Help with Pratt Burnerd set-true chuck

[JST]

Well, they do say that... They did make direct mount chucks.

But, if the chuck is a direct mount type, the screws should be what the factory put in, and ought not to have changed since then. Maybe someone "fixed it" later with some bodge or other.

The factory screw setup, whatever it may have been, would allow the desired amount of adjustment. That may only be 10 thou or so, and so no large clearances may be needed.

This all would be cleared up with just a couple pictures.

Are these flat-head screws put in from the front of the chuck?

Are they in factory countersinks, or are they bodged into counterbores, or crudely countersunk by some later person?

If the screws appear to be original, then the odds are pretty darn good that they are not the problem.

 

[Froneck]

Removal of the 6 screws will allow the one piece back plate to be removed. But before I bored the ID that fits over the hub that has holes for the scroll pinion shaft and supports the scroll the plate would not move when replaced with the marker I put on before disassembly marking the back plate to chuck body orientation. But as I said before rotating the back plate a slight movement was felt when in one of the 3 possible locations was used. The as casted slots are about .10 larger than the pinion shaft as seen by eye. After boring the center from being .025 larger than the above mentioned hub to .100 same as the the difference mentioned before for the adjustment "Tabs" the back plate would easily move in all directions. I did turn the as casted surface that would contact the pinion just enough to clean the surface and make it concentric with the ID fitting over the body hub.in doing so I faced the front faces of the "Tabs" just enough to make them all the same length allowing me to turn as much of the OD mentioned above as possible.
The flat head screws are mounted in the back are about 1-1/4 in length and the back plate is countersunk so they are slightly below flush. No holes in the face of the chuck. Also they do not look as if counterbored holes were altered to accept flat head screws. A continuous angle from max OD to the 11/32 hole diameter. Lengths of various surfaces are very critical in that one surface keeps the scroll against the jaw teeth and the depth of the hole over the hub as to be kept to a minimum to prevent chips from getting into the scroll pinion area. It's very clean with no chips of any size visible therefor the lengths are correct.
I mentioned using hex head button head screws because they have the largest head diameter so as to cover the suggested 10mm holes.

 

[JST]

You need to do pictures.

 

[Froneck]

Here's a few photos, Notch for the pinion shaft is 5/8 wide, pinion shaft is 3/8. Red is original casting probably Red Lead paint. Area of the pinion notch without red was machined by me but was as casted prior to me cutting it the be concentric with the bore. The surface machined below the pinion notch that meets the chuck body with the bolt holes was not cut by be, Only cut I made was over the pinion notch and the opposite side that was bored.

 

[JST]

Interesting. It appears to be just as you say, that backplate is screwed directly onto the adjustable front section. The screw holes do have what appear to be original countersinks. With that setup, it could be very hard to adjust anything, as the countersinks would hold radially. But they could allow the screwheads to tilt, especially if the screws were made with rounded instead of straight conical heads.

Could the original screws have been made with a reduced shank and have been intended to be just flexible enough to allow the intended slight movement? That seems a bit odd, but this is a British chuck, and British designers had some "original" ideas from time to time.

The threaded holes in the "front portion" also appear to be much smaller in major diameter than the clearance holes in the backplate. That could be a "built-in" flexibility feature.

Neither option would make the movement "loose", but it could at least be "possible". A similar setup appears to be required for the variant having a separate adapter backplate, since the screws, if on the back, would have to be recessed, and would not be accessible for adjustment..

In the pictures, I do not see the adjustment cones, although I do see the slots to accommodate them. I assume they were actually present.

 

[L Vanice]

I only have one PB Griptru chuck at present. It is an 8" 3-jaw with flat back. It has never been used, though the world patents pending stamp implies it is pretty old. Anyway, I have no doubt that the standard type hex socket flat head screws holding the two halves together are original, as is the visible lube on the screw. The through hole in the back half is larger than the screw size.

When all three of the tapered adjusting screws are tight, they hold the two chuck halves together, along with the six flat head screws, so the assembly is rigid when holding a work piece.

Larry

 

[Froneck]

I think the back plate is one piece because of the L taper. Looking ar the first drawing of the back plate it is as mounted on the chuck with the screws removed. Looking at the center of the L taper you see the chuck body hub that is visible in the center of photo .number 4. If it were 2 piece it would add quite a bit of length. I'm thinking the 2 piece back plate is reserved for threaded spindles.
I see the chuck in L Vanice's photo has flat head screws, how can the rear plate move with flat head screws? The chuck also looks like it was built to be 2 piece back plate. Maybe because it was 8" and mine is 10"

 

[JST]

....................... Anyway, I have no doubt that the standard type hex socket flat head screws holding the two halves together are original, as is the visible lube on the screw. The through hole in the back half is larger than the screw size.
.............................................
Larry

That's interesting.

No matter what the size of the clearance, the flat head screws would pretty well resist movement, one would think. And, with the adapting backplate in place, they would no longer be accessible to loosen, nor would it do any good to loosen them, since they would try to find center when tightened.

This then suggests a few questions:

Does your unused chuck adjust the way it should?

If so, by how much is it possible to adjust it easily?

if it does adjust, what happens with those flat head screws, do they tilt or otherwise move?

If those screws are snugged enough to hold the halves adequately, and the thing does adjust at least by 0.015" or close to that, what is allowing the movement?

