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Questions about Z axis drive

Posted by Bobby-S 
Questions about Z axis drive
February 19, 2015 04:33AM
I am new to this... I am building a printer of my own design. I probably should have just built a kit for my first 3D printer but that would be the easy way....

Anyway.. The design I am using has an X and Y carriage near the top of the printer and the print bed will lower for the Z axis... It is nearly completed and I have been doing some testing and ran into a problem. The X and Y axis appears that it will work as planned but I am having a problem with the Z axis...

The print bed is square and is supported by 3 vertical acme screws. 2 in the front corners and one centered in the rear.. There is also a couple of vertical rods for positioning at the rear corners. As long as the bed is level nothing binds and the acme screws turn with no binding. The rear screw is directly driven by a nema 23 stepper the 2 front screws are timed together and driven by a 2nd nema 23 stepper using a GT2 belt..
I am using a Arduino and Ramps 1.4 a A4988 wire to drive these two steppers in parallel.

With no weight on the print bed the z axis moves up and down as expected.. But even with the smallest weight the bed occasionally binds.. What appears to be happening is the steppers do not have enough torque to properly drive the two acme rods at the front of the bed. Once that happens the bed is no longer level and the bed binds up completely. I need to make changes to it but I haven't figured out what to do.

One thing I probably should do is redesign the Z axis drive belt system so that all three acme rods are timed together. But, one of the steppers I have won't drive 2 of the acme screws reliably so I doubt one stepper will drive all three.. I don't know if I can drive the 3 screws with two motors without having the two fight each other.

The nema 23 steppers are ones that I had before starting this project.. They are rated at 24V and have 53ohm coils with 40 Nm torque (but that would be 40 Nm at 24V... I am only using 12V)..

One possible fix would be to use different steppers.... I would quess that there are other steppers available that are have more torque (but they would draw more current.)
Another option would be to add a third motor (I do have a third one more like the other two) ... but the front screws will have to be belt driven and would no longer be timed together and I don't know if that will fix the problem without adding others.
Or, I could try raising the voltage driving the existing steppers but again, I don't know if that will fix the problem

So... Do I need to worry about if the 3 acme screws are timed together? or If I am using steppers that have enough torque is this not needed? .. I know a lot of designs use two steppers on the z axis and the aren't timed together with a belt.
How about changing the pulleys on the acme screws from 20t to 40t this would give more torque but lower speed? Don't think high speed on the z-axis is needed

Would the Ramps and A4988 that I have drive three nema 23 steppers ? What arrangement would you recommend me trying?


Re: Questions about Z axis drive
February 19, 2015 04:47AM
If those stepper motors really do have 53 ohm resistance, then they are totally unsuited to the application. Look for stepper motors rated at between 1.2 and 1.7A and 2 to 4V.

Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].
Re: Questions about Z axis drive
February 19, 2015 10:46AM
I think you were wise to use 3 screws to move the bed. Not many printer builders will go to that trouble/expense, but once you've got it working right you'll have a very stable bed that will be eas
y to level but won't require frequent releveling/zeroing. Using one motor to drive the three screws means they will always be in sync and the bed will stay level. Using multiple motors is asking for more of the sort of trouble you're having now.

You definitely want to drive all three screws with one motor, but you need the right motor. As dc42 says, a 24V motor is definitely NOT the right motor. In general, it is best to drive a stepper with a multiple- up to 10X- the motor's rated voltage. You won't find 1.2V NEMA-23 motors, but you can easily find 3-4V motors that will do the job for you and will run happily from your 12V supply.

If you want to figure out the torque required to move the bed this site will help: [www.orientalmotor.com]

If your mechanism is well aligned so it doesn't bind (which is how it should be) the screws provide a torque multiplication effect which means a relatively small motor should work.
Re: Questions about Z axis drive
February 25, 2015 01:24PM
I have an update on my original post... Since then I have added a vertical jackshaft to my printer and I now have all three bed screws timed to the jackshaft.. my plans are to use a single motor to turn the jackshaft... I also mounted a small crank on the jackshaft to I can easily raise and lower the bed manually . The jackshaft turns at a one to one ratio with the bed screws.. This part works pretty good! All I have left is to install a stepper to turn the jackshaft...

