Re: Re: Old CNC controls

"D.F.S." wrote:

> The only use withing the lathe itself for 3 phase power is the motor
> itself. It is a 2 speed motor with a 2 speed transmission of some
> sort.
> I have 4 speeds, I'd like a variable speed setup of some sort.
> I have a 7.5 Hp DC motor, but am a bit concerned about keeping a
> constant speed under a variable load, and how that would screw up
> things
> like threading, no pun intended.

The threading does not use some fixed value for spindle speed, it
measures
spindle rotation with a spindle encoder, and each sampling interval it
computes
a new carriage position based on the spindle position. So, actually
some
substantial speed variation will be tolerated fairly well, as long as
the machine
has enough inertia so that the speed variations are not sudden.

There are 2 ways to go. The simplest is to get a variable speed drive,
and hook
it to the CNC controller for a speed command. This is open loop, but
most
of these units will keep the motor at very constant speed within the
motor's and
VSD's power ratings. (With a little more sophistication, the CNC could
correct
for load variations since it is reading the encoder already.)

The other way to go is with a big DC control, which will allow tighter
speed
control, and possibly even positioning control of the spindle. I should
warn
you that a 7.5 Hp DC control will be big, heavy and draw a LOT of power.

There are so many VSD's on the used market now, they can be had much
more easily, and therefore probably cheaper, than a big DC control.

Most newer (PWM) VSD's can be run on single phase, even if the
manufacturer
doesn't specifically say so. You just cut into the safety factor on the
rectifier
diodes a bit. If you had a 3 Hp motor on the lathe, for instance, you
might
select a 5 Hp VSD to improve the margin on that.

I have a 1 Hp Magnetek VSD, which is sold as 3-phase in only, but after
researching the power block inside it, I found out that the recifier
diodes
are VERY conservatively rated. I've been running it for over a year on
single-phase 240 V input, and have had no trouble at all with it. It is

the best and simplest solution, as you don't have to adapt a different
motor into the machine.

> Maybe a variable speed transmission would be a good project.

I really don't recommend these. The hydraulic ones are energy hogs.

> I hear even the DC servos run better run from a REAL 3 phase power
> supply, I wonder if it is even worth the trouble with a Pseudo 3 Phase
>
> power source.

If they are designed for 3-phase input, they may have storage capacitors
that are
quite insufficient for smoothing rectified single phase power. Upping
the size of the
cap bank would help, but could raise peak rectifier current. My DC
servos, of
couse, have a power supply designed for single phase, and they work just
fine.

>
> > I tried to hook up a 17 KVA
> > motor-generator set at a friend's home, and we had all manner of
> > problems.
> That was another option I considered, I have an actual 3 phase
> generator.
>
> > How heavy a
> > 240 V service do you have? If less than 200 A, you are in big
> trouble!
>
> That could be a problem, I think the feed to the shop is only 100.

Well, expect lights to dim GREATLY when you start a 7.5 Hp motor, AC or
DC. If you use
capacitors to start your 15 Hp phase converter, I think your service
will blow the fuses!
The VSD's have one other advantage, soft starting. You can set the
startup ramp to take any
time you desire, from 1 Sec or less to 15-30 seconds, depending on the
model. This will allow
it to draw much less peak current.

> I don't have any manuals, another reason to not use the controller.
> All I'm going by are the stern warnings and diagrams inside the
> cabinet.
>

Phew! Don't even try it! I had to repair about 15 things on my AB CNC
control before I got
it running the first time. It was a bear! I had broken traces on the
backplane, shorted out
caps on the logic boards, a bad power supply, bundles of
identical-looking cables that had to
be plugged into the right place the first time or ELSE, bad memory
boards, a broken keyboard
encoder chip, blown fuses by the handful in the memory backup power
supply ..........

Without manuals, it would have been so much scrap metal! With manuals
(and DAMN
GOOD ONES, I must say, complements to Allen-Bradley), and the fact that
I am
an EE and an old 16-bit minicomputer hacker, I was able to get it
working. But,
somebody who didn't have this level of experience AND good manuals would
be DEEP
in the jungle!

> Pretty modern as to a lot of the mechanics the way I see it.

Well, Iron doesn't seem to get out of style. I've got EMC running on a
1938 Bridgeport
round-ram machine. Kind of a funny combination, really! The X and Y
axis drives
came out of an old Excellon circuit board drilling machine, made in the
60's. The screws
are untra-precision ball screws, and the axis drive assembly is a bit
antiquated, but
competent. And, although it is nearly 40 years old, I had NO trouble
getting identical
replacement timing belts, sprockets, etc.

> I'd like to keep the good, and replace the old and unreliable.
>
> One real advantage, is that everything is already there.
> This thing was designed and built for computer control, it already
> has all the limit and sense switches, motors to run all the axis
> and encoders, I have not identified them yet, but they are there.

Yes, this all took me a LONG time, and some very HEAVY milling. I
started with
a 111 Lb chunk of hot-rolled steel, and finally cut it down to about 44
Lbs., for my
bracket to attach the X axis drive to the Bridgeport table. I still
don't have the limit
and home switches hooked up.

>I was glad to move to EMC at that point.

> How have you set up your system?

Hmm, well, I'm not sure what you are asking. I have DC servo motors on
X, Y and Z. The X and Y are from the Excellong drill, they are GE shunt

field motors, and I used them because they were already mounted. They
are 1/8 Hp (Continuous) motors, with the rating point at 1725 RPM.
They are rated at 1.54 A continuous, I set my current limit at about 5
Amps.
There is a 2.5 : 1 belt reduction to a 5 TPI ballscrew. The shaft
encoder
is driven directly off the screw.
I used a tape drive capstan motor for the Z axis, and it is a little bit
weak,
but works. I have a 4:1 belt reduction to a 10 TPI ballscrew there.

> What kind of motors are does your "Mill?" use?

Answered above.

> How are you driving them?

I built my own PWM servo amps, since I didn't know where to get them
surplus at the time. It was a great education in control systems theory

for me. They all run from a single 69 V DC supply, and supply 5 amps
peak to the X and Y motors, and about 8 to the Z. I can go up to
60 IPM right now, there is some minor quirk on the X amp that causes
it to trip out above that. I originally had it running up to 105 IPM,
which was actually pretty scary to me. 60 IPM is plenty quick to
get from one side of the machine to the other.

These amps are fed velocity commands as +/- 10 V signals from the
servo-to-go card.

Jon