Newbie requesting help with Z parameter setting

[Ryan Richardson]

Good Morning-
I recently purchased a second hand shopbot (older PRT with upgraded RBK control box) and have gotten everything set up, with a couple of exceptions.

For some reason, I'll zero my Z axis using the plate and as part of this process, the machine will move the Z axis to the 1" position, or what it thinks is 1", which is actually closer to 7/8". Based on reading threads like this one (http://www.talkshopbot.com/forum/showthread.php?p=139611#post139611) it seems like this is an issue with the Z parameter- X and Y seem to be fine, by the way.

I've done a bit of playing around with my Z parameter based on the ratio of displayed Z zero and actual dimension, but so far none of the settings work in every situation- some numbers work up until 1.5", then get out of whack at higher Zs.

I seem to have the 20 tooth Z Axis gear, 0.25 degree/step vexta motor model number A6488-9412KTG.

Any assistance or thoughts would be much appreciated!

Thanks!

[Brady Watson]

Ryan,
Your Z axis unit value will be exactly 2 times the unit value for the standard PRT Z axis, which had a 3.6:1 motor on it. Your Z motor is a 7.2:1, which was common on machines running a heavier Z like a Colombo spindle etc.

Your Z UV should be somewhere around 1832.xxx - Just take the 916.xxx number for the standard Z and double it. That should get you where you need to be.

-B

[Ryan Richardson]

Brady- thanks for your response- it makes sense that the Z setting should be double the X and Y. My issue is that the X and Y have been set at 1833.465 and seem to be working. Doubling the Z value from there to 3666.93 still results in the Z reading on the computer reading 1", but the actual dimension being 7/8". I've played around a bit with my caliper and have set the Z on 4130 and that seems to work up until about Z position 1.5", then it starts getting off again.

I guess my question is whether the ratio of these numbers is the important thing, or whether the actual values are the important part. If it's just the ratio, I could set my X and Y on the 916.xxx number and my Z on 1833.xxxx with good results- I understand that these settings are the "original" settings for my machine which is a PRT built around 2002 (I was told).

Thanks again for your help

[zeykr]

It's the value that's important, not the ratio or x and y to Z.

See if these charts are of any help: http://shopbotwiki.com/index.php?title=PRTInchUnits

Looks like from the charts, if you have a 1/10 stepping machine with a 20 tooth pinion on z, I'd try 4583.662 for the z unit value

[Ryan Richardson]

Guys- I can't thank you enough for your help. Setting the Z parameter to 4583.662 absolutely did the trick and i just cut my first test files on the machine! Thanks again!