Precision in the Onion-Skin Cleanup Cut Pass

Advice on how to prevent a tiny "lip" from being left on wood edges after the final pass when onion-skinning parts. October 16, 2012

I'm not sure if my subject line is clear or not but here is the issue: When cutting out small parts my software (cabnetware R1) cuts an onion skin and then comes back and cuts the piece out. I have noticed that the second pass is not exact and leaves a slight lip on the bottom of the pieces. I originally thought it was an issue with the software but after cutting some pieces direct through the CNC (Biesse Rover 7.40 FTK with Biesseworks and XNC100) I noticed the same issue.

Forum Responses
(CNC Forum)
From contributor M:
There's been a fair amount of discussion here over the years on this. This is a difficult one to see but not so bad to solve, I struggled a lot with this too. This can be caused by several factors, machine slop, or bit deflection (I use quotes because bits really don't bend much in the wood world) and part movement, which includes sheet stress relief. (Ever see a piece of wood close or open behind a saw blade? Sheet stock can do this too under certain conditions).

Let's say you nest tight, and two parts are right up to one another. Cut the first pass and the bit is hogging a lot of material, the bit and machine are under stress and the bit drifts away from the part a little. It can cut into the adjacent part, so when the second pass comes it runs a lot truer to path leaving a lip. When this is the case you usually notice more of a problem on center cuts than cuts outside.

Other situations involve some sheet stress being relieved after the second pass. One part can move into the path of an oncoming bit creating a lip. You can also get a lip if the very tip of your bit wears away from constant contact with the plastic face of a sheet of melamine or plastic.

I solve this problem by spacing my parts in a nest by .03 inches (1mm or so). Then I take the first pass in climb and the second pass conventional. The climb cut, while taking 95% of the material biases the cut away from the true path, and the conventional cut taking maybe 5% runs true, effectively trimming a tiny amount (if any) from the part all the way up on the last pass. Parts come out pretty sweet and clean. Adding a very small amount, maybe .005 of trim to the first pass can be helpful if your machine is a little sloppier in its movements but avoid this if not needed as it contributes to side forces on the second pass, which is what the onion skin method is trying to avoid to start with. Keep your tools sharp of course and the lip should be eliminated.

From contributor D:
This happens less often for me with long shanked tools which bottom out in the collet. Also ETP hydrogrip seems to do better than collets. As Contributor M says, the conventional cut as a cleanup pass solves most issues. This problem is especially noticeable for me, as parts for 1 3/4" - 2 1/4" small door parts butted up to each other won't join right if the cut is not absolutely square.

From contributor L:
If you are using cutter compensation have you confirmed that the code is calling for cc on both passes?

From contributor K:
Just to add one possibility, an out of square router can also cause this condition, but typically is worse in one axis direction. Knowing Grizzley, I would think that a crash has not occurred on his machine, so this is more a general point.

From the original questioner:
Contributor M: I don't think it's bit deflection due to spacing since our program has more than .03 set for spacing, the other suggestion of an initial climb cut sounds like a good idea.

Contributor D: Those hydrogrips look nice. I was not aware that there were other options and I will be doing some research on them and longer shank bits.

Contributor L: I don't know, I am going to have to look for that in the preferences and see if I can find that information in the G-code.

Contributor K: The lip is even on all sides, no crashes, not on my baby.