Will I ever need a reversing switch?

[EugeneParticipant @eugene]

Refurbishing a Myford M type lathe

[EugeneParticipant @eugene]

Lads and lasses, beginners question ….

I'm refurbishing a Myford M type lathe and I'm now at he point of fitting a new motor and it's associated switch gear. Given that the spindle is threaded and so chucks can only work safely in one direction, what use is the normally fitted reversing switch? (These are traditionally the Dewhurst type but I won't have owt to do with them.)

I have a tumble reversing gear, so the lead screw rotation can be changed at will.

So, what use a reversing switch to me? Isn't just wiring the motor through a simple NVR switch enough?

Eug

[JasonBModerator @jasonb]

There are ways to stop the chuck unscrewing and it can be handy to have reverse for machining on the far side of the work for things like threading away from a shoulder or cutting tapers. I reversed my lathe today to cut a male taper and then cut the female without altering the topslide angle.

[Andrew TinsleyParticipant @andrewtinsley63637]

I have a Myford with screwed chuck and I frequently use reverse, for the sort of reasons that Jason outlines. Maybe you don't need to do reversals for your usage, but I would not be without the facility.

What is wrong with Dewhurst switches? They are excellent. Show me another switch that you can service! I made sufficient spare contacts for the half dozen that I use! I don't know if Dewhurst exists anymore or whether spares are available. But correctly set up they last a very long time , much better than the cheap Chinese switches that a lot of people use.

Andrew.

[NJHParticipant @njh]

Well Eug

There you are Jason has given you two good reasons…… however I bought a reversing switch some time ago and thought “I’ll fit that when I need it” ….. but so far I haven’t ! ( This is not really a fair comparison as Jason spends loads of his time in his workshop and produces prodigious and wonderous results… I pootle about, procrastinate, and produce little I’m afraid !)

…….having said that I’ll probably find a need tomorrow then all I have to do is find the switch……

Norman

Edited By NJH on 19/11/2017 20:04:05

[not done it yetParticipant @notdoneityet]

For most metric screw threading on an Imperial lathe reversing the chuck to drive back to the start position is a useful option (the same for a metric lathe cutting Imperial threads, too). It would be idling, while only driving the lead screw in reverse, and low speed – so safe. The alternative would be to wind the cutter back by hand.

[john swift 1Participant @johnswift1]

sooner or later you will need to be able to reverse the motor

all you need is a reversing direct on line starter

 

the starter contains 2 interlocked contactors

 

John

PS

the starter has  the same NO VOLT RELEASE   found with the more common start / stop direct online starters

Edited By john swift 1 on 19/11/2017 20:24:57

[OldironParticipant @oldiron]

I have a Boxford with a threaded spindle, it is also fitted with a VFD. I often used the lathe in reverse for threading etc. I only use it in reverse at low speeds and not under big loads. The VFD ramps down the speed then restarts in reverse. So no chance of a sudden change of direction. I do have one chuck with a locking screw and a copper plug to lessen the likelihood of it unscrewing in reverse.

As I was told many years ago " if a manufacturer thought the chuck would unscrew in reverse under normal use, why the heck did they fit reversing switches?

regards

[BazyleParticipant @bazyle]

Don't forget it is needed for typical metric threading methods.
Probably obvious but beginners picking up this thread the Dewhurst switch must not be used for the on/off and only thrown over when the lathe is not turning. Use an NVR or equivalent for the on/off.

[EugeneParticipant @eugene]

What is wrong with Dewhurst switches?

My first lathe, also an M type, had a Dewhurst fitted, and nothing else. So there was no NVR facility which I now regard as fundamentally unsafe; so too did the late Sir John of Blldgeport. Using it I managed to brush my hip against the lever and start the motor as I was attempting to change speed by shifting the Vee belt. Thus my body was against the revolving chuck and I had my hand on the belt. Not a comforting thing to experience.

And don't believe a chuck won't unscrew itself; I've had it happen. Circumstances …. big face plate on the spindle with an end mill in the spindle MT1 taper. I was using the face plate simply to ensure that parts held in the milling slide were square on, there was no work or tooling attached to it.

Inadvertently through tiredness or inattention I selected "Rev" rather than "Fwd" ; the face plate promptly unscrewed itself and dropped onto the end mill, so it didn't fly off the machine, but the cast iron thread was demolished, and the plate rendered useless. Fair to say I was running at warp factor 10 to get the mill closer to it's proper cutting speed.

