Wasteboard Help

Need some ideas or suggestions…I bought some 1/2 inch MDF and fastened it (20 Screws) to my X Carve waste board. After some carving I noticed that I had a low spot in the center of my sacrificial board. So I bought a 3/4 inch mortising bit and set up Easel to do a 29" x 29" pass. After 1mm, the low spot was still there. 2mm still there. Had to go to 4mm to get rid of the low spot. So now, I think my wasteboard is flat. On my 1st project (I do a lot of work with acrylic) I was able to engrave the letters ok (1mm deep) on the front of my piece, but the further back on the Y axis , X Carve would miss the letters again.
Any ideas or suggestions anyone might have?

I maintain 3 machines in an educational setting (at a middle school and at a college maker space). I’ve chase this problem regularly, but mostly have it under control
Make sure your machine is secured to a flat surface. If it sits on a warped bench top, no amount of resurfacing will solve the problem and it will change from day to day.

Check that the ends of the y axis rails are all coplaner and parallel to the waste board. When I built the first machine, the rear corner of the right side y axis rail was a bit higher than the other corners and this created a shallow cut as the spindle moved to higher X and Y coordinates.

Later the factory waste board started to sag in the middle, leading to shallow cuts in the center of large work pieces. At the same time I figured out that the factory waste board tends to flex, twist, or otherwise distort when work pieces are clamped down too tightly. Kids are terrified of work pieces coming loose, so they over tighten the clamps which distorts the waste board and often the workpiece too. I also found that a securely clamped, warped work piece will distort the carefully surfaced waste board. Before I figured out the problem I kept trying to surface mill the waste board which just made it worse. Repeated efforts to skim the waste board made it thinner and thus more flexible.

Besides educating the kids to not over tighten the clamps, I built torsion box tables ala Ron Paulk to support the machines beneath the waste board. I cut spacer blocks to add support between the table top and the unsupported areas of the waste boards.

I now have nearly new factory waste boards on all three machines and I never let a mill touch them. I have shimmed them as flat as I can get them with winding sticks and a laser level and secured them to the underlying work table with counter sunk screws passing through shim blocks into the torsion box table below. They are each overlayed with a sacrificial waste board of 1/4 MDF that is GENTLY fastened to the factory waste board. I then used the machine to drill through the sacrificial waste board at each clamp location. Users are not permitted to remove these.

Because these are school machines, I have to put up with some inaccuracies because kids will crash the spindle, try to drive the machine outside of it’s envelope, skew the gantry or lean on the table top causing it to sag or twist by a few thou while clamping. Oh, and then there was the time a janitor stood on the machine to change the clock battery…

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I added extra extrusion to support my waste board. It’s not that expensive to buy. I think I have 3 extra in both directions evenly spaced. I had extra extrusion here from my Shapeoko2 machine.

You need some angle brackets, screws, washers, and of course the extrusion which I think Inventables still sells. Think of how many floor joists support a floor…