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Seastar
06-11-2013, 05:39 PM
I rarely have occasion to use a four jaw chuck.
Today I needed to drill a 1/2' hole through a 3/4' square piece of steel 3' long.
No problem, I thought, I'll get the four jaw out of the cabinet (first time in 3 years) and set it up.
45 minutes later and a lot of swearing I finally got the &@%# 3/4 inch piece centered.
There must be an easy way and I probably knew it when I first started messing with a lathe.
My 80 year old brain is forgetting all sorts of things and this must be one of them.
HELP!
Bill

gcude
06-11-2013, 06:00 PM
Try this video:

http://www.youtube.com/watch?feature=player_detailpage&v=HSjQfnXuhEE

firbikrhd1
06-11-2013, 11:06 PM
I'm guessing you had so much trouble because it was square and not round. One option is to put a piece of springy thin steel like banding tape between the indicator and the square work. You'll have to devise something to hold it in position but after it's there as you rotate the chuck with the work in it the indicator plunger will move in and out freely as the springy steel rides over the corners of the square stock, like a lifter follows a cam in an engine. Then it's only a matter of checking the indicator for high and low readings at the appropriate places, either opposite corners or opposite flats and adjusting the jaws as necessary.

becksmachine
06-12-2013, 06:13 AM
Don't beat yourself up too bad, it is a very "fiddly" operation to center a piece of square stock in a 4 jaw. :)

You can usually get fairly close using the circles scribed on the face of the chuck body as an aid for the initial set up, but the subsequent spinning of the chuck by hand and running the cross slide in and out while rocking each face against the indicator tip can be a bear.

If your lathe has a threading stop feature on the cross slide it can speed things up somewhat as then you don't need to watch the cross slide dial when backing off and re-positioning for each face.

Dave

cameron
06-12-2013, 07:28 AM
I find it a lot easier to mark the centre on the end of the square bar. Then centre the bar in the chuck using an indicator against a spring-loaded bar held between the punched centre mark in the square bar and the tailstock centre. Can't remember what those bars are called, don't know if they can be bought, but they are definitely worth the effort to make, IMO.

EVguru
06-12-2013, 08:08 AM
I know several pople who have self centrering 4 jaw mounted on their lathe most of the time.

Doozer
06-12-2013, 08:26 AM
I know several pople who have self centrering 4 jaw mounted on their lathe most of the time.

How would that hold round stuff accurately?
Potentially 2 of the 4 jaws exerting pressure at one time!?!?

--Doozer

sch
06-12-2013, 08:38 AM
Try this video:

http://www.youtube.com/watch?feature=player_detailpage&v=HSjQfnXuhEE

Although square is more finicky than round the part I found most pertinent was at about 6:20 in where he pops up with his mini chuck key, having one of those makes dialing in the last few hundreds
of setting much faster, you no longer have to fust with the switching of one key back and forth and it is obvious which key moves the indicator which way. Make a second shorter chuck key to use on the
back side of the chuck, it really helps. It is not used for final torquing, but to loosen/tighten the slack while zeroing in.

Boot
06-12-2013, 08:43 AM
I find it easy to center squares or any rectangle by finding the center of square object by drawing or scribing diagonal lines from corner to corner,adding a punch mark there and then putting a pointer in my tailstock drill chuck and centering it simply by pointing that intersection in. If it has to be precise then I would indicate the four corners of the object. Plus the center punch mark aids in starting the center drill or common drill for the hole. I very seldom use any other method. I hardly ever take my 4 jaw independant chuck off the machine. I do also have a self centering 4 and 3 jaw chucks. I always say KISS it. Keep It Simple Stupid. Boot

Dr Stan
06-12-2013, 08:52 AM
Start by putting in a piece of round stock the same size. Dial it in, remove it by loosening 2 jaws 90 degrees apart, put the square stock in and re-tighten the appropriate jaws. This will at least get you close.

If it needs to be dead on, finish indicating it in by using a surface gage & a test indicator off a flat surface on the cross slide or the compound.

gizmo2
06-12-2013, 09:28 AM
I find it speeds things up if you center them in pairs, off the flats, and not try to do all four at once. F'rinstance, let's say jaws 1 and 3 are over 100 (thou) out. I'll figure out which way to go, and tighten one jaw in 50 and the other out the same. Next go round, let's say they are out 12. In 6 the high side. Now we're within a couple thou, I'll loosen those a little, just enough so the part can slide, and go after the 2-4 flats. Once that pair is close, go back to 1-3 and start snugging them up at zero. The last little bit can be had just with tension. I hope this makes sense, it sure speeds things up for me; trying to dial all four at once is just too confusing.

Optics Curmudgeon
06-12-2013, 11:56 AM
I find it a lot easier to mark the centre on the end of the square bar. Then centre the bar in the chuck using an indicator against a spring-loaded bar held between the punched centre mark in the square bar and the tailstock centre. Can't remember what those bars are called, don't know if they can be bought, but they are definitely worth the effort to make, IMO.

It's called a pump center, a web search will yield details.

dian
06-12-2013, 01:11 PM
im always surprised, how well square stuff centers in a three-jaw.

rohart
06-12-2013, 09:07 PM
With square stock I take a while to get it even roughly right first. Once I'm there, getting it spot on is almost as easy as round stock. It's remembering what the indicator said on the other face that's the problem.

Like gizmo, I do a pair of opposing jaws, till I get them right, and then I do the other pair.

I have a suggestion. Don't put the 4-jaw back in the cabinet. Leave it on. Withing a week you'll be mounting round stock in a couple of minutes flat.

Seastar
06-13-2013, 08:37 AM
With square stock I take a while to get it even roughly right first. Once I'm there, getting it spot on is almost as easy as round stock. It's remembering what the indicator said on the other face that's the problem.

Like gizmo, I do a pair of opposing jaws, till I get them right, and then I do the other pair.

I have a suggestion. Don't put the 4-jaw back in the cabinet. Leave it on. Withing a week you'll be mounting round stock in a couple of minutes flat.

Well, you got that right about remembering but your advice on leaving the 4-jaw on the machine is too late.
I've already hidden it in the deepest recess of the most inaccessible cabinet.

There are some excellent suggestions here. Thank all of you.
My favorite is the two key trick from the video.
I'll probably forget it in three years when I need to use the 4-jaw again.
Bill