Chuck master pinion

[mklotz]

I'd heard and read about this for years. Basically, the premise is:

---
On a self-centering three jaw chuck, there's one particular pinion (the
so-called master pinion), which, if used to tighten the stock being held, will
center it with better accuracy than would be the case using any of the other
pinions.
---

So, I decided to test this out.

Now, understand, I do not consider the 3jaw chuck a precision tool. Its real
advantage, if it has any, is its ease of use and wide range. If I'm turning
something where all the cuts can be made with a single chucking, thus
guaranteeing coaxial features, fine, I'll use it. If the work requires several
chuckings and/or reasonable accuracy, I'll use collets or the four jaw.

The premise goes on to say that good chucks will have the master pinion marked.
My Bison (Polish) 6" 3jaw has a small '0' (zero) stamped next to one of the
pinion sockets so, if this is all a myth, it's one that's permeated as far as
the Katyn forest.

So, select a piece of 5/8" stainless shafting that mikes the same all round,
shove in the 3jaw and tighten down using one of the unmarked pinion sockets.
Set up the DI and read 0.002" TIR (i.e., peak-to-peak) error in one
revolution. Pretty good for a 3jaw but proof of my wisdom in using
collets for anything important. Loosen chuck and retighten using other
unmarked pinion socket. TIR still comes in at 0.002".

Ok, now repeat the experiment using the manufacturer-marked pinion hole. Lo
and behold, now the TIR is down to 0.001". I'll be damned. Being the
untrusting soul that I am, I chalk it up to a fluke. Try the experiment again
varying the pinion-tightening sequence... master-x-x, x-master-x, etc. as well
as rotating the shaft slightly between each clamping. Same results each time.
Much as it annoys me, there seems to be some truth to the premise.

I'll leave it as a puzzle for our experienced members to explain *why* there
is a master pinion in the first place. I don't have a clue nor do I have the
patience to research it. Offhand, my guess would be that the master is the
pinion used to preload the jaws when they were ground on the ID but that's
just a SWAG.

Those of you who are trying to squeeze the last thou of accuracy out of your
3jaw might want to try this with your own chuck. According to the premise,
even if your chuck doesn't have a master pinion marked, there is one pinion
that will provide better centering accuracy - your de facto master. Once you
identify it, you may want to consider marking its location for future
reference. If you don't want to stamp marks into the chuck, get one of those
paint pens (like a Sharpie, but filled with bright colored paint), clean the
metal with brake cleaner and paint a spot/line, whatever.

One side note here. The books tell us, if chucking a heavy piece, to tighten
the 3jaw using ALL THREE pinions. This raises the quandry ... Do we tighten
the master pinion first or last? Personally, I don't care. I'd use the 4jaw
if I was worried about the part slipping or coming loose.

[Paul Jones]

Marv,
I too discovered this by accident several years ago and now use the technique. I found it was more than a coincidence when this technique worked for my 80mm, 100mm and 6 1/4" 3-jaw chucks. I used a center punch to permanently mark the pinion with the least TIR. I found this comes in handy when transferring machined parts between lathes or flipping parts around in the 3-jaw chucks.

Also, the 6 1/4" 3-jaw chuck uses a D1-4 camlock and after some experimenting found the best mounting position on the spindle with the smallest TIR. I put a prick mark on the spindle D1-4 collar and the chuck body for this best mounting position. Even though the chuck is well used, it can consistently run out by 0.001" TIR when using the same master pinion for the first tightening and the same D1-4 mounting stud positions.

Thanks for the chuck pinion advice, Paul

[Okapi]

I've mounted new chucks on all my lathes a day in teir life with me, and always used this hole as a reference for outmounting the chuck with the key, and as the first hole when I tighten a piece, on my own experimentation, not using all holes on non concentric chucks as Landner can make easily 5 hundreds of a milllimeter false round.
When I have to center a piece, with a feeler(the right word ? ) it's always false before tighten all holes and you can make fine corrections easily using this technique.

[Paul Jones]

Okapi,
Thank you for the advice and insight. The addition of a shim (piece of feeler gauge) sandwiched in between one of the chuck jaws does bring the part into better concentrically when using a 3-jaw chuck. I sometime use the thin metal strips from the store RFID tags for these shims (see http://www.homemadetools.net/forum/t...fid-tags-19067) when I don't want to remove the 3-jaw and mount the 4-jaw for this level of precision work.
Thanks, Paul Jones

[mklotz]

One way to turn eccentrics on a 3jaw, though not my preferred way, is to put a shim between one jaw and the workpiece. Calculating the size of the required shim is non-trivial. There's a program on my site, ECCENT, that will calculate the shim size required.

