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Possible Myford 7 safety issue

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John Haine02/02/2022 10:08:32
4714 forum posts
273 photos

In another thread I mentioned that I'd found a potential electrical problem on my Myford S7 lathe, bought new about 15 years ago. I thought I'd mention it in a separate thread to alert Myford owners to the issue.

I am in the process of fitting a 3-phase motor/VFD and preparatory to this I removed the swarf guard from the back of the motor. I found there was a short green/yellow wire connected at one end to the motor frame and the other connected to - nothing! The screw tag on the free end had fractured and there was no obvious place that it should have been connected. It seems that the machine earth enters the starter box and is directly connected to the motor cable, and the machine relies on this little wire tail to earth the who thing. As it's hidden behind the swarf guard the break wouldn't be noticed.

So it may be a good idea to check your machine and make sure that it's properly earthed. Looking at the wire it was designed to fail, being very short and subject to the motor vibration and shocks on starting.

ega02/02/2022 10:44:17
2565 forum posts
203 photos

I'm an electrical ignoramus but it occurs to me that the reason for the additional earth was the rubber resilient mount of the motor preventing the standard earth from earthing the whole machine?

I have the Newton Tesla package on my S7 and wonder what effect this has on the issue you have identified.

John Haine02/02/2022 10:45:04
4714 forum posts
273 photos

It wasn't an additional earth, it was the only one!

noel shelley02/02/2022 10:47:31
1442 forum posts
23 photos

EGA has it ! The motor is resilient mounted, so the link wire eaths the motor to the rest of the mount and therefore the whole macine ! Noel.

ega02/02/2022 10:58:24
2565 forum posts
203 photos
Posted by John Haine on 02/02/2022 10:45:04:

It wasn't an additional earth, it was the only one!

I see what you mean, although I suppose the motor itself was earthed.

Ironic that the highly desirable swarf guard (which I don't think was standard on earlier machines) prevented you from noticing the fault.

Clive Steer02/02/2022 11:03:26
99 forum posts
5 photos

Most modern electrical products are considered to be stand alone devices and do not require an extra earth (supplementary bonding) either because of where they are allowed to be used or because of their design.

There have been several forum articles about users wanting to change/upgrade the electrical features of their machines which they will do without compromising electrical safety.

My main point, although I may not have explained it very well, is that an extra earth wire attached to a machine is relatively easy to do by someone with little electrical understanding and can make a machine safer if for any reason they get it wrong.

Things have changed in 50 years with site electrics needing 110V equipment and isolation transformers, houses needing RCD's and MCB's, kitchens and bathrooms having special electrical installation regulations, the re-introduction of earth stakes because gas and water pipes can't be relied on to provide an earth, the introduction of more stringent product design requirements etc.

Whatever happens in a machine by way of failure or modification just makes sure that the main bits you are likely to grab hold of are earthed.

Clive S

 

Edited By JasonB on 02/02/2022 11:04:36

Philip Rowe02/02/2022 12:40:09
230 forum posts
31 photos

One thing that I believe a lot of contributers may be overlooking, is that for many years a Myford lathe even when new was not supplied with a motor, it was always an extra. Consequently a lot of machines that are around today only ever had an owner fitted motor and who knows what sort of standards where applied in those instances. Phil

KWIL02/02/2022 13:29:33
3562 forum posts
70 photos

Of course the Brochures show "machines (less motor)", the option was always for the Purchaser to specify a single or 3 phase motor dependent upon the intended use. The Dealer would then oblige and supply with the motor as desired.

Dave Halford02/02/2022 13:34:43
2091 forum posts
23 photos
Posted by ega on 02/02/2022 10:44:17:

I'm an electrical ignoramus but it occurs to me that the reason for the additional earth was the rubber resilient mount of the motor preventing the standard earth from earthing the whole machine?

I have the Newton Tesla package on my S7 and wonder what effect this has on the issue you have identified.

What does the supplied installation document say?

Dave Halford02/02/2022 14:19:57
2091 forum posts
23 photos
Posted by Dave Halford on 02/02/2022 13:34:43:
Posted by ega on 02/02/2022 10:44:17:

I'm an electrical ignoramus but it occurs to me that the reason for the additional earth was the rubber resilient mount of the motor preventing the standard earth from earthing the whole machine?

