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Dewhurst switch!

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Jacob Williams11/03/2020 08:41:28
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5 forum posts

Good morning Gentlemen, i have my first  Myford 4 inch precision lathe, when i got the lathe the was no switch just a 240v household plug.

ive been using the lathe like this for a little while but reliase that i need a proper switch for it. it has the mount for a dewhurst switch , so does anyone have a spare laying round they would want to sell??

 

im also in the market for a small bench mounted mill or even just a pillar drill to get me started.

cheers

Edited By Jacob Williams on 11/03/2020 08:44:49

noel shelley11/03/2020 08:55:46
1435 forum posts
23 photos

YES ! I have a dewhurst switch of the type you need, new never been used and the wiring diagram. Send me a PM or call on 07860 711926. I may be at work so it may be this evening before I reply. Noel.

Swarf, Mostly!11/03/2020 09:06:48
678 forum posts
78 photos

Hi there, Jacob,

I strongly urge you not to depend solely upon the Dewhurst switch to control your lathe motor. You should also incorporate a 'No Volt Release' switch to control the start/stop of the motor and only use the Dewhurst switch to reverse the direction of the motor - switching to the direction you need next while the motor is de-energised. By modern standards, the Dewhurst switch is not capable of disconnecting the motor without long-term contact damage.

The no-volt release property ensures that the motor will not restart without warning if it has stopped due to temporary loss of power 'up-stream'.

I use a Dewhurst for reversing but it is wired in conjunction with a single phase 'Memdol' direct-on-line starter that also has no-volt release and overload protection.

Best regards,

Swarf, Mostly!

Edited By Swarf, Mostly! on 11/03/2020 09:07:21

Mike Poole11/03/2020 09:33:09
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Moderator
3374 forum posts
76 photos

+1 for Swarfs comment, although the Dewhurst has long been a popular fitment on Myford lathes it does suffer from contact burning as it does not make and break quickly. It also has no overload protection for the motor. They make a useful reversing switch but with a screwed chuck reverse should be used with caution. Reverse is handy for activities like metric screw cutting where you don’t want to disengage the halfnuts but a heavy cut in reverse could unscrew the chuck. Myford did move to using a motor starter rather than the Dewhurst probably because it offers better protection for the operator and the motor which are both absent with the Dewhurst.

Mike

Clive Foster11/03/2020 09:37:08
3172 forum posts
113 photos

#2 to what Swarf Mostly says.

The Dewhurst is a direction control and switch with a central power off position. And a good one too. But the relatively slow make and break action of the contacts leads to arcing which will eventually destroy the switch. Home Shop guy (or gal) is unlikely to live long enough for that to happen tho'.

However the central off position is a mere sprung detent so fairly easily inadvertently knocked which will set the lathe running with consequences from Ooops, naughty words upwards. Its not that easy to quickly go back to off in an emergency either.

Proper NVR switch has nice button that are hard to hit by accident and can easily have an emergnency stop with nice big mushroom button added. Along with other safety interlock switches if need be.

NVR switches automatically go off and stay off if the power goes down. Sothings won't start up unexpectedly when it comes back. The common DoL (Direct on Line) motor starter box is an NVR switch.

Much to be said for connecting the lathe work-light to the same power input as the NVR switch so if the light is on you know there is power available and the lathe can be run. Sometime you need to be sure Turned Off really means Turned Off.

Clive

Speedy Builder511/03/2020 11:04:53
2642 forum posts
217 photos

Question. Why does a Dewhurst switch have a centre OFF position if it is not meant to be used as an ON/OFF/Reversing switch? I agree that arcing does take place however after 20 years of intensive use I find the damage to the rollers to be minimal. My switch is used on a Boxford fitted with a 240V 3/4 Hp motor.

I agree that there should be an NVR switch in the circuit - perhaps more than one so you can have emergency stops as well.

Perhaps next week it will fail !!

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