question about centering in the 4-jaw

[chkz]

Hey Guys, I'm curious here so would like to hear from "the pro's" on this one (or anyone else with an opinion...it is the internet right?).

Since both of my 3 jaw chucks have more or less packed it up...i've been nursing them along for years, finally sick of 'em and on the hunt for another one I can afford. I've been using my trusty 4-jaw lately.

I've been, of course, centering things up as near as I can by eye, and using the ref rings on the chuck face, then...here's what I do: stick my Starrett last word indicator...think its 0-15-0 on its holder in the tool post. Bring that up to the work and gently feed it in 'till I get about half the "dials worth" registering on the gage. Zero it. I try to have the gage contact the work inline (roughly...i just eyeball this) with one of the jaws. Rotate the chuck by hand, checking the gage and trying to slack one jaw and snug the opposite one 'till I get it reading around .003 or so, then go to work! I thought I'd be getting quicker at this but occasionally it takes a good deal of time and basic "screwing around". My question: Is this the right way to go about this? Should you try and center it in sort of an x and then y manner? (i've also tried, if I've noticed any eccentricity to move it diagonally, seems to just throw everything "out"), also...even though I know everybody TRIES for zero. And occasionally you can get it on the dial gauge...one bit of finger pressure on the end of the workpiece will register on that same gauge. WHAT is an acceptable level of accuracy? I know it depends on what you're doing....but for typical day to day type stuff...including maybe threading? Guess what I want to know is whats the typical accuracy of a 3-jaw (i've never measured it on mine) and how long does it take an "actual machinist" to center-up a workpiece using an indicator?

thanks
C.

[Magic9r]

I'd still say get a decent new 3 jaw for quick & easy (dirty?) work.
4 jaw is great for offsets & funny shapes.
Get a collet chuck & set of collets (ER type) for repeated accurate work.
Don't tighten your 4-jaw then adjust, adjust while finger tight & tighten while checking & tightening in the right places to correct your readings,
Regards,
Nick

[Sprocket]

I'm not a pro, but I do like my four jaw. If you are using a dial test indicator, that may be overkill for initial centering. I use a dial indicator, like 1" overall motion, and can usually get within .001" pretty quickly. John Stevenson had a good explanation (check the archives). He suggested thinking of it as a pair of two jaw chucks. Center jaws 1 and 3, then 2 and 4, check again and your done.
Unless you are re-centering something, if you're under .001" ought to be close enough.

[JCHannum]

It has been mentioned previously to think of the four jaw chuck as two two jaw chucks.

Roughly center the part, and then indicate. The indicator should be as close as possible to the centerline of the work.

Start with jaws 1&3 Set the indicator to zero on jaw 1, turn to jaw 3 and note the reading. Adjust the part to 1/2 the difference. Repeat, and it should be zero zero.

Now do the same procedure on jaws 2&4. You will probably have to go back over jaws 1&3, but the part should be well centered at this point. It should take only a minute or two to accomplish.

Look for some of the posts by Forrest Addy on the subject. The four jaw chuck is definitely not just for non-round or odd shapes.

[BobWarfield]

I like using my height gage for the job when the workpiece is square:



Otherwise, I use an indicator just as everyone has said.

Best,

BW

[dalee100]

Hi,

I've never thought of using a height gauge for square parts in a 4-jaw. May have to try that sometime. I use a level placed on the jaw to "index" the indicator back to the same spot on the part. A square-head works well for this, but I've got 4 small magnetic levels that work even better.

dalee

[TGTool]

One more wrinkle. You mentioned that you've got a 0-15-0 indicator, which means if you moved the toolholder to about half travel on the indicator and zeroed it, you might not have enough travel when you rotate the chuck 180 degrees. That is, if the part is off center by more than .015, you don't know exactly how much to move.

Check high/low on two jaws, turn the high side to the front move the indicator into the work until it registers and zero it, THEN set the crosslide dial to zero. You can now rotate the chuck move the crosslide until the indicator reads zero and read off the crosslide the double error. For example, if you have to move the crosslide in .080 to get the indicator to zero again, back the crosslide off and set it to .040. Now slacken the front jaw and tighten the back jaw until the indicator zeroes again at your .040 mark. Now look at the other two jaws, again select the low side to the front, slack and tighten to again zero the indicator and you should be quite close. Recheck, and tweak if you need to be very close, but this sequence will minimize the turn/move/check/turn business.

JM

[torker]

Forrest is going to like this thread
I use my four jaw for most everything. I hate my 3 jaw. It's accurate enough but it packs with chips far too easy and is a PITA. Always trying to clean it etc.
I rough in the four jaw with chalk, then finish off with a test dial.
I always try for "zero" or awful close.
Takes maybe a couple minutes...except for that "once in awhile" when I'm all thumbs and can't feel the wrench right. Get to fighting it and it can take awhile. Just a dumm thing but it happens.
If I keep this lathe I would like to buy a good 3 jaw chuck for it so I can start using soft jaws.
Russ

[miker]

chkz, take a look at this thread.

http://bbs.homeshopmachinist.net/showthread.php?t=20974

Rgds

[Jack Burns]

Setting a Four-jaw Chuck in Two Revolutions

Hi C.

Here’s the most efficient method I know for quickly and accurately aligning work in four-jaw chuck. I learned this technique from the late Robert Bastow of r.c.m. fame who used it for centering huge steelmill rolls in a 108-inch swing lathe with a top jogging speed of 2 minutes (yes, Minutes!) per revolution.

1. After rough aligning to the chuck rings, using a dial indicator on the work-piece, rotate the spindle through one complete revolution noting the highest and lowest indicator readings;

2. Continue rotating the spindle and halt at exactly Midway between the above two readings, then zero the indicator bezel to the needle;

3. Rotate the spindle to bring jaw #1 ‘on plunger’ and adjust jaws #1 and #3 to re-zero the indicator; finally

4. Rotate the spindle 90 degrees and adjust jaws #2 and #4 to zero the indicator once again.

That’s it – you’re all done! -- and accurately set to whatever tolerance your indicator will allow.

Hope this little trick helps you as much as it has helped me.

Kind regards,

Jack