125mm HBM Chuck too heavy for Myford ML7???

Hello,

Just wondered if anyone else is using a 125mm HBM 3-jaw chuck on a Myford ML7 lathe?

Since purchasing mine from RDG a few years ago i've always been a bit concerned about its weight with respect to the size of the spindle bearings on the ML7, however as the chuck was advertised as being suitable for my lathe, I assumed that it must have been tested and certified as fit for use.

Also several internet searches at the time of purchase did not reveal any issues with using this particular size/manufacturer of chuck so I took no further action.

However, I was recently looking at chucks on the Myford website when I noticed the following warning about using the Pratt Burnerd 125mm 3-jaw chuck - "THIS 125mm CHUCK IS TOO HEAVY TO MOUNT ON ANY MYFORD WITHOUT THE M42.5 x 2mm 4MT SPINDLE".

Since reading this warning I was once again concerned about the weight of the 125mm HBM chuck so I contacted RDG whom I purchased it from. They responded with the good news that the 125mm HBM chucks are much lighter than the 125mm Pratt Burnerd ones and as such should be fine to use on my ML7.

I was happy with that until I weighed the 125mm Pratt Burnerd (PB) chuck we have fitted to our Boxford lathe at work - the PB chuck weighs approx. 5kg and the HBM chuck weighs approx. 5.4kg? Obviously the two chucks I weighed have different back plates, but the HBM chuck is about the same, if not slightly heavier!

Anyway, I have used my chuck quite a lot without any problems and RDG said they have sold lots of these chucks without a single complaint (and to be fair they are in my opinion a very good and accurate chuck for this price bracket) so what do people think - are these 125mm 3-jaw chucks too heavy for the standard ML7 spindle bearings or am I being over mechanically sensitive?

Thanks,

Ross

Hello Ross,

Welcome to the Forum, I hope you find it both useful and informative.

To answer your question. I have two 125mm diameter chucks, both Pratt, which I fit with happy abandon on my 30-40 year old Myford ML7R.

One is a 4 jaw self centreing chuck weighing in at 5.1kg, the other a 4 jaw independent which weighs 5.8kg. Neither have given me the slightest concern in use, nor has the lathe shown any distress of any kind with them fitted. They are both well balanced and to my mind, as long as they will grip work without the jaws fouling the bed or parts of the carriage, it has never occurred to me that there might be some weight limit I should not exceed. I believe firmly that chuck mass is a bonus [within reason] and better work will result than might be the case with a lightweight chuck gripping a heavy job.

So, in summary----I think you are being over sensitive in this case.

Happy turning

Brian

Brian, OP has ML7, ie earlier light bearing model,

HOWEVER, the use of the Pratt 4 jaw independent chuck you refer to is equally acceptable for use on the ML7, as well as on ML7R and Super 7 series lathes. Myford supplied my ML7 with such a chuck.

I agree OP is being oversensitive but also the HBM chucks are in my opinion chunky.

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Ross,

This does seem rather worrying ... especially the inconsistencies in the advice.

Like Brian, I would be surprised if there was any practical problem ... But I would just mention that the 'Screwed Body' 6 inch 4-Jaw that Burnerd supplied for the Myford was of much lighter construction than their standard [backplate mounted] version.

Very interesting ...

MichaelG.

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Edit: Link to Myford page.

Edit: Link to 600 Group catalogue ... see p5

Edited By Michael Gilligan on 07/10/2015 09:44:31

Edited By Michael Gilligan on 07/10/2015 09:50:53

I have 4 & 5 inch Burnerd 3 jaw chucks for my Super 7 plus I have a 6.5 Colchester, the 4 inch chuck is the regular chuck in use, the 5 inch is only used for those lighter ,larger diameter jobs which are too small /delicate to be set up on the Colchester,I would not use the 5 inch in the higher speed range of the super 7,or do too much parting off, I think thats asking too much of the lathe bearings,the 5 inch is backplate mounted so it does stick out a long way from the spindle nose,but can be useful for the occasional job which does not easily fit the 4 inch chuck or the 6 and 8 inch chucks of the colchester. Myford supplied threaded body chucks with the prime reason of keeping the chuck from extending too far from the bearings, The MlL 7 has lighter bearings than the super 7 so I would obtain a lighter 4 inch chuck for regular work,and keep the 5 inch for work that will not fit in the 4 inch,respect your machine and do not strain it.

I have read in rather old books that chuck weight is a major cause of bearing wear. I think it is a fallacy. Cutting forces have a much greater effect. I did have an ML7 with 125mm chucks and know from reconditioning the head stock bearings that there was hardly any wear at all in the lower bearing halves that could be caused by chuck weight.

I think the concern from the manufacturers end is size of work and torque but all that I am aware of offer a small chuck that may come as standard and a larger one at extra cost. That includes Boxford. I have the original 100mm chuck that came with my ME10, also a 125mm one that some one has fitted themselves. I am looking at fitting a 200mm 3 jaw to it. That will have a very significant effect on chuck weight due to the piR^2 relationship plus other factors relating to strength/size of parts and maybe their should be some concern about weight with a jump like that. Chucks of that size are only usually fitted to very hefty lathes - I wouldn't include the modern boxford in that range. Those do look like they would take a much bigger chuck than they are usually fitted with. A lot of modern lathes are like that. I'd guess that they largely base the chuck size and fitting on the bore of the spindle and that's about it really.