 

[Froneck]

I would think that if flat head screws were used and tightened to hole the chuck securely the only way the adjuster can move the chuck body on the back plate would be to stretch the screws. Being alloy steel having 120,000 psi tensile that will not work well. Looking at Fig 4 cap screws hole the rear back plate to the middle on and L Vanice's photo shows flat heads holding the middle plate to the chuck body it is opposite the way I would think it should be done!

 

[JST]

Both the cases, with the extra back plate adapter, or with a direct mount, are equivalent. In each case, the plate with the "wedge screws" for adjustment must be able to move relative to the plate having the 3 bosses that the "wedge screws" work against. And, in both cases, those two pieces must be held tightly enough to avoid axial movement, while allowing "enough" radial movement for meaningful adjustment.

The "stretched screw" type fixing would seem to be capable of a decent amount of radial movement, and certainly prevents axial movement.

 

[Froneck]

Yes I assume that they are using flat head screw in the :movable plate. That;s why I think it odd that they use flat head in the "movable" plate and cap screw in the "spindle" plate that is fitted into the movable so it can't move and as I mentioned would think it should be the other way! My other chucks that use 4 large set screws to move the chuck body on the back plate have oversize holes for cap screws that are tight yet will allow side movement.
I might be wrong but it's my chuck to screw-up so I'm going to counter-bore the 82° holes. to 16mm or 5/8" use 10mm or X drill to open-up the 11/32" holes. Those in the chuck now are flat head screws taper from .656 to .312". I'll use button head hex drive screws with head diameter .546". But since I check 3 rimes and cut once "ll have to measure the back plate to see if I have enough material to counter-bore the holes 5/8". I guess there is more than .050 wall but may have to trimm the button head screws OD and use a smaller counter-bore.

 

[Froneck]

I forgot the camera at my shop yesterday. I did make a few more photos. I did read the instructions and it states not to use heavy force. I can loosen 2 taper screws quite a bit and 3rd will not screw in no matter which 2 are loosened. I actually removed 2 and 3rd would not tighten. Though I haven't tried after boring the center of the back plate.
The end view of the adjustment leg the face (OD was not touched and taper groove and surrounding surface was not cut by me) that I faced is not totally complete, part of original casted surface is visible.

 

[JST]

Were the backplate screws in and tight at that point? If so, that would certainly nix the idea that they allow movement in any way.

With the taper screws going into taper recesses, there is a decent possibility that they may be enough to hold the chuck together by themselves, with no backplate screws. It would be a wedge fit, but I'd not be really confident that they really would do it.

Plus, if you slack two of them, the chuck could tilt. Seems undesirable.

It's a great workaround vs the Buck patent. But I have no clue how it was supposed to work if those flat head screws were tight, unless some provision for movement was included. I see nothing but the possibility of slacking the screws off a bit

 

[Froneck]

I agree! The flat head tapper will stop any movement, I use them when fixed location is desired and dowel pins are a bit more than needed or can be used. When I got the chuck they were very tight, I tried snug but couldn't get adjustment loosened them and still nothing but didn't try after boring the center hole in the back plate. With them tight so as to hold the chuck firmly against the back plate that's attached to the spindle they would have to be stretched to get any movement. Mounting and Operating instruction paragraph 12c states not to us force on the adjustment screws. No where does it say to loosen the back plate screws that will be covered if a 2 piece back plate were used.
Simply put the bolt head they seem to be using is opposite what I think is correct! So I'll modify the 82° countersink to counter bored holes.and button head screws.

 

[guythatbrews]

Something about using buttons heads just doesn't sit well with me. If you don't mind the bolt heads sticking out you can make washers bigger than the countersinks and still use socket head cap screws.

 

[Froneck]

I wanted to prevent the heads from sticking out, also to have as much cast iron at the bottom of the hole as possible. In addition I want to use a washer with the button head screws. I can get hardened washers from McMaster that have 5/8"OD to fit snug in the 5/8" counterbore and open the ID to 10mm or 11/32". Now I'll have alloy steel to alloy steel rubbing rather than alloy steel to cast iron. Wrench size of cap screw is 1/4", head OD .469" Button head wrench is 3/16, OD .547". I'm not sure which one is stronger but head length will not add to strength. All the screws I've seen that broke were not broken at the head. Usually at the start of the first thread away from the head. Or I seen cap screws pulled thru the head hole. I'm thinking that the button head screw will be strong enough and possibly as strong as cap screw. In addition the only lathe I have with L0 taper is a 12" light duty Hendey No.1 It's not gear drive, the type that is driven by flat belt on the motor flat belt pulley with a shifter to change speeds driving a Vee belt to the spindle. All the other lathes from 12" Hendey, 14" Hendey, 16" P&W and 16" L&S have D1-6 spindles. Main function of light duty Hendey is metric threading and have the 128/120 teeth compound metric conversion on it.
But something came to mind, I have to look at the hole the tapered adjustment screw is in on the chuck body, if the taper is at the limit and taper can't go any farther then no adjustment is possible! I might have to braze the tapered slots in the back plate legs!

 

[Froneck]

I'm sure I found the problem! I screwed in the taper pins as far as they will go and measured the increase the taper reduced the ID radius, .10, gap is .050" and best as I can the depth the taper pin wore a groove in the leg was .050" So very little adjustment is possible! I can braze the tapered wear slot in the leg but worried about the heat distorting the back plate. Any suggestions?

 

[JST]

Set in a new wear plate on each?

 

[Froneck]

I was thinking of drilling flat bottom 5/16" holes with an endmill and putting Aluminum Bronze in the holes, probably only have to be 1/4" deep. maybe mill a key slot and make a key to fit the radius on each side. Make them hammer fit the turn the back plate.