Smart & Brown Questions

Hello I am John and have just joined today - I am a mechanic in Glasgow and have just bought a Smart and Brown lathe model A (Mk1) - I have a few questions about it, such as:

1. obtaining original used parts.

2. Wiring schematic for it (3 phase motor and switch controls)

My experience with forums is minimal; but I know there are specific rules to posting; perhaps someone could keep me right, so that i learn the accepted ways.

I am sure there is a wealth of information here, and I have also been drawn to forum participation because I see people with like interests really helping each other.

Cheers, John

Edited By JasonB on 26/10/2021 20:58:40

Welcome John

What are your interests and what do you plan to use your lathe for?

Don't worry too much about rules on here. Just be polite and not abusive, (goes without saying really) and make half a dozen postings before you try to buy or sell anything and you won't go far wrong. Oh, and don't try to post links to things for sale from eBay or the Far East.

Rod

Edited By Rod Renshaw on 26/10/2021 20:52:02

Edited By JasonB on 26/10/2021 20:59:03

Thanks Jason, what did I leave out or get wrong?

Rod

Thanks Rod,

I am an 'old school' mechanic, and love things built to last - old tricks of the trade - general engineering - but i've been a motor mechanic most of my life. I also have a hobby of making electronic circuits. I want to use the lathe mostly for making sleeves to press out bushes and bearings - and also to have fun making any special tool I need for automotive repair.

At the moment my lathe has a 3 phase cable going into what i think is called a 'drum' switch of the lathe (for forward and reverse) - and the original on/off latching push-button switches has its internals missing. I only have single phase in my garage, but the seller aso supplied me with an inverter which i will also be wiring up in due course. my preference would be to fit an original on/off switch (3 phase) if i can source cheaply enough - but failing that would consider fitting an emergency off switch integrated with on/off switch unit.

So that's the reason for wanting:

1. Sources for original used parts (on/off switch)

2. Wiring schematic (to know how drum switch as well as on/off switch was originally wired up)

So will appreciate any help - thanks again,

John

I moved both posts to a new thread as they were tacked onto the end of one about classified access

John,

Welcome. There are lots of experts on the forum. I’m not one of them.

Supplying your lathe motor with a 3 phase supply can be achieved in a several ways - have a three phase supply from the grid, a static (or rotary) converter or a variable frequency drive (VFD). There are other options.

The better way, IMO, is to fit a VFD. They plug into the usual single phase socket, provide lots of programming options, do not use (much) leccy while idle, are generally reliable - and reasonably inexpensive.

The one thing that most VFD installations do not like is switching between the VFD and motor. That may be why the 3 phase starter has been removed. Soft start, current protection, etc can be set, as desired, by the user. Even the forward reverse should not be used while the motor is running (ie only selected when the VFD is not supplying power to the motor. The VFD would have a directional selection, anyway.

I would be really miffed is someone was stopped from linking to one of my British made items on Ebay if they thought it would help out others on the forum. I do not think there is an overall ban on links to Ebay items (or is there) Only links to far eastern items, items that encroach on sponsers territory or items that may not conform to EU/UK standards are banned as far as I know. But I stand to be corrected if I am wrong.

regards

Jason,- thanks for that I was trying to spot the changes your edit had made and didn't think to look at the heading.

Oldiron - Yes, your explanation is clearer than my brief note.

John - I agree with NDIY about VFDs being the most useful drive for machine tools. Worth investigating. Usual advice with VFDs is "no switches between VFD and motor" because any switches might cause spikes which may cause the VFD to fail. This usually means one VFD per machine.

As NDIY says the VFD will be able to control the motor, which makes the old switchgear redundant. Some workers wire up the VFD low voltage control cables to the original switches to give the appearance of using the old switchgear, in which case you don't need the internals of the switchgear, only the actual switches.

There are experts on here who will advise on any specific aspect of your wiring but I think they will ask for more detail of the switchgear, VFD and motor.

Rod

Putting a switch between VFD output and motor is fine, if the switch is never operated while the VFD is running.

The output 3-phase bridge in a VFD will use MOSFETs or IGBTs as the switches. These transistors are pretty robust, with the exception of over-voltage. Voltage spikes are the quickest way to destroy the devices. The issue arising from switching the output while the VFD is running boils down to the law of induction:

v = -L(di/dt)

The voltage across an inductance is the self-inductance, L, times the rate of change of current. The minus sign indicates that the generated voltage is such that it tends to oppose the change in current. The coils in an induction motor have inductance. So if a VFD output is disconnected while running the coils in the motor are suddenly open circuit, with a current flowing. The voltage (back EMF) across the coils will rise rapidly as the current decreases in an attempt to counter the change in current. it is quite likely that the generated voltage will jump across the switch contacts and into the VFD bridge.

A secondary problem is that the motor currents are also flowing in the 3-phase bridge. The VFD busbars have inductance, so when the output is disconnected from the load the same problem of back EMF generated by the busbar inductance and change of current may also be enough to over-voltage the output switches. Never mind what the control algorithm may do when it sees the output currents decreasing.

It's best not to court disaster by wiring the VFD output direct to the motor. Direct wiring also makes it simpler to control radiated emissions from the output cables. The only controversial issue then becomes ground the output cable shield at one end, or both ends.

Andrew

i too have a smart and brown lathe a model m. best of luck finding parts there is not a lot out there. there is a smart and brown forum which might help i was on to but i have lost my link. some stuff turns up in the bay of thieves. i have been looking for a 95 tooth change gear for some time now and have decided the only i will get one is to make it which will be fun as i have no idea where to start yet

Thanks all for your help and advice: now I need more than my lunch break to think things over!

Cheers, john

I made a 127 tooth gear out of aluminium plate as a hobby machine will probably not wear an aluminium gear out in a hurry. A 95 tooth gear should be relatively easy as the 127 was about as large a disk as I could easily turn in the lathe. Since then I have converted my model m to CNC so I have the gears but never use them. However I have kept all the parts removed and all the gears to stay with the lathe when it moves on.

I still have the cutter and I have a suitable disk of 10mm steel plate so I will have a go at the 95 tooth and if it looks ok I will let you have it, It would just need a few washers either side to space it nicely. Then you can cut the 19tpi and 38tpi threads it is for.

Martin C

Welcome, John, there are a few of us using S & B model A lathes, I use the Mk 1.5 at the Helicopter Museum at Weston Super Mare. There are very few spares available, but Lathes UK do a useful set of manuals and parts lists. I always keep an eye out for listings on ebay, but most of the sellers want an arm and a leg for old parts. I gave up on getting changewheels and have made new ones out of metric mod 1 blanks which are similar in size to the originals.

If I remember, I will take a photo of the wiring schematic and post it on the forum for you.

Here are the wiring diagrams, I hope you can make them out.