Re: Stepper torque determination

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "troubadour31"
<troubadour31@y...> wrote:

> Hi everybody!
>
> As a newcomer to home cnc machining I'd like to know
> how to choose stepper motors.

Of course, the size of the stepper that you will need is determined
by the amount of mass that you are moving, how fast you expect to
move it, and the type of material that you are cutting. Some amount
of over-design is necessary to overcome transient loads applied by
the material to the tool while cutting. If you are planning on
contouring, an additional amount of over-design is necessary to
overcome inertial forces connected to the essential instantaneous
shift in velocity that occurs at the transition points between
interpolated paths.

Most successful applications use some rough-cut methods for sizing
the motors, without breaking the budget. If you are using a lead
screw drive (as I expect you are with your milling machine and
lathe), then the amount of linear pushing force (in ounces) will be
6.283 times the torque applied at the screw (in ounce-inches),
divided by the distance from thread to thread.

In other words, if you were direct driving a ten pitch (per inch)
lead screw with a motor capable of 100 oz-in of torque, the pushing
force will be 6.283 x 100/ 0.1 = 6,283 oz = 392 lbs

Other factors are involved, of course, such as the inertial mass of
the lead screw and friction. However, this is rough cut, and as you
can see, some tremendous forces are at your disposal - at least as
far as pushing force is concerned.

So - I suggest using an economical approach. Make a reasonable guess
and buy one motor and one drive. Experiment with your heaviest axis.
It is going to move it around, but perhaps not at the speeds that
you want. If you are not far off in your estimates, at worst I think
you can rescue the motor and drive that you chose for use on a
lighter axis, such as your lathe's cross slide.

That's my suggestion for getting your feet wet without wasting money.

If you are planning on contouring, smoothness does make a difference
with regard to motor size. Since steppers must follow the (near)
exact geometry of the cut, when contouring shapes a certain amount
of essentially instantaneous acceleration occurs at each transition
point. Ability Systems Indexer LPT software looks ahead through the
entire contour to determine where it must decelerate to avoid over-
stress at any transition. This simplifies your selection of motor
sizes, because it substantially reduces the over-design otherwise
required. More of the theory is explained in the "Frequently Asked
Questions" section of our web site at
http://www.abilitysystems.com , and I would be pleased to send you a
manual that describes it in detail if you wish. There are other
advantages to smooth, look-ahead technology. Motor sizing is one of
them.

Best regards,
Arthur Volta
motion control / CNC Control
motion@...
http://www.abilitysystems.com

> The 2 machines I'd like to convert to cnc (a lathe and
> a milling machine) are no desktop ones.
> They are of the same category as the retrofitted milling machine at
> http://www.bmumford.com/cncmill/. B Mumford says he
> used 1400 oz/in of torque steppers. After I e-mailed
> Pierre Rouzeau (http://otocoup.com/), it seems this is
> an enormous torque for a stepper.
> Even if the machine of the latter is a router and not a lathe,

there

> is a great diiference between the torque of his steppers (

6.2kg/cm)

> and those of b Mumford (1400 oz.in = 103 kg.cm).
> How can I determine what torque would be best suited
> for the machines I intend to convert so as to be as performant and
> cost efficient as possible?
> Thanks for help!!!!