Shopsmith Forums

While in the midst of a rip cut in 3/8 pine, my MK V up and quit. Just like the switch was turned off. Checked power from the outlet. All good. I suspect a loose wire internally. Hoping a visual inspection reveals the cause because my diagnostic tools are still packed away...in one box or another.

My money is on the switch itself. Believe it or not, my original one on my 520 failed CLOSED -- which is pretty scary when you think about it.

I suspect a loose wire(or something else loose) also.

BuckeyeDennis wrote: ↑Thu Jul 15, 2021 8:35 pm My money is on the switch itself. Believe it or not, my original one on my 520 failed CLOSED -- which is pretty scary when you think about it.

Yer right, Dennis! Broken terminal on the switch.

Start at the switch. They are prone to failure after enough years of use and on-off cycles. It is easy to check the switch, and it will tell you if the problem is there or farther inside.

Makes me wonder if I should buy a spare switch one of these days to keep on hand just in case. All three of my Mark headstocks are safety switch "C" headstocks.

chapmanruss wrote: ↑Fri Jul 16, 2021 11:42 am Makes me wonder if I should buy a spare switch one of these days to keep on hand just in case. All three of my Mark headstocks are safety switch "C" headstocks.

I have two thoughts on buying the spare. 1) The "C" headstock switch that came in my PowerPro upgrade has a much more solid feel than some of the earlier ones. So my thinking is the current version from SS is excellent. 2) For my shorty I use a non-safety rocker that fits in the same hole. IIRC this is endorsed by Jacob A but I bought a motor-rated rocker switch someone else on the forum found. I like it fine and it is much less expensive. Not a good choice if you use the safety feature.

- David

When I replaced my failed switch, I wired both contacts in series, like Shopsmith used to do. When you open the switch contacts (i.e. turn off the switch), the inductance of the motor windings forces the current to continue arcing across the open contacts, just like the coil in an old-fashioned engine distributor forced an arc across the spark-plug gap. The arc continues until the inductive energy is dissipated, mostly as heat in the contacts themselves. So I figure that with two contacts sharing the energy-dissipation duties, they should last much longer than if using just one contact.

I saw a switch specification the other day that tends to support that theory. It was rated for 1 hp if using just one contact, but for 2 hp if both contacts were wired in series.

BuckeyeDennis wrote: ↑Fri Jul 16, 2021 1:43 pm When I replaced my failed switch, I wired both contacts in series, like Shopsmith used to do. When you open the switch contacts (i.e. turn off the switch), the inductance of the motor windings forces the current to continue arcing across the open contacts, just like the coil in an old-fashioned engine distributor forced an arc across the spark-plug gap. The arc continues until the inductive energy is dissipated, mostly as heat in the contacts themselves. So I figure that with two contacts sharing the energy-dissipation duties, they should last much longer than if using just one contact.

I saw a switch specification the other day that tends to support that theory. It was rated for 1 hp if using just one contact, but for 2 hp if both contacts were wired in series.

I believe the coil and spark plug are still in style on relatively modern gasoline engines. I've also read this theory on switched neutral extending the life of the switch. I haven't seen actual test data that proves it but I think it makes sense. I think Shopsmith also went back to it. The headstock I did my PowerPro mod on was wired that way before mod. I'd have to confirm the year, seems like a 2001 model. From the 50th anniversary years, for sure.

- David

Not sure what switching the power pro power on/off effect occurs. Likely less 'kick' than an induction motor.