Re: Newbie question re stepper size for small mill

There are others on the list that may have more experience with some
of your questions but I will take a shot at some answers:

<snip> but don't

> quite know how powerful steppers I should be looking for and what
> kind of controllers I will need. I have been looking on Ebay but
> don't really know what to choose for steppers or controllers.

The steppers are rated in oz/inch of force. How many oz/in of force
you need depends on several factors: (a) The natural frictional
forces of your mill (what does it take to move each axis with no
cutting load). (b) The type of materials you plan to cut. (c) any
mechanical torque gain in the transmission (different sized pullys,
etc). Do you plan on replacing the current lead screws with more
precision units? You can measure the forces involved by using a
piece of rod and a small handheld scale (fishing scale?) that will
measure from 10 oz to 50 oz. A small torque wrench will work but you
will have to come up with the interface to the lead screws. The
numbers are exactly what they say. The length of the rod from the
center of the screw to the point where you use the scale in inches.
A 200 oz in motor only applies 20 oz of power at 10 inches. Or vise
versa 20 oz of force at the end of a 10 inch rod creates
approximately 200 oz on the shaft. SO...you can measure what it
really takes to move your table around with and without a cutting
load. Once you have that you have one raw number you need. If you
do a direct connect from the motor than you will need a stepper with
a MINIMUM of the oz/inch you have measured. If you gear things down
you sacrfice speed (IPM) for torque (oz/in) and then you motors can
be smaller. Gut feeling is that you can get by with 300 to 400 oz/in
motors (34 frame size) on that size mill especially if you do a 2:1
reduction in the pullys.

After the conversion will I still be able

> to use the mill in a manual configuration if I use dual shaft
> steppers?

yes. you will need to remove DC drive to the motors and let them
free wheel. Just be careful of rapidly spinning hand wheels while
you are in CNC mode!

> Also... is it feasable to convert the drive motor (1 1/2 hp) to DC

in

You cannot "convert" an AC motor to DC. If you want to go through
the hassle of replacing the AC motor with a 1.5 HP DC motor (90 to
130VDC is typical) you can get inexpensive DC speed controls. A
better way might be to look at the VSD units for AC that will make
your current motor variable speed with fairly constant torque by
varying the frequency of the AC to the motor.

Last question ... are the motors used on the old sattalite dishes to

> move the dish a stepper type motor? If so, can they be used to

drive

> the X, Y, or Z axis on my mill?
>

I don't know what kind of motors the old satellite dishes used but I
doubt it. Someone else may be able to answer this one.

Hope this helps. I am in the process of converting a large Gorton
mill to CNC (130VDC servo motors) and have already installed a VSD to
give me 3 phase AC and variable speed.