Troubleshooting a "Lip" on the Finish Cut after Onion-Skinning

      A CNC owner learns how to get a clean cut on his final pass with onion-skin cutting of parts. March 28, 2010

Question
I am onion skinning my parts on my CNC machine, and when the machine makes the second pass, it leaves a lip on the part that is about 1/32". We are mostly working with melamine, but I have the same thing occur with plywood. I am using a single flute compression bit and leaving a skin of approximately .03". The accuracy of our machine is good, although the machine is lower end, made by a company called Practical CNC that went bankrupt last year and reformed as precision CNC. I read of another guy using a standard upcut bit for his second pass on the onion skin to help reduce dust and am thinking this would be a good thing to try and see if it would eliminate the problem. Any suggestions?

Forum Responses
(CNC Forum)
From contributor E:
Try a couple of different things:
- If you are using a 3/8" bit, leave .6 in between parts
- Instead of starting at the corner of the part, start in the midpoint of the longer side.
- If the problem persists, make the start point cross over the end point by 1". Although you won't have a closed polyline and it is more work, it will make the problem go away.

I would try the first suggestion first. What type of software are you using? It may be ramping in and out automatically without you even noticing, and you may be able to adjust that as well.



From the original questioner:
Thanks. Our software at the moment is Bobcad. Unfortunately we jumped into this thing without much knowledge of what we were doing or how we were going to accomplish it, which is why we are operating with Bobcad at the moment. We have just purchased Cabinet Pro and I am in the process of learning how to operate that software now. Bobcad is not ramping in and the lip is along the entire edge, but is not always consistent. I am currently leaving .5" between parts. How will leaving an additional .1" alleviate the problem?


From contributor M:
My thought is that you are taking 0.72" depth of cut on the first pass. The torque on the tool is pulling the bit into the part. On the second pass, you are only taking 0.03" depth of cut, meaning that the tool deflection is not equal. Try this - on your onion skin, leave some stock amount around your perimeter, maybe 0.015". On your second pass, remove the 0.015" around the perimeter and the 0.03" skin. See if that eliminates the problem, or reduces the lip you are talking about. If the lip is only reduced by some amount, you should be able to find the right combination of roughing/onion skin to make good parts.

From contributor E:
Contributor M's suggestion is good. Also look at your code - see what the value of Z is. When you onion skin, I typically leave 1/16" and then cut the rest out.
I.e. on .75" thick part, the value should be:
first pass Z0.0625
second pass Z-.005


From the original questioner:
Thank you, contributor M - your suggestion explains what I have been experiencing and I am going to modify my code to allow the additional material to be taken out on the second pass. Appreciate everyone's help.


From contributor B:
If you were able to do the first cut in the climb direction, then the second cut in the conventional direction, this too would eliminate the issue.


From contributor D:
No one mentioned checking your tool diameters. Two different tools, one measured a tad inaccurately, will cause this. Brand new bits where the shaft is the same as the cutting diameter never cut the diameter they are supposed to. Or maybe I just haven't tried all the different tools, but all the bits I've gotten in the past that had the same diameter shaft as cutting diameter were off. Like half inch bits cutting approximately .48". I would wager this is the problem.


From contributor H:
There is no solid way to hold our hardwood parts that would be nearly as fast as our vacuum pod system. As such we live with part deflection on our first pass. To solve the problem you are experiencing, we simply climb cut on the first pass as mentioned above. The second pass completes the cut and skims away about 1/64" to 1/32" left from the part deflection. In your case it might be part deflection or spindle deflection. In either case a climb cut on the first pass should eliminate the problem.


From contributor K:
You said you just purchased Cabinet Pro. Go to your material setup screen; at the upper right side of the screen, find the "final pass offset" box and enter 1/32 or a value that you think will take care of the lip. Cabinet Pro gives you lots of options for operating your CNC.


From the original questioner:
Thanks to all for the advice. The issue has been rectified now and it was due to the deflection of either the bit or spindle. Leaving an extra .01" in the initial cut has taken care of the problem. I also now realize that the same thing could be achieved by cutting the first pass in a counter clockwise direction, which I have done and discovered after experimentation leaves more material due to the deflection. I assume this is what is being referred to as the climb. Is this correct?

As for the Cabinet Pro, I have not yet gotten to the point where I am able to use it. I am still drawing my library of cabinets and learning.



From contributor H:
Climb cutting is just simply cutting in the direction of the bit rotation. Picture the bit spinning up against the edge of your part. If the rotation of the bit against your part is the same as the rotation of your car tire on the road, then it is a climb cut. The bit is rolling along the edge of the wood just like the tire is rolling along the road.

Another way to look at it is if you held a loose bit between your finger and thumb. Now put the side of the bit up against a finished part edge and let it roll along the edge. That is the climb cutting direction. If you had the bit in a hand held router it would want to pull away from the wood while making the cut. Conventional cutting is just the opposite direction.

The hand router is actually a good analysis. When using one, you quickly figure out which way you have to move the router along the edge of a board, because if you are climb cutting it will take off on you.



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