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Reset switch

hman

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#1
While running my mini-mill a couple days ago I had a wild hair. Wouldn't it be nice if, after turning off the motor (with the e-stop) you could return to the same speed you were using without having to back the speed knob past the click, then forward to (maybe) the original setting?

I tend to always use the e-stop because I dislike the whine of the motor when it's "idling." I knew the switch click was sending some kind of a signal to the control board. But was it going on or off??? If I could figure out what it did I could add some kind of switch in parallel to serve the same function.

It turns out the switch is not a simple on-off, as the click might imply! It took quite a bit of playing around, but I finally figured out that the switch very briefly connects the two leads across a resistance just as it clicks. I was not able to measure the resistance with any semblance repeatability, let alone accuracy. Durn digital meters are absolutely hopeless when it comes to momentary events, and my analog meter just wasn't fast enough.

Then I looked at the rest of the wiring and noticed that the green indicator light is connected in parallel with the switch. This light has a neon bulb and runs off line current. I recalled from a dim past that a neon bulb is always wired in series with a 33KΩ resistor. That gave me an upper limit for the switch resistance. I first tried a 10KΩ resistor, but it didn't work. Then I tried a 1KΩ and it seemed to work fine during testing, so that's what I went with. I suppose it's possible that the switch actually shorts the two terminals together (zero ohms). But as the 1KΩ worked, I decided not to even try a direct connection. Why risk letting out the magic blue smoke?

CAUTION ... CAUTION ... CAUTION!!! If you want to add this switch to your own mini-mill (or possibly mini-lathe) be SURE to unplug it before opening the switch box!!! There's line current all over the place in there and most of the terminals (including those on the back of the speed control switch) are not insulated. Whatever switch you add, be sure to insulate the terminals (heat shrink is good), and use connecting wires with adequate voltage rating on the insulation.

The wiring is dead simple. You'll need a 1/4 watt 1KΩ resistor, a pushbutton momentary mini-switch, soldering iron, some wire, and a short length or two of heat shrink tubing.
kHPIM4841.jpg

Solder one end of the 1KΩ resistor to the "NO" (center) terminal of the switch. Solder a length of wire to the other end of the resistor. Solder another length of wire to the "C" terminal. Insulate the connections with heat shrink tubing. Solder the other ends of the wires to the terminals on the back of the speed control switch.
kHPIM4844.jpg

The mini switch mounts through a 1/4" hole. I located mine next to the speed control knob. Before closing the switch box, check that all your connections are secure and that there are no stray strands of wire floating around that could cause a short. Then plug the mill in and check for correct operation of the switch.
kHPIM4845.jpg

PS - When operating the reset switch, push it very briefly. I don't know the circuitry the switch goes to, but whatever it is, it's "used to" a short pulse. So give it what it knows and loves.

PS #2 - The "shielded" toggle switch in the background is a DPDT I added between the motor leads. It lets me reverse the motor - very handy for tapping! The shield snaps down and insures that the mill runs forward for normal operation.
 

Hukshawn

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#2
Oooh. I like the reverse switch. How did you wire it? Is it in the circuitry or are you just reversing the dc motor leads?
 

hman

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#3
The reverse switch is just a DPDT connected to the DC motor leads that already run through the switching box. IIRC, the existing leads have spade connectors. So I just put the mating connectors on the wires that I soldered to the switch.

The belt drive is a good thing to have if you're tapping - helps prevent motor and gear damage if the tap stalls. But then, if you run the motor SLOWly, it doesn't develop too much torque, so gears may be OK.

Another change I made was to cobble up an extended draw bar for use with the drill chuck. The R8 drive pin in the spindle will continue to turn the chuck even when the chuck is hanging ¾" below its normal position. The new (tapping only) draw bar is ¾" longer than the original. I did this so that the tap wouldn't have to fight the weight of the mill head or the friction of the dovetail. The only weight on the tap is that of the chuck. Maybe a bit paranoid on my part ... but I've never boogered up a thread in soft material, or broken a tap in tough stuff.

Worst comes to worst, the tap might turn in the chuck, the belt slip, or the motor stall. If that happens, I release the tap from the chuck and continue with a regular tap handle. By that time, the tap is well into the hole and known to be straight. I've used taps as large as ⅜-16 with this setup.