Lathe chuck master pinion

I didn't know that 3 jaws and probably other scroll chucks had a master pinion for years. I think I read it in a chuck manual but have subsequently seen it elsewhere. My 4 chucks, Pratt-burnerd, Bison, and TOS all have the master pinion marked with a 0 next to it. It is due to the clearances internally which have to exist, one pinion is set-up so that what is held will run true or close to it when tightened. The other pinions close the clearnace in another direction and so won't run as true. This may change during the life of the chuck due to wear.

I know know if I buy into a master pinion theory. All of the pinons are tightening the same scroll.
I think a crappy chuck is a crappy chuck, that that's probably what the original poster got with his mini-lathe.
Tony

It doesn't seem to be common knowledge at all. Some seem to believe it's just a theory.

Well, you're sadly mistaken. The finest of universal chucks display this very quality, it's just a matter of the degree. The original poster may or may not have a crappy chuck, but this isn't necessarily an indication.
If you have worked with soft jaws, it all comes into focus instantly. Due to clearance of the scroll on its bearing, which pinion tightens the chuck has an influence on the scroll as it relates to its bearing surface, which in turn relates to how the jaws tighten as they relate to the centerline of the spindle. It has nothing to do with the quality of the chuck unless you have one that has a ton of clearance. Buck chucks display the same thing, and they had the respect of the industry.
Harold

They may or may not be marked. Without fail, there is always one pinion that will tighten the jaws closer to concentric than the others.
Harold

Did you intentionally put that in past tense?
I ask because I have 2 very nice (virtually new) Buck adjust true chucks for my Rockwell lathe. Both can be adjust for zero runout at one diameter and have 3 thou runout at another diameter. Some say this is normal but I am a bit disappointed because my bison adjust true for my Myford lathe will only have 1 thou change diameter. I consider that MUCH better and it is 1/2 the price to boot!
chuck

I have been under the impression that Buck is no longer made, so I may have been in error if they are still on the market. That would account for the past tense comment.
Thanks for the great report on the Bison chuck. I've long wondered about their quality. I recently bought a new 10" 4 jaw for my Graziano to replace the one that came with it from the factory. It was always no damned good, what with the body being completely out of balance and a rather poor slide system that was easy to damage simply by chucking. After complaining about it for over 35 years, I finally replaced it with a new Burnerd, which I found on ebay. I'd have gladly bought the Bison had I known they were of decent quality, and they made a 10" D1-4 mount. I've found that they are hard to find in the 10" size with that mount, although they are available.
I have never been too concerned about how well a universal chuck runs, but then I've always used soft jaws and have created my own level of precision. The 3 jaw that came with the Graziano seems to be a reasonable chuck, unlike the 4 jaw, so I've been happy with it. Both sets of the hard jaws that came with it tend to have a little runout, but I expect that from a chuck. I've yet to see one that ran close enough to make me happy.
Harold

Another thing that doesn't seem to be mentioned much is using a dial indicator to center the 3-jaw chuck during installation. Even after finding the best of the usual three possible mounting positions, it's a good idea to check runout on the chuck body itself and center it using the DI. A runout of .002" for the chuck body itself was typical when just bolting my chuck on. Using the DI, I'm able to bring that down to less than .0005". It may be difficult to find any "master pinion tendency" unless the chuck is centered properly. Furthermore, for absolute precision using a 3-jaw chuck, centering the chuck body has to be a prerequisite IMO. Did somebody say use only a 4-jaw chuck for absolute precision? Well, I ordered one yesterday.

I have a 4 inch 6 jaw on my myford. Bought it at NAMES from New England brass and tool. I also bought the myford backplate which required some fitting. The register way so close, I had to open it up with some fine sandpaper. It was so close it was a matter of bluing it to find the high spots and sanding off the blueing to make it fit. I also trued the back plate by taking a light cut on the face. After dialing the chuck in at one diameter (1/2 I think), I checked it at several other diameters and it was less than 1 though off. Take that with caution because it was just a standard last word indicator that is graduated in 1/2 thou. Anyway I'm quite happey with it.
I used the same procedure on my 6 inch buck 6 jaw. I get 3 thou TIR when changing diameters. My buck 3 jaw is about the same. My burnerd collet chuck on the other hand is quite good and I use it far more than either Buck chuck!
chuck

My last Bison plain back chuck that I bought from J&L UK clocked .0005" TIR when checked with a piece of 1" silver steel after fitting to the backplate. I also bought from them the Bison part machined backplate and carefully machined it to suit. The backplate register was machined about 0.001" oversize then fitted to the chuck after the chuck was warmed and the backplate cooled in the freezer. It is a D1-4 fitment for a Harrison M300.
Harold & Susan Vordos wrote:

[ ... ]
I've got a Bison with the two-pieced jaws, and the corresponding Bison backplate for the L-00 spindle on my lathe.
I typically find it to have about 0.001" runout with the hard jaws -- close enough so I save the soft jaws for special tasks where I truly need to grip on an already machined surface and machine the other end. Usually, I am starting from stock a bit oversized, and set up to turn all diameters in one chucking, and then part off from the stock still held in the chuck.
Agreed -- though having the 3-jaw start out that close means that my stock can be closer to maximum finished diameter, thus potentially saving some money in the purchase, and reducing the volume of chips which I have to shovel out. :-)
There is that. :-)
Enjoy, DoN.

