Myford / Dewhurst

[Rod AshtonParticipant @rodashton53132]

Once again my venerable old ML7 has arcing problems in the Dewhurst switch. Reading about the inherent lack of safety in these units, could anyone recommend a direct safe replacement please. (Will need a lego-brick wiring diagram for the electrically challenged.)

[Rod AshtonParticipant @rodashton53132]

Dewhurst switch arcing

[davebParticipant @daveb17630]

You can fit a DOL starter before the Dewhurst switch. If you clean the contacts it should be OK. The problem is caused by arcing when switching off. Bear in mind that these have been around for 50 years or more and have worked trouble free for most of them. I doubt if they satisfy modern design requirements but the only safety issues I've heard of relate to the possibility of accidentally knocking the lever and unintentionally switching on the machine and that in certain wiring configurations, one pole remains live.

If you fit a DOL starter, the machine is switched on and off with this, the Dewhurst switch is the used to select for/rev only so is switching no load.

Dave

Edited By daveb on 23/10/2016 14:51:22

[Les Jones 1Participant @lesjones1]

Hi Rod,
Arc Uro Trade sell some spares for Sieg lathes that will serve your purpose. They have a reversing switch (C6-205) that will replace the Dewhurst switch and they also have an NVR switch (SC3-104) that would suit.If you look at my entry in this thread at 10:47 on 17/4/15 I have posted a circuit of how to connect them. Your motor may have different labels on the connections to that shown in the diagram. (In the diagram U1 and U2 are the run winding and V1 and Z2 are the connections to the start winding via the capacitor and centrifugal switch.) You will have to buy or make an enclosure to mount these switches.

Les.

[Glyn DaviesParticipant @glyndavies49417]

A MOELLER T0-4-8213 cam switch will fit in the Dewhurst switch housing

There are sixteen terminals on the switch body, four pairs of which were already linked by Moeller. You need to add additional links as follows: 1-7, 3-5, 9-11, 13-15, 4-10 and 8-14. The switch wiring is then Live in to 2, Neutral in to 6, motor start winding Live to 9, motor start winding Neutral to 13, motor run winding Live to 1 and motor run winding Neutral to 5. You may need to swap 1 and 5 to get the motor to run forwards with the switch set to forwards.

Edited By Otley on 23/10/2016 17:18:23

Edited By Otley on 23/10/2016 17:18:45

[EmgeeParticipant @emgee]

Otley, wouldn't it better to change over the motor start winding ends to reverse directon ?

Emgee

[Rod AshtonParticipant @rodashton53132]

Thank you all for your suggestions.

[Jon GibbsParticipant @jongibbs59756]

Hi Rod,

+1 for daveb's comments and for a cheap DOL starter you can do a lot worse than the Toolstation one for £20 delivered…

**LINK**

I have one on my ML7. It's big but makes wiring easier and works just fine.

HTH

Jon

[john fletcher 1Participant @johnfletcher1]

On the Dewhurst switch there are contacts at the lower part which are not used normally, you maybe able to swop your burnt contacts for them, I have done it for my friends in the past. When taking switches to pieces, do it a large clear plastic bag, that way you don't lose that important clip, spring or contact. As others have said, you should use a proper motor starter for the On/Off, and use the Dewhirst for direction only. The starter from Toolstation look to be a good buy with free delivery. John

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