size of stepper motors needed

[Wyndham]

I'm brand new to the CNC world but have been studying different machines here and other sites. I'm starting to learn some from just reading the post but have a ways to go.
I most likely will try and build a small cnc with router in the future from an available kit. I may try the Zen 12x12 as what i need is a small router for some lithophanes masters I wish to make.
My general question( if this is the right sub forum) is about the size of the stepper motors vs the size of the machine it fits on.
The Zen has a stepper motor kit with I believe Nema 13 motors and drivers w/ps.
Is there a rule of thumb to approximate the size of the motor for the size of the machine you want and match it to the driver and ps you want to build.
In this case I'll ask what motors would be on a woodworking cnc router table that could make a sign 1ftx2ft and be reasonably powered.
I'm only trying to get a rough idea so I can visualize when I see 20 different Nema motors and amps as well as PS's and driver boards what these numbers mean in a real working envirnment.
This is something i don't know anything about.
Thanks for any comments.
Wyndham

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[louieatienza]

You would need to figure out how much each axis weighs, how much force is needed to "push" each one, the speed you need to drivev the axis at cutting speed, and the drive ratio of each axis, plus some headroom (maybe about 10-15%) From there you should be able to deternine the "torque" you need to drive each axis. The torque ratings of different motor frame sizes can overlap. There are a few useful articles on the geckodrive.com website that you should read to get an idea of motor sizing as well as power supply sizing.

[Wyndham]

Thanks for the reply. I basically understand what you are talking about. I think it most likely be more useful(till I learn the math) to look at the specs on different machines and compare the hardware till I get a better understanding of the math as well as read comments from folks that have the machines.
Wyndham

[r00t4rd3d]

Most people use Nema 23's. The higher the oz. in. rating the better off you will be. The power supply you will need is dependent on the steppers you choose as they all have different ratings. My machine is 36"x16" and I use 3 Nema 23's that are 125oz inch and feed them with 18v. My oz inch rating is pretty crappy too. Most people go with 200-300+. Also, if you just go with a 12x12 machine and you get into CNCing, you will want a larger machine eventually so picking a little larger steppers then you normally would need is not a bad thing, you can reuse them on a bigger machine.

[Al_The_Man]

Re NEMA 25.
Pacific Scientific have a NEMA25 line that goes from 45 oz-in to 253 oz-in, just the stack length differs.
Al.

[louieatienza]

Generally, the finer the lead on your leadscrew, the smaller the stepper you can get away with, while a higher lead screw may require a larger stepper. A smaller stepper may drop off at a higher RPM than a larger stepper, and a smaller stepper can usually achieve a higher RPM.

There is a certain tradeoff between speed and resolution with steppers, since their torque drops as their speed increases. So another thing to consider is the mechanical resolution you require from your machine, whether you can hold those same tolerances with the materials and tools at hand as well.

While it may seem to reason that a smaller stepper and finer pitch leadscrew may be a viable option, keep in mind that, unless you're using a ballscrew, part of the motor's output will be overcoming the inefficiency of the finer pitch screw, the stepper will have to spin faster to achieve the same linear speed at the nut as a higher lead screw, and faster speeds comes at a decrease in torque. This may also tax the nut, many her are made of delrin or other forms of plastic.

On the other hand, a high lead screw can give you faster feedrates at a lower motor RPM, which can mean more avaiabe torque and headroom. The higher lead usually means more efficiency due to less friction. Of course, larger motors cost more, and the higher lead could mean less smoothness at slower speeds. The efficiency of the higher lead screws could cause them to backdrive as well, which is another reason the larger motors are needed to hold position. Modern stepper drives however have features like micosteppings, which smoothes steps out at lower speeds, and morphing, which transitions the drive to full step at higher speeds (since microstepping comes at a penalty of reduced torque.)