ML7 3jaw pratt burnard

[Alan JacksonParticipant @alanjackson47790]

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

[old martParticipant @oldmart]

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

[Sam StonesParticipant @samstones42903]

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

[old martParticipant @oldmart]

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.

[Sam StonesParticipant @samstones42903]

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

[Adrian RawsonParticipant @adrianrawson27430]

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?

[Adrian RawsonParticipant @adrianrawson27430]

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

[HopperParticipant @hopper]

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.

[Author]

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