I got this stepper, which matched the Solidoodle parts list exactly for NEMA/angle/voltage/current/resistance/inductance (but it's a bit shorter so the screws stick thru).  It was called 1.8 Degree 42mm NEMA17 2 Phanse Hybird Stepper Motor SC42STH33-1334A.  It worked out well that this was shorter, it leaves a bit more space for the coupler on the shaft without reducing the print-bed travel (the coupler takes up space where the stock motor is threaded right to the body).

For the connector, I soldered on a 4-pin header similar to what a computer's motherboard uses -- think old CD-ROM or USB plug.  I put the wires in the same order as the old connector (by color) and then painted a "Z" like the factory since my connector isn't keyed I know it faces out.

I got 5/16-18 threaded rod from Home Depot, they had 12 inch pieces for just over $1 and picked 2 that looked perfect as near as I could tell rolling on a piece of ceramic tile from the flooring department (presumed to be truly flat).  The stuff at Home Depot was far superior to Lowe's (most of which were 2-3 foot and bent or damaged threads) and sold in smaller pieces as well.  I got a 5/16 "coupler nut" (about 1.5" long) and ran it up and down the threaded rod a few times to ensure smooth even movement along the threads (especially where I cut it), and then used it to run up and down spreading lithium grease evenly before putting it in the printer.  I still have the second rod I got as a spare.

For the motor-to-rod coupler, I ordered one of the "set screw" type from Amazon and realized it was what I was told not to do after it came.  I started to use it but then had a mishap and dropped the motor+rod assembly (doh!) on the floor and bent the aluminum coupler fairly badly (but rod and motor were fine!).  It was a PITA getting it off the motor due to a burr in the shaft where the set screw was.

I'm now ordering a new coupler though I was able to straighten it enough to determine this fixed my printer -- I'm going again and no more banding!

Thanks everyone!

I have had great experience with OMC-Stepperonline.com
After a couple of minutes, I found this http://www.omc-stepperonline.com/nema-1 … -p-13.html This virtually mirrors the specs on the stock SD motor and it is cheap. Personally, I would purchase a few extra motors should you like to experiment or be prepared for another potential failed motor.

They ship from China, but I haven't had to wait longer than a week.

+1 for this seller, I have bought several motors from them.  They also have an ebay store which ships from the US and will get it to you very fast and cheap shipping: http://stores.ebay.com/StepperOnline?_t … 7675.l2563

mdrVB6 wrote:

For your new rod, if you want to ensure that you get a high quality, very straight rod, I recommend Misumi-USA for either a 5/16 or M5 rod.  It will be more expensive than other places, but you can order it cut to length and it will be high quality.  Other options include ordering from Folger tech, they sell M5 rods and I'm not sure about other sizes.  They also sell quality parts but I don't think they cut to length.

I'm having difficulty using the Misumi-USA site...everything looks metric?  I do want to go with a 5/16, everything there looks metric.

I do see some nice looking stainless-steel threaded 5/16-18 rods on Amazon, would that be suitable?

I will confess I did not actually look for that item on Misumi, just assumed they had it because they have a very large selection.  If they don't have it then one off amazon is likely fine.

For your new rod, if you want to ensure that you get a high quality, very straight rod, I recommend Misumi-USA for either a 5/16 or M5 rod.  It will be more expensive than other places, but you can order it cut to length and it will be high quality.  Other options include ordering from Folger tech, they sell M5 rods and I'm not sure about other sizes.  They also sell quality parts but I don't think they cut to length.

I'm having difficulty using the Misumi-USA site...everything looks metric?  I do want to go with a 5/16, everything there looks metric.

I do see some nice looking stainless-steel threaded 5/16-18 rods on Amazon, would that be suitable?

They ship from China, but I haven't had to wait longer than a week.

That stepper linked on Amazon is not a 12v stepper, look at the datasheet in the photos. It is a nearly identical replacement for the existing motor.

Oh.  I totally didn't see the datasheet in the photos, I had been looking for one by the number (and not having much luck).  That makes a lot more sense now.  I'd only seen the couple specs in a customer question.  Oops.

I guess that only leaves the one question about which material to look for in a threaded rod (if I have a choice)?  Should I look for a stainless-steel 5/16" threaded rod if available, or just a plain zinc-plated-steel one?

The motor can't pull too much power, the stepper drivers are current limited.

