HELP! Z depth changes as machine carves

[mr_logan]

:mad:I have a PRT bench top ShopBot that is not working well. While trying to carve some text using a V bit into some cedar it will carve the first couple of letters fine and then begins to carve each letter slightly deeper and deeper by about .125 each time. I have only seen this one machine so I do not know them well, but here may be a hint, when you move on the Z the motor it is jerky, like when setting the zero z, it will moves jerky on down stroke. If I command it to travel up and down the z axis it is not smoothe at all like the carriage moving in the x or y. I did exchange the wires from the x motor and connect them to the z and the proplem was still there. Any hints before I start drinking? Thanks in advance

[steve_g]

Sounds mechanical... the only other thing I know that can cause those symptoms are the "Resolution Multiple" being off in the setup.
Of course I assume the collet is good shape and not slipping...

SG

[mr_logan]

Sounds mechanical... the only other thing I know that can cause those symptoms are the "Resolution Multiple" being off in the setup.
Of course I assume the collet is good shape and not slipping...

SG
Thanks for the reply.
The collet is in great shape and I also think it may be mechanical??? while the machine is carving at several times its assigned depth, the Z depth always show the correct depth. So it is losing track of the actual Z depth and then the jerky movment I am sure a hint.

[CNYDWW]

My PRT has springs hooked to the bracket on the bottom on the z to help compensate for the router weight. Are yours still there, broken, etc. ?

[mr_logan]

Is it normal for the Z motor on my PRT bench top to be jerky while moving up and down, much more so going down? The X and Y are smoothe as silk.

[Brady Watson]

Scott,
It sounds like you have a 'tired' Z motor driver. With the control box OFF, put the Z motor cable into the A connector on the control box. Just unplug the Wago connector for the Z (white stripe) and put it in the bottom-most plug. Turn the control box back on. Go into SB3 and type in the VI command. Then where it says Channel 4, put in the letter Z. Now see how it works...

Just make sure you ALWAYS turn the control box OFF before you do anything with the wiring, or you'll damage your motor drivers.

-B

[mr_logan]

Brady,
Thanks for the reply and I will do those check and changes when I get back to the shop.
What are motor drivers and can they be replaced? and $?

On another note, I sure do love looking at the work you do and art you produce.
Thanks
mr. Logan

[Brady Watson]

Hey Scott,
Thanks for the kind words. I've been working on some tribal artwork for a friend near Juneau these past few days. I think that's a few hours South of you.

The instructions I gave you are for a PRT Standard - which has an ATX computer case control box (not Aluminum sheet metal). The motor drivers are integral to the control board, and are not easily replaced unless you are handy with a soldering iron. Usually the diodes go bad, not the driver itself.

The motor drivers (one for each motor) translate all those XYZ moves in your ShopBot part file into electrical pulses that command each motor to move a specific distance and direction.

ShopBot should have an upgrade path for that machine. I am not certain if they still sell the "4G Upgrade" for your tool, but it would sure add a lot of performance to your existing iron and give you more options when it came time for a repair. It would be worth a call to ShopBot to see what is available for your tool.

Hope that helps!
-Brady

[Christian]

Scott,

I'm not sure if you had talked to anyone here this week, but I thought I'd check in. If you run the Z motor on the X driver and cable, and the problem remains on the Z axis, then it sounds like the problem exists at the axis instead of the driver. Try running a known good motor from the Z driver to confirm.

If the problem is at the axis, there are other factors to consider besides the motor. Remove the motor from the axis so it's just sitting on the table, then give it some move and jog commands. If it's still running rough then it's safe to assume that it's getting ready to quit on you. If it runs better, you might have a loose/worn out pinion gear or some **** in the gear rack. Hope this helps.

Christian