Newbie- stepper motor torque

[CaptKidd]

I hope this will be my last question with the "gosh that's so stupid" tone to it.

I'm looking at stepper motors for my home design. I just figured out that NEMA is more of a size for mounting than any other indicator of motor parameters. Although, I guess the larger the number, maybe the more powerful motor, but not always.

OK, to the real question. I'm fairly certain that torque requirements are related to any individual machine. Since mine will be unique to the world, and I've never owned one of these things, can someone give me an idea of what the minimums might be for a non-optimized-designed CNC PCB 3 axis mill? I've been all over the place trying to see what commercial machines use just for reference, and it looks as though something in the 300+ oz-in range would be a starting point. I'm going to design a moving gantry machine (stationary table) and the spindle will more than likely start out as a Dremel or tile routing machine. So I'm guessing the gantry will need the most torque, followed by the Y axis Dremel holder, and then the Z axis Dremel height. The motor will more than likely be connected directly to an Acme screw type drive, but I find a few Ebay motors with a gear already mounted, so should those be a consideration and what are the limits of gear ratio if I should purchase one of those? I'm guessing that the torque drops or rises based on the gear ratio at the expense of speed.

And lastly, is holding torque the parameter that I should be considering as the main criteria. I've been trying to get a handle on all those torque and inertia definitions, but have to admit, I've not put the time into "study hall" as much as I should have.

Again, any help and patience would be greatly appreciated as I plow through this learning curve.

[RotarySMP]

The loads on a PCB mill are small, but you want speed. A 300inlb Nema 23 is probably going to provide more torque that you ever use due to the limited loads, but be slower on rapids.

I'd use low inductance Nema 23's like Kelings KL23H256-21-8B with a Gecko 540.

To do the motor sizing properly, you need to calculate the cutting force require by your most severe job, then factor in table friction, then work back through the leadscrew and drive reduction pulley (if any), factor in leadsrew/pulley friction, and you will then know what your maximum required torque is. Multiple it by a factor of safety, and you know what motor you need.

[stevespo]

Yes, your needs for the PCB mill should be fairly modest. The configuration mentioned above seems pretty ideal. The 2.4mH inductance would indicate a 48VDC power supply, and provide maximum speed/torque for those motors.

Assuming 10 TPI acme screws, I'd imagine you could manage 1000 RPM, perhaps 1200+, so rapid speeds of 100 IPM which would be great for a small machine.

Steve

[irving2008]

Originally Posted by CaptKidd I hope this will be my last question with the "gosh that's so stupid" tone to it. I'm looking at stepper motors for my home design. I just figured out that NEMA is more of a size for mounting than any other indicator of motor parameters. Although, I guess the larger the number, maybe the more powerful motor, but not always. OK, to the real question. I'm fairly certain that torque requirements are related to any individual machine. Since mine will be unique to the world, and I've never owned one of these things, can someone give me an idea of what the minimums might be for a non-optimized-designed CNC PCB 3 axis mill? I've been all over the place trying to see what commercial machines use just for reference, and it looks as though something in the 300+ oz-in range would be a starting point. I'm going to design a moving gantry machine (stationary table) and the spindle will more than likely start out as a Dremel or tile routing machine. So I'm guessing the gantry will need the most torque, followed by the Y axis Dremel holder, and then the Z axis Dremel height. The motor will more than likely be connected directly to an Acme screw type drive, but I find a few Ebay motors with a gear already mounted, so should those be a consideration and what are the limits of gear ratio if I should purchase one of those? I'm guessing that the torque drops or rises based on the gear ratio at the expense of speed. And lastly, is holding torque the parameter that I should be considering as the main criteria. I've been trying to get a handle on all those torque and inertia definitions, but have to admit, I've not put the time into "study hall" as much as I should have. Again, any help and patience would be greatly appreciated as I plow through this learning curve.

HI, this is one of the most common questions and the least effectively answered... so its isnt a stupid one by any means.

I wrote a tutorial on this subject for the UK CNC forum. You may find it useful.

[CaptKidd]

Great to you all. Let me go over these a little to clear some confusion.

RotarySMP, you listed the motor as 300 inch-pound as a recommendation, but when I viewed the specs at one site listing the motor you suggested, using the site's torque curve and rated at 3Amps and 30Volts, the maximum torque was 0.9 Newton-meters. The conversion I used converted this to 7.9656711909 pound inch. Am I using the right parameter? Where does this rating come from in the datasheets?

stevespo,

I'm getting a better feel for this. Ratings are so misleading sometimes.

irving2008,

Now that's a tutorial!! Putting it all in a nice step-by-step solution makes it a lot easier to not get lost in the individual calculations.

Guess the best thing to do is build the mill, and take all of the measurements. Then start figuring out what's needed based on what I have built. You guys can poo-poo this idea up front, but I'm thinking of building the frame and gantry out of foam and fiberglass. I think that by using proper I-beam type construction, I can make this stiff enough to avoid distortion. If anyone has tried this before, I'd like to hear about your experiences.

And irving2008, is there anyway I can get that spreadsheet without signing up on another website? It just seems like I'm signed up everywhere for this and that. I guess one more wouldn't hurt, but seems like a lot of overhead for the site to manage if it's not necessary.

Thanks all for everything.

[judleroy]

Your convertion is correct. I believe he intended to say 300 oz-in not in-lb. 300in-lb would be a large industrial motor and very very expensive. Also way bigger then you need.
Judleroy

[irving2008]

Originally Posted by CaptKidd ... And irving2008, is there anyway I can get that spreadsheet without signing up on another website? It just seems like I'm signed up everywhere for this and that. I guess one more wouldn't hurt, but seems like a lot of overhead for the site to manage if it's not necessary. Thanks all for everything.

Attached it to my post above...

[CaptKidd]

Thanks for the file irving2008.

I appreciate the reply judleroy. Firstly to affirm I'm sorta going in the right direction and secondly for anyone that passes by this thread later as a rookie for reference. Simple mistake, I'm sure, as I do it all the time.

[RotarySMP]

Sorry in-oz as Judelroy pointed out.

[MadMachines]

Quote..CaptKid,
I'm thinking of building the frame and gantry out of foam and fiberglass.

I am interested in your idea, I have seen craft-board and fiberglass used successfully before on a foam cutting mill, a pcb mill should have similar light loads, perhaps a flexi shaft on a dremel might cut down some of the weight, keep us up to date with your idea. I'm interested.

[RotarySMP]

You want mass, think newtons third law. If you are accelerating a dremel around quickly, and those forces are reacted by a super light frame, your frame is going to jump about.

[CaptKidd]

Originally Posted by RotarySMP You want mass, think newtons third law. If you are accelerating a dremel around quickly, and those forces are reacted by a super light frame, your frame is going to jump about.

What about mounting the frame to a bench top? I think I can make the frame components fairly rigid.