Sieg lathe motor conversion

If you look at the last photo in my original post, you will see that the lathe controls are on a pendant, with a cable which connects to the inverter. This is located on the bench at the tailstock end of the lathe, so well out of the way. Before investing in the 3 phase inverter system, I had the original motor fail. (shorted windings on the armature). Bought a replacement motor (about 100 beer tokens) and fitted it. A few weeks later the control board died. (probably damaged by the motor failure). Rather than pay for replacement board, which are notoriously unreliable anyway, I decided to splash out on a full 3 phase inverter system. I selected a half horse power motor (about 320 watts), which was as near as I could get to the original 250 watt motor.
The lathe is actually raised up to a comfortable working height, by mounting it on two pieces of thick walled 80mm square tubing. This also enabled me to fit a home made coolant system underneath. I fitted a longer tube at the headstock end and made and fitted a 10mm plate to take the motor (see photo) The whole mod was done for reliability as I was fed up with unreliable ,expensive Chinese crap.

Good stuff. Well thought through. I like the idea of the controls on the pendant - I missed that. Are the 2 square section steel supports joined other than by the lathe? All the components joined together make a much stiffer support that helps keep the lathe true (I know the problems of weak supports because my lathe is poorly supported!).
I think you will enjoy your new motor?
K2 ☺

Steamchick said:

Nikhil, FYI: recently I was cutting threads on some brass and bronze fittings - really easy to machine, but reached the limit for the lathe.

Click to expand...

HI,
The last time I run my lathe was about 1 month back. I made a backplate for my new 5" 4 jaw self centering chuck. The lathe was cutting the steel plate quite easily. DOC was about .3mm and rpm 200 if I remember correctly. Now after reading your post I have to plan and fix the lathe to something stiff. It is only fixed to the chip tray it came with originally and is kept on top of a 30mm thick stone slab table top.

Regards
Nikhil

Hi Nikhil, Good idea. It is what I should have done many years ago - before I joined this site and learned about things I didn't learn at school, or during my apprenticeship (because the machines were already installed and set-up).
There are "proper people" on this website who can advise and help better than I can, so when you want to ask I suggest you raise a new thread?
I may know some stuff, but not all, and not all that is "best practice".
K2

Hi Nikhil, I have a larger lathe, that very crudely, I can stop the motor with a wrench using less than the torque I apply to car wheel nuts (80ftlbs).
But doing some very approximate modelling of the stiffness (actually deflection under load calcs.) of the lathe bed versus 2 in square and 3 in square steel tube, I conclude that the lathe bed (cast iron, from China, is only about as stiff as the 2in. square tube. So to get a GOOD stiffer base I need at least a 3in square steel tube (rectangular) welded frame bolted securely to the feet of the lathe bed. This should halve the deflection of the tool when cutting at the further part of the lathe, approx 20 inches (0.5m) along from the headstock.
I guess you have a smaller lathe, less torque? Shorter bed? slimmer and therefore the bed is not as stiff?
But if you can bolt it securely, or clamp it to the stone slab, then that will make a big difference.
You can do some checks. Use a torque wrench on a bolt fitted into the chuck, to measure the stall torque from the motor. - Don't power the motor stalled for more than a couple of seconds! A quick "ON and OFF" is all I do... with controller at lowest speed. Then apply this torque (using your super muscles!) to the chuck while holding the lathe on the table at the furthest setting of the tailstock. You will probably "feel" the twist as it lifts a foot off the surface? - BUT you need to be able to lock the mainshaft (safely) somehow.
You can of course do some sums, or look-up the motor's torque details and do the sums on the gear (pulley) ration to the mainshaft to see what torque you can get at the chuck...

Enjoy!
K2

Steamchick said:

Hi Nikhil, Good idea. It is what I should have done many years ago - before I joined this site and learned about things I didn't learn at school, or during my apprenticeship (because the machines were already installed and set-up).
There are "proper people" on this website who can advise and help better than I can, so when you want to ask I suggest you raise a new thread?
I may know some stuff, but not all, and not all that is "best practice".
K2

Click to expand...

