Every answer

Has anyone else noticed that you can find any answer you like on the internet?

I have been debating for months on whether to fit a three phase motor on my Myford ML7 lathe. Apparently the bearings will seize and the lathe will be a worthless pile of junk inside half an hour or it’s the best thing I could do and will give me years of wonderful service at high speeds.

Who does one believe?

Just because it could go faster than Mr Myford deemed wise doesn't mean it has to, it's all in the hands of the operator

I fitted a 3-phase motor + VFD to my ML7 about 14 years ago and have been very happy with it. I don't run it at higher speeds than normal though (lower speeds, yes, and it's very handy).

Running faster than normal would be taking a risk, I think, with the plain bearings used on the ML7 - which maybe the vibes you are picking up on the internet.

At very low speeds (for tapping and die-threading usually) I limit the duration since I'm not sure how well the motor cools. Could be worrying over nothing there though.

My ML7 has had the Newton Tesla drive on it now for over twenty years. It has not seized yet. Of course the way that it is set up, the new maximum is only about 20% higher than the old maximum, and it rarely does that since most jobs don't need to be turned at maximum speed. Only tiny diameters need to be turned fast, and I have a unimat that is better adapted for that kind of thing. I did do a mod to the Myford part way through its time with the VFD, it now has a four stage pulley setup, using poly V belts, instead of the three stage V belt setup it had from new. The poly V belts actually put less load on the bearings, since the belt tension needed is less. It also gives me a lower bottom speed. Even with the VFD, it is useful to have a wide range of belt ratios, since the large diameter slow jobs tend to need more torque. The poly V belts are great, they don't heat the pulley up the way the old V belts did, so much less power is being wasted in friction.

As Peter has said, the VFD is especially useful when low speeds are needed.

Hemingway do a kit for putting Poly V belts on the Super 7, but if you want to do it to an ML7 you are on your own. Maybe I should make some drawings of what I did!

John

The latter. Or at least, the former are talking rhubarb.

I would not be surprised if the doom-mongers had thrown some second-hand 3ph 3HP umpteen-rpm motor and inverter, both of dubious origin and condition, at their poor little machines, think they can now hack off high-tensile steel at factory rates, then wonder why they've ruined the lathe.

That's both electrical units, not electrical set plus owner, that are of dubious origin, I mean. Though.....

These conversions are not for trying to make an elderly, relatively small lathe emulate a modern industrial machine on piece-rates. They are for much smoother running and variable speed control enhancing the original 6-speed transmission, which I advise you keep; and should not be of higher power and maximum speed than those originally specified for the lathe.

Allegretto ma non troppo, and all that.

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I fitted a Newton-Tesla (others are available!) motor + inverter set on my ML7, and following that experience, to my Harrison L5 lathe, Myford VMC milling-machine and BCA jig-borer.

For each case I told the supplier which machine, so the conversion is appropriate. The motor for the two Myford machines were straight swaps but I needed make an adaptor plate for the BCA, and built a completely new motor frame for the Harrison anyway.

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Also, I refused to fall for "You can now throw away the countershaft". The speed controller allows running the motor at speeds well below its best, if only best for cooling, but that is bad practice, and loses the torque advantage of the mechanical transmission.

For example, the spindle on my Harrison, with its all-geared headstock, can amble round at about 70rpm with the motor happy at somewhere approaching 1000rpm - the pointer on the speed knob well inside the green sector on the scale.

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Having so fitted these machines, they all run very well indeed.

The first gain for the ML7 was that the cabinet does not resonate as it did with the original 1ph motor. Is it fast enough with a 3ph motor? Yes: I think the motor is of the same top speed as the original 1ph (I keep as spare); but I rarely need use top pulley speed, even with carbide-insert tooling that "received wisdom" says only works at terrifying speeds - just because it can!

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You must shield the motor without blocking the air-flow if an open-frame type, because if swarf enters it and causes a short-circuit it will wreck the electronics before the motor. As I found when a long ribbon of bronze sneaked round the somewhat crude shield....

I append the photo, pre-installing, of the motor-guard / splashback I fabricated from 3mm PVC sheet.

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Also note that although the Newton-Tesla inverters for the Myford lathes are enclosed (at least, mine is) some of the other they sell are a bit more generic with much more open cases. These should really be installed in proper enclosures. (I'll do that..... one day.. I did ensure they are well away from any swarf etc. though.)

