Test bar for new 6 in 3 jaw Buck chuck

I have a piece of drill rod 1" diameter and about 9" long that I consider "good and straight". I throw up in my 3-jaw chucks once in a while to check them for runout. If there's runout, I adjust to get as near as I can to "0" runout.

One thing you have to remember, if you have runout out at 6" from the chuck, what do you do? Most of the time the runout is within .002" TIR. This is very common runout for a 6" to 8" 3-jaw chuck. Everyone will get varying results. If too excessive, you probably have a wore out chuck!
 
I believe an important factor is the repeatability of the jaws at a given diameter. If you remount the same test bar and you get significantly different results, it means that the journals (slots and grooves) that the jaws ride in may be worn excessively. When that happens, even if the jaw teeth surfaces are concentric and even, the jaws get cocked at unrepeatable angles when tightened each time. Its time for a new chuck.

If you have a 3J that works repeatably at one diameter but not at others, it means the scroll plate or the mating threads on the back of the jaws are either burdened with swarf or bent. A malformed scroll is a good sign it's time to rebuild or retire the chuck. It's been my experience that purchasing new jaws for old chucks is not easy and is expensive.

I've been though all this with used equipment.

BTW: I know of at least a few people (some on this list) who've had good experiences with this rebuilding service: http://www.worldwidechuck.com/


Ray
 
Old post, but not big list of options, suiting those of different means.
Ground and Polished is good of course, but unhardened.
Drill rod an equivalent of G&P, but tool steel. Usual lengths are 12" and 36". Choice is Oil hard, Water hard or grades of HSS.
Drill blanks have those qualities, but un-centered.
The right reamer blanks have all those features plus considerable length to diameter ratio.
One alternate; larger wrist pins, the hole will engage live center diameter, instead of the tip.
Another; linear bearing shafting, very hard case, center can be drilled, not too large for fit small spindle bores, available online in convenient [short] lengths.
Off direct topic; mills. I often set up 2 vises. Table them with clamp screws in place, clamp a long [I use a 35 x 1000 mm linear shaft] bar in each, on which you run the indicator. Instead of endless transit from end to end, just indicate somewhat more length where the operation will take place, and clamp.
 
lrsglory said:
I just want to present a different approach. I read a lot about 3 jaw chuck problems. They are real. Seems like most of what folks are doing is just one of a kind. one part. A 3 jaw has its limitations, even top drawer. A story. Years ago I started in the toolroom of a large jack mfging co. First thing I did, based on what the production foreman told me was take that ( dammed ) 4 jaw off and put the 3 jaw on. I did and it was ragged. oh well. Next day the Chief Engineer strolled into the tool room. Looked at the lathe and said take that ( dammed ) 3 jaw off and put the 4 jaw on. And so I did. He was my boss. That 4 jaw stayed on that lathe for 5 years. I got so good at indicating a piece of stock in that chuck , you can't imagine. There is no trick to it. Every once in a while the production foreman would come in the toolroom dragging a hank of round steel and want to do it himself. Every machine he supervised , production lathes, had oviously 3 jaw chucks. Sorta his mindset. I rarely made two of anything, only if I scrapped it. Crap happens. Don't writeoff the little 4 jaw. Try it, you might like it. For what its worth. Believe me chuck run out will never be a problem.
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Man! There is a trick to indicating a 4 jaw and I wish you guys would spill the beans. I'm like a special needs kid playing whack a mole when I'm doing the aforementioned...really sad.
 
GunsOfNavarone said:
Man! There is a trick to indicating a 4 jaw and I wish you guys would spill the beans. I'm like a special needs kid playing whack a mole when I'm doing the aforementioned...really sad.
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Just throw it on use it, it will come to you.
 
GunsOfNavarone said:
Man! There is a trick to indicating a 4 jaw and I wish you guys would spill the beans. I'm like a special needs kid playing whack a mole when I'm doing the aforementioned...really sad.
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No trick. Watch Abom79 [Adam Booth] do it. They even stage races!

The 'trick' is visualizing, translating what is going on, toward the intent.

