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ok my cnc friends, what is causing this?

Dan Noren

Dan Noren

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falcon heights, minnesota
ok guys, what is causing this? i'm working on something for my cousin the cabinet maker. it is a vent cover, 1/4 inch mdf core red oak (the real one will be white oak) plywood. running a pocket toolpath, 1/8 inch bit, 15 inches per minute, and 6 passes. it makes these ghost images, not very deep, but you can feel it with a fingernail. tried sanding them out, but the veneer is thinner than scott tissue. any ideas?
20251009_165550.jpg
 
Looks to me like you need to tighten up something, maybe the cams on the tracks or have something loose on a rail? Belt could have skipped too, so make sure it is tensioned.
 
I know 1/4 inch thick plywood seems relatively stable but like so many thin plywoods it may not be "perfectly" flat. Maybe not noticable but it may not be flat.

It does look like it may be moving under the clamps. I don't know how you are clamping it.

If you are clamping with hold down clamps along the edges make sure there is no space at all under the plywood at the clamping point where the clamp is applying pressure. Even with no space the plywood can buckle up in the middle.

Double sided tape as a hold down is a decent option especially whereas you cutting at .05 DOC and your 15 IPM is so extremely low. With Double sided tape controlling the hold down in the middle of the plywood is better than hold down clamps. Don't use both methods.

If the double sided tape still does not stop the ply from moving you can use stop blocks on the 4 sides. Just any old block securely clamped on the table up against the plywood on all 4 sides.

I would also recommend a down cut cutter. The upcut pulls up the plywood.

I also have trouble with large sheet goods flexing in the middle. That is why vacuum tables are a good choice.
 
leo, i tack it down in the corners with small nails from the air gun. i think i have some double sided tape around here, and use a spot of it in the middle of the next test blank, and speed it up again.
 
It is possible that the tack it down in the corners is causing the plywood to bow up in the middle.

Use the double sided tape, downcut cutter and nail down the stop blocks around the perimiter. Don't take down the plywood.
 
Sorry Leo, no double sided tape, or down cut bits here. Replaced the sacrifice board, set up boundaries again (no need for limit switches), squared up everything, new bit. Changed speed to 40 ipm, and 8 passes. Still using pocket, but went from climb, to conventional, and getting good results (so far). Doing it good side down, as I saw that the ugly side was clean. 20251010_134358.jpg
 
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