Not enough holding power for z axis?

[kakapoopie]

Hi all, I'm putting together a monster top down dlp and have run into a problem with my z axis. The z travel is 13", carriage is much heavier than normal and the motor it came with was nema 23 with an e brake. I've tried running it with the NEMA 17 motor + RAMPS I was previously using for my small printer and now the z axis will drop as soon as the motor is turned off. It seems to be fine moving, very smooth seems to have enough torque but as soon as it finishes a movement the carriage drops from gravity.

Any ideas?

BTW Here's the z stage in question, using a nema 17 instead of the supplied NEMA 23/brake ( I don't think standard pololu can handle it?)


Best,Vaughan

[Phife]

that seems perfectly normal, there was a brake on there for a reason! Your Z is heavy and there is little friction to keep it in place when the motor is not powered so of course it will drop.

My suggestion, switch back to the NEMA23 and brake, the RAMPS can handle it no problem.

[kakapoopie]

Hrmm thanks..makes sense. What motor driver do you recommend for that motor? I think its out of the voltage range for the pololu

Also, how does the brake get connected to ramps? And how to control it in CW?

Vexta Stepper two phases C9936-9212K DC2, 8 V 2 A phase 1.8 degrees
Shinko brake 24 V 4 W Type BBS-4-3B

[dangre]

Looks like the brake is electronically separate from the motor? I suppose you could turn it on/off with a relay controlled by the aurduino and gcode. Us CNC mill guys often use counterbalances to offset a heavy spindle on the z-axis. Is that a ball screw that turns so easily when the stepper is deactivated?

[Phife]

I was running 2 Nema23's with my RAMPS 1.4 and the Pololu's. I was only using a 12V 5A power supply and they seemed to work with no problems. You can bump it up to I think 30v if you need more but not sure why you would. The SLA printers dont really have to move the axis very fast. 12V is probably fine. The Pololus can run up to 2A/Phase if you have some good cooling.

As for the brake, Im not exactly sure how yours works but typically you want it to be powered when the stepper is powered so it releases the stepper. The brake should be preventing the stepper from moving when its not powered and allows it to turn when powered.

All you really need is to be able to provide 24v to the brake when the system is turned on, you might need a bit of a delay as your stepper drivers get powered up. Or you could just use one of the outputs on the RAMPS board to turn a relay on and off, that in turn will connect and disconnect power from the steppers, you can easily program this into your firmware or CW.

For example I use output D9 on the RAMPS that is typically for the FAN as a relay output. You can use Gcode M106 to apply 12v to this output and M107 to turn it off. Connect that to a relay and use the relay to apply power to the brake.

So you'll need a 12v and 24v power supply, 12v for ramps and 24v for your brake.

[cncbasher]

[kakapoopie]

[kakapoopie]

[cncbasher]

put simply try and get as high a voltage as possible within the spec of the stepper driver. ( i often run steppers on a 68v supply with approprate stepper drivers )
polou's i believe can take up to 35v , however i would not go above say 32v max , polous have a habit of instant death if over 35v .
depending on your power supply , the main critera is not to exceed 2a max drawn by the stepper motor , as most modern stepper drivers are current limiting this will also look after the voltage
steppers are not like normal motors they drive max current when stationary and not whilst moving .. steppers are also designed to run hot , so long as you can bear your hand on it it's fine , if it's at the point of frying eggs then no

the benefit is the steppers will run faster .. volts = speed , current = holding power or torque , the higher the voltage the lower will be the amps ( ohms law )

the main problem is polous are cheap but low current ... Leadshine are seperate and more expensive however they allow higher voltages and current
my rule of thumb nema17's are fine with polous , nema23's and above use Leadshine or similar . ( i.e black boxes with step direction inputs ) they come in many disguises and brand names , but they are just about identical
( apart from the price some times )

[kakapoopie]

Something like this?

or maybe this

Thanks for all the help. I will try with the pololu when I get home and if that doesn't work I'll go for the bigger drivers