ML7 3jaw pratt burnard

[fastdaveParticipant @fastdave]

Need a best as opposed to a best as for this one – help please.

I have even watched u tube on grinding out a bellmouthed chuck – I have one which is old and has seen some abuse, and grip is poor. To eliminate the bearings and the spindle, I plugged in the 6" 4 jaw chuck – and it turns beautifully and parallel – even looking at the jaws, they're in a poor state – so also are the outside set that came with it.

It just feels crude to insert a round grinder and spin the chuck, also someone suggested using a boring bar.

Both are within my scope, but I can't help but feel that some ingenious engineer is going to come up with a tried and tested method, or at least confirm the use of the above method/s.

Any takers?

Dave, Fife, MM7ASN

[fastdaveParticipant @fastdave]

chuck problem

[HopperParticipant @hopper]

Dremel grinder mounted to the toolpost works just fine. As long as you make some kind of spacers that push the outer ends of the jaws OUTWARDS while the grinding is being done. Several of the YouTube videos have it all wrong and are actually pushing the jaws inwards by using rings around the outside of the jaws in various places. Below is the traditional spider or clover leaf plate used for many years to true up chuck jaws. Worked a treat for me on two old chucks so far.

 

Edited By Hopper on 19/07/2020 12:52:43

Edited By Hopper on 19/07/2020 12:54:52

[RoParticipant @ro]

Hi Dave, I've literally just had the same issue. Some very helpful folk here (including Hopper) helped me out with loads of advice: here's the thread

ro

[fastdaveParticipant @fastdave]

Thanks Ro, I had actually looked at this thread, and that made me think there might be a more precision way to true the chuck, but if it's a tried and tested method, I shall go for it.

Two things though, –

one – I don't have anything like the spider device which you display, Hopper, and I am a bit baffled, for if it is possible to overpush the jaws in one direction, how do we determine the precision of the other – not too sure I got my head around that one, although I can see how making them rigid works, what if they're rigid in the wrong direction, opposite to your criticism af holding the outside?

two – Is the dremel spinning? and if so – do we align it to one jaw before entry? or simply gauge it and run it through? – still sounds a bit iffy, or am I just overthinking this whole approach?

Many thanks for your thoughts,

Dave, Fife, MM7ASN

[Martin KyteParticipant @martinkyte99762]

All that the spider does is push the back of the jaws into close engagement with the scroll as if they were gripping a workpiece in the normal way but leaving the faces of the jaws clear for grinding.

Yes the dremel is spinning. The grinding head is introduced to the centre of the chuck and a cut slowly put on. The length of the jaws can be ground then. Increase the cut and repeat untill happy.

regards Martin

[fastdaveParticipant @fastdave]

OK, I think I'm getting the idea – the jaws are pushing against the sides of the holes.

I just made an attempt, encircling the middle of the jaws with a bronze ring which I knew to be round – and each cut seemed to do little, or make it worse – so I gave up and came back to the forum.

Now racking my brain to find something to use in trad spider form!

Ho do you align the dremel, or is the concentric motion the instrument fro self centring the grind?

Many thanks, Dave

[Alan JacksonParticipant @alanjackson47790]

I made three equal length spacers cut from a steel ring and insert them as shown . Works very well

Alan

Edited By Alan Jackson on 19/07/2020 18:21:56

[fastdaveParticipant @fastdave]

Wow, you must be nimble fingered Alan – I'll have a go.

Dave

[Martin KyteParticipant @martinkyte99762]

You may like to stand out of the firing line with that one.

regards Martin

[Lee RogersParticipant @leerogers95060]

Posted by Martin Kyte on 19/07/2020 19:10:36:

You may like to stand out of the firing line with that one.

regards Martin

It has that sudden TWANG look about it .

[Brian WoodParticipant @brianwood45127]

What these methods will not be able to correct is in the case of a badly worn chuck whose jaws are sloppy in the guides, by that I mean capable of being rocked backwards and forwards in the guide. Another situation that defies such correction is in the case of a strained and damaged scroll which will affect the jaw closings in an unpredictable way

The fix for these basket cases is to replace the chuck I'm afraid.

Clearance along the guide is OK, the methods work in that case

Brian.

[old martParticipant @oldmart]

I started grinding the jaws for two 5" Pratt Bernerds yesterday at the museum. Doing them properly is a long tedious task. First, they have to be dismantled and cleaned, or any precision grinding a chuck full of muck is a waste of time. Then a method of tensioning the jaws is required. I chose to drill the jaws with a solid carbide drill. Tensioning rings of just the right size for each diameter of chuck are needed. The optimum diameter to set the jaws at for grinding is just inboard of the hole in the chuck body, that is also best if you also decide to do the outside steps at the same time. With holes drilled in the part of each jaw as the photo the ring is attached and the jaws tensioned inwards to grind the inside surfaces. If the outside steps are also to be ground, the jaws must be tensioned outwards. As our chucks have loose registers, the first step is to get the chuck body running true and fully tightened using the six mounting screws for the backplate. The lathe was running in reverse backgear at about 60rpm, and I set the carriage feed to move to the right (check direction in a safe position) and took 0.0005" deep passes, about three passes between adjustments. The first jaws were true in about 0.002" of cut depth. The outside steps trued up in only about 0.001" and purely by luck with the jaw settings, the whole surfaces cleaned up straight away. You could try checking the radii with a dti to get the easiest clean up.

