Z axis speed (only at print start)

Hi.
My printer is working mostly fine (own design) with the z never missing a step. Doing a Z home is never a problem.
However, when I start a new print there is some Z positioning up and down and the last ‘up’ seems to be at a very high speed. I often get missed steps and motor buzz at this point. It seems to be the only time it tries to drive the Z that fast. It looks like its doing a double Z home with the second one being very fast.
Where can I turn this bit down without affecting the whole print?
Thanks.

Since it is not your homing it must be a gcode command you send. Find it and you get the reason. Check your start gcodes in host and slicer, which may be resonal position where that could be.

On the other side, you firmware should limit z move speed anyway, so you should also check the maximum z speed you allow.

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Thanks for that.
Im not much of a gcode expert so haven’t tried to look into that.
I have noticed that the second of the two Z homes, just at the print start is affected by the firmware Z max speed, not the Z max home speed. I have turned it down from 5 to 3 and all seems OK. There is no apparent slowing of the Z movement during print but it has fixed this rapid moment.

BTW, is there another Z lift option somewhere, apart from the one associated with the filament retract in Sli3er? I often get a horrible rasping noise as the nozzle drags across the hardened plastic. I think this may also be the reason for occasional massive X slips.

Edited 1 time(s). Last edit at 12/23/2013 05:23PM by Vince.

No the one in retract options is th eonly one. You should be aware that slic3r does not set any z speed and always relies on the firmware to reduce to maximum speed, so these are always very fast. Normally you do not get these noises even without lift option, except if you print too much filament making the filament higher then wanted. This gets worse if your z axis/extruder has clearance. If your acceleration is too high you may also loose height in z axis. You should make a test print and measure if the height matches to be sure you loose no more steps in z direction.

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I ran some tests by sending the Z to max travel and back to 0.1mm of home (on the repetier screen) while the head was set up with a dial gauge. It was spot on every time not losing any obvious steps, even when I weighted the bed down.
I will try increasing the Z lift to 1mm (now 0.5mm) but as you say it should clear anyway.
My bed relies on gravity to keep it on the lead nut but as its 2 layers of 6mm glass 700mm x 400mm it should be Ok.

I did suggest it would be great to have a post print missed step check on all axis by returning the carriages home. Also some routine that thrashes the carriages and checks for missed steps at increasing speeds would be good.

Thanks for your time with my little issues.

Vince