Z axis homing issue, but ONLY at the start of a print job

Hi.

I just completed my first RepRap this weekend, and I tried to print a 20mm cube after calibrating all the movements.

All axes home without problems when I "manually" home them.
However, the homing that happens after the heat bed has warmed up (and before the hot end starts to warm up) at the beginning of a print job, presents an issue:
The Z axis moves to trigger the endstop, then it moves up a little bit, and then the motors stall instead of moving the Z down again to the correct height!?

Can anyone please help me figure out why this is happening? I've tried to adjust the pot for that axis, but it happens no matter what the pot settings are.
As mentioned earlier, all manual homing are fine..

Any help is greatly appreciated.
Thanks.


\Martin

Try reducing the maximum Z speed in your firmware.

Usually it's just the software telling the machine to go too fast for your motors/mechanical setup. Since the firmware has too high a limit, it allows this figure. The firmware should limit the movement to the max speed your hardware can handle.

Thanks Cefiar!

I'll give that a try when I get home from work tonight.


Anyone else with a RepRap Prusa i2 and RAMPS1.4? What max Z-speed have you got on yours?


Thanks again,


\Martin

Quote
netmav
Anyone else with a RepRap Prusa i2 and RAMPS1.4? What max Z-speed have you got on yours?

It doesn't matter what others have. They may have different steppers. Or their steppers current is slightly different than yours which makes comparing values pointless. What matters is what YOUR hardware can handle.

If indeed your existing feed rate is too high, just run some simple tests. Take the existing feed rate and cut it in half and try moving the z-axis. Do the steppers stall or skip steps? If so, decrease the rate by half again repeat. If it ran fine, then increase the rate by half the difference and repeat. Keep doing this a few times until you find the point just below the maximum that you can get it to work.

Cdru, you are absolutely right.

Anyways, I just tried to lower the speed now, and that helped! The Z doesn't stall during inital print homing anymore...

BUT, that presented a new problem.
During homing at the beginning of the print process, after the Z endstop is egaged, the Z axis moves back up about 5-7mm (instead of moving a small amount up, and then slowly moving down to the Z 0.00 possision??

When I home "manually", it's ok (Z moves down until the Z endstop is engaged, moves up a bit, and then slowly down to Z 0.00), but the inital print homing doesn't.. :-(

So, the stalling that occured earlier (before I changed the Z speed) "covered up" this problem.. I thought that it stalled when it was about to move slowly down..

Any more ideas, guys?


\Martin

Quote
netmav
During homing at the beginning of the print process, after the Z endstop is egaged, the Z axis moves back up about 5-7mm (instead of moving a small amount up, and then slowly moving down to the Z 0.00 possision??

That sounds correct. It will "bounce" on purpose. The initial descent (I'm presuming you have the switch at z-min) is a rapid descent, it raises slightly, then descends down again more slowly until the switch is triggered. The initiail faster descent is to save time, the slower speed is for positional accuracy so that the switch is tripped at exactly the right point and not overshoot it.

Quote
cdru
That sounds correct. It will "bounce" on purpose. The initial descent (I'm presuming you have the switch at z-min) is a rapid descent, it raises slightly, then descends down again more slowly until the switch is triggered. The initiail faster descent is to save time, the slower speed is for positional accuracy so that the switch is tripped at exactly the right point and not overshoot it.

Thanks cdru

I agree that it should move down "fast" untill the Z endstop is triggered, then back up slightly, and then move slowly down again to accurately position the nozzle at Z = 0.00.
And that's exactly what happens when I manually home the Z axis in Pronterface (click on the Z axis home button).

But the homing sequence at the start of the print job (after the heat bed has warmed up) doesn't do that.
It rapidly moves the Z axis down untill the Z endstop is triggered (so far so good), and than it moves back up 5-7mm and stops (not good). Then the hot end starts to warm up (with the nozzle hangig 5-7mm up from the heat bed). When the hot end reaches the set temp, it starts to print at that height.

I can't figure out why the "print homing" behaves differently than the "manual homing"...


\Martin

What does the gcode at the start of your print job look like? Look for the line in particular that has G28 in it.

Quote
cdru
What does the gcode at the start of your print job look like? Look for the line in particular that has G28 in it.

Thanks cdru!

The G28 ; Home all axis is there, but the next line got me thinking:
G1 Z5 xxxx ; Lift nozzle

I removed that, and that solved it!


Thanks for all your help, guys!



\Martin

Quote
netmav
The G28 ; Home all axis is there, but the next line got me thinking:
G1 Z5 xxxx ; Lift nozzle

I removed that, and that solved it!

Well that would definitely cause the issue you were having. Glad it was something simple.