nails. The table isn't super-solid. I think some areas had a gap between the layers, and either milled the top piece down thinner than intended and when actual plywood stock was fixed, it pressed flat and was too low, or vice-versa- the cutting bit pressed it down flat and it popped back up with a gap that plywood stock doesn't flatten back down, so it's too high (more likely case).

Is there better info on the best way to do a sacrificial MDF table here?

[ger21]

I'd permanently bolt the first layer down with steel bolts, as it will be more rigid. Counterbore the holes so the bolts heads are below the surface.

Then glue the second layer down. Use screws to hold it tight (remove when glue sets), and a slow setting glue to get it really tight without gaps. Generally, rather than constantly replacing it, you just resurface it as needed. When it gets really thin, just surface it, and glue on another layer.

[nainino]

Why aren't you bolting both layers of MDF from the top with longer nylon bolts ?

[minicncuser]

I use an MDF table and I do surface it when I put it on and the I use dry wall screws to hold everything stops and held with them but every time I go though the top layer of mdf I do get bumps in the middle so it changes the height of the board. If I was cutting something its not a huge deal but it I wanted to do some light engraving I would need to resurface the top to ensure a flat work area

[MechanoMan]

Why aren't you bolting both layers of MDF from the top with longer nylon bolts ?

Well that would defeat the point of having 2 layers at all. I could strike through the nylon boltheads right off the bat. And the composite Raptor nails won't hold if penetrates into a hole with a nylon bolt. The point is to have 3/4" of buffer that can be made flat, nailed into, and accidentally gouged without major consequences.

[whimsical]

Listen to ger.
I used to have an 8x4 with an mdf sacrificial table. We would glue it to the base with lots of the white wood glue and lots of weights while it dried, no screws and never had a bubble issue.
This machine ran 8 or 9 hours 5 days a week and the surface would get cleaned up as needed. The 32mm mdf lasted 2 years before needing a new one gluing on the top of the old, never routed it down to get back to the original bottom layer, just glue on some more thickness.
We did a lot of engraving on large panels of thin material, ally and engraving plastics. Even though the surface was nice and flat the material never bolts down that well if it is getting large so I would recommend a proper floating engraving head. We adapted a Gravograph, New Hermes, spindle that used 11/64 cutters.

[louieatienza]

I agree with the others... GLUE IT! You can use polyurethane glue (Gorilla Glue) and regular sheet rock screws. When the glue dries just REMOVETHE SCREWS! Very simple. If you need to resurface, you can just fill the deep areas with Bondo before surfacing, then you don't have to resurface so deep. Very simple!

Best thing to use for the first layer are carriage bolts. The larger flange helps hold things down better.

[rocketflier]

Drill and countersink 3/8 or 1/2" MFD, screw that down to the table. Glue 1/2 or 3/4 to the adapter board. Surface as needed.

If you use the CNC to drill the holes in the adapter board, it then knows where to drill if you ever want to replace the sacrificial surface.