4 jaw independents are generally bigger than the 3 jaw supplied with the lathe, 200mm on a Boxford for instance but they are much lighter chucks.

John

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Ross,

Is this the HBM Chuck that you have ?

MichaelG.

Just to put a bit of context into the equation:

My 4" 3 jaw Burnerd with backplate weighs 2.8Kg

My 6" 4 jaw Burnerd independent with screwed body (no backplate required) weighs 4.5Kg

Both as supplied with my (super) 7 lathe. The 4 jaw has swung some pretty hefty out of balance castings in it's time!

Rod

I did think about mentioning rpm but didn't bother. Figures can usually be obtained from the chuck manufacturer. Where that might just be a problem is when rather large very old chucks are being used due to the gradual increase in lathe speeds over the years. These days it's not unusual to see lathes that can take 250mm chucks have speeds nearing 2000rpm even from the far east. I don't think this is even a thing worth considering on a 125mm 3 jaw chuck. 2,500 rpm is fairly run of the mill now on lathes that take that size. The actual ratings will be much higher than that.

One thing for sure. Bigger chucks help at home especially on lathes with relatively small spindle bores - less short dog ends left lying about because a useful length of excess bar can be held within the chuck up to a larger diameter.

John

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That summarises it nicely Rod. I'll add one other aspect concerning work size. Say something is so big and hefty that the 3 jaw shouldn't really be used - out comes the independent 4 jaw as they WILL grip work far more tightly than a self centring 3 jaw. That includes longer lengths of large bar.

John

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Edited By John W1 on 07/10/2015 10:47:37

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John,

Just for info. The manufacturer's speed rating is given in the catalogue that I linked.

MichaelG.

Doesn't it rather depend on other factors - including how fast you run the lathe, how well you lubricate it and how heavy the work pieces you fit in the chuck are. The most important being the quality of lubrication.

What matters is not exceeding the PV value for your bearing which is calculated as the product of pressure and velocity and a low maximum for plain bronzes is 35,000.

Assuming a lathe top speed of 1000 rpm and bush diameter of about 1 1/4" then the maximum surface speed is about 350 feet per minute.

That gives a conservative maximum pressure loading for the bearing of 100 psi, now you must have at least a few square inches of bearing area in an ML7, so a 300lb load seems reasonable.

As JW1 says cutting forces will greatly exceed 5kg/11lbs, perhaps by a factor of 10 or twenty, but even so I would say that as long as you ensure the bearing is always well lubricated you will be fine.

Naturally, on startup you will be getting more wear before proper lubricating conditions are established, but I assume you aren't talking endless stop-start six days a week.

Neil

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Not according to 'Myford'

Opps, of course ML7s have plain white metal bearings. Babbit has a max pressure of 800-1500 psi (according to my Machinery's), it doesn't give pv values for babbits (although it does for wood, teflon etc.!)

The net suggests pv values from 12,000 to 30,000, still plenty high enough that chuck weight shouldn't be an issue, especiually as you will be using lower speeds with such a big chuck (I hope).

Neil

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... But Myford's emphatic warning is not limited to the white metalled ML7

MichaelG.

Edit: I'm guessing they might be worried about the strength of the nose thread ... 42.5mm is significantly larger than the original diameter. ... The nose adapter shown on this page, graphically illustrates the difference!

Edited By Michael Gilligan on 07/10/2015 12:44:39

Edited By Michael Gilligan on 07/10/2015 12:54:28

Edited By Michael Gilligan on 07/10/2015 13:01:34

Why would you use lower speeds with a larger chuck? If you were boring say a 1/2" hole in a big bit of metal you should be running at a speed to suit the cut

Edited By JasonB on 07/10/2015 12:44:16

Many years ago I bought a 5" Polish 4-jaw self-centring chuck, mounted it on a backplate on my S7 and then thought "Oh dear, it's a bit big." It just looked wrong. It wasn't weight so much that jarred, as the distance of the front end of the chuck to the headstock. Large bending moments seemed likely! By an amazing coincidence, they were about to buy an identical model at for use at work, so I persuaded them to buy a 4" one and we'd do a swop. Never regretted that choice.

.

Who are now owned by if my understanding is correct a company of 'box shifters' not engineers and designers.

Nick

Thank you KWIL for correcting me on that point.

In my reply to Ross, I should have added that the heavier 4 jaw independent also saw duty on my previous lathe, an ML7, without giving me the slightest concern. Work speed has always been appropriate to the job in hand and using that chuck ~600 rpm has been the maximum. I still stand my what I said.

Brian

It's interesting to look at Pratt Burnerd. 125mm 3,700rpm for the standard range 5,000rpm for precision.

100mm 3,800 rpm and 5,000 again.

200mm 3,200rpm standard range.

Part of the difference between the standard and precision will be down to precision and it's effect on the balancing of the chuck.

My Chinese 200mm has a stated max body run out of 55um or 2 thou in old money. The info that came with it doesn't state a max rpm but it's usually fitted to a lathe with a max speed of 1800 rpm. Their web site shows 3 grades, 2000,2500 and 3000rpm. Weight is 14.1Kg. It's a chunky chuck just like the Polish ones tend to be. I always feel Burnerd are more refined and probably stronger too.

John

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