Re: [CAD_CAM_EDM_DRO] What Size Servo Motors?

Servo motor sizing involves some calculations regarding desired
speed and acceleration as well as machine mass, moment of inertia,
transmission ratios, friction, etc. I usually use softare programs
that calculate these parameters.

I can do this for you given those parameters, but you are probably
looking for a rough idea of what is required first.

For a Bridgeport type machine a typical servo might be about
3 inches in diameter and 10 inches long. It would use a few amps
(perhaps as much as 20 peak) and run with about 90 volts.
With this you could get (depending on configuration) table
forces from 1000 - 2000 lb peak and speeds to 360 ipm.
Is this a lot? No, not really.

For a mill drill servos could be smaller; perhaps half that size
or less. Speeds can be high but forces need not be anywhere
near as much; the structure is very light.

Amps to drive these might be inexpensive Gecko or Copley
PWM units.

Servo control units can be Master5 or emc in step mode with
Gecko servo conversion or full host software control with
Servo to Go or Pico Systems and emc.

The motors are available surplus from a variety of sources
including ebay, dealers, and list members. I sell new SEM
servos sized for typical Bridgeport mill applications at a
very low price. Others here have smaller ones for mill
drills etc.

Obviously I am shamelessly biased, but I would go for an
old (small) Bridgeport. It does not take up much more floor
space than a mill drill. It's just heavy. Rent a small forklift and
no problem. Old machines are very low priced as well,
I guess due to the economy and deindustrialization.
Don't be fooled by the prices you see from machinery
dealers. If you are in a big city and look around the
things can be had for a song. Often the moving costs
are the only consideration.

And just so I don't irk the mill drill guys- yes you can
make a very nice light duty cnc machine from one.
Many have. Most use steppers. Are they inferior
to an old industrial machine? Well no, not for hobby
stuff. They can be just as accurate (if you work on them).
Of course they are smaller capacity and you will not be taking
any heavy cuts due to the lack of rigidity.

Les


Leslie Watts
L M Watts Furniture
Tiger, Georgia USA
http://www.alltel.net/~leswatts/wattsfurniturewp.html
engineering page:
http://www.alltel.net/~leswatts/shop.html

----- Original Message -----
From: "natchamp_87" <mark@...>
To: <CAD_CAM_EDM_DRO@yahoogroups.com>
Sent: Monday, October 14, 2002 6:51 PM
Subject: [CAD_CAM_EDM_DRO] What Size Servo Motors?


> I am new to the list and have a couple questions and need some high
> level guidance. My plan is to convert a mill (which I haven't bought
> yet) to CNC. The mill will either be an older Bridgeport type or a
> smaller bench type. Thanks in advance for any help.
>
> 1) If I go with servo motors because of the higher torque at speed
> and the continuous feedback. A) what size servo motors should I be
> looking for? Are there any other things to look for, or keep in mind
> with a servo setup? Are there any good sources for used ones?
>
> 2) Am I assuming correctly that the Harbor Freight import type of
> bench top mills are significantly inferior to the older models? I
> don't really need the size of a Bridgeport and have space
> considerations. Any recommendations?
>
> Mark
>
>
>
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