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Rostock with nema 14 possible?

Posted by nikbjo 
Rostock with nema 14 possible?
July 22, 2013 05:58AM
Has anyone tried building a rostock (or derivate) using nema 14 steppers? I have 3 spare ones from a huxley, and it would be nice it it would be possible to reuse them.
Re: Rostock with nema 14 possible?
July 22, 2013 10:45AM
I haven't tried it but I know it would be possible. The NEMA14's I looked at had 1/3rd the torque of a NEMA17. (Note: NEMA14's come in all shapes and sizes and so do NEMA17's. The NEMA17's we use are on the top end of the spectrum so you will have to get a NEMA14 from the top end of the spectrum to get 1/3rd the torque.) That means to accomplish the same acceleration you will have to modify the mechanical advantage (MA) by 3.

Ways to increase you MA...
1) Instead of attaching to the carage directly, loop over a idler and then attach to the frame. This will double the mechanical advantage which will probably be enough. [www.thingiverse.com]
2) Decrease the size of your drive pulley.

It is not necessary to increase the MA by a factor of 3. It is a balance between top speed and top acceleration. If you replace NEMA17's with NEMA14's and change your MA by 3 then you will get all the same performance during normal printing except you top speed will be reduce by a factor of 3. You may want to keep some of that top speed but sacrifice some acceleration by picking a smaller MA.

I was hoping to use NEMA14's for Wally or Simpson but they are slightly more expensive for me. For a Rostock, weight isn't really an issue so I would discourage going that route unless you already have some on hand or you found a cheap source. If you found a cheap source for some NEMA14's with decent torque please share. :-)
Re: Rostock with nema 14 possible?
July 23, 2013 03:41AM
I already has some nema 14 motors (rated as 1400 g-cm (20 oz-in) when driven with 1000 mA / 2.7V) so they are much cheaper than anything else...
Many of the nema 17 i have seen are speced for higher amps but most ppl seems to run them at 1A (with pololu) so the torque would most likely be lower than the specified.
Re: Rostock with nema 14 possible?
July 23, 2013 10:56AM
I don't know about everyone else but I run my steppers at 90% current. This gives you almost all the power and they run cool.

Note: You can run your NEMA14s at 1A unless you are mounting them to plastic. In that case, you need to drop that to around .9A so the steppers will stay cool.

Good luck. I would like to see more people use NEMA14s. It will eventually make them cheaper.
Re: Rostock with nema 14 possible?
July 23, 2013 02:59PM
You probably run at 90% of 70% of the motors rating. The rating is for 2 coils on and 80C temperature rise. The Allegro datasheet current calculation for Vref is the one coil on value. The two coil value is 70% of that. So if you run a "1A" motor at "0.9A" you get 63% of the torque and 40% of the 80C temperature rise, I.e. about 32C making it just about comfortable to touch.

NEMA17s give more torque per amp but they are limited by the drivers. I worked out I can get about half the torque with a NEMA14 by running it hot.

People that use 2.5A NEMA17s get less torque than the recommended 1.7A motors. They do run cooler though. Not really an advantage unless you use PLA brackets.

Re: Rostock with nema 14 possible?
July 23, 2013 03:13PM
Great tip! I actually tweak the power until I get the highest steady state temp I will accept. I do mount in PLA so this is important. I will have to do some calculations to see what I am actually running my steppers at.
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