Adding NVR Switch

Hello,

I am setting up my first lathe, a Myford speed 10 which has what I believe is called a Dewhurst switch. Some articles I have read have suggested I should have an NVR also. Is it just a case of wiring this in before the Dewhurst? And is it really necessary (I imagine not for it to work, but I would like to be as safe as possible)

thank you

A NVR (No volt release) is a good idea for any/all workshop machines. Should the electrical supply be interrupted, even momentarily, the NVR will open and the machine cannot restart until you do it manually, i.e. knowingly. The most dangerous situation (IMHO) is when the supply fails for a few seconds and is then restored. In that few seconds, one is working out what is going on and has time to place a hand where it should not be, just in time for it to restart. Not good.

My workshop has two RCDs on the supply, one each for the light and power circuits. The power circuit also has a NVR. In this way, every machine is protected against unexpected restarts. NVRs on each Individual machine is another alternative.

I recently bought an Emco Compact 8 lathe which just had the standard Fwd/Off/Rev switch, so I added an NVR switch AND a big red emergency stop button before the standard switch. Cheap enough to do and makes me feel happier when using the machine.  Here's a simple way of doing it - http://www.warco.co.uk/accessories/346-nvr-switch.html

Edited By Gordon Tarling 1 on 13/08/2015 08:31:59

This is only my experience, but apart from the safety advantages of having an NVR, if you use your lathe a lot you may find the pins and springs inside a Dewhurst switch eventually burn. If the Dewhurst is backed up by an NVR, or even a more simple on/off switch, then you can just use the Dewhurst for selecting direction of rotation with the power switched off. The result is no internal arcing inside the Dewhurst, which results in a very long life.

This has been covered a couple of times before Richard - but NVR switches are not expensive and very easy to fit. I find mine very convenient in use and have retrofitted most of my larger machine tools now - as well as having a couple mounted on ply blocks with extension leads (that I use with portable tools) typically clamped to a Workmate or similar. I get them from Axminster.

Regards,

IanT

Another good reason for fitting a NVR between the incoming supply and the Dewhurst switch is to relieve the Dewhurst of having to act as the on/off switch and just leaving it to select forward or reverse (with the power off). Dewhurst switches are from a long ago era of electrical engineering and although they were robustly made they are susceptible to contact arcing when used to make and break a circuit driving a motor which eventually causes the contacts to fail due to burning.. Sooner or later they will fail. With a single phase motor this failure often shows when the motor buzzes but fails to rotate because the start winding is not being energised. Over the years I have experience several such failures with Dewhurst switches failing through burnt contacts.

When "fed" via a NVR (or other switch) they will last forever as the contacts will not be subject to arcing.

The ones IanT shows in his photo are fine - ebay etc. There is another type that has a sort of plastic latching part that fits over the off button. Personally I am not to keen on those even though they may be cheaper. They are irritating to use as an on off button as the latching part gets in the way.

John

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Edited By John W1 on 13/08/2015 12:29:53

Yes Dewhurst switches are from another ear - built to last. They do too but like most things how ever well they are built can wear out.

John

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DOL starters do the same job and are cheaper FWIW...

**LINK**p95364

I also added one of these to my lathe as it's a latching stop button which prevents the machine being turned on until the stop button is unlatched.

**LINK**

HTH

Jon

Stationary Engine Parts do a good quality replacement switch similar to Dewhurst for £28.20.

The nvr switch on my miller had starting to act up so I bought one of these which are nice and compact.

**LINK**

I've also used them on machines that I no loner own.

John

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Thanks for all the advice, including about saving wear and tear on the Dewhurst switch. I shall definitely get one fitted.

I got a new Myford VMB, the only thing that ever failed was the NVR....

I would suggest that the NVR's will fail over a much shorter time period than a dewhurst switch. I've had several lathes with them on and not one failure. My mill has only seen hobby use and not much at that - the nvr has failed. An industrial one as well.

John

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