Desktop losing Y zero position
Hi. I'm having a problem with my Desktop losing it's Y zero position. I've worked with Jeremy at ShopBot and will continue tomorrow, but I thought that maybe someone here has seen this problem before. If you have any thoughts, I would appreciate hearing from you.
Before this problem arose, the Desktop had performed well with moderate use for the 25 months since it was purchased new from ShopBot. Last fall I adjusted all stepper motor driver pots per ShopBot instructions to reduce motor resonance (turn fully CW, then back off 1/16 turn), and I also installed the 5.5” Z mod; the machine worked even better after those changes. For the past four weeks, however, during 3D cutting the Desktop gantry will occasionally and sporadically fail to move fully in the negative Y axis on a move command, causing the Y zero point to shift toward the back of the machine (i.e., a shift in the positive Y direction). When this happens, the Y stepper motor makes the lead screw assembly noisy and jerky, and the bit will continue cutting the pattern in a new location toward the positive Y direction. The machine can cut several 3D plaques without issue, then suddenly in the middle of a job shift its Y zero point. The failure is always when the gantry attempts to move in the negative Y direction, and I can watch the SB3 position readout on my laptop move while the gantry moves slow and jerkily, or not at all. Movements in the positive Y direction are always normal. Tracks and bearings are fine, as are the lead screw components, attested to by their smooth movement when this problem is not actually happening. In all, this failure has happened approximately five times. Various corrective actions have been taken, summarized below, but have not effective in resolving the problem.
Corrective actions, in chronological order:
1. After the first incident (cutting HDU), I thought the problem might be in the utility power, so I ordered a 1400va CyberPower LCD uninterruptible power supply (UPS). I did not suspect a ShopBot problem. After re-zeroing from this incident the machine continued cutting normally.
2. After the second incident, I looked for mechanical causes. I cleaned the machine thoroughly, checked tightness of all fasteners and electrical connections (all were tight), and installed the UPS.
3. After the third incident I reinstalled the firmware and tried running the machine without the UPS. It did not seem to matter what the power source was, straight utility or UPS.
4. After the fourth incident I called ShopBot Technical (Jeremy). After troubleshooting involving swapping the motor leads for the X and Y motors and seeing the problem move to the X motor, he suspected a faulty Y driver. During the troubleshooting the machine got to the point that it would not move at all in the negative axis direction for whichever motor’s leads were attached to the normal Y terminals on the control board, but the motors would always turn in the positive axis direction normally. Motor temperatures were normal (warm). The actual movement of the gantry on the tracks was smooth when (slowly) manually moving it with power secured. I ordered a new driver and installed it.
5. After the fifth incident, I again swapped the X and Y motor leads at the control board and the problem again moved to the X axis. I returned the leads to their proper location, then changed the laptop to a new one running Windows 7 and SB3.8.26. I also changed the USB cable from computer to Desktop. I reseated all pins on all cards, except for the Control Card, whose attachment screws I could not budge, even with the control board off but I did wiggle the Card to ensure the pins were seated fully. All wiring looks good, and the motors never get beyond warm. The problem can be reproduced with keypad movements, and it seems that jog speeds are worse than move speeds. I let the machine rest overnight and this morning ran an air cut of a file. Half way through the cut, the problem reappeared. It seems the problem happens when something (a chip?) warms up. To get further data on the stepper motor drivers, I removed the X driver and replaced it with the old Y driver that I had replaced earlier. After that change, the X motor seems to work fine, but the Y motor is still noisy, jerky, or immovable in the negative Y direction. This seems to indicate that the drivers may all be fine, even the one replaced after the fourth incident (which is now driving my X motor), and that the problem may be in the circuitry to which the Y driver is connected.
Andy
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