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  #1  
Old Mon 09 September 2013, 04:54
Red_boards
Just call me: Red #91
 
Melbourne
Australia
Missing steps

I have had an intermittent problem for the past few weeks where the Y axis misses steps or stops moving altogther. I'm guessing that they could both be the same problem, but they could be two problems. Sometimes the cutting path ends up off by an inch or so in Y but the machine continues normally. Other times the y-axis stops moving so the cutter traces a line in Y.

I've read other posts where people were missing steps and see that parallel cabling was a common cause. In these cases were the missing steps intermittent? What other symptoms were present?

Here are things I have checked:
  • Steps still generated by Mach3 according to screen readout of steps
  • Steps appear to be at BOB because onboard lights are flickering
  • I have checked pinion is tight and the rack/pinion interface seems fine.
  • I have replaced wiring from control box to y-motor
  • I have checked that all connections are sound.

I'm considering:
Could motor be flakey (would it be intermittent in this way)?
Could the Gecko be flakey (ditto)?

What other things could I consider?
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  #2  
Old Mon 09 September 2013, 05:43
bradm
Just call me: Brad #10
 
Somerville(MA)
United States of America
Red, when I saw a symptom like that it was because 1 of the 4 motor wires had a loose connection. Even through you've replaced the wiring, I'd try metering it out, possibly
while moving and flexing the cable chains. In my case, it was a connection at the motor end. Of course, a failure in the motor or Gecko could cause a similar effect; you could swap Geckos with another axis to see if the problem moves or stays put.
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  #3  
Old Mon 09 September 2013, 06:09
sailfl
Just call me: Nils #12
 
Winter Park, FL
United States of America
How long is your printer cable going from the computer to the controller box? That can cause a problem.
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  #4  
Old Mon 09 September 2013, 06:55
Red_boards
Just call me: Red #91
 
Melbourne
Australia
Thanks, Brad. I thought I had found a loose connection at the motor. I think I've checked this out, but I'll have another look at it. Thanks for the idea.

Nils,
The cable is about 1.5m.
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  #5  
Old Mon 09 September 2013, 09:16
sailfl
Just call me: Nils #12
 
Winter Park, FL
United States of America
Just over 3 ft should not be a problem.

Last edited by sailfl; Mon 09 September 2013 at 09:19..
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  #6  
Old Mon 09 September 2013, 12:28
Richards
Just call me: Mike
 
South Jordan, UT
United States of America
What do the motors sound like? Do they sound smooth or can you hear a "pop" or other unusual noise?

When a stepper motor looses steps, often those lost steps are caused by the load being too great for the motor to handle. As the motor tries to move against too great a resistance, the magnetic field is broken, and you'll normally hear a very distinct "pop" or other loud noise. That does not damage the motor, but it ruins the cut.

Be sure that the cutter is sharp. Try cutting the same pattern at 1/2 the depth and making two passes. Decrease the acceleration so that the motor has more time to ramp up to speed.

On my Shopot PRT-Alpha, when I was running too close to the "missing step speed", I sometimes had a problem with different batches of MDF. Perhaps the glue used in one batch required a heavier load than another batch. I solved the problem by finding the speed that consistantly caused missed steps, and then by reducing that feed speed by 25%.
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  #7  
Old Mon 09 September 2013, 21:38
Red_boards
Just call me: Red #91
 
Melbourne
Australia
Thanks Mike,
I'm cutting about 2mm of polystyrene foam with a burr cutter - basically no load. Feed is 6000mm/min (236ipm) so pretty slow.
I'll do an air cut and listen to the motors.
(betcha the air cut is flawless and the first real cut drops steps again )
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  #8  
Old Mon 09 September 2013, 23:21
KenC
Just call me: Ken
 
Klang
Malaysia
Seen this problem before. A few times actually. For me, it is the printer socket went loose. gently un plug & re-plug the connection will do the trick.

As for 6000mm/min, I consider it as very fast for MM weighty gantry & car. At this speed, acceleration become dominant in motion behavior. Would suggest re-assessing your acceleration setting.
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  #9  
Old Tue 10 September 2013, 08:59
Richards
Just call me: Mike
 
South Jordan, UT
United States of America
I think that Ken is correct. The acceleration (ramp speed) is very important. On my Shopbot PRT-Alpha, I could jog at 30" per second without loosing steps. The Shopbot program was able to change the multiplier for the step rate when it was in jog mode, so high speeds were possible; however, ramping was critical. I had to use a long ramp time (slow acceleration) to get those jog speeds.

The Mach 3 program has a visual indicator in the motor tuning section which makes changing acceleration fairly easy. I would double the time that the motor takes to get to speed. If that works, then you can play with the acceleration settings until you get the best performance. Some people recommend that all axes be set at the same acceleration settings so that multi-axis moves ramp at the same rate.
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  #10  
Old Tue 10 September 2013, 23:00
Red_boards
Just call me: Red #91
 
Melbourne
Australia
Thanks, Ken for the thought. I keep returning to wiring connection issues as a potential source of problems.

I called in the big guns to check the wiring and connections. She put the baby to bed and then gave the machine the once over and said she sensed one of the plugs at the Y motor connections was a little (very little) loose. Tightened it up and cut 60 000 lines with no problem, then lost 20mm in Y. I have to go back and see how long this equates to in steps and time.

I dropped Z 2 mm to shave off any wobbles, restarted the job and ran the 30 000 lines of code without error. But it sure is boring watching the machine do its stuff for an hour!

Thanks, Mike. I had not considered speed and acceleration may even be an issue. The Y axis moves about 40mm (2") over 1.8m (6ft) of X travel, and Z moves about 80mm (5"). I'm reducing speed by half in the cutting file at the X ends where there is a direction change. The Y action is concentric.

I'll go back and look at the acceleration settings. I recall I was quite conservative, but I'll report back on my findings.
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  #11  
Old Fri 11 October 2013, 09:37
silverdog
Just call me: Sergio #70
 
Rome
Italy
I had many times problems on most of the motors due to the gecko connectors ... touching the gecko conector I noticed that the motor engage disingage ... those conector are not soldered to the board but have pins that sometimes get loosy, you can lift off the connector remove the screws and tight the connection ..... if this is your problem ....
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  #12  
Old Sun 13 October 2013, 20:31
Red_boards
Just call me: Red #91
 
Melbourne
Australia
Hi Sergio,
Thanks for the thought.
Since my MM, computer and control box sit in a drafty uninsulated shed, I guess conditions are ideal for corrosion of contacts.
In speaking to a friend who does electronic wiring for a living, he confirmed what everyone here recommends - that the most likely source of intermittent errors is in contacts, particularly parallel cable and pin-type connections.
I've pulled and blown most of my connections and the problem appears to have stopped.
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