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Pratt Burnerd 4 jaw Chuck jaw alignment

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DC31k22/10/2019 07:26:38
177 forum posts
Posted by Hopper on 21/10/2019 23:20:53:

Number of jaws makes no difference. You still need to preload them to hold them in the working position and to stop them moving about during grinding.

Perhaps I stressed the wrong word in my previous reply. For a four jaw INDEPENDENT chuck, what is the working position? Please compare and contrast with the same concept for a three jaw self-centering chuck and suggest a practical method of setting up your four leaf clover.

Independent chuck jaws (more specifically the faces of the ones under discussion) do not necessarily need to be ground while in the body of the chuck.

lfoggy22/10/2019 12:27:11
133 forum posts
16 photos

I was planning to position all four jaws the same distance from the centre using a dti, then set the chuck rotating and grind all four jaws at once with a pneumatic die grinder held on the cross slide and rotating in the opposite direction to the chuck. I was assuming that centrifugal force would hold the jaws in position. I only need to remove 0.1mm.

Does that sound reasonable?

SillyOldDuffer22/10/2019 12:35:19
5607 forum posts
1153 photos
Posted by lfoggy on 22/10/2019 12:27:11:

I was planning to position all four jaws the same distance from the centre using a dti, then set the chuck rotating and grind all four jaws at once with a pneumatic die grinder held on the cross slide and rotating in the opposite direction to the chuck. I was assuming that centrifugal force would hold the jaws in position. I only need to remove 0.1mm.

Does that sound reasonable?

Probably not - I don't think centrifugal force is strong compared with the forces applied by a cutter or grinder ploughing into four hardened steel jaws. Never tried it myself but the book method suggests forcibly opening the outside of the jaws into a steel ring. Then the jaws are firmly held whilst leaving plenty of room to get at their gripping faces.


old mart22/10/2019 12:36:33
1516 forum posts
136 photos

Something I forgot to ask is whether the jaws have been ground flat on the faces, or axially with a wheel. If the surface is curved, they will have to be checked while sitting on a cylindrical surface. The ideal diameter of cylinder would be about half the width of the jaws, with a shim to hold it on the jaw centreline.

If that 0.1mm slope is just made parallel, you will definitely end up with bellmouthing when the chuck is used, there has to be clearance in the slots for the jaws to move and all jaws have a degree of flexibility.

The last time I ground internal jaws, which are the same way round as yours, the only way to pretension them was to drill holes in them for the tensioner pins. A solid carbide drill was required to get through the case hardening.

Edited By old mart on 22/10/2019 12:43:55

lfoggy22/10/2019 13:02:02
133 forum posts
16 photos

Quick 'back of the envelope' suggests that at 1000rpm there will be a 50N radial force on each jaw, i.e. 5Kg . Cutting forces will generally be in the same direction as well. Maybe a bad idea though ! I'm sure a rummage through my scrap box will unearth a suitable ring against which to clamp the jaws.

lfoggy22/10/2019 13:04:51
133 forum posts
16 photos
Posted by Pete Rimmer on 22/10/2019 00:43:31:

I have I think 3 sets of 4-jaw chuck jaws of unknown provenance. Put up some basic dims of your jaws and I'll check the ones I have. In the unlikely event that one of those set match, you can have it.

Very kind offer. Let me try the grinding first but if that goes pear shaped then I may be in contact !

old mart22/10/2019 13:57:50
1516 forum posts
136 photos

Grinding this type of jaw would be best done individually in a vise. There still needs to be a slope on the jaws, and it would be best to try reducing the slope from the present 0.1mm to 0.05mm initially and then find out if that solves the problem.

The reason for drilling the jaws is because they have to be tensioned inwards while leaving the inside edge free for the grinding wheel, a near impossibility with four independent jaws, which is why nobody grinds them in situ.

I have a 6" Toolmex four jaw independent at the museum, and will try measuring the jaws to see what slope they have.

Pete Rimmer22/10/2019 14:00:46
683 forum posts
49 photos

If you're going to grind then then turn a ring concentric on OD and ID and clamp it in the jaws on the ring's ID. Clock the ring in the 4 locations directly over each jaw and adjust until all read the same then you can grind them very gently at slow rpm.

Use the biggest grinding wheel you can sensibly fit in the hole so that when you dress it true you get the maximum amount of grinding before the wear on the grinding wheel reaches full span. You might do well to make a roughing pass, dress the wheel then take a very light finishing pass.

old mart22/10/2019 14:42:36
1516 forum posts
136 photos

If you do that, you are tensioning the jaws outwards, which defeats the whole object of tensioning them in the first place. They must be tensioned in the same direction as they are to be used.

Hopper23/10/2019 11:18:00
4381 forum posts
92 photos

Normally when cylindrical grinding, the job is rotated at more like 50rpm or even less.

