Z axis randomly plunging

I've had some problems with my Sherline (running EMC 2, and all the A2Z upgrades). Today, I was watching the final passes on a test piece of aluminum, and watched the Z axis decide to take a 1/4" plunge into the piece before I shut off the driver. It took the required .015" Z axis movement, and when the X axis began to move (on the screen only..the X axis never began to move).. the Z axis kept cutting downward, ruining the piece. Almost like it just "hung up", and continued to do what it was doing regardless of what the software was showing. I've had problems like this before that I attributed to motor couplers loosening on the X/Y axis.. now I'm thinking the Z axis has been the culprit, and causing most of the problems by taking deep bites on occasion.

Has this happened to anyone else? I'm tired of pouring cash into this thing, and having to buy new equipment to replace the stuff I paid alot for to begin with just recently. Time to build a new computer? Upgrade from the Sherline stepper drivers?

Perhaps you could specifically list the gear that you have running things. Specific drivers, power supply, motors things like that. The configuration of your PC as well might help. It's hard to diagnose the problem with only the symptoms given and no look at the patient.
There are some on this forum who use EMC, but a lot of us use Mach3. Is there a possibility of you trying the demo version of Mach to see if the problems follow the hardware or the software?

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Originally Posted by escott76

Perhaps you could specifically list the gear that you have running things. Specific drivers, power supply, motors things like that. The configuration of your PC as well might help. It's hard to diagnose the problem with only the symptoms given and no look at the patient.
There are some on this forum who use EMC, but a lot of us use Mach3. Is there a possibility of you trying the demo version of Mach to see if the problems follow the hardware or the software?

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I am using the factory Sherline driver/power supply. The steppers are a carry over from the old smaller tables (130oz. I believe). The problem rears it's head every once in awhile.. not every time.

I would have to dual boot the computer for Windows to run Mach 3 correct?

Pc is (again) the standard Sherline equipment.. 1.8Ghz processor, somewhat older & unknown motherboard.. upgraded from 256M Ram to 1G.

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Originally Posted by Don-in-Japan

I am using the factory Sherline driver/power supply. The steppers are a carry over from the old smaller tables (130oz. I believe). The problem rears it's head every once in awhile.. not every time.

I would have to dual boot the computer for Windows to run Mach 3 correct?

Pc is (again) the standard Sherline equipment.. 1.8Ghz processor, somewhat older & unknown motherboard.. upgraded from 256M Ram to 1G.

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Ok, well to start if you don't have Windows on the machine (not sure of the sherline PC config out of the box) you would need to dual boot with Windows in order to run Mach. If you go that route I'd recomend a separate drive, that way it's easy to swap back in the second drive and be up and running with a known good config.
Barring that, those are smaller motors, and I don't know what the drivers are specifically. There are many potential issues for trouble with steppers. Not enough power, midband resonance in the drive, lower motors than are needed for the application are all potential problems. Even things like motor tuning, setting accel and max speed are important too. Couple that with fine pitch leadscrews and you have to spin the motor fast, which is harder on the steppers as well.
But losing touch with the machine and plowing down into material seems like software issues rather than motor tuning issues. Unless your drives are freaking out or getting noise somewhere. Try a different parallel port cable?

On advice from another gentleman, I cobbled together another PC from 4 different ones around my computer room.. Loaded it up with Linux/EMC2, and it works like a dream.

My factory Sherline computer was faulty out of the box, and I didn't know enough about CNC to consider it wasn't me or the mill itself to blame.

I have completed 4 test pieces with zero flaws..! Pretty good considering I would be lucky to complete one with the old computer. Apparently, the Sherline computer would add or skip steps, and enjoyed plunging the Z axis into the workpiece, causing the X/Y stepper couplers to undo themselves.. leading me to believe they were working themselves loose when they weren't the cause.