Z wont move only durring print

Hello, I've assembled my printer and at this point the X and Y axis moves without glitches, however if the Z axis is not at 0 the moment i press ''connect'' in Repetier host when i press the ''home'' button, the Z axis makes a noise as if it was stuck and is struggling to move, but when i hit the endstop with my hand only then it starts to move towards zero. And when i run a simple g-code that has noticeable Z axis movement, the Z axis only makes that same struggling sound, but remains at the same height.
I've looked through some of the other threads that had simmilar issue, and the sollution was reached by editing the Config.h file, however I've already doe that before even encountering this problem.
any ideas?

Thanks for the input

#define HOMING_FEEDRATE_Z (4*60)
#define DEFAULT_MAX_FEEDRATE {300, 300, 5, 25}

most I3s's have m5 rods, if you have m5 rods on Z then these are far to fast

try

#define HOMING_FEEDRATE_Z (2*60)
#define DEFAULT_MAX_FEEDRATE {300, 300, 2, 25}

NB eeprom setting can over ride these, check what is is eeprom with m503

Edited 1 time(s). Last edit at 12/27/2016 06:10PM by Dust.

How'd You know that i have an is? lol, but yeah i have a i3, however ive designed the x and z axis myself so that its a lot different from your usual i3. and in this ''reworked' model i used m10 rods I now know that it was a mistake, however currently i cant really do that over.

Anyways i'd like to ask in line:
#define HOMING_FEEDRATE_Z (4*60)
Why is there '4*60' why is it multiplied by 60? why not just '240'? and i've seen the same thing in multiple different lines

Also, in the line:
#define DEFAULT_MAX_FEEDRATE {300, 300, 5, 25}
are these mm/s? and how do you decide for z feedrate? any formula or sth?

Forgive me if these questions seem to mundane or anything. i just really cant read code
Thanks for your help

because for some reason HOMING_FEEDRATE_Z is in mm/min and and DEFAULT_MAX_FEEDRATE is in mm/s

so if you specify 2 (mm/s) * 60 (seconds) = whatever (mm/min)
make it more readable.. but you can use either

"decide for z feedrate"
experimentation... its related to torque of the motor vs frictions and mass, there is probably some horrible math for it..

Personally I just test it

set the max feed rate to something unbelievability high eg sent the printer a "M203 Z100" (100 mm/s, can't use maths in here only in firmware, not is real command to the printer)
now try to make it move but specify the feed rate eg "G1 Z5 F300" where F is in mm/min (so in this eg 300/60 = 5mm/s

It will probably buzz and miss steps, but keep playing with the F value till it moves reliably, then set that value as the real max feedrate value so that the feed rate is always clipped to that value. (remembering to convert it back to mm/s)

Okay thank you,
now as i understand if i set the DEFAULT_MAX_FEEDRATE to for example 120, whenever the Feedtrate in gcode is above that, it will get clipped to 120, am i right?
wont it affect other axis? because i've tried moving the x and y at 1800 mm/min and it worked fine, did that with 120 mm/min and it took forever to just get to one position

Yes this is how it works, an it only effects the axis is is set on. (but if you where doing something weird like printing while moving up, the print speed would also be limited)


And yes that is slow... Z axis is slow on most I3 type machines, unless you upgrade the threaded rods.

Thanks You good sir. have happy new year!