It would seem that the number of people "who know how to set up things" is shrinking.
Ditto for me - - - when you're learning or working - - - machines are already set up - - - - setup - - - I think - - seems to be a different kind of art form.
(Hmmmmm - - - wondering - - - - anyone on here that has a background in machine setup and installation?)

ajoeiam said:

It would seem that the number of people "who know how to set up things" is shrinking.

Click to expand...

Hi,
We recently got a new lathe installed on the oil rig where I work. The shipyard people just made a base of heavy angle plates and bolted the lathe to it. No checking of anything was done.
Lathe was purchased only to fulfill some commercial requirements and to put a checkmark on some checklist. The lathe will be used once in a blue moon and that too for non precision work. We don't have a trained machinist to work on the lathe. All our lathe jobs are done shore side at a proper workshop.

Regards
Nikhil

Heavy angle plates welded to an oil rig sounds stiff enough to me, apart from the induced twisting distortion when they tighten down the last mounting bolt with heavy hands on the spanners to pull the last gap closed... "What? It must be shimmed? It'll do for what we want!"

K2

Nikhil Bhale said:

Hi,
We recently got a new lathe installed on the oil rig where I work. The shipyard people just made a base of heavy angle plates and bolted the lathe to it. No checking of anything was done.
Lathe was purchased only to fulfill some commercial requirements and to put a checkmark on some checklist. The lathe will be used once in a blue moon and that too for non precision work. We don't have a trained machinist to work on the lathe. All our lathe jobs are done shore side at a proper workshop.

Regards
Nikhil

Click to expand...

A precision level doesn't cost that much - - - - too bad.
After a while bolted down hard and not level the bed will develop a nice warp!
Precision - - - if there's enough stress - - - well it might even do some other funky things.

Maybe you could offer to cross train or what ever its called so that you could also run the workshop equipment.
(If you're comfortable running a 8" x 14" lather you can certainly run a 18" x 40" one - - - its not that different. )

Nikhil Bhale said:

Hi,
We recently got a new lathe installed on the oil rig where I work. The shipyard people just made a base of heavy angle plates and bolted the lathe to it. No checking of anything was done.
Lathe was purchased only to fulfill some commercial requirements and to put a checkmark on some checklist. The lathe will be used once in a blue moon and that too for non precision work. We don't have a trained machinist to work on the lathe. All our lathe jobs are done shore side at a proper workshop.

Regards
Nikhil

Click to expand...

I think what is shrinking is the number of people who imagine that the manual might tell them anything is striking. And the number of people who can read, understand, and follow instructions is shrinking to zero

ajoeiam said:

After a while bolted down hard and not level the bed will develop a nice warp!

Click to expand...

Hi,

A ship or Rigs deck plating are never rigid. There is always some flexing, bending or twisting going on depending on sea condition or loading condition. The engines and propeller shaft are supported by additional strengthening frames to minimize this. Additionally there are always some vibrations transmitted throughout the structure. So unless the lathe support are very rigid it will develop some warp. Or the lathe may be supported on some stiff spring to reduce this, I have no idea.
Maybe someone who has worked on ships where lathe is used extensively can shed some light on it. Some ex navy machinist maybe

Regards
Nikhil.

Having just experienced my second failed motor and controller... I have a sorry tale to tell.
It started with the 750W DC PM motor making odd "slow" moments, then it stopped. I guessed the brushes had lost length and contact.

The original motor brushes lasted about 10 years, but hardly as much use as since a 2021 rebuild and new brushes. So as I use the lathe a lot more (now retired) I expected more brushes needing to be serviced. I measured the last worn ones, 12 mm and 15 mm long.

