Stepper selection, torque and inductance

[richard-at-home]

I know that is may not be in the correct section, but

Having read this long build log, https://www.usinages.com/threads/cnc-mecanosoudee-type-portique-mobile-pour-laluminium.63531/page-35


(Which is in french by the way)

The builder, rider63, is initially using 34HS6850 2.2Nm nema34 steppers to move a very chunky gantry router. The z is at least 35kg, the gantry, at least 75kg. (it's welded steel plate filled with resin)

There are videos showing it cutting aluminium and steel at pretty fast feeds.

Later on, he does switch to 400w servos, but he selected the steppers to get low inductance, in order to get fast feed speeds. Torque didn't seem to be too important to him.

He also used drivers that allow the drive current to drop when the motor is stationary, to reduce heating.

Is there a tool/formula for estimating required stepper torque for a given mass and acceleration requirement?

Cheers!

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[ger21]

Is there a tool/formula for estimating required stepper torque for a given mass and acceleration requirement?

Yes, most motor manufactueres have motor sizing programs.

But with steppers, it gets a bit tricky, as steppers lose torque, the faster they spin. So you need to know how fast it will be spinning at max speed, and then figure out how much torque it will have at that speed. That depends on power supply foltage, microstepping, and the specific drive you are using. In an industrial environment, they'd probably have about 50% more torque in reserve, to prevent lost steps.

[NIC 77]

Is there a tool/formula for estimating required stepper torque for a given mass and acceleration requirement?

Cheers!

http://www.cnczone.com/forums/linear...118-cnc-2.html

There is a sample calculation in post #21 and links throughout with equation sources.

I haven't found a suitable computer program that gives relevant results for a CNC machine application. Not saying it doesn't exist.

I've also made some graphs for other people (ballscrews) like here:

http://www.cnczone.com/forums/diy-cn...45126-cnc.html

post #27

http://www.cnczone.com/forums/vertic...m-posts-2.html

post #12

And R&P here:

http://www.cnczone.com/forums/cnc-wo...-forum-12.html

post # 247 & 248

post # 246 has a sample calculation

[NIC 77]

And as Ger21 mentioned, mentioned, max speed is where the torque is important.

This is because in software programs like mach 3, you can only input a single value for acceleration, and that is used across the entire speed range. So you can have more available force for cutting at lower speeds, but you can't use this for better acceleration at lower speeds unless you limit your maximum speed to a lower value.

There are some programs where you can have a different input for acceleration for rapids, for two in total.

[richard-at-home]

Thanks for the links. I'll keep reading!

I found it quite hard to find low inductance steppers with a torque figure over 2Nm. But this one is was listed on a UK site:

SY85STH65-5904B Nema 34 Stepper motor


It's a Chinese made motor from CHANGZHOU SONGYANG MACHINERY & ELECTRONICS. It has 3.4Nm and 1.7mH.

Not sure if there are any downsides to this motor though?

I guess I might get similar a lot cheaper via aliexpress?


Assuming I use 20mm rails and 1610 ballscrews for x&y and 1605 for z, for a moving gantry machine of approx 1500,750,500mm built from steel box section, with a 2.2kw spindle for cutting ali, plastics, some wood and maybe a little steel now and again... (just spec's plucked from the air at the moment, but you've got to start somewhere!)

Once I get something drawn up, I'll try and get some idea of mass worked out.