This suggests that you don't have a register turned on the backplate to truly center the chuck on it. On any quality chuck, if this is done, the chuck body should run very close to true.
The exception, of course, is a chuck with the "adjust-tru" feature, which must be set up to move a little bit, so it can be adjusted to true the workpiece.
Yes -- but for a non adjust-tru style, that should be done by the register, turned in place on the lathe spindle where it is to be used. The chuck should not be capable of being moved that much if properly mounted.
Good. It is a little slower (though not much once you have practice), but is far more certain to be true. (Of course, so is a set of soft jaws on a universal which have been bored in place to fit the current workpiece.)
Enjoy, DoN.

Either that or the tolerance resulting from the register/chuck combo in my case is around .002".
It's not a quality chuck but capable of less than .001" runout on round stock provided it's centered using a DI and the jaws are tightened based on a master pinion.

snip----- Yep, that's a serious concern, one that I address with soft jaws as well. I have one set that is bored for gripping straight material with no steps involved. By setting my "spider" behind the jaws, on the master jaw, which is well below the gripping surface, I can bore straight through. I end up with jaws that run within less than .0005", and generally run most any size stock with them. They real advantage is that springing, which is so common in 3 jaw chucks, is not an issue. Long pieces of stock seem to run true, so long as they aren't bent, anyway.
Using these jaws used to be critical due to my manufacturing precision metering pumps for BD, which were made from 1-7/8" 303S stainless. American made stock in that size used to be between .002" and .003" oversized after centerless belt sanding. That permitted a finish cut on the exterior, which I held at 1.875" for the various components that stacked on the pumps.
Interestingly, as time went on and material began being sold from foreign countries, I had to specify US made stock, for each time I did not, I ended up with cold finished material that was, at best, nominal in size, and didn't permit a finish cut. My shop aid tooling relied on the 1.875" size for locating in most instances. It stands to reason that going to 2" stock for a finish cut wasn't in my best interest.
Harold

I didn't learn it until just last year when I bought a new (Made in India) 3-jaw chuck and noticed that the final inpector's report had numbers written in recording the runout of an unspecified diameter test bar grabbed in the chuck. (At two distances, close to the jaws and about 6" outboard of them.)
There were separate lines on the form for entering the test data, one designated "Master Pinion" and the other "Any Pinion". The runout filled in for the "Master Pinion" was about 1/3 of the runout for "Any Pinion."
Clearly, the "better" the chuck the closer to perfection the scroll will be (when new). Runout can vary when the jaws are grabbing different diameters due to deviations of the scroll from a perfect spiral. That can be because of imperfect manufacture or just plain wear, among lots of other things of course.
Jeff

Your "Three jaw lathe chuck..."Master Pinion"??" post back in March, 2003 is one of only five relevant threads returned after a Google search on >"master pinion"

Most scroll chucks have a little clearance between the circumference of the scroll and the body of the chuck. Otherwise you could barely turn the scroll. Therefore the scroll carrying the jaws and the work is free to float within the bounds of the clearance "even after the chuck has been tightened". (What is there to stop it, other then friction?)
I "think" that the so called master pinion is the name assigned to the pinion that has been found to push the scroll in the direction that displays the least runout. The pinions only exert force while they are being operated after which they hold nothing in place.
If a scroll were so worn that reasonable centering could not be obtained, then I think it would be possible to drill and tap the body to accept adjustment screws to act on, and to center the scroll. This would seem to be far superior to trying to remember which pinion would push the scroll to a particular position ??
Bill D my ($.02) (O:

Lets put this in perspective. A 3 jaw is designed for fast setup with reasonable centering accuracy. My 3 jaw is only only good to about 3 thou. I simply don't use it if I need better accuracy.
An adjust true chuck allows you to zero in a scroll chuck. Its only worth the effort if you have a lot of parts to make that are the same diameter. I can center up a 4 jaw in 1/4 the time I can dial in my adjust true chuck. Especailly that damn burnerd chuck on my myford which has 3, not 4, adjusting screws.
I use a 3 jaw chuck because it is fast to setup. If you have to use a test indicator to check it every time you use it, you are expecting more that the tools was designed to give or its time to get a better chuck!

snip---
Chuckle!!
Or learn to use soft jaws.
Harold