ML7 refurbishment

Hi folks, I got a backplate for my ML7 from RGD, and at the same time, a Pratt Burnerd 6" inch chuck form the guy who sold me it. - it is a 1966 model, came with a 4" chuck, which glides on and off - the other devices mentioned don't. - They screw on easily until about 8mm from the full fitted position, then seize to a halt. I did read in the manual about new devices needing scraping - does this mean I clamp the on and take a skim in my other lathe?

Next question is perhaps rendered rhetorical by my 'suck it and see (Electronics background) - I have installed 1 hp motor with a VFD - connected up and ran beautifully - altered motor mounting plate slightly, and running off a 65mm pulle, rather than 50mm original. runs beautifully, and people talk about the danger of using too great H.P. - As long as I am careful, I can only see that a jamming disaster would cause as much trouble with high or low H.P. - any comments? Dave from Fife.

Sorry about the typos, didn't check to send - concerned about using up my 240 characters allowed. One more thing though, does anyone know what the normal feed gearing for a smooth turn is? I intend to do a bit of thread cutting, but at the moment, it looks as if I am set to cut 8 TPI!

I shall try to do a refurb article for ME in the near future(if they need it).

Dave, Fife.

Hi Dave, I have an ML7 of a similar age as yours. Presently having the gear cover off I thought I would put a picture in my album of the range of gear pitches. However I cannot seem to upload a picture try as I might. I use the following gear train for normal smooth turning: MANDREL, driver 20T. 1st STUD, driven 65T, driver 25T. 2nd STUD, driven 70T, driver 20T. LEADSCREW, 75T. This gives a feed of 0.0037” per rev. There is a finer feed giving 0.0018” per rev but you need a 12T drive gear on the mandrel, as I don’t have one I have never tried it. Hope you can make sense of that lot. If you would like the picture I tried to upload I will email it if you PM me.

Malc.

1 HP on an ML7 isn't insanely powerful, just over-the-top for a machine not designed for that much oomph.

The first problem with it is the 'As long as I am careful' statement. Nothing matters when being careful, it's accidents that do the damage! For example, if lubrication fails a big motor will rip more metal off the bearing than a small one. Whatever the accident it does more damage all round simply because there's more energy in the system, whether running the saddle into the chuck or ripping a ring finger off.

Second problem is the slow temptation to use the extra power. Satan whispers every day. Because all seems OK, the operator will gradually start taking faster deeper cuts than the lathe is happy with. It's unlikely to fail spectacularly. Rather a bearing that would have lasted 15 years only lasts 3 whilst the extra stress quietly wears the bed prematurely, twists the frame, and fatigues the keyways etc.

Generally not smart to up the power of machines without considering the whole system. For example, putting a sports engine with Nox into my bog-standard Corsa wouldn't be a good idea unless I also uprated the clutch, brakes, tyres, suspension and cooling system. Plus a roll-bar if I'm driving it!

There are reasons why Myford didn't put huge motors on their lathes. It's not because their engineers lacked imagination or cojones!

Dave

Edited By SillyOldDuffer on 06/07/2020 19:02:50

A very good reason to put a bigger motor on a machine is when it is operated on a VFD. It gives back the torque you loose when operating the motor at lower speed.

Temptation is overcome by current limiting the motor via the VFD for normal operation.

Andrew.

Not that far over the top - While the instructions suggest 1/3 to 1/2 HP for the ML7, 1/2 to 3/4 for the Super 7 and Tri-Leva ML7s were recommended 3/4 HP so with a VFD it is probably the top end of normal.

I would be concerned about the backplate only screwing partially on the spindle !

I remember buying a new back plate for a 4 jaw chuck to fit my ML7  ,this would not go on all the way.Faceplate, catch plate and 3 jaw chuck all spun on easily by hand.

I bought a tap ,tapped the new back plate and it fitted alright then.

