Help with Pratt Burnerd set-true chuck

[Froneck]

I bought a Pratt Brunerd 10" set-true chuck with LO back plate. Chuck is in like new condition but I'm having problems making adjustments. I removed the taper screws one at a time so as to not mix them. Looked in great shape, put copper anti-seize on them. But chuck moves with quite a bit of torque on long hex wrench, I usually add a cheater bar. I tried loosening back plate screws but still very difficult to move chuck. I have other set-true chucks, type with 4 set screws and the type with 2 piece jaw so the jaw can be moved similar to a 4 jaw chuck. But all my other chucks are D1-6.
Any suggestions? I might disassemble it to see if I can fin the problem, have like new non adjustable Bison 8" to use. It runs with about .002 or .003" runout.

 

[WHHJR]

I would do the obvious and take it apart and clean and perhaps polish the sliding surface.

 

[BobH]

Take it apart and clean it. You may have to stone down burrs where the adjusting screws touch if someone has been reefing on then that hard. There isn’t a lot of room to start with and raised burrs may take up all your adjustment room. Bob

 

[Froneck]

OK, thought so! I disassemble the chuck and report what I find when I fix the problem. That way someone else will learn what I found.

 

[L Vanice]

Pratt Burnerd used the name Griptru for their design that evaded Buck's patent for adjustable chucks. That is the chuck with three tapered tangential adjusting screws. When the Buck patent expired, PB came out with a nearly exact copy of the Buck design and called it Setrite. There never was a PB set-true chuck.

Here are the instructions for the PB Griptru chucks. The important part is to never put a cheater bar on a long hex wrench to adjust the tapered screws. Follow the instructions.

Larry

 

[JST]

Heh............ instructions very reasonable. Never had a 3 screw type, but that's the way I'd have approached it.....

People complain that with three you will chase your tail forever and learn new curses. What PB suggest is basically treating each screw as if it were on a 4 jaw, and the other two like the opposite screw on a 4 jaw. That simplifies the issue and makes it manageable.

 

[L Vanice]

Heh............ instructions very reasonable. Never had a 3 screw type, but that's the way I'd have approached it.....

People complain that with three you will chase your tail forever and learn new curses. What PB suggest is basically treating each screw as if it were on a 4 jaw, and the other two like the opposite screw on a 4 jaw. That simplifies the issue and makes it manageable.

Yes, the PB Griptru chucks do work OK once you learn how to use them. I think the main advantage of the Buck Ajust-Tru and PB Setrite (and other copies) is they are cheaper to make. Plus, the four screws that are exactly opposite each other are, as you said, pretty much like a 4-jaw to adjust. But I made the mistake of buying a new Bison adjustable chuck years ago. It had four screws, but they were arranged in an X-shape that made you back off two screws in order to tighten one. That was a pain to adjust, worse than the PB Griptru.

Larry

 

[Froneck]

As I mentioned I do have the adjustable jaw scroll chuck in 3 Jaws (4 Jaw too) but after adjusting the scroll is difficult to move with the wrench due to the high pressure exerted by the screws. When the item is removed the setting is no longer good due to the changing pressure exerted by only the scroll. The 4 jaw is easier to adjust but will have the same effect when putting another part in the chuck not to mention added weight when installing/removing the chuck.
Thanks for the instructions L Vanice, didn't intend to use the exact name but the function. I know now that the PB is a Griptru

 

[Limy Sami]

I never liked those 3 screw PB Griptrus, give me a 4 screw or better still 4 jaw independent anyday, ........oh and learn how ro use soft jaws.

 

[Froneck]

As I mentioned I have all kinds of chucks including a few that just have soft jaws. I do have a Buck 8" Six jaw chuck with soft jaws and 4 set screw set-true feature. Only chuck I don't have and don't want is a 4 Jaw Universal. Soft jaws are OK but I don't do production work so a set true feature is quicker than boring soft jaws!
BTW I removed the back plate of the PB chuck. I think I found the problem. From what I see someone made the back plate wrong. Yes it's the correct back plate for the PB but the "Tabs" where the tapered adjustment pins engage have almost no clearance.! Without the tapered adjustment pins installed and no screws to hold the back plate I can NOT move the back plate, if it does move it's only a few thousandths. In addition the back plate is held on with flat head socket head screws! There is only one back plate not 2 as per 1 thru 8
The clearance as shown in the top cross section is at the most a few thousandths. so combined with flat head screws the chuck will not move only a few thousandths on the back plate.
Anyone know what the clearance should be as shown in the top cross section between the chuck and the back plate attached to the spindle??

 

[JST]

No experience with the Griptru, but I do have a Buck type. The adjustment is only a few thou, maybe 10 thou or so. With that tapered adjusting screw, I would reckon the total adjustment might be a bit smaller for the PB type.

It really is only for setting true, not like a 4 jaw, so a few thou is all that is needed.

 

[Froneck]

I too have a Buck and a Cushman set-true type 6 Jaw and a Buck 3 jaw and can get more than a couple of thou adjustment, I know I can get at least .015 being my last word indicator will swing max + to max - (.030) from time to time and have no problem getting .000. Nor do I think I'm near max adjustment. In the top drawing there is a noticeable gap between the chuck body and the back plate, lot more than a couple of thou! Plus a couple of thou is guesstimate since I feel no movement between the 2 parts. Nor do any of the other chuch that have the 4 set screws for adjustment have flat head screws holing on the back plate.
I plan on putting the back plate on the spindle and take about .050" off the diameter using the lathe dial since the adjustment tabs are 3 legged. I'll have to figure out what screws to use to hold the back plate on since plate is countersunk for 5/16" - 24 flat head screws..

 

[JST]

You might want to do a little checking on the tapers, and how much adjustment you can get.

