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.