The latest ones were also 12mm and 16mm long.
At 12mm, the brush does not contact the commutator any more. The spring fouls the holder and can't push on the brush, that is disappearing down inside the holder..
Having fitted new brushes and cleaned the brush holder (a place where I had experienced many flash-overs and consequential blown thyristors previously 2021) and added some extra silicon sealant where I suspected there may have been new flash-overs, or probably just old scars (Silicon has 3 x the dielectric strength of air. Sharp corners and pointed parts on brush holders cause arcing where air gaps are <1 or 2mm.).
Then the newly brushed and clean motor flashed over! - Repeat - 3 times. Used the AVO meter to check no shorts t earth, - Check, check, check = seems OK. Then the fuse blew, and domestic Circuit breaker popped.
Now found an arced track on another circuit board (~2 Ohms! live to Neutral!) for a low voltage DC supply for the speed meter and sensor.

- Cleaned that and it is now OK,
Then the Start-Emergency Stop button would not latch... replaced that. (Why are all these bits failing?).

So I tried the variable speed - without motor wired in - but using a 60W filament bulb as the load. Ok at "low speed" on the rheostat, but as I reached "half to 3/4" full speed,
BANG! - The fuse holder exploded and pieces of plastic and glass hit me like a bullet.
So my expert electrician tested everything. He has a 1000V Mega tester, and it showed the faults to earth at 250V...
All the thyristors blown on the variable speed board. Last repaired 2021. Motor armature fault to Earth - all windings.
So I surmise that the worn brushes caused some overload as the controller raised voltage to try and keep the motor going, as it was suffering poor contact of the worn brush. The excess voltage had flashed over - causing maybe 1 or 2 thyristors to fail. Subsequently, the other thyristors failed until the last one gave an adequate short circuit to explode the 10A fuse and holder.
So please beware of any strange noises from DC motors, ("phut", "spop", etc. - in Motor language that is a little cry for HELP!)) clean the carbon from brush holders frequently, and replace brushes before they get down to 12mm long.
I am now risking an alternative, having ordered a servo motor and control board from China. The only option to avoid major engineering of fitting a proper IEC frame 3-phase motor and controller...
When I receive it I shall advise how it fits, works, etc... so watch this space for the next exciting episode of "Ken playing with mains electricity!" - Hope I survive to write it, and not end up as "fried Chicken"!
Wish me luck?
K2

In my opinion, those motors and electronics to allow variable speeds on these off shore manufactured and smaller machine tools are another item where your locked into replacement parts from the factory. And that's if they still even support what you have with those direct replacement parts in the future. Also fairly expensive for most of what seems to be some failure prone factory parts unless your lucky enough to have a good back round in electronics, trouble shooting and individual component replacement. That sure isn't me. Using locally sourced and higher quality vfd's and motors are always going to have replacements for one or the other if there ever needed.

So for a bit more money, I'd just replace anything that's powered with a variable speed single ph AC motor over to 3 ph and a vfd since that type of drive system is so much superior. However................some may not yet know this. European and North American motors would generally be built to the most common metric electrical motor standards for frame size, mounting, shaft diameter and exposed shaft length. Some motors are still available at least in in North America using those older imperial motor standards. But what doesn't seem to be common knowledge is that there's a third set of electrical motor standards in use for many or possibly most of the electric motors built in Asia. And those simply don't match up to any of the imperial or metric standards. So trying to find something that's a direct drop in replacement that might be commonly available and off the shelf in your location with one of those European or North American built motors probably isn't going to happen without some extra work. Afaik, motors built to those Asian standards using foot or flange mounted dimensions are also different enough that some machine work might be necessary. I'd also check any shaft dimensions for usable length, diameter or keyways for re-using any OEM drive pulley. A bushing or re-boring it larger, and / or new keyway might also be required. Hopefully this might save someone from an unwelcome surprise.