I made the crank so the handle is exactly 1 inch from the jackshaft. I don't have a fish scale or I could measure exactly how much torque is needed to turn the jackshaft... so I have to guess... to get an idea of what is needed I put one finger on the crank handle and another on a digital scale and pushed about the same amount... So... it feels like it takes about 15- 20 in.oz. torgue (defiantly less than 30 in.oz) to turn the jackshaft. This is with no weight on the bed.

I have a 36 tooth, a 20 tooth and a 16 tooth pulley that I can use for the jackshaft drive .. So, I can have a reduction of 1.25/1, 1.8/1 or 2.25/1 between the stepper and the jackshaft Since this is the z axis I am not worried about speed too much. I am thinking I should use the 36T and 16T for a 2.25/1 reduction...

Now I just have to choose the stepper and this is where I get lost..

Without purchasing anything new I have a choice of using the Nema 23 motor discussed earlier but it is unsuitable for the job..

I also have a NEMA17 stepper whos' specs are: Holding torque: 40N.cm, Current: 1.7A, Resistance 1.5 Ohm, Step angle 1.8
If I understand the specs and proper useage correctly, this one might do the job better but it may not be ideal...

Would a similar stepper rated at something like 76in/oz, 1.5A, and 2.8 Ohm be better??

I am driving it with an A4988 on 12 volts...

Another question I have is I currently have the RAMPS board strapped for 1/16th steps.. In Marlin I had to set the #define DEFAULT_AXIS_STEPS_PER_UNIT for the z axis to 1600 to get the z axis to move 10 mm when commanded to move 10 mm.. What would be the pros and cons of going to half or full steps and reducing the steps per unit?

Re: Questions about Z axis drive
February 26, 2015 02:28AM
Well I answered some of my own questions...

As noted I have a NEMA17 stepper whos' specs are: Holding torque: 40N.cm, Current: 1.7A, Resistance 1.5 Ohm, Step angle 1.8
Apparently I am missing something.. I have a 16 tooth pulley on the stepper and a 36 tooth pulley on the Jackshaft. I am glad I made the change to drive all three screws together but now I need to figure out how to get this setup to work properly. I am wondering what I may have missed. The motor and the heatsink on the driver module are staying cool to the touch and I have a fan blowing over them and I would have thought they would be getting hot (or at least warm) if I have too big of a load..

On first try with this setup It would lower the print bed but when I try and raise the bed, the motor would make a rattling sound but the shafts do not turn. If while the motor was rattling I push lightly on the crank I installed the shafts will turn and the bed will go up... So I believed that for some reason the motor is just not delivering enough torque to raise the bed...One thing I had done was I read a procedure that says I need to adjust the pot on driver module so that I see .4V between ground and the wiper of the pot to limit the current.... When I first installed the A4988 they were set at .6V So I adjust the pot on the driver to .7V... Now the bed goes up and down just as it should except if I click to "Home" the z axis... Appearently on my printer Marlin is currently set to "Home" at a faster speed then when you are clicking to move the z axis in pronterface...

I know I could go into the configuration and slow down the rate at which the z axis moves when homing but I'd rather make it work at the higher speed..
I figure one option might be to turn up the current limit pot a little more.. Nothing appears to be getting hot yet...

I an still trying to learn about what effect changing the strapping options for full steps would do. Currently to get the z axis to move the correct distance I have the DEFAULT_AXIS_STEPS_PER_UNIT set to 3600..
Re: Questions about Z axis drive
February 26, 2015 07:51AM
The main thing you need to worry about with 3600 steps/mm is the maximum speed you try to move the Z axis- can the controller send pulses fast enough to move the axis as fast as you're telling it to go? If you're using Arduino/RAMPS, the max step rate is 40k/sec. 40k/3600=11.11 mm/sec maximum travel in the z-axis. That should be faster then you ever need it to go. OTOH, since it sounds like the torque is marginal, you might want to reduce microstepping because microstepping makes a motor run smoother but also reduces torque. In the Z axis you're not really concerned about smooth operation because most of the time just moves 100-500 um at a time and the machine doesn't print while Z is moving. Increasing motor current can also increase the torque, so try bumping it up a bit. The motor's torque should be highest without microstepping.

Steppers and drivers don't care what the load is, only how much current you're applying to the motor. Too much current and mator/driver get super hot. Too little and the torque will be too low. Overload on the motor will not cause the driver or motor to work any harder- the motor will just slip.
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