There's 3 things at play here:

Stepper voltage - the maximum voltage you can supply to the motor with a non-current limiting driver.
Stepper current - the maximum current that can pass through the motor without overheating it.
Reference voltage (Vref) a reference for the stepper driver to set the current limit, for example a 0.5Vref might mean 1 amp of stepper current

Driving a stepper with a higher voltage than it is rated for improves torque at medium and high speeds. Doing so is safe is the stepper driver is current limiting. If you drive a 12v stepper with 12v, you'll have less torque at speed than you would driving a 2.8v stepper with 12v. Has to do with inductance, too much to explain here.

So considering the bed is essentially a "worm gear" type ratio, I would imagine torque on the Z-axis isn't as important then...so if I understand correctly even the 12V will still run at "the right speed" to function properly and replace the 2.8v one?

There's 3 things at play here:

Stepper voltage - the maximum voltage you can supply to the motor with a non-current limiting driver.
Stepper current - the maximum current that can pass through the motor without overheating it.
Reference voltage (Vref) a reference for the stepper driver to set the current limit, for example a 0.5Vref might mean 1 amp of stepper current

Driving a stepper with a higher voltage than it is rated for improves torque at medium and high speeds. Doing so is safe is the stepper driver is current limiting. If you drive a 12v stepper with 12v, you'll have less torque at speed than you would driving a 2.8v stepper with 12v. Has to do with inductance, too much to explain here.

I will look into those leadscrews/rods, I am content with the 5/16 rod (and since I half-unscrewed the old stepper motor from the frame so it can flop around, all banding and Z-artifacts have gone away completely).  It sounds like there may be a few options, though I think I'll have to order one as my experience has shown most at my local Lowe's and Home Depot are very inconstant and bent or damaged in various ways.  I've read running a thread die down it may help if it's just rough spots on the threads so I may snag one of those too.

Being unaware of the types of couplers apparently I got the wrong kind (one with double set-screw) but I can return it later, that's the easiest part to swap out.

Is steel (stainless or plain/zinc-plated?) the best material to look for when buying threaded rod for the leadscrew?

I'm very new to understanding differences in stepper motors, so please bear with me.

That one seems to say 12V in the specs but when I search the model number printed on my Solidoodle motor in my printer, the datasheet says 2.8 V (and I recall the calibrating-drivers you use the 2V scale on the meter).

I had similar confusion when I tried to search Amazon -- not understanding what most of the specs mean I was trying to match every single thing to the datasheet I found for the SM42HT33-1334A.

EDIT: I gather the proper one is something to this effect from the datasheet above:
-NEMA 17 (mounting hole location)
-1.8 degree or 200 step (ticks per rotation)
-"rated voltage" of 2.8 (controller or power?  no idea.)
-current 1.33A (how much power it can draw?)
-resistance 2.1 ohm (related to current/voltage?)
-inductance 2.5 mH (no idea but sounds important)

I'm terrified of frying the old-style Sanguinololu mainboard (esp not being able to buy it anymore?) if I plug in a motor that pulls too much power.  I'm fairly comfortable with electronics but I'm also used to only worrying about the inconvenience of buying a replacement part if I break it -- I don't normally work on stuff that is unavailable to get replacements.
EDIT2: Actually it looks like China has plenty of the Sanguinololu boards available, and I have an ISP programmer so maybe it's not that serious after all.  I still want to understand what I need to buy and why, for learning sake if nothing else.

Can someone help me understand what this means and why it does/doesn't matter with these stepper specs?

edit: bolded questions for readability/clarity

... just make sure you get one that pinches the rod instead of holds it with a set screw

Thank you! I forgot to mention this. It is absolutely mission critical that you get this right or you will have no end of problems.

http://www.soliforum.com/topic/12500/qu … ng-zscrew/

For your new rod, if you want to ensure that you get a high quality, very straight rod, I recommend Misumi-USA for either a 5/16 or M5 rod.  It will be more expensive than other places, but you can order it cut to length and it will be high quality.  Other options include ordering from Folger tech, they sell M5 rods and I'm not sure about other sizes.  They also sell quality parts but I don't think they cut to length.

A quality 5x5mm coupling will not induce wobble, just make sure you get one that pinches the rod instead of holds it with a set screw, as mentioned in post #3 in the above link.  Make sure the ends of the motor shaft and the rod touch inside the coupling.  Good luck.

You can find cheap stepper motors on Amazon and EMCStepperOnline. One like this will work just fine:
http://www.amazon.com/ZJchao-57oz--Step … ds=nema+17

You will then need a coupler. For this application you should use a flex coupler, like this one. Here is a 5 to 8 mm coupler which will work for coupling the 5/16" rod to the 5mm shaft:
http://www.amazon.com/5x8mm-Motor-Coupl … le+coupler

When you install the coupler, pre-load it by pulling down before tightening it on the motor shaft. This way the rod can't "bounce" and cause skipping. Hope this helps.