Pete. Exactly what I learned decades ago. long before I bought this lathe about 15~20 years ago. In terms of swing, wear, accuracy, "compact size" power and everything else it was a huge step forward from my 1960's Myford ML3 that visibly twisted when I was machining a tough stainless steel shaft one day... - That had been fitted with a 3/4 horse motor with clutch from a sewing machine... with hugely more torque than the ML3 was built to handle. Aftermarket "fixes" don't always "fix" things.
£ for £ the model I bought (at the price I paid) was half what a "better" lathe would cost, so deferred the cost of motor etc....
I do NOT regret buying this lathe as I have more machine (and new, un-worn precision) than I could get from any old lathe. I had owned 3 previously - all modeller sized and worn beds, etc.
But you are right about Asian standards being different from European or USA based standards. So much so, an IEC framed 3-phase motor will not fit in this lathe and in my 5 ft x 6 ft workshop I can't make space for the motor hanging outside the lathe frame, without major bench surgery. So a Chinese small motor is the only practical solution. £ do not make this decision.
I still have my re-motored Unimat SL3 lathe - Slow, adequately accurate, and with the re-motoring was able to spend long periods (20~30 mins without stopping the motor) turning some brass yesterday - The original 90W motor burned-out after 15 mins continuous work one day - 50% longer than the book said: "Do not run for more than 10 minutes then stop to let it cool down"! - Fortunately you can't get original motors now. Just smaller and better and cheaper from China. (£5 instead of £180). Technology has moved-on... The Chinese motor also cost me £5 for a variable speed... £2 to convert AC to DC and a £2 cooling fan (computer type). The expensive parts were £8 for a project box made in Europe. and a £3 rheostat, etc..
But I'll advise of the practicalities of the "Brushless servo-motor" conversion in a month or 2 when it arrives and has been fitted.
Incidentally, for anyone owning a similar Chinese lathe with 180V DC motor and variable speed control, parts are very easy to find on 3&@y, etc. - look for Treadmill motor and variable speed board. £120 for a 750W motor, and £120 for a replacement board. Cheap enough on what is now a $2000 lathe. But they are very prone to failure initiated by the carbon dust around the brush holders, - and when the brushes simply wear down to 12mm long! More frequent cleaning would have prevented mine from blowing-up!
On a lathe where you can fit $400 of "proper" 3-phasemotor and controller, it is surely worthwhile, but both need double the space to fit compared to the original. - I.E. No good in a shoe-box sized workshop.
Hope you understand why I have not gone for "the best" change here.
K2

Many of our machine tools have pretty specific motor sizes that will actually fit within the factory location when that motor is bolted directly to the lathe. But I've been studying how a great many past manufacturer's designed there equipment and doing some semi logical guessing at the reasons behind some of those choices for a long time. You can learn a great deal that way. As long as a that servo rotates smooth enough and it should, then your choice might end up a whole lot better, smoother and much quieter than the OEM motor. Today there's a fair number of industrial level lathes using high cost servo motors to drive the spindle exactly like your planning on doing. Although with some of those, the spindle is part of the motor itself.

Without personally seeing your shop and lathe bench, I've no idea what may or may not work for you. But a large number of lathes were purposely designed and Weiler to name just one still does it today with at least one of there lathe models. They remotely mount the drive motor and pulleys below and inside the stand the lathe is mounted to. Given there very high price, Weiler could have easily used a geared head stock, instead they chose that much smoother belt drive. Afaik some of the worlds best and most accurate lathes do exactly the same to remove possible motor and gear noise from affecting the surface finish lathes like this are capable of producing. A simple slot cut into the lathes chip pan and bench top for the main drive belt would allow remote mounting below the lathe any motor or even multi step pulley ratios within reason. Well it's possible as long as your shop size and lathe bench would work with that type of motor location.

Yes I'd agree, you can easily over power anything, but as far as using higher HP / torque motors than what the factory decided was appropriate? Without some means of a mechanical torque increase with a variable speed motor such a back gear or multi step pulleys, those higher hp motors are still a method of gaining more available torque at those lower rpms right where you need it. How much total horse power and torque you have at the more average operating speeds is almost incidental. You use what's appropriate for the machines design, size and rigidity.