I assume you backed out the opposite side screws somewhat before adjusting the one......... since you have the Buck etc, you know the drill with them.

 

[Froneck]

I backed 2 taper screw out so that they did nothing and tightening the third to the point it would not move using a cheater on a long hex wrench, would not move the chuck body. The spindle back plate will slide in but with the taper screws removed not move sideways. There is a taper that fits the screw taper only about 1/2 way accost the tab so there is a lot of adjustment available plus if I remove .025 the taper is still there, just less of a radius. Yes I know the 4 set screws on the other set-true type chucks have to be turned like 4 jaw chuck. one side loosened before other side tighten.

 

[JST]

OK. I learned, back when I was younger, and more foolish, to be darn careful when "redesigning" something that ought to work.

Sometimes the obvious solution is exactly wrong, and there is something different that is creating the problem.

 

[guythatbrews]

Look at step #4 in post 5 courtesy Larry. "Clearance around backplate." If flat head screws were holding the chuck to the backplate, a bad choice, it's not a stretch whoever made the backplate didn't understand there needs to be clearance to allow sideways motion. The OP says there is no sideways motion with wedge screws removed so there must be no clearance around the backplate.

Edit: After somebody pointed it out pretty obvious that the clearance in fig 4 doesn't have anything to due with griptru motion. Looking again I believe axial clearance on backplate face is being referred to.

 

[Froneck]

Measuring the diameter of the three legs is hard to do Maybe I can measure the OD of the back plate put and indicator in the tool holder and determine the differences in ODs. That will give me a close OD dimension of the 3 adjustment legs. I'll have to look close, it's possible who ever fitted the back plate by facing the mating surface didn't cut all the way and there is a slight flange limiting sideways movement. As always I'll measure 3 times, cut once. But I do have to change the flat head screws that are definitely wrong!
Update; Measuring the OD of the 3 legs was easier that I thought. When the 3 adjustment leg were turned the was a slight undercut in the mounting face plus about .100 of the face was cut so a full round OD same diameter as the leg diameter. Using a Starrett 123 vernier caliper I measured the ring, checked the ID of the chuck body and it was exactly .100 larger. I put the back plate on the chuck again and found that if I rotated the back plate to each of the 3 possible locations 2 would have no sideways movement and the third about .015 so the casing is hitting the pinion and it is as casted so I turned it round. The inside hub bore ID is .025 larger than the hub OD so I bored it to the same .100 difference as mentioned above. Now the back plate moves quite a bit on the chuck body! I put the flat head screws back in but only a couple of turns so flat head does not come near the countersink. Being the holes for the 5/16 screws is slightly more than 11/32 movement is again limited. If I use hex button head screws that have a large head diameter I ca use 10mm drill and 5/8"
counterbore to make the hole size

 

[JST]

The movement has nothing to do with the adapter backplate, look again at Fig 4. The movement is all within the chuck, not like the usual adjust-tru, where it is between the chuck and the adapter.

There is the front of the chuck, and then an adjustment plate. That's all chuck parts. Then an adapter backplate goes on the back of the adjustment plate, and can be solidly screwed to it, no movement. Flat head screws there should make no difference.

If the screws there are too long, they will go through the adjustment plate and foul the front part of the chuck.

Froneck says there is only one backplate, which would mean the adaptor is also the adjustment plate. Seems like that would be very odd, seems more like there must be a somewhat invisible seam.

If actually true, the previous owner must have made his own combo plate that is both the adapter and adjustment plate, and tossed the original PB adjustment plate. Why that would be done is puzzling. A lot of work. Maybe it was done when the original factory adjustment plate got screwed up, and the previous owner just tried to make a usable chuck out of the remainders, not bothering with the adjustable feature.

We need a picture or three to clear this mess up.......

 

[Froneck]

I think there are 2 types. Looking at Fig. 1 thru 8 there is 2 back plates shown. I have only one. The back plate fits the L0 spindle and is attached to the chuck with 6 screws at about 40° spacing. I'm assuming it's a 9 hole pattern using 6 screws in 2 screws, skip one, 2 screws, skip one, 2 screws. skip one. The 2 screws used are on each side of the jaw. There are no other screws but the back plate containing the taper mount is a one piece casting that has the pattern shown in the top diagram. It extends into the chuck body and has the 3 legs for the adjustment. Simply put my back plate is one piece casting that extends into the chuck body with a section that reaches the scroll plate. 6 screws hold the back plate on in the pattern above, 3 set screw that hold each scroll pinion in and the screws that hold the 2 piece jaws together are all the screws in the chuck. What I assume is PB sold a plain back chuck that a back plate could be attached probably for threaded spindles so the user could machine a back plate for their threaded spindle.while another type with one piece back plate was available for the more popular L or D type spindles. My chuck looking at fig.4 cross section is one piece not 2! The user didn't facricate a plate, it's casted and sections that are not machined are still as cast. For example the front face and sides of the adjustment "Tabs" are as cast in addition the area between the "Tabs" are also as cast and the OD that extends beyond the pinion shaft to the scroll as well as the opening for the pinion shaft is also as cast. What might have happened is the chuck had a different spindle mount and another one piece back plate was purchased from PB.
I'll get a photo posted in a few days. But there is NO invisible line, the 2 pieces are a single casting!

 

[guythatbrews]

The movement has nothing to do with the adapter backplate, look again at Fig 4. The movement is all within the chuck, not like the usual adjust-tru, where it is between the chuck and the adapter.

Right you are. I missed that. Way more complicated than the 4 screw type. Now I think note 4 clearance refers to axial clearance in that area.

My chuck looking at #4 cross section is one piece not 2!

So if you take all the screws out the backplate has got to move around of course.