Edited By Maurice Taylor on 06/07/2020 21:41:12

Much good data, won't need the photo Malc, that info is good,

Dave, you sound like me giving senior craftsmen, electrical and mechanical, a toolbox talk, and you're absolutely right - you musta been in H&S!

The others are right in their thinking, I do lose power with a 315 3 phase dual voltage, star connected - nice motor though, an Electramo - I've had it for years at the bottom of the bench - it was a spare wrongly diagnosed as bearing fail - I even bought spare bearings (SKF).

To put your mind at rest, I have always worked with precision instruments, am never heavy handed (sometimes to my detriment), and drive a 1991 Honda VFR like I drive my Honda CG125, proportionately that is. I had to reduce the 'jack the throttle and overtake that guy who just went passed' which I knew would eventually land me in the undertakers! in other words, I don't do speed any more, and although I am not a satanist, I understand too well what you mean, I was grinning away as I read your reprimand - if only everyone had that simple caution!!

I won't change it, and ordered two vfr's the other for my old Clark lathe - but that's another story,

Anybody need a perfectly healthy old AEI 240V 1/3 hp motor?

Maurice, Do I infer that it was the thread, rather than the smooth bore that was the problem, or was the thread on the backplate? The thread on the spindle nose is too big for a tap.

Thank you all - long live model engineers workshop!

Dave, Fife.

Fastdave

Tracy tools list a tap the same thread as the Myford spindle nose and taps can also be found to fit the Boxford spindle which is 1 1/2 x 8 IIRC.

Dave

Hi Fastdave ,regarding back plate thread ,try putting the back plate on the wrong round to see if it goes all the way on. This should show whether the threads are the problem or the plain bit of bore.

Hope this helps.

Excellent! Thank you guys - Malc, could maybe use the picture, Dave - [email protected] and before we get any comment, it applies to the old 56k connection on the internet - I got, I think it was T2 or T5 connection, which was actually 2 lines, and one could double up to get 128k if my memory serves me (and it usually doesn't) - in the olden days, this was lightning fast!!!

Dave, Fife.

Nick Clarke 3 - in your last, you noted that the TriLeva 7 was recommended for 3/4 HP. I have such a machine but I haven't seen that previously. Could you say what the source is, please? (and does it say why?)

rgds

Bill

You did make sure that both threads were clean?

The other day, one of my chucks stuck before going on all the way. Using an old, cleaned, toothbrush removed one piece of swarf from the chuck thread.

End of problem!

Now, I always check and clean both threads before fitting the chuck.

If the register on the Backplate is undersize, by a little, you could screw it onto the mandrel, with the register facing outwards. If you run the lathe as slowly as possible,and apply some fine emery to the register, you can remove a little metal. You are only looking for a tenth of a thou or so, maybe just removing a burr.

Cover the bed, to prevent abrasive dust falling on to it.

Stop, clean everything, and check the fit frequently, until you get a fit with which you are satisfied, (Tight rather than slack! ).

HTH

Howard

Edited By Howard Lewis on 07/07/2020 12:29:29

When I bought my (second-hand) ML7, the chucks that came with it fitted perfectly but a new collet chuck supplied by Myford wouldn't go on more than a couple of threads before jamming. Myford changed the chuck with no problems, and the new one was the same. I tried the new chuck on a friend's lathe, and it fitted perfectly, so the problem was obviously with my lathe. When I looked more thoroughly, the mandrel thread was very slightly distorted for the first couple of threads, presumably by earlier abuse, and I presume the existing chucks had enough slack in the threads to cope.

So I eased the thread with a small file until the new chuck fitted, and all has been well since.

George B.

About halfway down this post the two speed motor (3 phase) recommended for the tri-leva is mentioned as being in Myford catalogue 733A with the p/n 60/014A

**LINK**

True, but check the VFD isn't compensating for low torque at low speeds as well. Big motor plus smarty pants VFD might be too much of a good thing.

