Slight bed cup after surfacing

CNC Machines Shapeoko 5 Pro
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I have a Shapeoko 5 Pro that has been upgraded to a 5.1 with the upgraded base kit and leg kit. After surfacing the spoil board, I cut a part and I noticed a slight cup to the flattened spoil board - the middle is slightly lower than the edges.

The evidence is that the witness lines (I don’t know what else to call it) of cutting through the material are cut deeper into the spoil board on the outer edges than they are in the middle of the spoil board.

Some ideas I have seen about this are:

  • The gantry is sagging because physics.
  • The bed isn’t coplanar.
  • Ancient aliens.

What can cause this and what’s a person to do to correct it?

2

Which spindle do you have?

What tool was used to surface the MDF? Did you go back-to-front, side-to-side, or use a pocketing operation which worked its way in?

Photos showing the part in question? What toolpath was used for the cutout operation?

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3

I have an 80mm spindle from PwnCNC and I used an Amana RC2251 2-1/2" spoilboard surfacing bit.

Spoilboard surfacing was done using a an offset pattern - I’ve found this lets me spot tramming issues immediately and fix it before breaking out the magical piece of granite.

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The parts that I milled are fine, it was all sheet goods at a constant depth from AribaBox’s miter saw plans. To avoid being That Guy saying “trust me, bro”, here’s an example from Vectric of one of the sheets I cut.

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Parts were cut out using a profile tool path using a 1/4" compression bit from Cadence MFG. I used a separate last pass at full depth, leaving 1.5mm per pass that was removed on the final pass.

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4

How much time between resurfacing and you noticing this? What’s the humidity your shop?

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Oooh, great question. Maybe a week, but I live in coastal Maine, so humidity in my shop varies between “none” and “swampy boi”.

I’m looking for recommendations on sealing the MDF after surfacing. Looks like a 1-pound cut of shellac might work, so I’ll be trying that in the near future.

After poking the machine with an old screwdriver a few times, I think the issue is either:

A) the screws came loose and the slats have some spring that coincidentally looks like something else, or
B) the moron who tightened the screws securing the slats to the base didn’t do a good enough job.

I will be tightening everything and testing this tomorrow. I bet it’s a combination of humidity and not tightening things.

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Worked in the Maine area periodically, gets nasty muggy. I live about midway between Portland and Seattle in the mountains. I am lucky when I don’t have to leavel my spoilboard weekly due to the humidity.

Hint: I get MDF 1.5 or 2 inches thick. Hurts my part heigth, but I can go a month or two before I cut new ones.

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That’s a lot of flattening.
Have you considered PVC boards for the slats?

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I live about midway between Portland and Seattle in the mountains.

I used to live in Portland! I’m very familiar with the humidity out that way.

Hint: I get MDF 1.5 or 2 inches thick.

I bought 1 inch MDF and it’s been serving me well. The current plan is to flatten and seal the existing slats and then use sacrificial MDF with a bunch of dog holes in it for locating pins. Or I can use the MDF to make more specific jigs/fixtures for weirdly shaped parts.

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That’s a lot of flattening.
Have you considered PVC boards for the slats?

Flattening takes about 12 minutes, on my set up, with a 2-1/2" slab flattening bit. I usually wander around while that’s going on and clean the shop, sweep, collect trash, remove clamps from glue ups, etc.

I typically flatten in between jobs, for sure, and occasionally in the middle of a job depending on the state of the board and the mistakes I’ve made.

ETA: I did consider PVC, but I’m a bit of a stickler for mostly using supplies that I can acquire same day. Are you thinking of something like Azek boards that you can get at the big orange retail giant? Or is it something a bit more specialized?

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I get PVC trim boards from the blue place, but the orange has them as well. You can buy sheets also.

It is way cheaper than Azek. Look for the stuff from Royal.

To be fair, I have not used this for my slats yet but I have used it in a lot of projects.

The only real downside I can see other than a bit more cost compared to MDF is that you might need sunglasses to look at your spoilboard😊

Oh, and maybe more of a static concern.

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Oh dear, static is a huge concern in my shop right now. I’ll hold off on the PVC until I get that sorted out.

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After two flattenings, just because I could, I have concluded that the human is the problem.

Thank you to everyone with the great questions and suggestions!

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13

I am setting my Pro 5.1. 4x4 with an 80mm spindle, for production foam cutting. I have a dedicated 1200 CFM dust collector, into a cyclone, and a 4" custom dust shoe, soon to be a 5" shoe. I have the 4" hose hanging from a boom. A few things I noticed:

I had to tram the Z in both the X, and Y. I had to shim the Y 0.008". With a accurate tram, the weight of the hose will still cause the Z to droop up to 0.008. I am using a bungie cord to carry some of the load, but too much will pull the Z back. It is very sensitive to weight. I expect large cutting forces will affect it as well.

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14

Habitat for Humanity is close; old desks are cheap. About 1 hour each way to Home Cheapo or Lowest Quality.

Only takes 10 - 15 min to resurface due to warpage.

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I supported the hose by hanging a few loops of paracord from the ceiling. It’s not as classy as a hose boom, but it gets the job done and I had all the parts anyway.

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