I recently received my Altmill and noticed that my Z axis has 0.6mm deviation when homing. If I have to use the estop to stop a cut and then want to restart, I noticed the Z often will be off by quite a bit. What kind of homing repeatability should I be expecting for my machine?
I just noticed the option to change the amount of homing passes which is set to 1 by default. I set the value to 4 which causes it to take maybe 5 additional seconds to home but now the machine seems to consistently be within maybe 0.01-0.02mm. EDIT: I spoke too soon. After the machine sat for a while I turned it back on and homed it and it’s off by 0.5mm
Does this indicate any sort of issue with my machine given that it needs multiple passes to home accurately?
I have never run into that so it could very well be an issue. There are three reasons off the top of my head:
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the coupling between motor and z ballscrew is not tightened down (2 allen screws)
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There is a collar on the z ball screw. Is it tight?
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It could be that the Z sensor is in the wrong position - possibly too far away so that the sensor doesn’t have a strong enough signal
Of course there is also the possibility that the sensor isn’t 100%
I would contact Sience (after verifying 1 to 3 above) and maybe they have run into this before.
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Thank you for confirming this shouldn’t be occurring normally. I suspect it is the homing sensor since I initially had it installed so it was ever so slightly behind the aluminum plate but I ended up having to move it closer to the ball nut and now its slightly proud of the plate although still out of the way of the ball nut.
On my mill the z sensor is in a little recess and the front of the sensor is either at the level of the non recessed area or slightly below that. The sensor shows about 2 - 3 turns of thread in the little recess.
This is on an original Altmill.
Thinking about this whole issue a bit more, I can’t actually say for certain that I do not see the issue you are talking about. I set z zero before each job and then don’t touch it any more. There are no interruptions in the job that would require re-zeroing (or re-homing). From my experience I can say that once my z zero is set (top of spoil board) it seems it stays that way …. but I am not in the habit of using the estop button. I suspect using the estop doesn’t loose position but I can’t actually say that for certain.
I noticed my Z axis coupler which was factory installed I believe had a stripped screwhead on the part that connects to the ball screw. I replaced the screws and tightened it up with some blue locktite thinking maybe that was the issue. I haven’t noticed such a large discrepancy as 0.5mm but did notice maybe 0.15mm differences between homing.
It probably isn’t a major dealbreaker regardless. Yesterday I was cutting some 2mm thick bookmarks and I had some issues with my Z offsets so I stopped the machine and then when I went to restart the print job that’s when I noticed the offset seemed to be different then when I had originally set it. That has sent me on this wild goose chase, partially because I know the altmill warranty period is rather limited.
An update to this. The answer which seems obvious now was my homing sensors. Jens mentioned that the sensor may not be close enough and that was the solution. I notice now when homing Z that the bearing blocks always remain at least 3-4mm away from the end of the rail where before the bearing block was flush with the end of the rail when homed. I’ve ran about 10 tests and it returns back to previously set coordinates accurately.
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