My Bridgeport clone for example has a 2 hp 3 ph motor. With it's back gear, the available torque at the spindle is massive. Far more than the head, spindle taper and it's mounting design is really capable of using. With it's vfd and step pulleys, I seldom need to change the belt position due to having that extra available torque at lower rpms. But these BP type mills have always been fully capable of driving a large range of cutting tool sizes. I've used end mills as small as .125"and as large as 3/4". How much extra HP I have for those tiny end mills with this mill is meaningless as long as it's enough and I'm not trying to over power those tiny cutting tools, or the machine with those larger tools. Lathes are no different. Obviously a much smaller and lighter mill might give better and more tactile feed back through the hand wheels, so with mine it's using visual clues and sound instead of that feel since there isn't any.

Old school steam powered line drive shops were driven with engines having multiple times more HP than any one machine could ever use. Yet there seemed to be no issues since the machinists took appropriate depths of cut and feed rates for each machine. Although I suppose belt slip was also an indicator that limited just how far you could push things. I've used 500+ ton machines with over 3,000 hp in mining to move as little as a few hundred lbs into a pick up truck more than a few times. That's not what there designed to do, and it's a totally inefficient use of a very costly machine to operate. But if it's the only machine available at the time, they will do it. So adding a larger and more powerful motor to get more usable torque at the lower rpm's shouldn't be an issue. Our available rpm and HP with a lot of these off shore machines seems enough for most of what were doing even with carbide. But once you go to larger diameters anywhere close to there maximum swing or single point threading, a lot of them are either still too fast or that torque falls off to almost nothing.

Steamchick said:

Having just experienced my second failed motor and controller... I have a sorry tale to tell.
It started with the 750W DC PM motor making odd "slow" moments, then it stopped. I guessed the brushes had lost length and contact.
View attachment 157995
The original motor brushes lasted about 10 years, but hardly as much use as since a 2021 rebuild and new brushes. So as I use the lathe a lot more (now retired) I expected more brushes needing to be serviced. I measured the last worn ones, 12 mm and 15 mm long.
View attachment 157996
The latest ones were also 12mm and 16mm long.
At 12mm, the brush does not contact the commutator any more. The spring fouls the holder and can't push on the brush, that is disappearing down inside the holder..
Having fitted new brushes and cleaned the brush holder (a place where I had experienced many flash-overs and consequential blown thyristors previously 2021) and added some extra silicon sealant where I suspected there may have been new flash-overs, or probably just old scars (Silicon has 3 x the dielectric strength of air. Sharp corners and pointed parts on brush holders cause arcing where air gaps are <1 or 2mm.).View attachment 157998
Then the newly brushed and clean motor flashed over! - Repeat - 3 times. Used the AVO meter to check no shorts t earth, - Check, check, check = seems OK. Then the fuse blew, and domestic Circuit breaker popped.
Now found an arced track on another circuit board (~2 Ohms! live to Neutral!) for a low voltage DC supply for the speed meter and sensor.
View attachment 157999
- Cleaned that and it is now OK,
Then the Start-Emergency Stop button would not latch... replaced that. (Why are all these bits failing?).
View attachment 158000
So I tried the variable speed - without motor wired in - but using a 60W filament bulb as the load. Ok at "low speed" on the rheostat, but as I reached "half to 3/4" full speed,
BANG! - The fuse holder exploded and pieces of plastic and glass hit me like a bullet.View attachment 157993
View attachment 157994So my expert electrician tested everything. He has a 1000V Mega tester, and it showed the faults to earth at 250V...
All the thyristors blown on the variable speed board. Last repaired 2021. Motor armature fault to Earth - all windings.
So I surmise that the worn brushes caused some overload as the controller raised voltage to try and keep the motor going, as it was suffering poor contact of the worn brush. The excess voltage had flashed over - causing maybe 1 or 2 thyristors to fail. Subsequently, the other thyristors failed until the last one gave an adequate short circuit to explode the 10A fuse and holder.
So please beware of any strange noises from DC motors, ("phut", "spop", etc. - in Motor language that is a little cry for HELP!)) clean the carbon from brush holders frequently, and replace brushes before they get down to 12mm long.
I am now risking an alternative, having ordered a servo motor and control board from China. The only option to avoid major engineering of fitting a proper IEC frame 3-phase motor and controller...
When I receive it I shall advise how it fits, works, etc... so watch this space for the next exciting episode of "Ken playing with mains electricity!" - Hope I survive to write it, and not end up as "fried Chicken"!
Wish me luck?
K2

Click to expand...

