ML7 3jaw pratt burnard

Fastdave,

Apologies for hogging your thread.

Rather than leave it hanging however, I have to accept that my cheap method (brass bar honing) has its limitations. As you suggest Hopper, there could (would) have been errors associated with the fit of the jaws. The one certainty was that whenever I gripped a ½" diameter piece of material especially ground stock silver steel it was pleasing to see it running true, whatever that means. It was irrespective of whichever of the three chuck key holes I used.

I’m more convinced than before, that what I believed to be indications of case-hardening were nothing more than variations in the degree of ‘penetration’ between the (orange) ‘corners’ and the ¼" radius ‘channel’.

Old Mart, your comment about broken teeth adds to my probable error in that there was damage of that kind to one of the jaws, which may (or may not) have been damage I did or was the result of the previous owner. I certainly don’t recall anything going ‘with a bang’.

After a bit of Photoshop jiggery-pokery in the hope that I could see more of the honing, I was puzzled to see ‘deckled’ edges on both sets of jaws. I can only assume they were from single file strokes during de-burring at the time of manufacture.

Sam

I would be wary of honing jaws, as the hone could cause bellmouthing and also just follow any radial errors. If the jaws are not pretensioned first it is just a waste of time. Even in the most accurately made chucks, the same key should always be used to tension the jaws. Any key can be used to loosen them, however.

My take? Just accept that bell-mouthing can be reduced, but that running true (at all diameters) is never going to happen - unless you are exteeemly lucky!

Grind the jaws as per how the sensible posters have directed and accept what it is, from then onwards. It will never be perfect - it is only a worn (and likely over-strained) 3 jaw chuck which rely on both the jaws and the scroll being perfect to give perfect centring.

If I want something to run true I resort to collets, 4 jaw or turning between centres. Plenty of choice, so why worry about a 3 jaw that has a few thou run-out and will likely never be much better?

N D I Y , there are two other options for accurate workholding, soft jaws, and the griptru type of chuck. I use both methods. My take on the poor mans griptru with the reduced register between the backplate and the chuck is present on the chuck in my photos in this thread. What cannot be seen in the photos is the presence of three rear mount shcs, giving six screws hloding the chuck on. This method is frowned upon by some .

fastdave & Martin kyte, I am certainly not nimble fingered; it is not hard to adjust the jaws so that each spacer can be pushed in position with the jaws set to a push fit so to speak. Once this is set up I just tighten the jaws up more and have 'not yet' found that the spacers have moved due to centrifugal force etc. I run the chuck at about 300 rpm or so, no problem. If you are very safety orientated I suppose you should make the ring arrangement shown earlier, but its a lot of work for such a simple process.

Alan

Edited By Alan Jackson on 21/07/2020 17:10:15

Theres nothing wrong with Alan Jacksons spacers, except they are hard to make, I shall be making a pair of similar ones to grind jaws of a four jaw independent chuck. The method differs in that only one jaw will be ground at a time, the spacers tension the jaw to be ground between the adjacent jaws. I will endevour to set the position of each jaw in turn to the same radii.

Edited By old mart on 21/07/2020 20:43:44

Honing with a brass rod and lapping compound, I acknowledge, was a cheap and perhaps less desirable method of three-jaw ‘rectification’ and, upon reflection, hardly necessary anyway.

Not wishing to labour the point, I would however suggest that given any degree of play between the jaws and the chuck body, and a respectable NGP in the vertical direction, the jaws would tend to rotate/deflect (anti-clockwise in this diagram),

such that there would be more metal removed from the back of the jaws than the front, i.e. the opposite of bell mouthing. I would not care to determine what the results would be from lateral (jaw) twist.

"Twist'ut jaw and lack'ut theen" was a phrase I recall, although my vernacular spelling might be a bit suspect.

Given the need to carry out such 'improvements', I would certainly favour Old Mart’s hole-drilling technique.

Incidentally, the use of collets for repeatability and accuracy was my preference too, OM.

These went with it when I sold the ML7.

Sam

NGP = Narrow guide principle

PS - Was there more than one version of the Griptru chuck? I can only relate to the version with tangential conical wedges. It was possible to centralise the chuck to within 0.0001", so long as you stayed with the same stock diameter, and in that case always selected the same keyhole for the chuck key.  

Edited By Sam Stones on 22/07/2020 01:18:00

There were several versions in the griptru theme by different manufacturers, certainly if you are rich, you can buy a Bison or a P B. The types with four adjusters are much easier to use than those with three.

Thanks for your reply Old Mart,

Only having used the three adjuster version of the Burnerd Griptru chuck, centralising felt odd when compared to using a normal four-jaw independent chuck.

The last time I used one was in '59.

Sam

A bit late on the scene here, but I also need to grind my chuck jaws true. All the advice made it clear that the jaws should be loaded during the operation. I looked around for availability of a Hopper's clover leaf plate but found nothing. Should I make one? Then I was inspired by Alan Jackson's solution, but accepted it must be a bit fiddly to set up. Clearly the jaws needed to bear on their angled faces allowing the 'bite' surfaces to protrude for grinding. Decided to try a design in plastic. No milling or drilling, just a bit of 3D printing. The plastic takes quite a load without any distortion. Oh! and the mount for the Chinese Dremel is plastic too. So before I spin things up, can anyone see a problem with this set up?

Tried this out today. Seems to work and hope it's improved things..

Adrian, 3D printing is putting a 21st Century twist on a traditional technique. Love it!

I reckon there might be a bit of a market for such a handy thing.