The torque produced by feeding a pure sine wave into a three phase motor falls off rather dramatically as the frequency drops towards 0Hz, so Andrew's point is a good one. But a VFD doesn't have to feed the motor a sine wave! It can punch more energy into the motor by changing the waveform. The ability to compensate means a VFD driven motor is likely to maintain torque rather better than might be expected.

Of course there's no such thing as a free lunch - the motor will get hot! One solution is a separate fan (always a good idea in my opinion), the other is soaking up the heat with a heavier motor. The main objection to cooling by buying big is the cost, which might not matter if the motor is spare. I think if a lot of low speed high torque work is intended, best look for torque from belts or gears rather than electronics.

VFDs have so many advantages in terms of speed control, torque management, and energy saving that it's becoming unusual for 3-phase motors to be connected directly to the 3-phase supply. Instead a 3-phase input VFD converts it to DC and squirts out 3-phase electronically optimised to the motor and it's application.

Who cares? I reckon an M7 would work reasonably well with almost any 3-phase motor between 150 and 750W out. 150W is uncomfortably weedy and 750 somewhat muscular, but neither is daft.

Another objection to fitting big motors without thinking it through is the power rating on the plate isn't the motor's maximum. It's how much power the motor can deliver without getting too hot. If a bigger load is applied the motor will deliver a lot more power than the rating implies until something melts! On a 13A fuse, a 750W notional motor could poke out more than 4kW for some time, and as a 13A fuse takes 100A for about a third of a second before blowing, it could go ten or more times over power - briefly! If the worst happens a big motor spinning a heavy job flat out can do serious damage to the machine, job and operator in the blink of an eye.

Do not try any of these experiments!

• Put a 20mm diameter rod through the headstock with a metre protruding out the back and spin it up to top speed so it whips like a propeller. Now walk into whirling rod!
• With a 3-jaw chuck turning at top speed push a 40mm square steel block in so it jambs between the jaws and the bed. (This can happen when work accidentally comes out of the chuck)
• Remove the cover protecting the change gears and defeat the safety interlock so the lathe runs at top speed. Then push your hand or sleeve between any pair of gears.

All these are nasty with a small motor. Big motors more so!

Accidents with home workshop metal lathes, saws, mills and drills are very rare. Safer than woodwork and DIY. I put it down partly to sensibly powerful fixed equipment and the above average intelligence of metalworkers who enjoy working accurately. I can't imagine a clockmaker taking guards off and fitting monster motors so he can work faster.

Dave

Edited By SillyOldDuffer on 07/07/2020 17:25:24

Very interesting Dave - I tried all of these experiments before I re - read to find you said - DON'T - so insurance are coming back to me about smashed Window, dented mill, wrecked pillar drill, brickwork damaged, and I can't handle all of this from the ambulance - no idea what the lathe itself looks like! - (reaches for vallium and Vodka to intake Laphroiag from bottle!!!

Seriously, being from an Electrical/ electronics background (Everything from TV experiments with MOD projects, to HV electrical, and after many apprenticeships, Gas, air and petrochemical A.P. (writing safety programs for all of these disciplines)) I have found, in the past twenty years, that Electrical people are not always the superior beings that they give themselves credit for.

If one reads from any of the Electrical regulations, 15th, 16th, 17th and so on - then pick up just about any article from MEW, the eloquence, precision and interest which is and remains with people from a mechanical background, becomes quite patent.

Yes, I fully acknowledge Logie Baird, Watt, Henry, Faraday, Tesla, Westinghouse, and many electrical geniuses I have met in my career, but I stand by that statement. I was disappointed in Electrical engineering - not Mechanical.

I am overwhelmed by you guys, electrical or otherwise, on these pages - no - one is supercilious, sarcastic, or smartassed - try the Linux forums, and you'll see what I mean!

Many thanks to all of you, oh, and Malc, I have that screw thread chart - I was looking for a photo of your gear arrangement - on my old clark machine, it gives a different set of gears for normal running aside from the screwcutting tables

Dave, Fife.