I have the Newton Tesla package on my S7 and wonder what effect this has on the issue you have identified.

What does the supplied installation document say?

Apologies for the pointless question, the other Newton thread says there isn't any.

Dave Halford02/02/2022 14:51:53
2091 forum posts
23 photos
Posted by ega on 02/02/2022 10:44:17:

I'm an electrical ignoramus but it occurs to me that the reason for the additional earth was the rubber resilient mount of the motor preventing the standard earth from earthing the whole machine?

I have the Newton Tesla package on my S7 and wonder what effect this has on the issue you have identified.

If you have the package with the moulded plugs then there's no effect at all.

If you then modify the installation with lights and or a DRO with separate plugs then you bring extra live feeds that may supply risk of shock if the Newton unit is unplugged and the extra feed cables have been damaged and touch the lathe..

not done it yet02/02/2022 14:52:47
6887 forum posts
20 photos

Unless this machine was switched only at the plug socket, to which it was likely connected, there will be a switch between the mains and the motor - so likely the machine is connected to an earth in there and therefore protected.

The only part not earthed would be the motor (due to that insulated carrier) so only the motor will not be earthed. Not a safe situation, even if the rest of the machine is earthed. Relying on RCDs, etc is not a good way to avoid a 240V electrocution is not too good where an initial shock could cause contact with revolving machinery.

ega02/02/2022 15:07:50
2565 forum posts
203 photos

Dave Halford:

No apology necessary! I have the PDF manual which, on searching, turns out not to include the character strings "earth" or "bond".

My unit has the moulded plug, its steel enclosure is screwed to the steel lathe stand and the NT motor (three phase, of course) has a solid foot mount ie not the resilient mount favoured for single phase motors.

Dave Halford02/02/2022 15:32:56
2091 forum posts
23 photos
Posted by not done it yet on 02/02/2022 14:52:47:

Unless this machine was switched only at the plug socket, to which it was likely connected, there will be a switch between the mains and the motor - so likely the machine is connected to an earth in there and therefore protected.

The only part not earthed would be the motor (due to that insulated carrier) so only the motor will not be earthed. Not a safe situation, even if the rest of the machine is earthed. Relying on RCDs, etc is not a good way to avoid a 240V electrocution is not too good where an initial shock could cause contact with revolving machinery.

I need to try harder.

If, whatever is suppling the lathe with power is unplugged from the mains it will remove the earth point . My lathe gets unplugged to run the compressor. This much like the waterpipe in our houses leaves a large lump of partially bare metal unearthed which is why plumbing is earthed.

John Haine02/02/2022 15:59:21
4714 forum posts
273 photos

Just to clarify.

  • On my lathe, bought new in the early 2000s complete with motor and switchgear fitted, the mains comes in to a grey plastic box mounted on the left hand side of the cabinet. This houses the NVR/start-stop switch, estop switch, and reversing switch.
  • The box is mounted by being itself bolted internally to a substantial steel plate that in turn is bolted to the cabinet.
  • Inside the box, the incoming mains goes via a large connector plug and socket (the kind that's used for caravan mains connections) to the switchgear. L and N go to the reversing switch and NVR, while the only connection to incoming earth is the earth wire on the motor cable which comes through a strain relief gland.
  • At the other end that cable is terminated inside the motor where presumably the earth wire is connected to the motor frame.
  • That frame is then held in the mounting foot by the resilient mounts which insulate the bracket from the motor. The short green wire has one end fixed by a crimped on tag to the motor frame, and the other end was presumably connected to the foot to complete the earth connection. However, the crimped tag to do this has fractured so there was no connection.
  • This was invisible when the swarf guard was fitted so until I set about getting ready to fit the 3 phase motor the fault couldn't be seen.

I'm relieved to note that the 3-phase motor shouldn't have this problem as it doesn't apparently have a resilient mounting.

So a simple message - if you have a 7 series lathe with factory fitted single-phase motor and switchgear it might be wise to check the earth connection to the lathe metalwork is sound.

old mart02/02/2022 19:12:28
3892 forum posts
268 photos

While the motor in your modern mains powered electric drill will be double insulated and not require an earth cable, machinery motors either single or three phase are not double insulated and must be earthed and an eathting point will be provided.

