Fitting a Reversing Switch to a Motor on a Myford Lathe

Thanks chaps - I knew I could rely on you all to help me out!

There is an assembly with springs at the back of the spindle, inside the motor. This definitely moves when the motor starts up, so I assume this is the centrifugal switch. Do you think I would still be able to reverse the motor?

Mike

My ML7 was reversible, (The dreaded Dewhurst Drum switch! )and you could hear the centrifugal switch cut in again as the motor slowed when the power was cut. So you should have no worries there

I was lucky and never spun the chuck off in reverse, but there is always a first time. Do take care!

Howard

Hopefully you have a multi meter or some thing to measure Ohms./ resistance of the two coils. Since you have a couple of picture you have already a record of where the wires go and to which terminal. Make sure the lathe is completely dis connected from the main electric, UNPLUG !. disconnect the wires to A & AZ and measure the resistance across those two terminals, it should be low around 6 Ohms area. Then disconnect wires to Z, and with one lead of the Ohm meter connected to Z probe around with the other lead to find the other end of the start winding. This winding will have much higher resistance around 60 Ohms. Hopefully you have located both end, if so your motor can be reversed. The start winding circuit consists of the winding, the switch on the end of the rotor and the capacitor, often A & AZ, both connected in parallel with the run winding, and then to the supply. I've had a Super 7 for about 30 years and have never spoken to a person who has had the chuck wind off, much spoken about though. John

I have warned one person of the danger in very recent times. He was actually building a mechanical brake to stop the spindle on a non-myford lathe with a screwed chuck. He did not finish the build/installation and nor did he thank me for the warning, as I recall.

Yes, explained in my previous post.
See the third link.

Disconnect the start winding wires from the paxolin plate, extend them to the input of your reversing toggle switch. Take the wires from the other end of the toggle switch and terminate them where your removed the start winding wires previously.

N.B. Start and stop the lathe with the normal no-volt-release contactor, using the reversing toggle only when the lathe motor is stopped.

Bill

Edited By peak4 on 01/02/2019 21:09:39

You can actually get a reversing NVR contactor, my 254S had one before I converted it to 3 phase, if you're interested I could photocopy the wiring diagram from the manual, but if you've already got a reversing NVR and a Dewhurst I'd use them

No, don’t have two flexes - buy a new flex with the correct number of cores. A far safer option. Proper electrical suppliers will stock a sutable flex - probably not your local hardware store such as Wilco, homebase or similar.

A couple of points regarding this.

Whilst its not the best idea to change the reversing switch on a single phase machine capacitor start machine while the lathe is running it won't do any harm because the start winding is switched out (by the centrifugal switch). Any time you change the switch it won't take effect until the motor is started again. N.B, three phase motors are different and reversing the switch will have immediate effect !!!

You can use a simple two pole on/off/on toggle switch as a reversing switch. You wire a pair of crossover wires between the two end pairs of contacts and then feed live and neutral to the middle pair. The feed to the capacitor and start winding goes onto either of the pairs of end contacts. Switching the switch from one end to the other will reverse the motor.

My own lathe does not have centrifugal switch built into the motor, instead it is supposed to use an external current sensing relay to switch out the start winding once the current drawn by the winding allows. Not having this fancy relay (they are still available but they seem to be used/available only in the U.S.A,) Instead I have an extra set of contacts on the back of the start button that power the start winding. Pushing the start button for a second or so holds the start winding energised during start up, once you release the button the start winding drops out leaving the main winding held on via the NVR contactor

Martin

That's pretty much what I did when the centrifugal switch died on a very old motor I had on my ML7. It was so old that in the 1990s there was no-one at the makers (Crompton Parkinson) who was working there when it was made. Despite this they sent me some capacitors to reduce the start current and a diagram of how to connect it as Martin has. Worked a treat, just need to use some intelligence, hold down the start button till it's got going. You don't get that sort of after sales from Banggood

Pleased to hear you have the motor sorted Mike. Martin those current sensing relays are fitted to most Fridges and I'm sure with a bit of Model Engineering ingenuity one could be adapted. They have three external Lucar type terminals and three internal in which three pins on the outside of the motor/compressor fit in a triangular formation, the top pin is the common.