Sir John would no doubt have applied the epithet "Clumsy bastard" and he'd have been right, but I was a very raw hand then and knew jack.

I see the metric thread point now, I didn't before, We need to keep the gear train together to ensure the thread geometry stays put, (the thread indicator dial being of no use) but the tumble reverse negates that. So the need is to reverse the whole shooting match, not merely the chuck ….. got it.

I think I'll fit a reversing switch, just in case I ever do need it, but not that cursed Dewhurst thing! Once bitten and all that. Something like the Crompton in John Swift's reply looks to be the thing.

Like I said, it's a beginners question, thanks for the help and input.

Eug

 

 

 

 

 

 

 

Edited By Eugene on 19/11/2017 21:26:44

[John McNamaraParticipant @johnmcnamara74883]

Hi

I have clutch reverse on my lathe and use it when threading in Metric to reverse the carriage, the imperial thread dial indicator does not work with metric.

If I was setting up another lathe I would look into a VFD to provide the reversing function, they can be set to speed up or slow down over a specific time, you can also set the maximum reverse speed. Less likely to undo a screwed chuck. if you gently ramp the speed up.

And variable speed for general work as well. you will still have to change belts for high torque loads in particular but a lot less often.

change

Regards
John

[HopperParticipant @hopper]

I've never had reverse on my M type and very rarely ever wished I did. For metric threads etc I just disengage the back gear (yes you can do that without disengaging the gear train between spindle and leadscrew) and wind the lathe backwards using the leadscrew handwheel. A bit slow but no slower but not too bad.

I even bought a reversing switch and mounted it but have not got around to wiring it up as I so rarely need it.

[not done it yetParticipant @notdoneityet]

very rarely ever wished I did….. I even bought a reversing switch and mounted it…

If that does not indicate indecision or contradiction, I don't know what does!

Clearly you wished for one enough at some point to go and get one mounted. Not wiring it in might indicate something other than not actually wanting it operational? I sometimes have a problem of "getting round to it".

[geoff walker 1Participant @geoffwalker1]

Hi Eug,

Simple answer with the dewhurst. Remove the handle, shorten it and then refit. No chance now of switching on accidentally and still plenty of leverage to operate the switch.

I would always have a reversing facility as I have no milling machine and require more versatility from the lathe, also an M like yours.

These set ups are a slitting saw on a morse taper milling arbour and on an arbour between centres. Reversing the spindle is essential here as when the work is fed forward into the saw via the cross slide you needs to up cut mill rather than down cut mill as would be the case with spindle running forwards.

Keep the dewhurst Eug. Just wire it in alonside your NVR

Geoff

[john fletcher 1Participant @johnfletcher1]

Dewhurst switches are for reversing once you have stropped the motor, then use your starter to restart the motor again. The contacts are flimsy and are not really adequate for continually starting and stopping the motor especially on an ML 7 without a clutch. I haven't seen the Chinese switches which Andrew refers to but have seen other electrical products and all so far have been good, especially their inverters for around £100. John

[Neil WyattModerator @neilwyatt]

Posted by Andrew Tinsley on 19/11/2017 19:52:52:

What is wrong with Dewhurst switches? They are excellent.

Show me another switch that you can service! I made sufficient spare contacts for the half dozen that I use!

One could argue that if it was up to the job you wouldn't need to keep making spare contacts…

Neil

[SillyOldDufferModerator @sillyoldduffer]

Posted by Andrew Tinsley on 19/11/2017 19:52:52:

…

What is wrong with Dewhurst switches? They are excellent.

…

Andrew.

No experience of them myself but they pop up in correspondence in old Model Engineer magazines quite often – usually people asking how to fix them plus a scattering of reversing under power and safety scares.

Here's what John Stevenson said about them on MadModder:

'I cannot see the fascination to use these drum switches. Is it just because they were fitted as OEM equipment ?
Whatever the answer technically they are illegal in that they are a not a no-volt switch.

Power cut for any reason and the lathe stops, power is restored and the lathe starts unattended.

Even Myford were forced by H&S to be dragged screaming into the 19th century on later models due to the inherent danger of these switches.'

Cheapo modern switches may not look the part but at least they force you to turn off before reversing. They're also cheap to replace.

Dave

[NJHParticipant @njh]

Dave

I don't think there is an argument IF they are fitted correctly.

