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Is my uno/gshield dying?

Posted: Sun May 11, 2014 10:25 pm
by awesomename
So it seems that there is something going on with my uno/gshieldv5 combo.

Every job I run in the last week does the same thing, at some point or another, it will overplunge on the z by about .25 to .5inches and start trying to rip away .

I have verified that the g code seems to look fine but I can't get through a single job without this happening . I flashed .8c again to the uno but the problem persists.

I am using Meshcam v5 and v6

any ideas?

Re: Is my uno/gshield dying?

Posted: Sun May 11, 2014 11:01 pm
by WillAdams
Have you adjusted the current of your Z-axis?

Re: Is my uno/gshield dying?

Posted: Mon May 12, 2014 7:04 pm
by Schruminator
To go along with WillAdams, it could be a couple of simple things:

-The acceleration on your z-axis is too fast (depending what version of grbl you have, your z-axis acceleration may be tied to the x and y as well). If the stepper motor tries to raise the z-axis too fast, often times
things will bind and seize. So it thinks it moved it up the full amount, but maybe it moved 20% of the way and seized. Next time it plunges, the z-axis won't be as high as it should be, thus it'll plunge way too deep. Also check to make sure that your z-axis threaded rod is not wobbling as it spins. If it wobbles, then something is either bent or not lined up straight-- both of which cause more friction issues and can lead to seizing

-The current going to your z-axis is too low. Not enough power to move your spindle up (unlikely, but possible)

-More likely than low current, the current/trim pot setting is too high. Your z-axis is overheating and going randomly into thermal shutdown (to protect itself) and missing steps.

Re: Is my uno/gshield dying?

Posted: Mon May 12, 2014 8:42 pm
by cvoinescu
What Schruminator said: your Z axis does not over-plunge, it under-retracts. This could be due to too little current, or too much current with too little cooling causing thermal shutdown, or mechanical problems (too much friction).

Re: Is my uno/gshield dying?

Posted: Mon May 12, 2014 9:12 pm
by awesomename
This is definitely what it was! Is there a way to independently control the z axis speed on retracts?

Re: Is my uno/gshield dying?

Posted: Mon May 12, 2014 9:19 pm
by WillAdams
Not that I've seen. There are some Communication / Control programs which will limit all Z-axis moves --- different speeds for up/down would be an interesting request.

One could manually edit G-Code and always use G1 moves, setting the F value to the desired speed though.

I don't think that you want your acceleration set so close to the limit that whether or not the machine is going up/down results in different behaviour --- instead, you should adjust your current so that both directions work reliably.

Re: Is my uno/gshield dying?

Posted: Mon May 12, 2014 9:24 pm
by awesomename
I will try adjusting the pot tonight. I was able to max out around 750mm/min before the z started binding. I was hoping to run around 5000mm/min on x and y


Not a huge deal since I am cannibalizing my shapeoko for another, bigger and stronger build and will be using the USB CNC mk2/4 controller soon.

Re: Is my uno/gshield dying?

Posted: Mon May 12, 2014 9:52 pm
by cvoinescu
GRBL 0.9 will allow you to set the traverse speeds separately on X, Y and Z.

Re: Is my uno/gshield dying?

Posted: Mon May 12, 2014 9:55 pm
by Schruminator
Adjusting the pot is a balancing act-- you want more current to try and muscle through things, but too much current and you'll overheat the stepper motors and they'll shut down for short periods of time to cool off-- and when they do that you'll miss steps and have even more problems.

Find the sweet spot on the pot adjustments and then you'll just have to tone down your z-axis acceleration settings and live with that until you upgrade hardware/software/firmware in the future.