View Full Version : Dummy's three-jaw chuck question

10-09-2005, 08:28 PM
Obviously, my three jaw chuck doesn't give great concentricity when I clamp it down on a piece of drill rod, or any thing else for that matter. Is there an easy way to figure out how to make a rod run true in a three-jaw? Or do I just give up on it and use a four-jaw?



10-09-2005, 09:07 PM
try a test using all 3 of the chuck pinions and see if one does a better job than the others, if you find one thats the best, mark it as a master pinion and always use that one.

after that its a brass hammer or dead shot job, thump it strait.

hope that helps.

10-09-2005, 09:16 PM
What Tattoomike68 said is true, and it sure helped me! Check out http://bbs.homeshopmachinist.net//Forum1/HTML/008265.html and read rheng's post.


Forrest Addy
10-09-2005, 09:27 PM
If your three jaw isn't performing no sweat. Use the 4 jaw. Adjustable concentricity is what it does best. If you don't have much experience using a 4 jaw youre not getting any younger putting it off.

If I had my way (Puff! Puff! Harumph!) I wouldn't let new lathe hands use anything but a 4 jaw for their first year.

A three jaw chuck's only virtue is that it's quick. Those fortuanate enough to have an adjustable 3 jaw chuck are only one minor wreck (like a chip ball that busts the parting tool) away from a sprung jaw or a local deformation in the scroll.

10-09-2005, 10:13 PM
I only use the four jaw. I dont like removing the chucks often possibly causing more were or inaccuracies due to unseen debris.

But when I did use the three jaw I always use the number one screw for tightening and made sure to rotate the stock while tightening to relieve friction.

Now, if you completely turn ODs and cut off parts a three jaw will always be very accurate. JRouche

Paul Alciatore
10-09-2005, 10:50 PM
I'm not too sure I want to start hammering on any parts mounted in my new 3-jaw. Seems like a good way to make it even less accurate then it already is. And it won't do the headstock any good either.

Use a 4 jaw or a tru-adjust style for better centering. Leave the hammers to the blacksmiths - for work mounted in a heavy bench vise.

Paul A.

10-10-2005, 12:29 AM
Welcome to the forum JC!

Is this an old or new chuck?

Has it been cleaned lately?

A small piece of swarf (or abrasive caused nick) on the scroll can throw the jaws off. A careful disassemblying and cleaning might do the trick. Examine the scroll and jaws carefully. It doesn't take much.

10-10-2005, 12:43 AM
JC, I just thought I would add that as long as your three jaw will hold work securely, it will turn out accurate work as long as you don't remove the work from the chuck before all diameters are turned and/or bored. If any of the diameters you turn or bore must be concentric with an existing diameter, then you will need to use a four jaw chuck.

[This message has been edited by Carl (edited 10-10-2005).]

10-10-2005, 12:46 AM

[This message has been edited by Carl (edited 10-10-2005).]

10-10-2005, 12:47 AM

Frank Ford
10-10-2005, 01:08 AM
"Use the 4-jaw."

Forrest, I'd only been reading this forum for a short while before I noticed how often you'd said it. After I read that bit a couple of times, I figured there must be a reason you keep repeating it. So, being a newbie, more or less, to machining, I figured I should give it a shot.

Armed with your simple instructions of how to handle the thing, I found it was not only easy and reasonable to center work with precision, it was also a quick illustration of the simple reality that the four jaw was useful for something other that what I'd originally thought. I 'd just figured that it could hold things that were square, irregular or otherwise not round or hexagonal.

Thanks to your advice on four jaw chuck use I ended up cancelling a deal for a "set-tru" Buck. Seems I already had that feature in the "that other chuck" that came with my lathe.

While I still consider myself a novice machinist, I've worked full time at skilled craft all my life (you can see that stuff at my web site: http://www.frets.com). I, too, moderate a couple of fora (had to use that plural after four years of high school Latin - yes I am old enough to have been through that), so I dish out my own littany of repetitive instruction.

Thanks again for your guidance!

[This message has been edited by Frank Ford (edited 10-10-2005).]

10-10-2005, 09:54 AM
Thanks to everyone for the comments. I'll add a bit more info.

I was told of the proper way to tighten the jaws, using the marked pinon (as mentioned in this thread and others), and always use it. The chuck is not new out of the box, but, since I rarely use the lathe, it has very little operating time on it. I checked this morning and got .007 TIR on a 1/2" drill rod at about 1" from the jaws.

My recollection is that it performed better out of the box, so, based on CCWKen's comment, I am suspecting that there may be some dirt in the scroll. I'll try cleaning it to see if it helps.

I've been using shim stock randomly placed on the jaws to reduce the TIR, but haven't found a good formula for how much to use or where to put it. Any thoughts on that?

I have heard that the jaw grinding technique works well but only for the particular diameter that was ground. Is that true? Would the cause be a function of scroll variablilty from one part of the scroll to another? Any way to predict the result over a range of work diameters?

Since I am such a novice, I'll take the good advice and start learning the 4-jaw. But I would still like to make the 3-jaw be all it can be.



10-10-2005, 11:46 AM
I s'pose old farts like us belong in musea http://bbs.homeshopmachinist.net//biggrin.gif