If it needs to be said, for the unsure, a Clark machine, was probably made in the far east will have a Metric Leadscrew, where most Myfords will have an Imperial one, most probably 8 tpi.

For anyone interested, don't make the assumption that the Leadscrew or gears on your machine are applicable to one of another make.

Some manufacturers made machines with Imperial or Metric Leadscrews, depending on the model.

Leadscrew pitches vary whether Imperial or Metric. What works on one machine may not on another.

My old ML7 had a 8 tpi Leadscrew, with 14.5 Pressure Angle 20 DP gears. Other Imperial lathes use other pitches, DPs, and Pressure Angles.

One of my current machines has a 3 mm pitch, and the smaller one a 1.5 mm pitch, Leadscrew, and the Changewheels are different modules. Some far eastern lathes although ostensibly the same have different sized key ways for the Changewheels, depending on which factory produced the machine.

My mini lathe, judged by the DROs, appears to have Imperial Leadscrews for Top Slide and Cross Slide, but a Metric main Leadscrew!

It is worth checking, rather than assuming!

Howard

Further to Daves comments on VFDs and motor overheating at low RPM its worth remembering that modern VFD boxes come in two flavours. Old style V/F (voltage/frequency) control devices and modern Vector drives.

Basic V/F control VFDs merely vary the input voltage and frequency to get the motor running at the right speed. The motor sorts out the actual current drawn itself using the same self generated back emf opposing applied voltage mechanism that occurs when it is simply connected to utility power. For all sorts of reasons this works best when the motor is running at the speed its designed to run at. Once you shift significantly from that speed things start to go wrong and you get lots of excess current running round inside the motor that just makes things hot. The going wrong once too far off the design speed issue also affects torque and power output. Hence the old advice of staying within ± 1/3 rd of the design frequency because things are generally well behaved in this range. High quality, expensive, VFD boxes are generally better engineered and have internal programming to mitigate such problems. Economy range ones won't. Motors vary considerably in how the actually behave too so apparently equivalent pairs of motor and VFD of different breeds may behave rather differently. Performance over a wider speed range is something of a lottery when mixing any random pair of affordable v/f drive VFDs and affordable motors. Especially if the motor is old.

Vector drive VFDs attempt to give the motor exactly the power it needs at any given speed and load. So the pair is much better behaved, less prone to overheat and torque tends to hold up better at lower speeds. We are most likely to encounter self tuning vector drives which measure the motor characteristics on set-up and select an appropriate drive characteristic from a pre-programmed set. The sophisticated (aka expensive) breed having feedback from the motor can do pretty spectacular things but are not for the likes of us. Generally vector drives have eliminated overheating worries in sane use and also overspeed / over power worries. If you set the limits properly of course. Ultimately performance with a self tuning drive is limited by how closely the drive it provides matches what the motor wants. As ever better brand and higher price tends to mean better performance. I'd be suspicious of the over cheap imports. Many modern motors have a highest speed rating on the plate. This generally refers to the highest speed at which rated torque is available at standard voltage. Above that speed torque falls off unless voltage is raised to compensate.

Properly engineered VFDs can put out a voltage higher than the nominal line input. Which helps keep torque up. Although the 220 / 320 or similar rated VFDs will run a motor in star (420 V) configuration the actual output voltage is always lower than the motor should have so performance is inevitably limited, especially at higher speeds. Although they do work to run a motor that cannot be rewired into low voltage delta configuration the input rectifier design is severely compromised and power somewhat limited. Best thought of as the VFD equivalent to a static converter. Works but only use when there is no alternative. There are properly engineered ways of making a voltage doubling VFD work but the market seems to have decreed that the price-performance ratio doesn't work.

Clive

Edited By Clive Foster on 08/07/2020 13:25:51

Edited By Clive Foster on 08/07/2020 13:26:26