Making a Push-to-Stop Switch

Controls for my Taig lathe

Part of the Making Stuff collection
by Douglas W. Jones
THE UNIVERSITY OF IOWA Department of Computer Science

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The Basic Switch

The switch
illustration showing the switch
In building a base and control system for my lathe, one feature I wanted was a big red push-to-stop switch where I could bash it if anything went awry. There are plenty of these switches on the market, but most of them are too big to fit into the base of such a small machine tool, so I hit on the design shown in the photo. This has the following elements:

The knob itself has a red face. It took me a while to figure out how to do this. What I hit on was using polymer clay, which is actually PVC. Sculpey and Fimo are two common brands. I turned a hole in the front of the knob with concentric rings to grab the pvc, then pressed a gob of red polymer clay in place. To get a nice smooth surface, I pressed the result against sheet of plastic wrap on a smooth clean countertop and then peeled off the wrap, leaving a mirror-smooth face. The final step was to heat the knob with the polymer clay in place in my toaster oven, following the time and temperature instructions for the brand of clay used. Here's a scale drawing of the knob I made:

The switch
scale drawing of the knob

The Interlock

Interlock locked off
photo of the front panel
The official rules for emergency stop buttons say that pressing an emergency stop button should latch the system in the stopped state, so that there are two distinct actions required to restart the system, one to unlatch the stop button, and one to start. This is why many of the commercial emergency stop buttons have spinning arrows on them. You push the button to stop the system, and then you must twist it to unlatch it before you can push the start button. I wanted this kind of two-step restart, and furthermore, I wanted an interlock with the forward-reverse switch.

I built an interlock between the push-to-stop button and the forward-reverse switch. The interlock is based on a slider. When you push the on-off switch to turn off the lathe, a spring pulls the slider to the right, latching the on-off switch in the off (pressed in) position and freeing the forward-reverse switch. Setting the forward-reverse switch to its intermediate off position locks the slider to the right, preventing the lathe from being turned on.

The slider and the track in which it slides are both made of brass, with a tungsten carbide pin (the shank of a broken micro-drill) sticking out of the left end of the slider to interfere with the forward-reverse switch. The screws that attach the interlock to the front panel are all worked from the back of the front panel, as is the set screw that holds the pin in place.

Interlock locked forward
photo of the front panel
To run the lathe, you first select either forward or reverse, and then slide the slider left. When you do this, the pin blocks any change to the forward-reverse switch, and the on-off button pops out, locking the slider to the left. At this point, you can pull the on-off button to turn on the lathe, or you can push the button back in to return the lathe from ready-to-run (but still stopped) to latched-off.

On a different subject, when I was turning hard steel before finishing this project, I noticed that bits of swarf would sometimes land on the push-to-stop knob, and then wedge between the knob and the faceplate when I turned off the lathe. I solved this problem by putting a "porch roof" over the switches. The roof is made of 22 gauge (0.0299" or 0.76mm) steel, bent down to approximately the same angle as the chamfer on the lathe foot, and held on with the two front mounting bolts that hold the lathe to the pedestal. The porch roof is about the same thickness as the washers that had been there, and the 22 gauge steel is thick enough that I could round the corners and edges so it doesn't threaten to injure anyone who accidently touches the edge.

I described some of the material here in an Aug. 23, 2016 posting on Nick Carter's Taig Lathe and Milling Machine Blog