I only got the inside jaws finished on on of the chucks finished, so I have a way to go yet. The extended nose die grinder was very good and an improvement on the straight drill that I used to use before.

[Sam StonesParticipant @samstones42903]

I have a question … ‘Are your chuck jaws through or case hardened?’

Applying lapping compound to a ½" diameter brass rod, I honed the gripping faces of the (outside) jaws of my ML7 Bernard three jaw.

At a slow spindle speed and the bar gripped in the ½" tailstock chuck, I slid the tailstock and the brass rod back and forth while occasionally adding gentle pressure to the jaws. I continued this process until there was a clean line from the front to the back of each jaw.

Later, (after much use) I noticed the appearance of a pattern on both the gripping surface of the jaws and marks in material e.g. aluminium. The imprinted pattern showed how the core material of the jaws had been impressed, while the ‘skin’ had remained less so.

While I have no direct photographic evidence, I found that the jaws were not through hardening but case hardened.

I have mentioned this elsewhere but as I recall no one responded, presumably because no one else had observed the same effect or that it is not important.

At a guess, the chuck was new and came with the ML7 between the 40’s and 50’s.

Any comments?

Sam

[Sam StonesParticipant @samstones42903]

I should check my spelling …

It's Burnerd not Bernard!!!

Sam

[Sam StonesParticipant @samstones42903]

Pondering further on whether the ML7 Burnerd 3-jaw chuck jaws were case hardened or through hardened, I’m now wondering if the results of my (brass bar) honing exercise produced this (geometric) shape to the gripping area of the jaws, and that what I thought was an imprint of the hard skin (marked in orange) and softer core was incorrect.

Although it’s still an open question, I have no way of finding out because I sold everything in’007.

Sam

[HopperParticipant @hopper]

I never checked about the case hardening vs through hardening. Even if it is case hardening, you probably have a case thickness of maybe 30 thou or so? And even if you go through that, the chuck is still useable. I havent struck any problem with the Burnerd chuck I reground, or the UK Crown chuck of my Drummond either. Both have given some years of good service since regrinding. I think it only took grinding less than 5 thou off the worst jaw, so chances of breaking through the case hardening were slim.

I'm not sure about the lapping to a brass bar method. Firstly because not sure if the three jaws are worn so they are running not concentric, then the brass bar will move or flex that couple of thou and wobble around with the worn jaws, so lapping, like reaming follows the existing hole. And secondly, not sure if there is a bit of slack between the jaws and thier slots in the chuck, will not clamping on the brass bar as it spins set them in one position, but then when holding a short piece in the outer end of the jaws, the bellmouth will still be there?

That's why I like the grinding method and the spider plate used as far back as "Duplex" in ME in the distant past and probably before. Holds the jaws in the outward bellmouthed position and guarantees all three jaws run in the same concentric circle (at that particular diameter!).

Edited By Hopper on 20/07/2020 10:59:54

[RoParticipant @ro]

Posted by Hopper on 20/07/2020 10:58:41:

… and guarantees all three jaws run in the same concentric circle (at that particular diameter!).

Haha, yeah, that's what I found. With the state of my scroll, I have no measurable runout at the diameter that I trued my jaws at, but if I go larger or smaller, the runout returns!

ro

[HopperParticipant @hopper]

Posted by Ro on 20/07/2020 11:07:26:

Posted by Hopper on 20/07/2020 10:58:41:

… and guarantees all three jaws run in the same concentric circle (at that particular diameter!).

Haha, yeah, that's what I found. With the state of my scroll, I have no measurable runout at the diameter that I trued my jaws at, but if I go larger or smaller, the runout returns!

ro

Yes if the scroll is worn out it's time for a new chuck. Luckily both mine seemed ok.

[fastdaveParticipant @fastdave]

For me, Fascinating development of this thread. One can see why we would want to repair rather than replace, and I shall certainly be pushing on with lots of the above – one thing, though, no – one has suggested milling the jaws…..

Thank you all for great data,

Dave, Fife, MM7ASN (RSGB Callsign)

[old martParticipant @oldmart]

I'm sorry, I cannot remember if the P B jaws are case hardened, it would have been obvious when I drilled them. Unless you have a solid carbide drill, don't ruin an hss, even cobalt.

If I had to guess, I'd plump for all hard, as the recent posts by people breaking teeth while overextending their jaws would suggest. The teeth break with a bang, not a whinper.

Edited By old mart on 20/07/2020 14:17:40

[Sam StonesParticipant @samstones42903]

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

[old martParticipant @oldmart]

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.

[not done it yetParticipant @notdoneityet]

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?

[old martParticipant @oldmart]

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 .

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