It seem both motors life short live.
The motors I am use to has life over 20,000 hours.

Do you any idea why you had a short life?

Dave

Short life on these motors is because they do not stay cool at lower cutting speeds. LOW torque at low speeds is a big problem. So it is easy to overwork the motor, and after 2o years of using the lathe I managed to do that.
BUT carbon pollution around the brush holder causing arcing short circuits, across the commutator or to Earth, the non-fail-safe design of the voltage controller, worn brushes so the contact becomes intermittent, etc. all cause faults so the motor slows down - then the controller turns the voltage high to try and compensate - which in turn causes overheating when the contact becomes "normal".. added to which the high voltage spikes as the back EMF from the motor causes high voltages from the motor inductance = and electrically it is quite easy to either cook the Variable speed thyristors, or the motor armature. I had burned-out the insulating board the first time with arcing, the second time the armature insulation to Earth (shaft). - Then the thyristors!
Western motors of this type are not made... because they would fail the Standards. Western motors of this design - used at lower power density in hand tools - are double insulated - avoiding any possibility of the windings shorting to earth inside the armature. (Wire lacquer Insulation breaks down when the motor overheats - the Chinese motors then short to earth - the shaft - making the problem catastrophically worse! - as mine has done.). But the Chinese system is used on millions of cheap machines, so replacements cost a bit less (initially) than Western designed motors and controllers. Just like the cast iron (not cast steel) beds, etc, and bolts of low grade mild steel (do NOT tighten with the Western torques, only use about HALF!), etc.
"Ya pays ya money and takes ya choice!" - But precision is adequate for modelling, and with more care than I managed they will last a long time.
K2

Steamchick said:

Short life on these motors is because they do not stay cool at lower cutting speeds. LOW torque at low speeds is a big problem. So it is easy to overwork the motor, and after 2o years of using the lathe I managed to do that.
BUT carbon pollution around the brush holder causing arcing short circuits, across the commutator or to Earth, the non-fail-safe design of the voltage controller, worn brushes so the contact becomes intermittent, etc. all cause faults so the motor slows down - then the controller turns the voltage high to try and compensate - which in turn causes overheating when the contact becomes "normal".. added to which the high voltage spikes as the back EMF from the motor causes high voltages from the motor inductance = and electrically it is quite easy to either cook the Variable speed thyristors, or the motor armature. I had burned-out the insulating board the first time with arcing, the second time the armature insulation to Earth (shaft). - Then the thyristors!
Western motors of this type are not made... because they would fail the Standards. Western motors of this design - used at lower power density in hand tools - are double insulated - avoiding any possibility of the windings shorting to earth inside the armature. (Wire lacquer Insulation breaks down when the motor overheats - the Chinese motors then short to earth - the shaft - making the problem catastrophically worse! - as mine has done.). But the Chinese system is used on millions of cheap machines, so replacements cost a bit less (initially) than Western designed motors and controllers. Just like the cast iron (not cast steel) beds, etc, and bolts of low grade mild steel (do NOT tighten with the Western torques, only use about HALF!), etc.
"Ya pays ya money and takes ya choice!" - But precision is adequate for modelling, and with more care than I managed they will last a long time.
K2

Click to expand...

I put temperature gauge on mind to watch.

I am surprised the carbon dust is inside motor it should have been cleaned by cooling fan.

Maybe that is why I saw on large industrial motor with the blower was over the brushes keeping motor cool.

