Re: [CAD_CAM_EDM_DRO] CNC Mill Conversion.

willybbca wrote:

>Hi Guys
>
>I am new to this CNC stuff and have just joined the group. I am
>planning on converting a bench style Mill/Drill machine to full CNC
>control. The factory feed screws will be replaced with
>re-circulating Ball type screws and driven with Servo motors. I am
>thinking of using the Gecko G320 Controllers and mounting everything
>inside a Camtronics enclosure. This enclosure is a nice set up and
>comes ready to use with all of the connectors. I have a couple of
>questions in regards to parts type and availability.
>
>(1) Looking for a good place to purchase Ball Screws?
>

Not the most easily obtained item, unless you are willing to pay list
price for new.
You can check eBay, or if in the northern California area, check out
Trinagle Machinery.
They had a TABLE covered in used ballscrews of all sizes and lengths!
(Hmm, had
a lot of servo amps and motors, too.)

>(2) Looking for a good place to purchase Servo Motors?
>

Surplus Center (Lincoln, NE), Herbach & Rademan, C&H Supply and many others.

>(3) What size of Servo motors will be required for one of these
>Mills?
>

It depends to some extent on what performance you need. For a mid-sized
machine like
this, motors about the size of 2-3" diameter and 6" long would be a good
place to start.
Depending on the winding, you get more torque at low speed, or less
torque at high speed.
That power can be brought into the right range by the belt ratio. A
motor with a continuous
rating of 75 - 100 W will probably be sufficient, since the peak rating
on such a motor will
be 1/2 Hp or better.

>(4) What ratio works the best between the Servo Motors and the Ball
>Screws? (I have heard both 2:1 and 3:1) Concerned that with 3:1
>ratios, the Servo motor will not be fast enough and the IPM will be
>slow?
>

There's no hard and fast rule. Some, low-speed, high-torque motors can
be run 1:1. Some low-
torque, high-speed motors might need 10:1. Most servo motors have LOTS
of speed, 6000 RPM
is not unusual for the 2-3" diameter motors. So, even with a 3:1
reduction, a small motor that
can reach 6000 RPM will give you 2000 RPM on the screw. With a 5 TPI
screw, that's 400
IPM!

>(5) Is the G320 Gecko drives a good way to go with the above
>combination? What other alternatives are there?
>
>

It depends. If you want to be able to switch smoothly between manual
and CNC control,
and use the CNC system as a DRO when in manual, then the Gecko and other
step-direction
to servo drives doesn't work, as the CNC computer never sees the encoder
feedback.
You can use Copley Controls or other brands of analog-input servo amps,
might check eBay
for that. Servo-to-Go makes ISA interface boards, and I make a parallel
port motion control
board set, see http://jelinux.pico-systems.com/PPMC.html for more info
on this. Both work
with EMC (see http://www.linuxcnc.org ).

Jon