Problems Cutting Shell. (CAM issues?)

[Brock Poling]

Hi.

First off apologies in advance for the length of this post / question, but I wanted you to have all the info to make it easier to help me spot my issues.

I am relatively new to CNC (but not brand new). I can get around in VCarvePro easily enough and I don't have any issues cutting basic shapes etc. However I am now working on cutting pearl for inlay on a guitar and I am pulling my hair out.

I am using a Shopbot DesktopMAX with an ATC.

The project I am working seems fairly easy. I am trying to cut paua shell rings out of Abalam sheets. I have the Abalam glued down to a hard board substrate using bottled hide glue, and the substrate affixed to the spoil board with tape and super glue.

The rings should be .047"

Here is what my design file looks like:

https://drive.google.com/file/d/1oGR...ew?usp=sharing

I double checked to make sure the size was the correct size all the way across, I checked on the left side, center and right side. All were the same as you can see here:

https://drive.google.com/file/d/1mCg...ew?usp=sharing

and

https://drive.google.com/file/d/1Uo8...ew?usp=sharing

Here are my tool path settings. I am using a .03125" 3FL UCEM, making a conventional cut with a separate last path.

https://drive.google.com/file/d/1g8B...ew?usp=sharing

Here are my feeds and speeds (I got this info from the folks at Precise Bits who have been a huge help in helping me get started on pearl cutting).

https://drive.google.com/file/d/1ats...ew?usp=sharing

When I ran the file the shell pieces ended up about .05" - .052" on the left side (larger end) and around .037" - .039" on the right side (smaller side).

I found this perplexing since our shopbot is usually so accurate. I changed the bit for a fresh one and recut the job with the same results.

Hmmm... could there be that much variance in my SBMAX? Not sure... so I cut the same file on my heavily upgraded Shark CNC. I got almost exactly the same results. So the problem's not the machine, it's something in my files.

From here I wondered if I was trying to cut the rings too close and was getting vibration that was causing the material to move and lose fidelity of the cut. So I took out half the ring segments and cut it again.

https://drive.google.com/file/d/1zG5...ew?usp=sharing

I got the exact same results as the first time, the left side was a touch big and the right side was a touch small.

Ok, let's set that aside for a moment and work on the pocket... I was struggling as well here. The pocket was small in spots. I will save you some of the things I tried here. Let's fast forward to another experiment where the lightbulb finally clicked on for me.

I have a piece of .047" ZipFlex which measures out to .057". I went to cut a pocket that was .058" and some parts of the zipflex fit perfectly and some were too tight to get it in the slot.

Here is what I was cutting with the tool path.

https://drive.google.com/file/d/1TRW...ew?usp=sharing

And here are the speeds and feeds

https://drive.google.com/file/d/1b9c...ew?usp=sharing

So finally I tried running two toolpath's. One cutting conventionally, and one running in a climb cut mode. And presto... that finally got the pocket right all the way around and now my zipflex fits perfectly.

So this leads me to believe the problems with my shell cutting are also CAM problems as well.

For those of you with more experience than me, would you agree with this conclusion?

And more importantly, what do I do about it?

I am not good enough in VCarvePro to figure out how to run half the part in a conventional cut, and half the part in a climb cut. Both ends of the pearl were off (one too big, one too small) so I am not sure what the solution here is.

Any advice would be very much appreciated.

Thanks in advance!