Re: Conversion Of Mill to CNC (LONG)

Wayne,

Let me jump in on the motor/drive aspect.

Ask yourself 2 questions: How fast do I want it to go (IPM) and how
much "push" (Lbs) do I want? Make the IPM a number you would be
impressed with and make the Lbs big enough to bend and break things.

(1) Multiply IPM and Lbs, then divide the result by 531. Your answer
will be in Watts need to do the job.

If the answer is 100W or less, think step motors. If the answer is
200W or more, think servo motors. If between 100W and 200W, consider
either carefully.

Once you know how many Watts you need, calculate the leadscrew torque
required.

(2) in-oz = (2.55 * Lbs * eff) / TPI where TPI is turns per inch and
eff is the screw efficiency. Use .3 for a really crummy ACME screw
and .95 for a really fine ball screw.

(3) REALLY IMPORTANT. If servo motors are used, reduction gearing
may/will be required. To figure the ratio, take the need screw torque
(from the above) and divide it by the servo motor's rated continuous
torque. Gear the motor down by that amount.

EXAMPLE:

Let's say you answered "500 Lbs" and "180 IPM". Let's also say you
have a 5 turns per inch (5 TPI) ballscrew.

From (2) the screw torque is 242 in-oz.

From (1) you get 169W; either a big stepper (NEMA 34) or a small
servo (NEMA 23) will do. Let's do the servo first.

You pick a nice 200W NEMA-23 servo motor rated at 54 in-oz at 5,000
RPM. From (3) you get a reduction ratio of 4.5:1. Everything will
work very nicely and you will get a top speed of 222 IPM instead of
180 (that's where the extra available 31W go).

The peak torque of a servo comes in handy at low speeds. This 54 in-
oz motor may have a peak torque (stall) of 400+ in-oz. That will give
you 4,000 Lbs of "push" for very short periods of time.

Now to step motors. No gearing required here. Simply pick a motor
that has a holding torque 2.5 times the needed screw torque; that
comes out to 600 in-oz which a NEMA 34 step motor can do easily.

Mariss



--- In CAD_CAM_EDM_DRO@yahoogroups.com, "camerashy44"
<camerashy44@y...> wrote:

> Although I have been monitoring this list for some time this is my
> first post. I am a complete novice at both milling and CNC so need

the

> help and advice of the very experienced and knowledgeable members of
> this list.
>
> I have recently purchased a GEM500G mill/drill from Busy Bee. Yes I

am

> in Canada. The mill has a 32 x 9.5" table and is a gear head mill

with

> dovetail column. It has a x axis travel of 18", y axis travel of 14"
> and a z axis travel of 18.25". It is my intention to convert this

mill

> to CNC. There are pictures of this mill in the photos section. It is
> the mill of Allan Reinhard who is also a member of this group. He

has

> also converted his mill to CAC using 720 oz/in steppers and Geko

drives.

> From what I have been able to pick up, servos seem to be a better
> alternative than steppers. Is this true and what are the advantages
> disadvantages of each? I want to have enough power to make sure

that I

> never have a problem and am inclined to ere on the higher power

side.

> I was looking at Sanyo-Denki P1 series servos, the 1kW size, 2000

rpm.

> This servo has a rated torque of 665.5 oz/in and a continuous stall
> torque of 835.5 oz/in. I was told that this was overkill and that a
> 425 peak oz/in 2500 rpm servo would work fine with a 3 to 1 gear-

belt

> drive. I am sure that this is correct at least for the x and y axes.
> However the z axis(Column)takes more manual power to move especially
> up and I like the idea of all axes motors being the same. I want the
> simplest cleanest(neat) and most trouble free system possible. Yes I
> want a lot!
> What are others opinions on this? Is gear-belt drive superior to a
> direct drive?
> I like the Camtronics kits and the idea of getting everything from

one

> place like Dan offers. However I have to consider that if I ever

need

> to replace or repair any part of the system, items that are

available

> here in Canada would be preferred.
> I am not going to go into ball screws yet but I know that these will
> depend on the motors that I choose (pitch) so comments on these

would

> also be appreciated.
> I am sure that I will continue to have lots of questions but will

stop

> at this for now. Thanks for your patience especially if you have

read

> this far!
>
> Wayne