z axis loosing steps

[jeffspin]

hi, I'm looking for some advise on my cnc that I just built. This is the first one i'v made or used and know very little, even if this a good forum for a novice. i'v made a bench top, sized router cnc with nem 17 motors, using a garble shield to drive it at 1/8 th micro steps.My problem is my z axis motor looses steps (quikly) so i'm thinking I have to light of gear carrying the weight of the router. Before I buy new gear I was wondering if the stepper should have some "holding torque" when not stepping? it can heave the router around quick but not hold it on a plane.Any advice would be appreciated . thanks.

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[msimpson99]

I have used NEMA 17 motors on small cnc machines for all the axis without issue. For these motors I am using single start 8mm screws that work fine.



You have not given much information about you Z axis setup. I think a single start screw for NEMA 17 motors. You wont be able to put much weight on it. Some type of rotary is about all it will be able to handle. See my KReduCNC build here:
KReduCNC Introduction — Kronos Robotics

[jeffspin]

my z axis is made up of a 4"x 5" x 3/8" aluminum plate screwed to my linier bearings.It has a compact router( Makita rt0701) fastened to the other side via a plastic clamp cut from 1" hard plastic . the whole thing is driven by my nema17 ,inline coupled to a 8 mm drive screw,with 8mm of travel per rev.
Just grabbing the router and lifting will make the drive screw and motor spin(via the drive nut of course) is the part that seems odd to me because for some reason I thought that the motor should have some" holding torque" when plugged in and g-code sender open. It dosnt feel any different powered or not.
I was thinking about switching to a course thread 8mm drive screw which should gear it down at least 6:1 from where it is now but I was worried it may be software related due to how easily I could make the linier drive spin, just hopeing someone could let me know what is normal torque while in a fixed position and powered, or if while running some g code from a file will the motor behave different .
thanks .jeff

[ger21]

Sounds like either the motor is way underpowered, or the drive is not configured properly. My guess would be a little of each.

[msimpson99]

The motor should have holding torque when its not moving. If (with the weight removed and powered up) you can turn the motor coupler easy with your fingers then you have something hooked up wrong or a bad component.

I still think the NEMA 17 is too little for that much weight though.

[jeffspin]

thanks for that, i'm going to see if I can find a way to measure how much torque is there.
I also don't think,just by looking at the whole machine that the 17 is big enough either but with a 5/16 " course thread leed screw instead of the one I have now ,should get me up and at least playing. I would like to get some time in using it even if it is really slow to maybe have time to research what I" really "want to buy or upgrade. for right now I want to get to the bottom of my torque problems , see if I can rig something to compare It with maybe . thanks . jeff

[msimpson99]

One thing you can do is add some springs to help with the weight. I have done this on much larger machines with heavy spindles or routers. It sounds like you should have started with a half or full sized NEMA 23.

[jeffspin]

yup i'm probably going to have to up grade to a nema 23 . I did how ever hook up an easy driver to an other nema 17 I have and used the x or y from my Arduino uno to drive it(used pins 2 ,and 5 if it matters). anyway with my bench power supply for power and g code sender booted up, (or off ,I found it dosnt matter)the motor will draw around .7 of an amp at 12 v and with a small aluminum coupler on the shaft for leverage was still VERY hard to make turn. Especialy compared to the motor on my cnc that can nearly be turned with one finger.
it seems as though the small drivers on my cnc shield may have got hot and aren't working properly ,is at least what i'm hopeing. I havn't used the machine enough to be able to tell when one thing is acting different so when I have time to assemble it like on the bench I will report what I found thanks

[jahnj0584]

Try using 1/4 stepping first and doing $1=255 in your GRBL controller.

Faulting that, I have a 270 oz in Nema 23 for sale if you want. 20$ and its yours.

[jeffspin]

jahnj0584, I will give that a try later today ,thanks . I thought I was on to something with the different step driver but it had to do with the enable pin(more stuff I don't understand yet).
As far as the motor is concerned I have already bargained one with all the couplings etc on ebay. Very good price tho, thanks for the offer and the advice.