which stepper motor

I am currently looking for which stepper motor to get for my reprap. My problem is that apart from knowing it needs to be a NEMA 14 or 17 or 23 (I am choosing nema 17) not much other information can be found. for example would this be ok stepper 6 wires even with 6 wires?
or should it be something like
also 6 wire
or 4 wire
because it seems i am able to get the 6 wire slightly cheaper then the 4 wire ones like the one from maker bot. Also does the size matter the last 4 wire stepper is a lot larger then the others and am wondering would weight be a problem. i understand it probably would for the extruder.

I'm not qualified to say whether or not the specific motors would work well for building a RepRap as there are many things to consider, but if your driver board will work with a 4 wire stepper it will also work with a 6 or 8 wire stepper (assuming the driver can deliver sufficient current). The reverse is not necessarily true.

A 4 wire motor only works with a bipolar driver board. 6 and 8 wire motors can be wired to work with bipolar or unipolar driver boards.

Did you check the Wiki ? There is a whole page on steppers. It is written for Darwin not Mendel - it lists 100 oz-in torque for example, need more like 20 oz-in for Mendel - but it has good information on wiring and such.
There is another Mendel-specific page with requirements along with links to specific motors+suppliers and if they're tested or not.

Basically what you want is a
- bipolar (4 wire, or 8)
- 1.8 degree or 200 steps (minimum)
- roughly 14 N-cm torque (minimum)
- 5mm shaft.

Many NEMA 17's will fit this bill. NEMA 14 generally will not be strong enough or if they are, run hot because they're being driven at a higher duty cycle. NEMA 23 will be overkill and I'd be a bit concerned over the extra weight.

There are some other factors to consider such as phase resistance and therefore how much current and voltage the motor can handle. There may be some odd low-voltage motor or something that would burn up or draw too much current, but most of the commonly-available steppers will be fine.

Depending on which extruder design / motor pulleys you will be using, it can be advantageous to have various shaft geometries - single flat, double flat, splined, etc.

[reprap.org] - Darwin stepper motor page, has wiring information

[reprap.org] - Mendel stepper motor page, lists a bunch of known-working motors and links to suppliers

FWIW: I think we need to either merge these pages, or make them properly distinct.

If they stay separate, we definitely need to get all the units sorted out. I'll have a go at it when I can.

4 wire motors are bipolar (which work)
5 wire motors are only unipolar (which won't work)
6 wire motors can be wired in bipolar series (which works just fine) or unipolar (which wont work)
8 wire motors can be wired in bipolar series (which works just fine) bipolar parallel (which will work but the amp draw requirement is high, make sure it would be less then 2 amps, otherwise wire in bipolar series) and unipolar (which won't work)

is it me or is there not much out there for unipolar stepper drivers, i have only seen one so fare but then again i am not really looking for them too.

i do wish there was more information out there has to what kinda torque you need on your stepper if you want to use a geared extruder

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[mike-mack.blogspot.com]

Actually there is a lot on unipolar steppers. These are really easy to bit bang. No H-Bridge is used, so fewer failure modes. Most drivers just use a current limiting resistor on the supply.

As far as I know, micro-stepping and current control does not work as well in unipolar mode. I think one can half step. The drivers I have seem to exhibit a resonance point which limits the useful speed.

A unipolar driver can be made with about 3 TTL/HC chips and 4 transistors that switch to ground. My piano roll scanner works with a simple Unipolar board.


-julie

i dont doubt you a bit, i just have not seen many on the sites i go to, but seeing how simple they are i am surprised i have not seen many controllers for them thats all.

i will have to look into them a little more,

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[mike-mack.blogspot.com]

Unipolar motors only have 70% the torque of the same sized bipolar, so once you get above a certain price motor the total cost becomes less to have the more expensive electronics. Tin can permanent magnet motors tend to be unipolar. Hybrid motors like the ones ones used for RepRap tend to be bipolar.

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[www.hydraraptor.blogspot.com]

For 6 wire steppers used on bipolar mode, is it better to use half-winding configuration (one end and the center tap) for each winding or the full winding? Apparently using the full winding means less current, more inductance and less high speed torque...

If you use half the winding the motor has the same performance as it does when operated by a unipolar drive.

If you use the whole winding you have to drop the current by sqrt(2) so that the total power is the same. I.e. power is I^2 R and you have double R so have to reduce I by 70%. Torque is proportional to current times turns so you get 40% more torque. Power is the same so your motor has become 40% more efficient as bipolar motors are.

Inductance is proportional to turns squared so will be four times bigger causing torque to fall off with speed 4 times faster.

So for slow moving high torque (extruder conditions) full coil might be better but for fast movement half coil is better.

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[www.hydraraptor.blogspot.com]

for a simple unipolar driver check out the stepgenie chip

[www.stepgenie.com]

i've used them and they are fantastic. no overheating what-so-ever. well obvisouly because the mosfets are running way below what they are capable of.

however only half step mode at most, i've used current limiting resistors with good success but getting really high speed will be an issue without a 'chopper drive'

here is another site from a guy that frequents cnczone.com

[pminmo.com]

some cool circuits that will peak someones interest.

that step genie lookes to be a good idea, almost in line with the reprap project, almost what it needs for stepper drivers too.

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[mike-mack.blogspot.com]