Yes the jaws would need to be set up so the outer tips of the jaws are tensioned outwards, as they are when clamping onto a job. You could still do this with a "four leaf clover" plate and then as you say set the jaws concentric with a DTI. Althoug, perfect concentricity is not critical on an independent jawed chuck. Few thou extra off one jaw is not going to make any difference when each jaw is set up in used independent of the other.

If you had access to a surface grinder it would be ideal to set the jaws up on the table and knock that few thou off that way.

I've reground several Myford (PB and Crown) chucks with clover leaf plates, ground them parallel with a die grinder in the toolpost, and they are still working well. Maybe in another 50 years they will start to bellmouth again but I won't be around to worry about that.

Whichever way you go with it, the jaws have got to end up better than they are at the moment. You could almost dress four thou off one end with a diamond file by hand and be better off than you are now. Almost.

Edited By Hopper on 23/10/2019 11:32:32

Pete Rimmer23/10/2019 11:41:07
683 forum posts
49 photos

You should be able to grind an essentially brand new chuck without fear of the jaws cocking. If they do cock then they wouldn't be able to clamp anything effectively inside and outside without suffering problems.

lfoggy23/10/2019 17:21:03
133 forum posts
16 photos

The jaws are a firm sliding fit in the slots with no discernible play so I am hoping that they will remain aligned during grinding.

I don't have a surface grinder but another option is to use a cupped diamond wheel in the milling machine with the jaws held in the milling chuck. I have used this setup to grind small amounts off hardened components in the past and it has worked.

I will try to get this sorted over the weekend and will post an account of success or failure....

old mart23/10/2019 20:13:54
1516 forum posts
136 photos

Making a four leaf clover tensioner seems like so much bother, and it only fits one size of jaw. That's why I drilled the jaws with a carbide drill and use any of several rings of different sizes bought cheaply on ebay. The rings are primarily for use with the serrated jaw chuck which requires tensioning every time the jaws are bored.

Howard Lewis24/10/2019 10:25:16
3132 forum posts
2 photos

Somewhere, I have some new chuck jaws for a 160mm chuck, still in original wrappings. Think that they are for a

P B, but can't remember if they are for a 3 or 4 jaw. Would need to go and find them!


Pete Rimmer24/10/2019 11:20:36
683 forum posts
49 photos

I just checked, my spare jaws are PB but they are for a smaller 4-jaw having a .200 slot and 8tpi screws. The jaws in my 8" 4-jaws and a bit bigger with coarser screws.

Hopper24/10/2019 12:25:42
4381 forum posts
92 photos

You can avoid the fiddly job of making a clover leaf plate and use four pieces of flat bar to put between the jaws and then tighten them down on. This video shows the concept, albeit on a three jaw, better than I can explain in words. The hacksaw slot and hole in each piece would be good on teh four-jaw as it would allow fine adjustment of each jaw to get them all concentric before starting. A suitable disc held in the tailstock might also help set the jaws concentric to start with.

I would use some thing like this even on a new chuck because there is going to be that tiny bit of working clearance between the jaws and the guides in the chuck body, which will allow a certain amount of movement. Also the slack between the thread on the jaw and the thread on the chuck key barrel will allow movement. Best to have everything locked down dead tight for the best possible job. 

Edited By Hopper on 24/10/2019 12:30:19

Edited By Hopper on 24/10/2019 12:33:28

old mart24/10/2019 19:16:36
1516 forum posts
136 photos

I set up one of the jaws of my Toolmex light 6" chuck, which is in almost new condition in a similar fashion except for having a cylindrical surface for the tip to bear against. The ground surface if the jaw tip is not flat, it has a radius. The jaw is exactly 16mm wide, so I used the shank of a 16mm endmill. The measured slope is much less than yours at 0.0005", 0.0127mm over approximately 40% of the length of your jaw.

lfoggy27/10/2019 17:57:29
133 forum posts
16 photos

I have now ground the chuck jaws using the setup below.


Grinding wheel rotating at 5000rpm (too low I know) and lathe chuck rotating at 1000rpm. Removed 0.02mm at a time. Lots of sparks but otherwise uneventful. The finish on the jaws was very good.

The chuck is new and the jaws are a very firm fit in the slots with no discernible movement so I don't think much 'cocking' has taken place although i fully acknowledge the logic of grinding them tensioned in the closed direction.

Testing the newly ground jaws as below shows that the clamping face of the jaws is at right angles to the slots to within <0.01mm as below.


I know some people suggested a slight taper on the jaws which is logical but hard to achieve so I just ground them straight.

I can now hold a test bar securely and accurately in the chuck - much better than before. I'm very pleased with the result and was surprised how easy this was to do.

Michael Gilligan27/10/2019 18:00:55
15450 forum posts
668 photos

Nicely done, Sir yes


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