Finally, they warn not to use the emergency stop-button as the routine off-switch, as repeated such use can damage the electronics. This likely also applies to safety treadle-switches.

'...

So by all means convert your ML7 - but go for a decent make and a kit specified for the lathe.

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Myford ML7 splash-back. The circular bulge is to give more room for cooling air. I pressed it by softening the plastic using a heat-gun, and pushing down between wooden former, using the bench-drill arbor.

Firstly, this is a case of ‘don’t believe all you read/see on the internet’. It helps to look for corroborative evidence.

This is basically nothing at all to do with the motor - only the lathe, by virtue of idiot operators. Myfords were limited to the original top speed to avoid early spindle bearing failure. A small increase in speed may be possible due to the design engineers conservatism on that front. Not sensible at all on a worn machine, mind.

Here is an analogy from fairly early in the last century.

Early petrol engines could operate on gravity/splash lubrication. Oil dippers on the big ends scooped up oil for the big end bearings and splashed oil around, that could be directed to the crankshaft main bearings. This system was OK for many engines of the day. Diesel engines (heavier loadings on the crankshaft bearings than petrol equivalents) were generally limited to lower engine speeds (of around 850rpm).

Later, engines were fitted with pressure systems from an oil pump. Some diesel engines were OK up to about 1600rpm without a pumped feed to the bearings but these employed a low pressure oil pump which sprayed oil to the crankshaft bearings from suitably positioned nozzles. I have one (1930/40s?) petrol engine that can rev to 2200rpm using that system.

Eventually all larger engines adopted a direct pumped lubrication system, allowing the engine maximum speeds to be steadily increased.

Of course, there were other design changes as well - replacement of cast iron (to Aluminium) for pistons is one such progression.

Drip-feed of plain bearings is reckoned only be good to about 1100rpm, tops. For Newton Tesla three phase conversions, engineers will have programmed the VFD to limit the motor maximum speed.

Few or no doom mongers will have actually done the conversion.

A good point, John!

Though it' s not unlikely that some of them had in fact ruined what had been a good lathe they may have obtained without its own motor, perhaps for heavy uses other than model-engineering, by cack-handedness and simple ignorance.

Think of the comments we read on here from time to time, warning of bad practice in some YouTube video, summed up as, "Oooooh. I'd not have done it that way!" .

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For example, my own ML7 had not been greatly abused by its seller, who had used it only for light work in aero modelling. So he'd not been too unkind to it.

However, he had bolted it to a wooden bench via a strange cast-iron "sauce-boat" of unknown origin and purpose - but crucially, its irregular as-cast surface had no machined areas for the lathe feet.

There are also several small nicks in the edge of bed below the chuck area, obviously by hacksaws dropping uncontrollably to the bed after sawing rather than parting-off the work. These may have been from some previous owner again.

Yes, you can get any answer you want - including here.
John Haine makes a good point - leading me to saying "it's best to listen to someone who has done it, rather than armchair pontificators."
So, here is what I did. I bought a longbed S7 on an industrial cabinet with EDIT: the standard Myford 3 phase motor.
I chose to keep the 3 phase motor, rather than changing to a single phase motor because I saw the advantages of the features of a VFD - mostly variable speed but other advantages like programmable soft-start etc.
I bought a VFD and fitted it inside, using the controls for start/stop and direction, already on the control panel, just fitting the speed control next to them. EDIT: I set the VFD to give the same max motor speed as the motor rating.
Peter Green makes a good point about cooling at low motor speeds - if you want to do a lot of it, and I mean a lot, then add some motor cooling.
This has worked fine for over 5 years, and I like it.
Note: if you have nerdish tendencies, wanting to constantly make adjustments to the VFD (perfectly fine), it is better to put it outside.
Other methods are available.

Edited By Clive India on 12/05/2023 11:08:31

In the interest of balance, let's ask the question, 'what could possibly go wrong?'

Not much, but there are two possibilities, both of which require the operator to be a bonehead, or - if you prefer - an optimist.

