Print Head falls down when off

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kittka
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Print Head falls down when off

Post by kittka » Wed Jan 29, 2014 7:19 pm

At the risk of monopolizing the troubleshoot forum, and failing to find any previous threads on this...

According to the Rostock wiki:

http://reprap.org/wiki/Rostock#If_Rosto ... ll_down.3F

My print head shouldn't fall down because of the permanent magnets. But it does slowly fall down when I turn the power off, also after a print using the sdcard.

I didn't really tension the belts aggressively, could this be the issue? I mean the belts are tight enough to run with decent results/no skipping. I haven't checked for belt slippage when off, but it moves slowly like the motors are just letting the platform go down slowly.

Anyone else run into this issue? This is just a standard Rostock MAX kit assembled using the seemecnc manual.

int2str
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Re: Print Head falls down when off

Post by int2str » Wed Jan 29, 2014 7:27 pm

That does sound like your belts are not tight enough.

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Captain Starfish
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Re: Print Head falls down when off

Post by Captain Starfish » Wed Jan 29, 2014 7:38 pm

Doesn't sound like belts at all. For it to slowly drop (mine does it too) due to loose belts without jumping you'd have to have the belts on the wrong way around - otherwise the teeth would catch.

Motors off mean no braking, so the carriage is pulling the motors and belts with it.

From what I read in Gene's manual the slow-falling-down is an indicator that you've got the tension on your carriages about right, if it stays put then the cheapskate bearings are too tight.

I generally just let an edge of the carriage (not the nozzle) rest on an old test cube.

When it starts to drop it's a timely reminder that the Max is going to sleep on an idle timeout and I should do something about it anyway, so not terribly fussed about it.

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Flateric
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Re: Print Head falls down when off

Post by Flateric » Thu Jan 30, 2014 2:45 am

Properly adjusted carriages should drop slowly after the motors are disabled at the end of the print. The trick here is to have them tight enough not to have any play but loose enough to offer no resistance.

You can prevent the drop by going into the printer settings in repetier host and deselecting the disable motors option at end of print.
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Flateric
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Re: Print Head falls down when off

Post by Flateric » Thu Jan 30, 2014 3:21 am

To further illustrate our discussion about adjustment of the carriages here is a youtube vid by one of our own showing correct adjustment.

Hope it helps.

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Re: Print Head falls down when off

Post by enggmaug » Thu Jan 30, 2014 4:09 am

If I don't attach the platform, my cheapskates won't fall after turning power off.

If platform is in place, with hot end installed, it does fall down to the bed.

When I need to access the bed with turned off machine, I bring one cheapskate down to the bottom plate, keeping the 2 others up. That way, the head will rest against the column, not fall down, and clear access to the bed.

But be carefull, and make sure all cheapskates are higher than the hot end when you turn the machine on, or at least before homing the machine.

One more thing, be gentle when you move the cheapskates by hand. If you move too fast, the motors will act as generators, and send current back to the board. I already saw the LCD turning on while the machine was turned off, because of moving cheapskates too fast. I don't think it could damage anything, but I don't wanna try.

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Re: Print Head falls down when off

Post by smiley » Thu Jan 30, 2014 9:21 am

enggmaug wrote:One more thing, be gentle when you move the cheapskates by hand. If you move too fast, the motors will act as generators, and send current back to the board. I already saw the LCD turning on while the machine was turned off, because of moving cheapskates too fast. I don't think it could damage anything, but I don't wanna try.


I have noticed this too. I can make the fan on the power supply spin by moving the platform with the power off. and it struck me as a huge design flaw, either in the wiring of the steppers or the design of the board.

What would it take to prevent current generated by the motors from the motors coming back to the board? Just a couple of extra diodes on the motor driver circuits, right? Any good reason not to have that in place?

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Re: Print Head falls down when off

Post by geneb » Thu Jan 30, 2014 9:42 am

The drivers aren't designed to be protected against current coming the other way and they shouldn't be. When you move a stepper motor fast enough by hand, you can generate enough power to blow a driver.

Even "professional" drivers don't have this kind of protection - the stepper drivers in my ShopBot run about $400 each to replace and the manual clearly says to not move the machine by hand too quickly. I'm pretty sure I mention this in the 'Max build manual as well.

As to the "drop" - a properly tensioned belt & cheapskate will allow gravity to overcome the cogging in the stepper motors. It may not be very much, but the "slump" as it were, should be noticeable.

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Re: Print Head falls down when off

Post by smiley » Mon Feb 03, 2014 11:25 am

geneb wrote:

Even "professional" drivers don't have this kind of protection - the stepper drivers in my ShopBot run about $400 each to replace and the manual clearly says to not move the machine by hand too quickly. I'm pretty sure I mention this in the 'Max build manual as well.

g.


Interesting- I think you did say that in the manual, but i didn't really think about it.

I have a set of Gecko drivers for my homebrew CNC router, I guess it never occured to me that I could blow them by moving the machine against the screws- since we built it on the cheap with Home Depot allthread, there is enough friction that you can't really move the spindle by hand without turning the axle. But a good reminder not to do it with the MAX!

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Re: Print Head falls down when off

Post by Apollyon25 » Wed Feb 05, 2014 2:51 pm

Depends on the output drive topology - most MOSFET output transistors have a body diode inherently. For a normal 4-wire stepper, the H bridges should enable full 4 quadrant operation and each FET should have this inherent diode. These diodes are biased towards the supply so when the motors are moved and current flows into the supply, its not regulated nor limited so potentially can damage the input stages as these typically don't have much protection from supply over voltage. Also when the system is off, all parts of are not correctly powered so can get some odd indeterminant states, potentially resulting in output transistors being turned on together shorting the supply. Then the "driver" 'blows'. It very much depends on the design of the driver circuitry.

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