On Thu, 26 May 2016 19:43:33 -0700, rangerssuck wrote:

A customer has a very large, very old (and pretty dirty) machine that
needs some help. It is a multi station drilling and tapping machine. The
production items sit on 4" square pallets spaced roughly 8" center to
center. There is a microswitch that is activated by contact with the
leading edge of a pallet. The switch causes the drive motor to stop, and
the drilling & tapping to begin.

Their problem is that the stopping point is somewhat variable, and they
thought that if I replace the microswitch with a prox sensor, all would
be well. So, I went down there and had a look, and found a good place to
mount the prox sensor and a brush to sweep off whatever debris was there
(and maybe an air jet for good luck). Then, I had a talk with the plant
electrician.

The electrician, who really knows his stuff, pointed out that the
controls stop the motor, but it's really a hydraulic motor that drives
the system, and maybe it coasts a little when the electric motor is shut
off. Also, he pointed out, that the offset changes during the day, maybe
as a result of the fluid heating up. So, my not-at-all educated in
hydraulic motors mind started thinking, what if, instead of shutting of
the pump, what if we installed, right at the motor, the hydraulic
equivalent of a DPDT relay that would disconnect the motor from the pump
and put a short across the motor?

Does that make sense, would it (as would a DC motor) stop on a dime? I
have googled a bit and I believe I have seen such circuits, but I'm
really pretty much in the dark about such stuff. If it DOES work, do I
have to worry about overpressure in the lines? I haven't seen the
machine run, but I'm told that a) it doesn't move fast and b) it doesn't
move far.

The pump motor is 3-phase, so I suppose I could install a VFD (they
probably have extras on site) and use a second prox sensor to cause the
speed to ramp down, but it would be better if I could just stop the
motor. And, there really isn't a good place to mount a magnetic brake.

As always, all thoughts (especially kind ones) are welcome.

Before I did anything else I would ask both myself and any old-timers if
the machine used to work better. It might even be worth a visit to the
retirement home of a former millwright or two.

Then I would check for signs that something on the table is there but
broken, or has been removed.

It may be that the only thing the "idiots" who designed it did wrong were
to retire and leave it in the hands of _real_ idiots.

The only way you're going to get the thing to have precision in line with
modern machine tools is to either have some positive closed-loop control
of position, no doubt with ball screws and motors and whatnot involved,
or to have some fixed positive indexing (like the "shot pin" mentioned
elsewhere).

Deciding to revamp it with electronic controls may just be jumping onto a
slippery slope leading to replacing the whole damned thing.

For the fixed indexing, you probably want a pin or whatnot to establish
the correct location and then something that holds tension against the
pin (else the whole pallet will be short of the pin, and your precision
will have gone out the window).

Sometimes the quickest and most reliable super-high-performance
modification to an older machine is to put it back the way it was
originally designed.

--
Tim Wescott
Control systems, embedded software and circuit design
I'm looking for work! See my website if you're interested
http://www.wescottdesign.com