Back gear and bull wheel for myford ml7

The back gears and the bull wheel on my Myford Ml7 are stripped.

The cost of relpacing them is huge.

Is it possible/practical to use a differnt motor with variable speed control?

Obviously the original Induction motor can not have its speed controlled.

Hi there, Mark,

It all depends on what you regard as huge.

Have a browse on eBay, for instance:

eBay ML7 Bull Wheel

Best regards,

Swarf, Mostly!

The answer to your question is yes, but I doubt whether doing it properly it will be cheaper than replacing your damaged gears. So you have to consider whether a variable speed system will give you any worthwhile advantages for the type of work that you are doing. I would suggest probably not - you are clearly interested in the back-gear speeds, and the top speed will be limited anyway by the plain bearings of the ML7. Others may disagree.

Edited By Redsetter on 17/06/2021 13:41:16

Redsetter is correct in that low motor speed means low motor power (and less motor cooling as well).

You say ‘stripped’. Quantify - they may be repairable.

IF the original motor is three phase, it may well be able to be speed controlled.

FWIW

My advice would be to find some means of replacing the original arrangement. This will probably mean buying secondhand parts, if the present ones cannot be recovered by some means, to reduce costs.

It sounds as if someone has resorted to the ruinously bad practice of engaging Backgear to lock the spindle while unscrewing a stuck chuck, with the inevitable damage that this practice causes..j

Back gear can be very useful, even with VFD.

My lathe has VFD, but when I wanted to cut a 4 mm pitch thread, ending part way along a bar, I used Backgear and low motor speed. ) The back gear provided the torque and low speed needed for screwcutting to a finite poijnt, and a low speed meant that the motion could be stopped as the tool entered the under cut.

Howard

Hi there, again, Mark,

If you do decide to buy a new bull wheel and back-gear cluster, I suggest the following modification:

Drill a hole (say, 6 mm clearance) through the bull wheel diametrically opposed to the sliding gear segment. (While the bull wheel is off the mandrel this is more convenient than having to disassemble the lathe. If I were to need to remove my lathe mandrel and bull wheel for any reason, I would perform this same modification before reassembly.)

Then cut a length of square bar to span the head-stock casting and fit a 6 mm dowel in such a position that the dowel can be engaged with the hole in the bull wheel with the ends of the bar resting on the front and rear of the head-stock casting.

Should you, for example, need to unstick a stubborn chuck, fit the bar and dowel to the hole in the bull wheel. This will immobilise the mandrel without any excess sress being imposed on the teeth of the bull wheel or back-gear. The torque applied to the chuck will be transferred from the mandrel, via the woodruff key, to the bull wheel, thence to the dowel & bar and then to the head-stock casting.

It might be argued that drilling such a hole in the bull wheel would cause inbalance but I do not think this would be serious in an ML7.

Best regards,

Swarf, Mostly!

I've used my backgear for years to remove and tighten chucks (drummond M)

If you get a bad one then put decent but non excessive load on it and wait

Go to the toilet, have a cup of tea, pick your nose etc

The same system has been used for decades with car ball joints

after 5 minutes or so it releases itself, no excessive force required

(edit: Ball joints are more exciting because they can make quite a bang when they separate)

Edited By Ady1 on 18/06/2021 10:04:17

Ady 1,

You probably get away with it because, as a recent post, showed the Drummond gears are getting on for half as wide again as the Myford ones.

For Myford owners, there are the various already suggested ways, Hexagon bar in chuck, socket and long power bar; bar through chuck jaws; drilled chuck backplate and long steel arm with pin in the end. (The method that I use with a larger lathe) and a smart tap on the outer end with a mallet..

A smart bash on the end of the arm usually frees the chuck, if not a second whack usually does the trick, without risk to the gears.

Howard

Now you mention it I think you're right, that's a really annoying deficiency for an ML7 user

My method on a Myford is to clamp a 1/2” drive 10mm Allen hex socket in the chuck and use makita 1/2” impact driver, never fails. I removed a chuck that had been stuck on ML7 since 1960 like this recently, unscrewed it instantly.