Edited By old mart on 02/02/2022 19:12:50

Howard Lewis02/02/2022 19:26:29
6305 forum posts
15 photos

The swarf guard is a most important part of the installation.

My early ML7 did not have one. One New Years Eve afternoon, there was am almighty flash and nothing worked. The fuse had blown.

Rushed the motor to the local rewinders. they said that swargf had tntered bthe centrifugal swict and shorted.

They tidied up the switch and got the motor running again.

Having refitted the motor, an Alumiiium guard was fitted to allow ventilation, but not swarf entry. No more problems.

So: the single phase motor being resiliently mounted needs to be nonded to the rest of the machine, vis its baseplate. Otherwise a short to the motor frame will render it live.

Having said that, as long as no fuse blows or RCD trips, unless we make a habit of caressing the motor frame, we are unlikely to be aware of such an event!

If the only plug supplying power to the machine is unplugged, other than any charge remaining in the start capacitor, there will be nothing live on the machine, and hence no danger, at that time.

Howard

Martin Kyte02/02/2022 22:01:57
avatar
2786 forum posts
53 photos

To my mind having read the original post and John Haine's clarification as the motor is earthed to it's metal casing that is sufficient to protect the user.

Howard Lewis wrote "So: the single phase motor being resiliently mounted needs to be bonded to the rest of the machine, vis its baseplate. Otherwise a short to the motor frame will render it live." However as the casing of the motor frame is earthed it cannot be made live without blowing the mains fuse or in these more enlightened time throwing the RCB on the workshop electrical distribution system.

The above in my opinion is valid if the motor is the only part of the lathe connected to the mains where the rest of the machine is merely a complex bracket to hold the motor electrically speaking. Earth bonding is a question of interpretaion, sometimes helpfull sometimes not. In the case of the Myford resilliant mount single phase motor having a seperate earth bond to the machine could in some instances make things worse. Suppose that the eath bond to the internal casing of the motor were to fail and the case become live them a hand on the metal stand and one on the casing would be possibly fatal. A good ground in one hand and a good connection to live in the other with the current through the chest. If the lathe is floating however the resistance to ground is much higher and therefore less likely to proove fatal.

A rough guide is if a live connection to skin can likely be made then bare earthed metalwork is not good news. If all mains souces are remote the earth bonding is benificial. This is why there are different rules for kitchens as opposed to bathrooms.

regards Martin

Ian P02/02/2022 22:51:26
avatar
2594 forum posts
114 photos
Posted by Dave Halford on 02/02/2022 15:32:56:
Posted by not done it yet on 02/02/2022 14:52:47:

Unless this machine was switched only at the plug socket, to which it was likely connected, there will be a switch between the mains and the motor - so likely the machine is connected to an earth in there and therefore protected.

The only part not earthed would be the motor (due to that insulated carrier) so only the motor will not be earthed. Not a safe situation, even if the rest of the machine is earthed. Relying on RCDs, etc is not a good way to avoid a 240V electrocution is not too good where an initial shock could cause contact with revolving machinery.

I need to try harder.

If, whatever is suppling the lathe with power is unplugged from the mains it will remove the earth point . My lathe gets unplugged to run the compressor. This much like the waterpipe in our houses leaves a large lump of partially bare metal unearthed which is why plumbing is earthed.

If the lathe is unplugged (therefore not connected to the mains) it does not need to be earthed! Its just a large lump of metal.

Ian P

Clive Steer03/02/2022 10:17:41
99 forum posts
5 photos

I'm not sure that some people understand even the basics of ensuring electrical safety.

The first rule is prevention. So at no time should anyone come into contact with a potentially lethal voltage.

If it does happen other factors will determine if the outcome is fatal or just a warning tickle or the RCD tripping if you have one. Arguing that if a machine is earthed it may make the situation worse is very questionable.

Safety improvements seem to be driven more by hindsight than foresight with train, car and aircraft safety being good examples of our failures of foresight.

Let us at least apply what hindsight has taught us.

Clive S

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