I dial in regularly, but often coarse, rough, and large diameter material. These are hanging from a boom or crane, no way I'll hold it while manipulating everything else. Those especially, are conducted in the following manner.

Note some position on the jaw meets or close to the rings scored into front of chuck. Using part or a scale, set a jaw opening where two opposing jaws [ie #1 & #3] look evenly positioned according to those rings. Unless part is very heavy, two jaws will hold now, while bringing in other pair. Just secure, not clamped tight. Again, on big parts, have the tailstock ready to hold part with center and a block of wood. One aid, is to paint mark two sides. Way easier recognizing paint, compared to grimy stamped numbers.
The rings are one way......another is measure part OD, divide in half. Bring tailstock close enough to jaw plane that a steel rule can show that half from center point to jaw face. 'My' lathe too big for that, and 12" scale often too short.
You are already halfway there.

Set a travel [aka drop] indicator perpendicular to material, on centerline, not touching. Rotate spindle [thank engineers for neutral] slowly while moving indicator in until highest spot touches. I like using cross slide or compound with indicator on toolpost, adding a little more indicator travel [aka deflection].
You are better than three-quarters done.

Watch the jaw number, lets say #3, that is obvious high spot. Spin to #1, gap signifies amount off. But the jaw movement is half that. Big stuff, I might even run cross slide in, and let lathe dial 'measure' amount needed, but jaw movement will still be half.
Back [loosen] jaw #1 in approximation of that half distance. #2 & #4 will hold, while rotating to #3. The indicator might be touching, or very nearly, but far less than before.
Observation will show what #2 & #4 need.
Figure yourself seven-eighths of the way.
 
There is a trick, it's called second nature. In martial arts it was called muscle memory. After you do something in the range of 3 to 5k times, it becomes second nature...you get a feel, a reflex. I will try to bust out the 4 jaw, but honestly, unless you have something NOT round or run out you need to compensate for (maybe a hole down the "center" of something)...I dunno, 3 jaw is faster than Abom or Fenner!
 
Fast is good. Secure and perfect? Better.
3 jaw good when everything's round, or hex, no question, running centerline.
4 jaw grips nearly any shape because the jaws are independent. Rare, but there are scroll 4 jaws too.
Guys (people) with smaller machines don't think twice about chuck changing, switching to collet closers or what suits next job. That tooling I can hold in one hand.
When scanning machine ads, bigger lathes are exclusively 4 jaw. As in can't even buy a 3 jaw chuck that big. Must be a reason.

One is overlooked, the physical size of chuck, especially thickness. 3 jaw require a front body, scroll, the chuck rear plate which may or not need a backplate [spindle adapter] too. The pinions are outboard of scroll, so increasing diameter of body, while unusable attaining extra clamping range.
4 jaw; chuck body, which may or not need a back plate, but usually direct mount, like A2 or D's. The screws, carried by forked retainers, with just half threads in jaw, are easily removed when needed, add little to thickness for a sturdy chuck.

To me ideal fixation on chucks aren't jaw count. Need one of each, minimum guideline.
3 jaws with a single pinion, adjust-true body, face slotted for mill clamps, and master jaws. Scroll can only be one way, common switch OD to ID by reversing top jaw. Some, and I can't illustrate why, use 2 separate jaw sets. Masters my favorite, adding use when soft or pie jaws required.
4 jaw, slotted for clamps, groove witness rings on face, solid jaws are usual, but had access to 4 jaw masters here and there. At either we reworked large, irregular pump castings without ideal flanges..........they had a clever process. A mill fixture, located a calculated distance from part and lathe spindle center. Each jaw [top, not master] was milled accordingly, angle-radius-undercut-bolt holes even. Of course, each had to return to correct screw. With casting hung in front of chuck, spindle rotated to match part features was not much juggling to get mounted up right.
Why? Pump builders [a production environment] have resources and space for big vertical lathes, easiest of all. Job shops, not so much. That's an ideal niche, because they're known knocking out those jobs, compared to less imaginative companies.
I'd guess 80%+ of my years are not production environment, let alone production work.

Some apology to readers, I don't do brief.
 
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