# Another VFD Proximity Lathe Stop



## kb58 (Dec 28, 2020)

I read through MKSJ's threads on adding a proximity stop to his PM1340, and think that I can contribute on the concept in general.

Backing up a bit, I bought a used Takisawa TSL-800 lathe which uses a 5hp motor. This is more than most VFDs on the market can handle, but I really wanted variable speed, so bit the bullet and bought one of the few that could, an Invertek unit (made in the UK). At the time of purchase, I had no thoughts of using it for anything other than speed control.

As-built, the Takisawa has a foot brake that both physically stops the spindle and shuts off the motor, but the brake parts were missing on mine. Since I was going to add a VFD (with braking resistor) I didn't worry too much about it. After the VFD was added, all the original electrical bits were removed and the motor was controlled totally through the VFD front panel, and at that time, the brake pedal did nothing.

After seeing the above thread, I started wondering how I could use the brake pedal, but the problem was that the pedal was momentary - step on it and the motor would stop, but take your foot off and it restarts - no good. How easy it is to enable extra features on a VFD very much depends on the manufacture. It turned out that of the many control configurations available, one used momentary switches for reverse, forward, and stop, with the stop switch being normally-closed. Normally-closed switches are used for stopping mills and lathes because they're safer. That is, if the circuit goes open because a switch or connection fails, the machine stops. This also means that as many switches as you want can be strung in series, where opening any one of them will stop things.

Between the above, and seeing the solid state proximity sensor that MKSJ used, I came up with a circuit that provides everything I wanted:
1. Momentary push buttons for reverse, stop, and forward
2. A functioning brake pedal
2. A big Emergency stop button
3. A switch and indicator showing when the proximity switch is enabled, stopping the carriage when it comes into range.

Having two stop buttons on the same panel may seem odd, but a proper (large) mushroom-style switch is much more likely to be hit during high drama. Additionally, they aren't intended to be used as a normal switch, hence a separate normal switch that's used most of the time. Note how it, the E-stop, and the normally-closed brake pedal switch are simply wired in series. The proximity sensor produces zero volts when it senses the carriage, and supplies voltage when it isn't. The switch allows using the proximity sensor output as the voltage supplying the stop switches, and there's an LED to remind me that it's enabled. It's actually useful in that it's possible to think that the proximity function is enabled when it isn't, and to start driving the carriage toward the spinning chuck - that can get exciting, and the LED helps reduce that! Hope this helps others.

[edit] The above works for any brand of VFD that allows using a momentary normally-closed push button as the stop switch.


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