Re: [CAD_CAM_EDM_DRO] Added a VFD to my CNC mill

Dan Mauch wrote:

>I am going to figure out how to use the analog signal from DeskwinNC to
>turn on and off my VFD and set the speed from computer via the G code. I
>think that would really be a nice to automatically turn on the spindle
>motor and set the speed via the program. So far I can heartily recommend
>the Mitsubishi FR200E series inverters. BTW I have been told these
>normally sell for around $800 but got mine New off ebay for a fraction
>of that. It sure is nice not to have to change pulleys all the time.
>Last I would like to know how to calculate the maximum safe speed that I
>can run the spindle at. I would think that the 3500 RPM 50 Hz motor
>would cause the spindle pulley to explode if run at 400 Hz.
>
>

Yes, VFD's are really nice! I'll never go back to a static phase converter.

I suspect these motors can be pretty safely operated at twice the rated
speed. There was some serious
discussion about higher speeds on rec.crafts.metalworking, and people
with contacts in the woodworking
industry indicated that it was fairly common to run 60 Hz motors at 400
and even 800 Hz, and that
motor manufacturers had told them that 400 hz (2 and 4 pole) motors were
built with the exact same
materials in the rotor as the 60 Hz motors. I am pretty skeptical of
all that, and would rather see
this first before believing it. I suspect a solid (meaning no spokes)
pulley is capable of taking at
least twice rated speed. Bridgeport J heads are available with dual
speed motors or the high-speed
option, to go to 4250 RPM. With the cost of good spindle bearings, I
really would not want to
exceed that. Now, taking that standard head, and running a 60 Hz motor
at 400 Hz, you'd be
running the spindle at 28333 RPM. While the spindle probably would not
explode at that speed,
the pulleys would, indeed, be at dangerous speeds for their
construction. I can't imagine the belt
would handle this at all, though. V-belts tend to flip over when run at
excessive speeds, and then
shred themselves in minutes.

Truly calculating the max safe speed involves computing radial loads
from centrifugal force, and
then also analyzing the natural vibrational frequencies of the parts,
and making sure you don't
operate at a speed where the rotational rate and the bending natural
frequency coincide. That
is the first critical speed, and resonance can develop into fantastic
explosive force in just a few
revolutions. It requires a pretty detailed mechanical analysis to
figure out where these resonances
will occur. 28333 RPM = 472 revs/sec, which is not an unreasonable
natural frequency for some
of these components.

But, all that is meaningless, because bearing life of some of the
smaller bearings in the head
would be reduced to minutes. Even with the best oil, the spindle
bearings would be toast
(literally) in a few hours, tops. I have heard of people running
Bridgeport and similar spindles
up to 7500 RPM or so on occasion, but it seems a bad idea.

If you have a real need for high spindle speed, why not put in an
auxilliary direct-drive spindle?
I picked up a Rockwell/Precise spindle that can run up to 45,000 RPM,
and it is DESIGNED
for that use. I got it for a song on eBay!

Jon