Z changes during cuts :-(

[mrlogan]

I am using a table top model shopbot that will total go nuts and lose the z cutting depth during a cut. Not every time. It will start out cutting correctly and then cut the z deeper and deeper on the next cuts. (next letter etc.) It lowers perhaps an 1/8" or so during the next cut. It has ruined lots of projects and the odd bit. Anything I can look at?

[ken_rychlik]

Scott, The cutter your use and speed of your Z dive make a difference on my machine. If you try to dive to fast and it gets in a bind, it will lose a step and then go deeper each time. Make sure the bit is good and not feeding down to fast. I can tell by the sound it makes when mine does it.

You can always move the machine up one inch and zero the cutter up there. Measure it and run an "air test" when the file is finsihed re-measure and see if it in the same place. If it is, I am thinkinking the z is hitting to hard.

Kenneth

[erik_f]

Grounding?

[jelpspeed]

What is grounding?

I have a similar problem, but my Z zero goes up like 1/8", my Z speed is 250mm/min, is it too high? I´m cutting 1/2 inch MDF, 3mm depth passes (4 times), at 5000mm/min. 1/4 inch spiral flute bit.

Using a PRT 96 Alpha, rack and pinion, I have this from a year ago, and this time is the first time I have this problem, never happened until today.

Thanks in advance from Mexico.

[myxpykalix]

Grounding-is where you ground the system from static electricity. If you have a dust collection system (or not)you build up static electricity which zaps your system and causes it to lose its coordinates and shorts out the system.

To fix the problem you need to take a bare copper wire and run it thru your dust collection hose attaching it to the carriage on one end and grounding it to the body of the dust collector.

I had some of those same problems and it would stop the file several times and since i grounded my system it hasn't happed once.
I don't know if this is the original posters problem but doing this will eliminate that as one of the possibilities.

[jelpspeed]

That´s a nice tip, I have the same setup as 1 year ago, nothing changed, but today I tried to cut some furniture for my daughters and the shopbot screwed the job. I measured the Z axis to find if the spindle was "sliding" or if the Router bit was sliding up, but no, the problem is the machine itself, because when I returned to the original zero, the X and Y where Ok, but there was a gap of 1/8 between the MDF and the bit. my mark also showed that the machine changed the height of the zero. I will take a picture tomorrow to make it more clear, because english is not my best language =) and I don´t know how to tell this technical problems.

thanks for the tip, and if you happen to know what may be wrong, please let me know.

Best regards

Ever.

[beacon14]

If yours is a standard machine and you have the Z jog speed set too high you might be loosing steps on the upward Z moves. This would cause the situation you describe.

Most people who have grounding issues have problems with 2 or all 3 axes, I don't recall one where grounding issues only affected the Z axis. Not to say it couldn't be the case, and if in doubt proper grounding can't hurt, but it's not necessarily the only possible solution.