• Worrying if the replacement motor is more powerful, in watts, than the original. Using the excess power, rather than just boasting about it at the club, will wear the machine out faster, perhaps much faster. Nothing spectacular, but a bearing that would have lasted 20 years is done after 5. Worst case is the extra damage done to the machine when a big motor running flat-out out crashes the saddle into the chuck.
• More subtle, an advantage of 3-phase with a VFD is the motor can be over-sped. This is useful, except the extra speed has to be used responsibly. Myfords are fitted with plain bearings. These are delightfully low cost and high-accuracy, but it's important not to spin them too fast. Plain bearings pump a thin layer of oil through the system as they spin, at enough pressure to prevent metal to metal contact between the moving parts. Several tons per square inch. Unfortunately, above a certain speed, the oil film starts to collapse allowing high speed metal to metal contact, which is bad, bad, bad. The danger speed is related to how the oil is fed in and out of the bearing - it's design - and the oil's viscosity. The amount of contact is proportional to RPM. Occasional high speed is safe enough, prolonged high-speed reduces bearing life, and very high-speed will seize a plain bearing in short order. A VFD is unlikely to spin a 3-phase motor fast enough for an instant seize, but it is fast enough to cause intermittent contact.

In short, nothing awful happens when a single-phase motor is replaced by a 3-phase motor of the same oomph unless the operator chooses to consistently run a machine not designed for it at high-speed. In the age of carbide inserts, there's a temptation to do this. the operator forgetting that a machine designed 80 years ago for HSS, isn't up to running carbide at optimum performance. Warts and all Chinese machines are better placed than Myfords for high-speed because they come with roller bearings.

I wouldn't buy a second-hand Myford fitted with a big motor of any type, and especially not one with a VFD. Too risky that the previous owner has thrashed the bearings because he believes all is well mechanically unless there is obvious smoke and horrible noises! I might go for it if he'd fitted an oil pump to pressurise the system and had a convincing technical reason for needing a monster motor.

Dave

From my armchair and Lathes.co.

ML7: Speed Range:
As supplied to the UK market (with a 50 Hz 1425 r.p.m motor) the ML7 had a speed range of: 35, 62 and 110 in the 5.78 : 1 ratio backgear and 200, 357 and 640 rpm in open drive. Although it is perfectly possible to raise the top speed by increasing the size of the motor pulley, it is wise to bear in mind that the maximum recommended speed of the original white-metal bearing spindle is 1000 r.p.m. The writer has known machines to be run at much higher speeds, without apparent ill effect, but these were in good condition, carefully set up and with an increased flow of top-quality lubricant from their oilers; a worn machine treated like this might not take at all kindly to the treatment.
At one time, in order to permit higher speeds to be reached reliably, the option of a kit containing bronze headstock bearings and a hardened spindle was available - though at a cost well above that for the standard items. However, in later years, replacements were always provided as bronze-bearings (because the oil feed to the bearings is "constant loss", it is important to make sure that, every time the machine is run, both oilers are topped up and opened to give a generous rate of feed - one drop per 30 seconds as a minimum). I have seen countless ML7 lathes that, despite having given more than sixty years of service, still had bearings that were "spot on". Of course, the oil has to go somewhere and runs down the front and rear faces of the headstock to end up in the chip tray. If your headstock appears to be leaking oil, don't worry. However, if it's not, do worry…...it's run out.

Has anyone killed an ML7 by over revving? Possibly, but then no one would admit to abuse. Myford would not have put together a kit to upgrade the bearings if there hadn't been a demand.

Even the slow speed overheat issue depends largely on the motor insulation type and wether it's rated for intermittent or continous use. The former may suffer from low speeds.

Most VFD's are designed to be controlled via a pendant, which is why some VFD have exposed live terminating screws similar to chocolate blocks .

Nice one Dave.

I have edited my post to emphasise the max speed on the VFD was set to the normal running speed of the motor and that it was the standard 3 phase motor fitted to the machine from new. It had never had a VFD fitted.

I didn't have an objective to squeeze any more out of the machine - just to make it easier to use.

I cannot see how a slight increase in max speed, especially if you use modern oils can cause any harm. Oils from the 50's are primative compared to modern synthetics. John Olson runs at plus 20% which is very safe indeed.

Putting a 3 phase motor and VFD on a ML7 (or S7) is one of the best modifications you can make.