The mains should enter via an NVR switch and THEN via the reversing switch. The sequence of operation should be to first select the direction of rotation ( by means of the Dewhurst) and then to operate the NVR switch to power up the machine. This prevents potential disasters in the event of a power cut and means that any arcing on power connection / disconnection is taken by the NVR switch contacts which are designed for this,

If the NVR is omitted then, in the event of a power failure, the machine will start up – with possible disasterous consequences – when the power is restored.

Norman

[DeanoParticipant @deano]

Reversing is one of those things you don’t need every day but when you need it, you need it. I’m glad of the facility on my 1960s Emcomat for some jobs, threading etc. The screw on chucks on mine have a locking collar so a bit more peace of mind. If you already have the parts, you might as well fit them. I had an M type a few years ago. Lovely things.

[David George 1Participant @davidgeorge1]

I have a reversing switch on my M type, if you reverse it cuts the power as reversing switch is interlocked so you have to start spindle after change over. I only use reverse when screw cutting where I cannot disconnect the lead screw because of metric etc.

David

[Mick B1Participant @mickb1]

I use reverse regularly:

• When sanding/polishing turned wood items – fibres get raised that can only be sanded off in reverse.
• When it's easier to chamfer something by projecting a tool past centre and cutting the chamfer on the far side.
• When backing a tap out of a hand-tapped hole.
• When I occasionally jam a drill in a hole in titanium or suchlke.

I could manage, but wouldn't want to be without it.

 

 

 

 

Edited By Mick B1 on 20/11/2017 21:55:57

[Andrew TinsleyParticipant @andrewtinsley63637]

I made some spare contacts once ,because all of my Dewhurst switches came from kit that had been used for many years. Most Chinese switches that I have seen, would not have lasted that long anyway! So I don't see making a few spares as proof that the switches are no good. Most trouble comes with idiots switching from forward to reverse instantly. I don't cater for idiots.

I have no problems with including a NVR switches, a good thing in my opinion. I brought my Dewhurst switches over a period of time and never paid more than £30, sometimes a lot less. The type of switch illustrated in one of the responses above, are excellent too, but considerably more expensive than my Dewhursts, even when purchased second hand.

Andrew.

P.S. My workshop is not approved of by Health and Safety and I really don't care.

[john swift 1Participant @johnswift1]

Hi Andrew

the reversing DOL starters like the one in my 19/11/17 post at 20:15 use a double contactor assembly like this

it could be incorporated with overload protection inside the machines electrical cabinet instead of the external box

on 3 phase machines the mechanical interlock makes it safer when you put the motor into reverse to stop the motor quicker !! – both the contactor & motor will have to be rated for this use

( I have known the brief application of reverse without stopping as "plugging" )

with single phase motors you will have to stop the motor before reversing

John

[HopperParticipant @hopper]

Posted by not done it yet on 20/11/2017 06:37:12:

very rarely ever wished I did….. I even bought a reversing switch and mounted it…

If that does not indicate indecision or contradiction, I don't know what does!

Clearly you wished for one enough at some point to go and get one mounted. Not wiring it in might indicate something other than not actually wanting it operational? I sometimes have a problem of "getting round to it".

Yes I need to get one of those "round tuit's".

Probably will happen when my need for reverse is greater than my reluctance to crawl under the lathe's bench to access the underslung motor and try to figure out the wiring on my Australian-made motor that does not match the terminal lettering/numbering protocol described in all the UK-based literature on how to wire up a lathe reversing switch for a non-capacitor type motor. Probably the next time I need to turn a long metric thread.

Meanwhile, I do my milling much the same way as Geoff W shows in his pics, but start the cut with the job behind the slitting saw and bring it forward to make the cut.

[Neil WyattModerator @neilwyatt]

Posted by Andrew Tinsley on 20/11/2017 22:30:09:

I made some spare contacts once ,because all of my Dewhurst switches came from kit that had been used for many years. Most Chinese switches that I have seen, would not have lasted that long anyway!

The one on my Mini Lathe is huge and was wired so it tripped the NVR as you went through stop. It's a failing of the Dewhurst that it doesn't do this.

The SC4-510 has low voltage 'tactile' forward and reverse switches. I pressed when running as an experiment – a bit more demanding with 1kW of motor… The lathe came gently to a halted, waited a fraction of a second, then accelerated up to speed in the opposite direction. A lot to be said for electronic control, although i obviously can't make any statements about longevity yet.

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