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Bad RAMPS?

Posted by lapo 
Bad RAMPS?
May 13, 2017 09:01PM
I"m hoping this is the right forum for this question. Apologies if it should be in the Delta section.

I am trying to get a delta printer working reliably but can't seem to remove occasional missed steps.
I am confident that the mechanics of the printer are good.
There is no play in the effector mechanism (magnetic arms), movement is nice and smooth and homing is still accurate after not having calibrated it or changed anything on it for a couple of weeks.

It seems to print small pieces without any problems, but as soon as I attempt something which takes more than 10 minutes or so, I keep getting the occasional small layer shifting which leads me to think it is missing steps.

The RAMPS electronics are of dubious provenance, (I was given the printer as an almost finished project).
The A4988 stepper drivers are on red PCB and have an Rcs of 200.
I've tuned the Vref accordingly and the stepper motors do get borderline hot (about 68 degrees after an hour of printing)
Stepper motors are Minibea branded, 1.4A. (I've also tried a different set of steppers and the problem persists).

I'm wondering whether it is possible for the RAMPS board or A4988 clones to be bad in some way and be the cause of this?

Anyone ever come across this kind of issue with a RAMPS board? My other two printers use a Melzi and a Maker Base so I'm new to the replaceable A4988s.

Thanks.

Lapo
Re: Bad RAMPS?
May 13, 2017 11:48PM
You could observe the printer to understand, why it is skipping steps.
Maybe your extruder is overextruding and cause the nozzle to crash into the part?
Re: Bad RAMPS?
May 14, 2017 01:04AM
Thanks for the suggestion.
I'm quite sure this is not the issue as, if anything, the extruder is under extruding at the moment.
I initially thought it might be related to acceleration values while moving so I've tried reducing acceleration and Jerk values in firmware right down (using Repetier firmware) as well as raising Z while traveling, but it hasn't helped.

Hence my question about the possibility of a bad RAMPS or stepper driver. I don't know enough about electronics to know whether this is feasible.
Anyone?

Thanks,

Lapo
Re: Bad RAMPS?
May 14, 2017 04:05AM
If the stepper drivers are "stepstick". then they probably need 0.8v


_______________________________________
Waitaki 3D Printer
Re: Bad RAMPS?
May 14, 2017 09:07AM
My next best guess is the ( often crappy ) 5A polyfuse. You could bridge it temporarily and make a test print.
How is your cooling situation? I never had steppers get to hot to touch, the drivers must be smoking hot then?
Re: Bad RAMPS?
May 14, 2017 09:56PM
With regards to the stepper drivers, I have them set to 1300 mV which should in theory give me 0.8A at the stepper. (8 x 200 x 0.8 = 1280mV)
Anything more than this and the steppers go over 70C.
The actual stepper drivers don't get over 25-30C measured on the heatsinks which are glued to them.

Is it possible to overdrive the stepper motors and cause missteps?


With regards to the polyfuse, I did have to change out the 11A one for the heated bed as it was tripping out after a few seconds.
I hadn't considered the 5A one being bad as I understood these fuses stay tripped until they cool down but I will try that out next.

Thanks.
Re: Bad RAMPS?
May 15, 2017 03:45AM
"these fuses stay tripped until they cool down"

this is what they are ment to do, but many from cheap suppers never fully recover so remain just at tipping point.
Re: Bad RAMPS?
May 18, 2017 10:23PM
I meant to also suggest previously - have you got both your printing software and your com port set to 115200?


_______________________________________
Waitaki 3D Printer
Re: Bad RAMPS?
May 21, 2017 11:09PM
I've finally found time to test shorting the 5A polyfuse but it has made no difference.

On the baud setting for the port, I'm running the printer via Octoprint and it and the Repetier firmware are configured to communicate at 250000.
Is this not reliable on RAMPS?

Thanks.
Re: Bad RAMPS?
May 24, 2017 04:21AM
Dunno about the effects of a baud rate of 250k but it would be easy to set the rate at 115200 and see what you get. Incorrect settings can definitely cause missed steps and communication probs.


_______________________________________
Waitaki 3D Printer
Re: Bad RAMPS?
May 24, 2017 01:55PM
I tried reducing the baud rate to 115200 but it made no difference.

I've now tested with a reduced maximum speed to 40mm / sec (for moves as well as printing) I didn't get any missed steps.

Given the mass of the mechanics, and the size of the steppers (up to 1.4A), I'm sure I should be able to go faster than this but if I increase Vref to 1.6V (which with my red Chinese A4988 clones should give me 1A) the steppers heat up way over 80ºC which I understand to be a problem. In order to keep them at a more same 50-55ºC I need to reduce Vref to 1.2V which should in theory give me 0.75A

Can the heating up of a stepper motor at relatively low amperage be caused by poor quality stepper drivers?

Thanks for any input.

Edited 1 time(s). Last edit at 05/24/2017 01:55PM by lapo.
Re: Bad RAMPS?
May 28, 2017 04:17AM
68 to 70°C after an hour is a fairly normal operating temperature for stepper motors, especially for a delta. Try looking at any end stop signal lines and make sure they are paired close to their grounds. Maybe try twisting them together to prevent any interference causing stray signals.
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