I cut a large but simple project. Single toolpath. I got a weird inconsistency on depth that repeated on a second board. Anyone have a guess why it would do this?
Here’s the file.
Monument Sign Logo PRO.c2d (588 KB)
What are you using for work-holding? Could the endmill be lifting the material as it cuts? Your DOC is 0.05", the pocket depth is 0.200, so your last pass is a full depth pass.
You could try adjusting either the depth of the pocket, OR the DOC of the bit, so that the last pass is shallow. If the depth was 0.210", then you would get another pass that takes off 0.01". That shallow a cut is unlikely to lift the material (if that was the original problem).
Or change the DOC on the bit to 0.0475". Four passes get to 0.190", so there’s another pass taking off 0.01".
OR, try a bit more aggressive DOC. A DOC of 0.063 would end up with four passes total, the last being shallow.
Could be lifting it. I’m clamping the edges hard but I can’t camp the middle of this 24" wide piece. The color core is a little flexible.
Out of curiosity, would a downcutting endmil be a decent solution for this soft of flexing issue? Or would that just turn the color core into melted garbage?
Use blue painter’s tape and cyanoacrylate glue to secure the field from underneath?
That’s a good plan. I’d guess a single strip down the middle of the underside would be plenty to prevent lifting. I suppose the sign might deform by the thickness of the painters tape but that probably doesn’t matter much. (could probably run a strip under the clamping areas to level it out if it does matter 
)
I would just recommend against using a whole role of tape and a bottle of super glue to cover the entire bottom side…
But only because the vapour would burn your eyes and you’d surely break the sign trying to get it off. It wouldn’t budge though, so that’s good 
For machining sheet material & maintaining consistent thickness/flatness, I’ve had good success with blue painters tape & cyanoacrylate glue - basically a strip of tape for each slat on the CNC bed. Putting some weights or sand bags on the sheet material to hold it down consistently in the middle for a few minutes usually ensures the glue is actually all contacting the tape.
Then stack of a couple tape layers on the bed to touch off on for Z-zero. After that there’s very little chance of the bit gouging into the bed while cutting fully though the stock.
If you do the double sided or painters tape remember to remove the plastic film off the ColorCore. I didn’t do that one time and it was a failure.
Also, I have had the sweepy cause the center of large boards pull up so I make sure to have a good bit of clearance between the sweepy bristles and the board.
Ran a 30" piece of painters tape down the middle and used super glue. Exact same pockets.
I could run a downcut endmill next, but I’m open to other ideas.
I would use multiple pieces of tape, and then run a final finishing pass which removes only a very shallow thickness of material — easiest way to do this is to adjust Depth per Pass to take into account Max Depth, say one has a Max Depth of 0.5" and a Depth per Pass of 0.125" resulting in four full-depth passes — reducing Depth per Pass to 0.124" results in five passes, with the final pass removing only 0.004" inches of material.
Just updated the file. Looks like I was taking 0.05" passes all the way to 0.2"
Hopefully that and a downcut do it. These are all roadside things where drivers won’t see the imperfections, but anyone who likes CNC can get particular on details for no reason.
Well it’s not for no reason…
We do what we must, because we can
I’ll be interested to see if the changes make a difference. I’ve only ever done small pieces so the upcut/downcut thing doesn’t come into play much. It’ll be neat to see if it seems a substantial difference to you 
You did a shallow final pass? I would have thought that would produce a lot of fuzzies.
I did, with a quarter end mill. Then I dragged a chisel to pull a few fuzzies off. Not terrible.
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