Of course, I almost always use the 4jaw for eccentrics. But, if all you have is a 3jaw there's a better, and far safer way to do this than putting a shim under one jaw...

Imagine a tube bored to be a sliding fit on the parent stock. A slot of sufficient width to pass one of the 3jaw chuck jaws is milled in this tube. The parent stock is inserted in the bore and the whole assembly clamped in the 3jaw chuck such that one jaw passes through the slot to seat on the parent stock while the other two jaws seat on the circumference of the tube. Obviously the centerline of the parent stock is offset from the lathe spindle axis and turning will produce an eccentric. If we know the OD of the parent stock and the amount of the offset required, it's possible to calculate the OD of the tube needed to produce just that offset when clamped as described above. The program ECCENTUB, included in the ECCENT archive, allows you to calculate the OD.

[Paul Jones]

Marv, this is an excellent suggestion for more safely turning an eccentric on a lathe without having to resort to using a shim when a 4-jaw chuck is not available. Thanks, Paul

[mklotz]

Marv, this is an excellent suggestion for more safely turning an eccentric on a lathe without having to resort to using a shim when a 4-jaw chuck is not available. Thanks, Paul

It's certainly safer than the single jaw shim disaster-waiting-to-happen approach but it depends on having the required diameter stock available (very unlikely) or needing to turn the stuff to size before milling the slot. A lot of time and effort compared to popping it into a 4jaw and centering (see my centering procedure elsewhere).

If you're interested in safety and accuracy buy yourself a 4jaw and learn to use it.

[Paul Jones]

Marv,
Over the years I have obtained 4-jaw chucks for my Unimat, mini lathe , and 12" geared head lathes and find them very useful. I am now working on a side project to build a small marine compound steam engine that requires four matching eccentrics. I also plan to build more than one engine. I could hold the small eccentrics with a 4-jaw chuck. However, I have modified some plans from Rudy Kouhoupt to build a micrometer faceplate attachment to make it easier. Its purpose is to hold the eccentrics during machining in a known position adjusted by a calibrated screw arrangement and be able to repeat the process. I will make the faceplate with more universal holding capabilities so it has more uses. I will publish photos of the tool.
Thank you for the interchange of ideas, Paul

[mklotz]

Marv,
Over the years I have obtained 4-jaw chucks for my Unimat, mini lathe , and 12" geared head lathes and find them very useful. I am now working on a side project to build a small marine compound steam engine that requires four matching eccentrics. I also plan to build more than one engine. I could hold the small eccentrics with a 4-jaw chuck. However, I have modified some plans from Rudy Kouhoupt to build a micrometer faceplate attachment to make it easier. Its purpose is to hold the eccentrics during machining in a known position adjusted by a calibrated screw arrangement and be able to repeat the process. I will make the faceplate with more universal holding capabilities so it has more uses. I will publish photos of the tool.
Thank you for the interchange of ideas, Paul

Sounds neat but isn't the 4jaw in effect a micrometer-adjustable work-holding device?

When I have to do eccenctics with matching offsets, I set up a collet chuck in the mill to hold the parent stock end-up. Center the stock and step off by the offset radius. Center-drill with a tiny (#00 ?) center drill. Mount stock in the 4jaw and, using a wiggler or pump center and a Di in the tool post, get the center-drilled hole running true.

After turning the first eccentric, repeat the process. (A vise stop re-positions the next one perfectly for center-drilling.) If you only slack off two jaws in the 4jaw when removing the first, the second will be almost dead-on when you mount the second.

Everyone has his own favorite approach to production work. All that matters is that you're having fun. And I agree, the interchange is stimulating and, hopefully, enlightening for some of the lurkers who I hope are reading.

[Paul Jones]

Marv, yes that is the way I have been machining eccentrics and other parts with offset holes by using a DI and a 4-jaw chuck (e.g., see the last photo in my article Homemade Lathe Carriage Locking Clamp ) but then there is the fun in making a new tool. I just needed an "excuse" to make the micrometer faceplate attachment in order to see how well it really worked. Regards, Paul