[mykrowyre]

Hi guys, I'm fairly new to cutting G10, or cnc for that matter. But Lately I have been cutting some parts from 1/16" G10 which need to be accurate within .002". Everything was going great until I purchased a new set of bits, .0394 bit here:

PCB Router Bits - Diamond Cut - Midwest Circuit Technology

Spindle is running about 20K, and feed rate is 14IPM.

Right away I started having issues.. all of my parts were too big and the holes too small by .006". I measure the bit and it measured at .0375. Strange that a bit is labeled .0398 but the cutter is actually .0375, that's a big difference.

So I figure no problem, I regenerate my g-code for a .0375 tool. Surprise all parts still to big with cutouts too small. Grrr... so I back down to .036.. .035.. .034 .. .033... and finally I am somewhere around .033 to get the right sized cuts but it's not consistent. Yes, the tool is cutting on the correct side of the path (cut2d does this automatically, and I did it manually with another package as well).

I figured it must be backlash. I measured the backlash on my X axis to be .001, and on Y to be .005. So thats it! I tighten everything up, re-measure and then enter that value into EMC backlash compensation. Now I cannot measure any backlash, the compensation works great.

Now with backlash fixed, everything snug, I set the tool size back to .0375, and surprise, parts are just as far off as they were before.

At this point I am sure it's a software problem, so I generate the g-code with a different program, but nope, same problem.

Getting desperate, I figure maybe it's tool deflection. So I slow down the cut rate to 4IPM, no improvement. I run a finishing pass, no improvement. Still need to set the tool size to .033 to get correct sized cuts.

Then I pull out a tool of a different brand... it's labelled .05" and measures .045". I regenerate the same g-code for this tool at .045 (as I measured it) ... cut the part. Bam it's within .001" of what it should be.

Now I'm really confused. They both appear to be the same design, but from a different manufacturer.

How can a .0375 tool cut a slot .032" in diameter? I thought maybe it was just a dull tool so I went through the whole set of brand new cutters and they all behaved exactly the same. Going back to a different tool, no problems.

Am I using the wrong type of bit to cut G10? Is it just a crummy bit?

[judleroy]

Can we see a picture of the offending tool. Is it possible that the tool is heating up the plastic then when the part cools the dimensions are wrong. This is something i've seen with alot of plastics but its been years since i've messed with glass filled phenolic. I do know that the tool is not shrinking during the cut. Also runout issues would cause enlarged cavities not undersized. Best bet is to go with the tools that you know work.
Good luck
Judleroy

[mykrowyre]

Here are the two bits... the darker one with the material in the flutes is the one which is giving me problems. The other one is hard to see but it looks very similar in person.. flutes with cross cuts almost like a diamond pattern, but not as pronounced as the other one.

[mykrowyre]

Talked to the midwest and they were nice enough to give me a call and send out a few different tools to try.. sounds like the one which was working for me was actually a chip breaker not a diamond cutter, so I will try that instead and see how it works out!

[svenakela]

You say EMC, you do know about Trajectory Control, don't you?
Make sure you run G61 to get the exact tool path (or run really slow).
EMC Documentation Wiki: TrajectoryControl

[mykrowyre]

Figured I'd update my own thread... after reading up on climb cutting I quickly realized what the problem was