### Author Topic: Stepper torque for linear actuation  (Read 4306 times)

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#### newInRobotics

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##### Stepper torque for linear actuation
« on: February 25, 2012, 02:18:44 PM »
Hello everybody,

hope everyone is all right

In the CNC I'm building, linear actuation on each axis will be achieved by threaded rod spun by stepper motor. I have managed to find only one article (Torque and Linear Motion Formula) showing how to calculate torque required:

torque = ((weight * distancePerRevolution) / (2 * pi)) * efficiency

Using this formula and ignoring efficiency that's what I get:

weight = 2121 grams
distancePerRevolution = 1.25mm
pi = 3.14

torque = (2121 * 1.25) / (2 * 3.14) = ~422.17gf.mm = 0.004140073N.m

That is ridiculously low! Am I missing something? Anyone who knows anything regarding this matter?
"Computer games don't affect kids; I mean if Pac-Man affected us as kids, we'd all be running around in darkened rooms, munching magic pills and listening to repetitive electronic music." - Kristian W

#### rbtying

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##### Re: Stepper torque for linear actuation
« Reply #1 on: February 25, 2012, 04:29:09 PM »
You need to account for the force needed to actually machine the part, not just the force needed to physically move the head back and forth. Also, efficiency is typically very low in a case like this, especially because of excess friction in the rod.

#### newInRobotics

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##### Re: Stepper torque for linear actuation
« Reply #2 on: February 25, 2012, 08:16:21 PM »
You need to account for the force needed to actually machine the part, not just the force needed to physically move the head back and forth.
Anyone's aware of formulas/methods to calculate force required to machine the part? Or maybe a rule of thumb to make a good (safe) guess for torque required?
« Last Edit: February 26, 2012, 05:51:09 AM by newInRobotics »
"Computer games don't affect kids; I mean if Pac-Man affected us as kids, we'd all be running around in darkened rooms, munching magic pills and listening to repetitive electronic music." - Kristian W

#### Resilient

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##### Re: Stepper torque for linear actuation
« Reply #3 on: February 26, 2012, 01:48:45 AM »
Well, I don't have any fancy maths for you... but I bought this motor:
http://www.sparkfun.com/products/10846

with this controller:
http://www.sparkfun.com/products/10267

for a CnC machine that sounds like yours and I think I will be alright. I am going to troubleshoot some more tomorrow because it is still snagging occasionally but that is mainly due to my shitty building more than the motor.

#### Soeren

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##### Re: Stepper torque for linear actuation
« Reply #4 on: February 26, 2012, 12:12:19 PM »
Hi,

You need to account for the force needed to actually machine the part, not just the force needed to physically move the head back and forth.
Hopefully not very much, or you'd need to sharpen/replace the router bits
Regards,
SÃ¸ren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Engineering is based on numbers - not adjectives

#### newInRobotics

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##### Re: Stepper torque for linear actuation
« Reply #5 on: February 26, 2012, 01:38:24 PM »
for a CnC machine that sounds like yours and I think I will be alright.
I'm not sure what Your's looks/operates like, but on mine table is actuated separately (on X-axis) and then tool is moved on Y-axis and Z-axis on different frame.

When picking motors, did You use any rule of thumb, or just had a lucky guess?

If I go with 4x torque than what I have calculated, would it be safe choice? Really have no idea what to aim for as I have never worked with CNCs before.
"Computer games don't affect kids; I mean if Pac-Man affected us as kids, we'd all be running around in darkened rooms, munching magic pills and listening to repetitive electronic music." - Kristian W

#### Resilient

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##### Re: Stepper torque for linear actuation
« Reply #6 on: February 26, 2012, 02:14:12 PM »
for a CnC machine that sounds like yours and I think I will be alright.
I'm not sure what Your's looks/operates like, but on mine table is actuated separately (on X-axis) and then tool is moved on Y-axis and Z-axis on different frame.

When picking motors, did You use any rule of thumb, or just had a lucky guess?

If I go with 4x torque than what I have calculated, would it be safe choice? Really have no idea what to aim for as I have never worked with CNCs before.

So on mine, there is an X and Y stage and the tool moves in the Z direction (so the piece being cut moves in X and Y and the tool can move in Z). I read up on CnC tutorials on Instructables and went with a similar motor to what I saw there.

#### Resilient

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##### Re: Stepper torque for linear actuation
« Reply #7 on: February 27, 2012, 07:50:00 PM »
Do you have you eye on any software?

I am working on getting my CnC machine working with my Axon and have been trying to use Grbl to do so... but it seems to be causing my motors to... how shall I put this... freak the F out. So I am looking for alternatives that I could download onto the Axon.

When they are turning too slow or too quick, they sort of just chatter and do not move. I have tried changing the driver from 8 micro step to the standard 2 step with no luck. When I just set it up with the default Webbotlib motor control setup it seems to work quite well. So I think something in the Grbl software is bad... but I don't know nearly enough about stepper motors to debug it.

#### newInRobotics

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##### Re: Stepper torque for linear actuation
« Reply #8 on: February 28, 2012, 01:18:19 AM »
Do you have you eye on any software?
I'm planning to write one myself
"Computer games don't affect kids; I mean if Pac-Man affected us as kids, we'd all be running around in darkened rooms, munching magic pills and listening to repetitive electronic music." - Kristian W

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