I've done it to my ML7. The difference in vibration and noise is astounding. I knew the torque ripple from the single phase motor increased vibration but was suprised by the difference. As others have said, this should not be about "more" it's about better. I actually fitted a 370W (0.5HP) motor abut it is plenty of power. I also used a slower 6 pole motor. This has a nominal speed of 910 RPM and is even smoother than a 4 pole. It also allowed me to fit a larger pulley on the motor. This reduces transmission losses. The motor is a Totally Enclosed Fan Cooled (TEFC) type. It is not really viable to use an open motor when there is swarf and dust around. Shields can not stop everything, the take time to make and can get in the way.

I can, and will, run the lathe at higher speeds using the VFD. This is not to use carbide tools on large workpieces. It is to machine small diameter parts and most of those will be in soft materials.

Robert.

When in doubt an experiment is called for! Anyone prepared to risk the plain bearings of their Myford putting by it through a proper high-speed stress test, whilst others do the same amount of work at normal speed?

After a rinse out and refill with clean oil, the lathes would convert four 4" diameter 12" long mild-steel billets into swarf. Half the sample would do so with the spindle running 20% over Myford's recommended top speed, the others at recommended speed.

Hopper assures us in the backplate thread that an ML7 will cheerfully take 100 thou deep cuts all day long, so we'll go for that Depth of Cut. Not done the sums, but the feed-rate will be calculated from the rule of thumb that 1 HP will remove 1 cubic inch of steel per minute. I'd expect the test to last between 20 and 30 hours.

During and after the run the oil will be collected and then sent for analysis. The amount of bearing metal found in the oil will put a number on the actual wear-rate occurring under known conditions. Much more reliable than personal opinion, where nothing is said about how many hours the lathe has clocked up, or hard it was worked in that time.

Machine tools have service lives measured in a few thousand hours, after which they need TLC or are replaced. Running a machine outside its specification reduces its service life. Light abuse will only reduce it slightly, heavy abuse might break one immediately; it's not black and white. Like playing Russian Roulette, it's a question of how long before the damage appears. Surviving a few goes at Russian Roulette does not mean it's save for everybody!

The over-speeding experiment can be tried with cars too. We need 30 members prepared to drive from Lands End to Skarfskerry and back at an average 40mph. 10 of them can only use 1st and 2nd gear; 10 can use 3rd as well; and 10 can use all the gears they have. Can we predict which group will have the most breakdowns? I think so!

Dave

Edited By SillyOldDuffer on 12/05/2023 19:00:44

Just for the record: an ML7 fitted with the TRI-LEVA attachment and original two speed motor had a maximum spindle speed of 1280 rpm.

The two speed ML7's with the higher top speed had hardened spindles that ran in bronze rather than whitemetal bearings, at one time these were available as a set from Myford to retro fit to the standard machines. I can't remember where I read it but the whitemetal bearings were considered ok to run up to 1000 RPM. I know of one ML7 that has an inverter fitted that is set to a maximum of 60HZ rather than the standard 50 and it's been like that for years (since the early 90's) with no problems at all. The owner uses it quite intensively on components for his G1 locomotives and has never had cause to touch or adjust the bearings, although he admits to maybe over oiling things a bit!

He didn't change the motor it came secondhand with a 3 phase motor fitted from new, fortunately it was easy to rewire to delta, and I used to get slightly used inverters from work so it was a cheap solution.

Come on folks, this whole thing is getting beyond the pale. The OP said he had experienced confusion when raising this elsewhere - this forum can only have added to the confusion.
Can we just say "yes, it is a good option providing it is used responsibly?"
SOD - let's be polite and say your last post was simply unhelpful. I think moderators are there to be helpful.

I spent ...years with myfords in production workshop( second op)

All 3phase machines.

Let me tell you a secret...the machine doesn't know or care what prime mover is spinning the wheels.

There are rpm limits to observe yes.. but nothing inherent in 3phase motor.

The VFD can allow a wide range of motor spindle speeds...

But belting for wider still , range is worth considering...yes too the "back gear"...VFD are not magic...

That said with huge torque comes huge responsibility

Nicely summed up Jason, got the point across much better than my rather rambly post. After fifty years in industry around machine tools I've never heard anyone proposed the kind of testing mentioned above, too busy making things on our rapidly wearing out machines just trying to make enough money to keep on keeping on!

Dave