On 2008-09-30, Vernon wrote:

[ ... ]

In the case of this mill it has an instant reverse switch on the left
side. Today KBC priced a single phase replacement motor at over $500
dollars plus almost a hundred for the replacement instant reverse
switch. Grizzly has some single phase motors that might work for
less than half of KBC's price.

Ouch!

It's beginning to sound like the advantages are tilting to a VFD.
That seems to be the most direct path to instant gratification.

Yes.

By the way, I probably don't understand what an "instant reverse" is
or why its needed.

Well ... picture using the mill to drive a large tap through
steel or some other metal.

Now -- assume that it is thick enough so you need to bury almost
the whole length of the tap in the workpiece.

And -- that you are using a rigid tap holder (most commonly used
in a CNC (Computer Numeric Control) controlled mill which can feed
downward at precisely the right speed linked to the spindle speed), or a
"floating" tap holder (which allows the spindle to feed a little faster
or slower than the thread wants to feed), but you do not have a
"relasing" tap holder, which will stop driving the tap when it is pulled
a certain short distance down from the spindle. Since as long as the
spindle rotates, the tap will keep feeding in -- or break when you run
out of threaded part of the tap.

So -- you want to be able to hit a button which will start the
tap rotating the other way, so it backs out of the hole -- *before* you
break the tap.

The "instant" will be a little slower than the instant from
commercial three phase power (the "plug" reversing which I mentioned in
another article in this thread), but it can be tuned to be fast enough
so your reflex time will be the true limiting factor. (And it can be
tuned a lot faster on a mill spindle than on a lathe spindle with a big
heavy chuck and big heavy workpiece adding to the rotational inertia.

Of course -- you could use something like a Tapmatic tapping
head, which will stop turning the tap when the tap gets a certain
distance ahead of the feed, and when you reverse the feed, the tap
starts turning backwards faster than it went in -- all while the mill's
spindle is rotating the same direction. I use a couple of these of
different sizes on my drill press.

But if I understood one of the explanations of the
pros and cons of VFDs if I buy a VFD I will need to remove the instant
reverse switch and start / stop switch wires from the motor circuit
and route them to the input side of the VFD.

That is right.

Provided I do this correctly and get a VFD of adequate capacity I will
have nearly perfect 3 phase, full horse power, and full use of the
instant reverse switch.

Yes -- except just very slightly slower in the reverse -- but
also will minimize the high current spikes which you would otherwise put
into your power panel.

Did I understand correctly? If so, I may pull out my credit card
tomorrow while it still works!

Take the time to pick one which is more affordable. Others here
can suggest a couple of sources which give good prices and good service.
IIRC, one is called "Dealer's Electric", and they have a web page.

Enjoy,
DoN.

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