Dave

Steamchick said:

Short life on these motors is because they do not stay cool at lower cutting speeds. LOW torque at low speeds is a big problem. So it is easy to overwork the motor, and after 2o years of using the lathe I managed to do that.
BUT carbon pollution around the brush holder causing arcing short circuits, across the commutator or to Earth, the non-fail-safe design of the voltage controller, worn brushes so the contact becomes intermittent, etc. all cause faults so the motor slows down - then the controller turns the voltage high to try and compensate - which in turn causes overheating when the contact becomes "normal".. added to which the high voltage spikes as the back EMF from the motor causes high voltages from the motor inductance = and electrically it is quite easy to either cook the Variable speed thyristors, or the motor armature. I had burned-out the insulating board the first time with arcing, the second time the armature insulation to Earth (shaft). - Then the thyristors!
Western motors of this type are not made... because they would fail the Standards. Western motors of this design - used at lower power density in hand tools - are double insulated - avoiding any possibility of the windings shorting to earth inside the armature. (Wire lacquer Insulation breaks down when the motor overheats - the Chinese motors then short to earth - the shaft - making the problem catastrophically worse! - as mine has done.). But the Chinese system is used on millions of cheap machines, so replacements cost a bit less (initially) than Western designed motors and controllers. Just like the cast iron (not cast steel) beds, etc, and bolts of low grade mild steel (do NOT tighten with the Western torques, only use about HALF!), etc.
"Ya pays ya money and takes ya choice!" - But precision is adequate for modelling, and with more care than I managed they will last a long time.
K2

Click to expand...

Do use the high belt setting or low speed belt setting?

Dave

The internal fan normally blew the carbon dust away from commutator onto the insulating mount of the brush holders, thus shorting the surface ... Check inside yours. Also some of the clearances between brush holder metal and earthed body metal were less than 0.020inch, 0.5mm. And there was evidence of arcing.
The attached are an old photos of the original brush holder that had suffered a lot of carbon dust pollution and arcing.
Plus a view of the replacement brush holder installed



Arcing had occurred between the Philips assembly screws (earth to body metal) and the aluminium of the brush holder.
Also on the under-side, directly from aluminium rivet heads for the brush holder to casting metal (earth) on the body. Photos are after the whole assembly had been cleaned of l the carbon dust that covered everything.
The arcing had caused the thyristors to blow. = expensive repair.

Diagram showing fault paths that short the Thyristor in the AC to DC bridge.

Explanation: The Live to + to motor to - to Neutral is switched ON and OFF by the thyristors, according to their trigger voltage. The wave capping of the AC input by the thyristors varies according to the feedback of the control trying to maintain a constant speed of the lathe, as the load varies. At very low speeds, the DC voltage is very low, thus current low, and speed low. At high speeds, the wave capping by the thyristors is reduced, so the DC voltage presented to the motor is high, current high and speed high. At a fixed speed, if the load is seen to vary, by a speed change, such as a cutting load, then the voltage is instantly increased by the thyristors to increase Voltage as a consequence until equilibrium of the speed is achieved. – This is almost instant and undetectable in normal use.

But if arcing occurs inside the motor, or the motor connection is interrupted by worn brushes failing to make contact properly, the motor slows, and the control turns-up the voltage using the thyristors. Rapid switching of the motor on and off when brushes are worn, or when arcing occurs (short to earth at a brush) will cause a voltage spike from the back EMF from the armature – which may cause major arcing, serious damage to anything else on the mains circuit, or cause sudden current spikes that blow the mains fuse. I have seen evidence of these damaging voltage spikes and over-current on mains devices on the same circuit. The original input mains filter had blown a copper track. The power board for the speed meter low voltage supply had a shorted track between the mains input soldered points. And a Mains fuse and holder had exploded with excessive over-current ( >30A estimated as a 30A over-current device had triggered => safe.).

If the armature winding fail to earth (as mine have) from overheating OR overvoltage from damaging voltage spikes, then the diode bridge is shorted and the mains will blow fuses. And earth leakage devices. And likely thryristors, etc.
I suggest you check and clean yours, if the same type of motor.

I hope some of this makes sense?

Dave, I use low speed setting. BUT that is a belt change, not an electrical matter.