I don’t have a makita impact driver, but the method looks like probably the best suggestion so far. My chucks usually loosen with the ‘palm of my hand’ force but, for unwanted reasons, they can occasionally be too tight. It would also provide extra duty for my under-utilised impact wrench these days.

Some powered screw drivers (with adjustable torque settings) might even be sufficient/applicable (but not my makita with a keyless chuck🙂 ).

Off the top of my head, I have no idea if the parts are inter-changeable but if they are I would suggest fitting as a replacement a ML7-R or Super 7 bull-wheel and back gear. The reason being that the 60 tooth 7R/S7 gear is so much more useful than the 55 tooth (I think) ML7 bull-wheel for dividing/stroke indexing purposes. A simple detent will give you 2/3/4/5/6/10/12/15/20/30 and a Radford/Thomas head-stock attachment will give you almost all the rest.

Just a further thought, if the cost of the replacement parts is prohibitive, may be breaking the lathe and selling the parts would finance the purchase of a ML7-R or better still a Super 7. I don't think you would ever regret up-grading even if you had to subsidise the purchase of a superior machine a little.

Edited By Chris Crew on 20/06/2021 08:35:09

Wouldn't putting a bar across the jaws, or clamping a hex socket/etc. all still require the mandrel to be immoblized? If you're doing this by putting it into backgear doesn't that present the same kind of risk to the gear teeth?

My solution was a short length of PTFE could be copper) which sits between the base of the spindle housing and the underside of the bull gear engagement bolt - no load at all is presented to the teeth that way. I use an adjustable spanner on the side of one of the chuck jaws, but all of the suggested solutions could work. FWIW I describe it in this YT video and my source is Steve Jordan:

Edited By William Ayerst on 20/06/2021 10:13:23

With a tight belt, there will be sufficient inertia in the system for a suddenly applied load to slacken the chuck. If not first time, but subsequently. It is akin to repeated hammer blows. This is why impact drivers work.

A suddenly (Instantly ) applied load has twice the effect of a gradually applied one. The area under the force / time graph is rectangular compared to a triangle.

The whole purpose of NOT locking the Mandrel by engaging Back Gear (Effectively engaging two different gear ratios at the same time, is to prevent applying a damaging load to the gear teeth.

The gear teeth may be adequate for the duties which they are normally expected to perform, but not the instantaneous loads required to overcome the frictional loads of a stuck chuck.

Simple mechanics shows that a screw thread provides a large mechanical advantage, so that the forces between the threads and any flange will be LARGE. (The thread used to jack up a car is smaller in diameter than then securing a chuck to a Myford lathe. Probably barely three quarters the diameter, although of similar pitch.)

A bearing puller can exert a force of several tons because it has a fine thread.

Mechanical Advantage = Distance moved by Effort / Distance moved by Load. The tip if the jack handle moves a long way compared to the amount by which the car is lifted.

Howard

Hi there, all,

Those of you who are familiar with the ML7 will have immediately realised that part of what I wrote, above, is sheer bunkum!! A plain bar will not span the head-stock casting - the bull wheel gets in the way.

However, while I was searching for a new book to read to while away lockdown, what should fall out of the book-case but 'A Man and His Lathe' by Laurence Sparey, published by TEE. Browsing its pages, I found what my memory had half remembered. Here's a scan of part of the relevant page:

My suggestion had the dowel in the bar whereas Mr. Sparey puts his dowel into the hole drilled into the bull wheel. I have to confess I prefer his scheme. The photos in 'A Man and His Lathe' are rather dark so I didn't scan the relevant photo of the C-spanner in action. Does anyone know if the book is still in print?

I apologise for reviving such an old thread but I wanted to correct my error. I am embarrassed by the web-site's rendering of my scan - moderators, please feel free to re-orient and reduce size.

I wonder whether the original poster has solved his problems?

Best regards,

Swarf, Mostly!

Sparey's idea can also be used for the Super 7.

The S7 has a spindle lock ! though it took me 10 years to find it. I didn't read the hand book !!!!! Noel.