z axis requires more force than x to move (?)

[forgetcolor]

i've got my gantry assembled, including the x and z axes assemblies. before installing the screws on both axes I aligned the pipes so that the assemblies glided smoothly. once adding the screws, coupling them to the motors, and nutting them to the bearings on the other end, i'm finding that the z axis screw requires noticably more force to turn than the x axis. this seems strange to me. i probably would have presumed the opposite to be true if I had guessed.

i pulled a pipe out of the z axis so that it hung freely and it didn't seem to change things much. could it be simply that the z axis hangs down and puts force on the nut and thus requires more force to turn?

i'm using dumpstercnc nuts, in case it matters. maybe I need to reverse the nuts? I have it installed on the underside, with the flange up and spring side down.

even if i can't figure this out does it matter? i'm using keiling 387 oz motors.

thanks!

[stangtjk]

With the z-axis you're fighting gravity and all the weight is being supported on the leadscrew and nut. On the x and y-axis the weight is supported on the bearings. People often have the misconception the the z-axis will require less force because it weighs the least, but that simply isn't the case. No one will be able to tell you how well those motors will work without knowing the specs of the z-axis, weight, screw pitch, etc. but unless your machine is really heavy it probably wont be a problem.

[ger21]

It's also possible that you have an alignment issue. A very slight misalignment will cause the screw to be much tighter.

[forgetcolor]

the weight idea really makes sense, but maybe it is misalignment. i laid the gantry on its back in an attempt to remove some of the weight from the equation. the result wasn't much different. i tried loosening up the rails and even just pulled one (with the gantry upright) and it didn't improve much then either.

any trick to aligning it right? what i did was to lay it on its back (before installing in the gantry) and keep playing with the rails until the bearings all touched and it ran smoothly. i'm finding it hard to get it so every bearing is touching and spinning all the time, even though i drilled quite precisely and have the holes aligned well. i'm ending up adjusting the rails towards the center almost as far as they'll go. i also wondered if my nut is not perfectly centered if it might be making it a little tighter? i was very careful in my drilling but maybe it's not perfect.

thanks!

oh, here's a photo for fun:

[forgetcolor]

i took things apart today and worked harder on alignment. i found that even though everything was carefully setup and drilled that it looked like the piece with the bearing blocks on it wasn't centered in regards to the motor and lead screw. so i adjusted the rails left/right to center it better and that made a noticeable difference. it's still not quite as easy as the x axis but it's better.

i wonder if the shortness of the lead screw exacerbates any misalignment between the motor, anti-backlash nut, and bearing end because its shortness prevents it from having much room to flex if it needs to. on a long screw it seems to me it wouldn't matter as much.

[Joham]

Hello - Here's a work-around I came up with for the alignment problem...hope it might be of use. http://www.cnczone.com/forums/showthread.php?t=76137