Re: Stepper Size Selection

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "dnaman43" <mevagissy@...> wrote:

>
> I want to convert a small-mill (large minimill) to CNC and have a
> couple of very basic questions:
>
> 1. How does one initially determine the size (torque) of stepper-
> motor required?
>
> I can check the static torque, on the leadscrew, to move the table
> but I imagine there's more to it and that dynamic torques are what is
> really needed.
>
> 2. What are the trade-offs between using a stepper directly coupled
> to the leadscrew versus one connect with, say, a stepup/down toothed
> belt drive consisting of a sprocket on the motor and another on the
> leadscrew?
>
> Obviously there's a torque/speed tradeoff here but are there any
> other considerations related to dynamic belt-stretch for example (or
> anything else)?
>

This does not have a simple answer. Everything in the equation is
dynamic:

What kind of milling are you going to do (steel, alum, light material?)

What is your leadscrew TPI and what speeds do you want to 1.) cut at;
B.) rapid at.

What is the torque required to move the empty table?

Do you have room to mount 34 frame motors? How about toothed pulleys?

You can start with the facts that:

Steppers spin about 800 RPM with optimum voltage and a good drive.
At 800 RPM they have less than 20% of their "Holding Torque"
At 400 RPM they have less than 60% of Holding torque (varies with motor)

You will trade speed for torque and resolution. Accuracy is
resolution times all errors (backlash, etc)

If you give more detail about your mill then others that have
retrofitted that type or similar units can chime in. As it is it's
difficult because the description of "large minimill" doesn't mean
much to us.

Pure guessing would have me suggest 740 to 900 oz-in motors with 1:1
coupling BUT that only gets you within a REAL rough estimate. The
leadscrew pitch will change the equation. If you use ballscrews the
backlash will be less and the force to turn the leadscrew less.

So everything factors into the equation. You might be able to get by
with 400 oz-In if you gear it down and run real slow. You may need
bigger motors if you want more speed AND torque and you may have to
use belt ratios to get the speed.

It most cases emphirical numbers from testing or from others that have
tested are the only way unless you know all of the numbers down to the
size bits and material you plan on using.

Sorry but if you can post better information then you will get better
answers.

TOM CAUDLE
www.CandCNC.com