Pitch on leadscrews from robotdigg

I am looking for ways to improve the accuracy of my Z axis. One of the things that has plagues me from the beginning is making sure the Z rod is exactly concentric with the motor shaft. I have used various forms of couplers in the past, but none has been totally satisfactory. It's not really noticable if you move fromone layer to the next, but if I move the axis quickly over large distabces an observable wobbly is noticable at the coupler.

I'm intrigued by these stepper motors from robotdigg that have the leadscrew permamently attached - actually from the photographs, it looks like the leadscrew actually goes into the body of the motor, so it might be incorrect to say they are "attached" - it looks as if the leadscrew IS the shaft.

In any event, what concerns me if the stated pitch of the screw. It's specified as Tr8*8, which I believe means 8mm diameter and 8mm/revolution. That seems high for our purposes here. Using the calculator, I come up with 400 steps/mm which is significantly less that my current screws. Should I be concerned about this, or is this pitch not too high?

Yes, in the case of those motors, the lead screw, and motor shaft are the same shaft. This should work well, as long as the screws are indeed straight. I've been meaning to try a couple of them myself.

What layer heights do you think you would print at? At 400 steps/mm, this is a resolution of 0.0025mm, which is still much higher than you could ever use, considering a typical layer height is not less than 0.05mm. It is however questionable as to whether or not that resolution is actually "real" since you are relying on microstepping to achieve it.

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Mike Anton
[manton.ca]
[laserlight.wikidot.com]

I can weigh in here to say that the pair of nema 17 motors with captive leadscrews that I purchased from Robotdigg are working out very nicely. The are concentric and dead-straight. I use them, with their anti-backlash nuts, on a Lulzbot Taz clone and have had great success with layer heights down to 0.1mm, but have never had occasion to try going lower. Should I build another printer with leadscrews, I would definitely use these again.

Thanks guys. I'm going to get a pair to see how well they work out. I'll let you know.

These are ACMEs, right? So, can they be used in a metric based system? I've read that their Z height wasn't a whole number so there might be some Z height calibration problems.

They are specified with pitch Tr8*8, which supposedly is diameter 8mm, lead 8mm (8mm travel/revolution).

I know people on this board get very passionate about acme versus trapezoidal, and I understand there might be some patent or copyright implications, but to me it's just a name. As long as I can verify that the pitch is exact I don't care what you call them.

They are likely a 4 start 2mm pitch screw, as others in the series are specified as such. This gives you 8mm of travel per revolution. I doubt that they are acme, even though that is what they call them.

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Mike Anton
[manton.ca]
[laserlight.wikidot.com]

Yes - I've posted a question to them about why the call them acme yet use trapezoidal pitch specs. I'll let you know what they say, if anything.

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jbernardis
Yes - I've posted a question to them about why the call them acme yet use trapezoidal pitch specs. I'll let you know what they say, if anything.

I was also wondering what their answer is as I also plan on buying some for my setup. I haven't been able to find any other manufacturer who can produce them on specification ( I need 380 mm long).

Edited 1 time(s). Last edit at 07/08/2014 12:59AM by drmaestro.

I asked why they are using trapezoidal specs on what they claim are ACME screws. I further asked if the specs were the manufactured specs or if they were truly Imperial screws and the specs given were approximations, or measured.

The reply I got back was:

Hi Jeff,

Your question about leadscrew is contradictory.
What I can tell you is that the integrated stepper motor with leadscrew is exact Tr8*8 which 2mm pitch 4 starts high quality stainless steel made.


I guess that means the specs are the manufactured specs?

Acme threads are trapezoidal shaped screws as designated by American National Standards Institute (ANSI) that uses imperial units. Trapezoidal metric threads are defined by the International Standards Institute (ISO) that uses the metric system. So there should be no debate on the issue the designation is done by definition. You cannot have an ACME thread with a TR*.* designation or vice versa. Hope this helps.

Edited 1 time(s). Last edit at 03/01/2016 11:16PM by 3DLTech.

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3DLTech
Acme threads are trapezoidal shaped screws as designated by American National Standards Institute (ANSI) that uses imperial units. Trapezoidal metric threads are defined by the International Standards Institute (ISO) that uses the metric system. So there should be no debate on the issue the designation is done by definition. You cannot have an ACME thread with a TR*.* designation or vice versa. Hope this helps.

The thread forms for trapezoidal and acme threads are similar but trapezoidal is 30 degrees and acme is 29, so they can't be used as direct replacements (unless the nut is replaced at the same time) .