what happened here

[Grahamd]

Things were going well when this happened. Z went crazy in the middle of a cut. The picture shows how things were great, then all of a sudden it went to trash. Any help out there.
Thank you, Graham

[steve_g]

Graham…

It looks similar to an issue I had when the down blast of warm air from my router dried out the damper interior of the board during a very long (timewise) cut. The solution, for me, was to re zero the Z axis a few thousandths lower after completing the cut and running it again.

SG

[Grahamd]

Thank you Steve. I was afraid the machine lost Z midway through the cut. 5 hour cut.
Graham

[tlempicke]

Thank you Steve. I was afraid the machine lost Z midway through the cut. 5 hour cut.
Graham

It appears that your Z went a little deeper and then corrected itself. If it had just lost, or gained, steps I would expect it to be uniform once it went wrong.

Loose pinion? Worn pinion? Rack need cleaned and lubricated? Static? Noisy cable connection?

[dlcw]

Collett slipping? I've had that happen a few times. A dirty collett can cause all kinds of problems. I clean colletts thoroughly after each bit change.

[Brian Harnett]

I am leaning towards steve g's answer, as the cut is made stresses change and the wood moves. I have that happen a lot with my wooden signs. I would do a roughing pass with a large bit and go to finish it will save a lot of time too.

[srwtlc]

Wood movement from stress and humidity....not on a 3D finish pass. You'd have to stop the machine and wait between each pass to make it show that much variation. Did your holddown become slightly loose? You don't say what type/model of machine you're using, but if it is rack and pinion, check the grub screws on the Z axis. Make sure they're seated and tight and that the pinion is firmly seated into the rack. Check for any play in the Z and Y cars. Debris under a V roller on the axis that is rastering. Loss of steps in both directions as it jerks the Z up and then back down. If you have a standard SB, this is quite possible if you don't run proper ramping and feedrates.

Spindle or router? If router, a bearing going bad might be possible.

5 hours!? How big is this? A model like that, depending on the size, shouldn't take anywhere near that long. If it's in the range of 4x6 or 6x8, you should be able to knock that out in no more than 1 to 2 hours (with 8% - 10% stepover). Again, ramps and feedrates apply here as well. Read up on it here (http://www.shopbotblog.com/2008/03/a-ramping-the-vr-command-and-how-to-tune-your-tool-for-maximum-performance/).

[Grahamd]

Thanks to all. Have been away for a few days. I am running a Buddy 32. It does have a spindle installed at the factory. Bought a computer just for the machine that is never allowed access to the outside world. The roughing pass is a good idea. 5.5 hours is a long time. I will check all the above mentioned areas.
Thanks again, Graham

[Grahamd]

The oval is 4.75by9.75. Bit is tapered 1/32 ball end carving bit. This is all new to me, so the learning curve continues.
Thanks, Graham

[Grahamd]

Wood movement from stress and humidity....not on a 3D finish pass. You'd have to stop the machine and wait between each pass to make it show that much variation. Did your holddown become slightly loose? You don't say what type/model of machine you're using, but if it is rack and pinion, check the grub screws on the Z axis. Make sure they're seated and tight and that the pinion is firmly seated into the rack. Check for any play in the Z and Y cars. Debris under a V roller on the axis that is rastering. Loss of steps in both directions as it jerks the Z up and then back down. If you have a standard SB, this is quite possible if you don't run proper ramping and feedrates.

Spindle or router? If router, a bearing going bad might be possible.

5 hours!? How big is this? A model like that, depending on the size, shouldn't take anywhere near that long. If it's in the range of 4x6 or 6x8, you should be able to knock that out in no more than 1 to 2 hours (with 8% - 10% stepover). Again, ramps and feedrates apply here as well. Read up on it here (http://www.shopbotblog.com/2008/03/a-ramping-the-vr-command-and-how-to-tune-your-tool-for-maximum-performance/).

Brady's article was great. Thanks. I have used him for a scan in the past.
Graham