Difficulties while homing Z

I have a minor problem which causes me a great deal of suffering. Whenever i have moved my printer i always have to recalibrate my Z axis, no biggie. However after the calibrationg / homing has been performed (using pronterface) and the print is initiated, the Z axis will move upwards a fraction of a mm leading to the filament not sticking, and so i have to calibrate again whilst the Z axis is at the wanted position.

Im using Repetier as firmware for my reprap prusa i2

Any suggestions ?

you need to put more information but is propably yours slicing software rising you hotend before print so high

How are you calibrating? If you are using a piece of paper or a feeler gauge, I found that a lot of times my hotend would end up with a small bit of hardened plastic at the nozzle. This would throw off the calibration just a bit making me think the nozzle was too close. After adjusting and starting a print, the plastic would melt when the hotend heated up and then the nozzle would actually be too far from the bed causing what you are talking about.

If you setup auto bed leveling, you take the nozzle out of the equation by using a probe for z homing.

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brucehvn
How are you calibrating? If you are using a piece of paper or a feeler gauge, I found that a lot of times my hotend would end up with a small bit of hardened plastic at the nozzle. This would throw off the calibration just a bit making me think the nozzle was too close. After adjusting and starting a print, the plastic would melt when the hotend heated up and then the nozzle would actually be too far from the bed causing what you are talking about.

If you setup auto bed leveling, you take the nozzle out of the equation by using a probe for z homing.

I always make sure that no plastic residue is present in the tip of the nozzle, ill just flick it off with a small knife, so its definitely not that which destroys the calibration, since the hotend is never heated whilst calibration is done.

As for how i do it, i use a piece of paper and rise the heated bed in the corners (by following the procedure in the reprap wiki which i cant find now..) until I feel a very slight drag on the paper. But i do follow your train of thought. I was thinking it maybe had something to do with the firmware, because when the print is started it will home z, and will hit the height perfectly, then move up 1 cm and down again to "home" but it never hits the endstop again.

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borupdk
As for how i do it, i use a piece of paper and rise the heated bed in the corners (by following the procedure in the reprap wiki which i cant find now..) until I feel a very slight drag on the paper. But i do follow your train of thought. I was thinking it maybe had something to do with the firmware, because when the print is started it will home z, and will hit the height perfectly, then move up 1 cm and down again to "home" but it never hits the endstop again.

Hmm, ok, you might want to check and make sure your z axis nuts are not loose in any way. If there's any play at all, it could make a difference when moving the z axis up as opposed to down. I've also noticed something similar on my i3 and it's mostly because the x-axis motor is on one side. When I would go to calibrate the bed on the right side (opposite the motor), if I just moved the z to 0 rather than homing, it would also not hit the endstop unless I gave it a little tap on the top of the x-axis, then I would hear the endstop click. The left side seemed to be no issue. No matter how tight your x-axis is, having the weight of the motor and the extruder on one side can make a small difference.

Someone here helping me once said to temporarily remove the z threaded rods from their couplers and make sure you can move the x-axis up and down freely with no hangups or binding anywhere.

Now it's getting really interesting, out of curiousity i tried to switch to another interface (repetier host) and hey presto ! no homing difficulties !