How to tighten a 3 jaw?

I tighten, or at least try to tighten all three holes on lathe and drill chucks. I was taught that way originally, but I continue to do so because I always CAN tighten the chuck a little on each hole. Now if I found that the other key holes were already tight all the time, I wouldn't bother, but that isn't the case.
 
Many moons ago when buffalo still roamed, I , as an apprentice was taught to tighten all three screws. I still do because I was taught that way and it is a habit. I have found no difference in run out tightening one or all three. Run out amount is determined by wear and fit in the scroll and jaws. If you set your chuck for .0005" run out at chucking on 1" diameter (this is what I did because most work is close to that size) it should repeat relatively close AT THAT SIZE and tightening one or all three screws seems to make no difference. As you chuck different diameters , the run out may ( and most likely will) change some. This is due to wear, fit , and accuracy of all the parts. My beat up Union 3 jaw on my SB 9 never shows more than .002" run out anywhere, but at the 1" or there abouts it is always best because that was where it was set to .0002" at. Tightening all three screws , I believe, will give a little more clamping pressure ( and also makes deeper marks in your part).

So after all this crap.... er ...scientific explanation.....I would say "It does not matter. Tighten one or all three, as long as it makes you feel good."

Note: the preceding was completely my own observations, opinions, and the way I was taught. :))
 
The 0 mark is for putting the chuck back on the back plate the same, it is called a witness mark. As far as tightening i have always found one hole that is best of the 3, called a sweat spot.
 
I always go around the three pinions, even on drill chucks. You can always get a bit more grip by tweaking each hole.

Notice that the zero mark on the chuck has a companion zero on the backplate. My thoughts are that those are witness marks, so if you ever take the backplate off the chuck, you can reassemble it the same way it was built.
 
I was taught to choose one pinion & stick with it to account for wear & fepeatability over the years (for new chucks). Not sure if actually makes a difference though, all my chucks started out as new, first time owner for me. In shop clas we we taught to use multiple pinions when more grip was needed & mainly because all the chucks were heavily worn. On my drill chucks I just use any hole but then again I don't use keyed chucks as often as I do keyless.
 
mark_f said:
.....at the 1" or there abouts it is always best because that was where it was set to .0002" at. .... :))
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Mark,

What is the trick to set a 3J scroll chuck for 0.0002 run out? I would like to explore how to do that on my 3J using a 1 inch test bar.

Thank you.
 
Bamban said:
Mark,

What is the trick to set a 3J scroll chuck for 0.0002 run out? I would like to explore how to do that on my 3J using a 1 inch test bar.

Thank you.
Click to expand...
I can dial in any of my lathe 3 jaw chucks to within .0005". Here's how I do it. I'm might catch flack for saying it, but don't knock it till you tried it!

I Chuck up the work piece and tighten all three scroll screws tight as if to hold the part for cutting. Now take a dial indicator and put it to the work piece and spin it by hand noting the run out. Now, take a brass hammer and wack the jaw that is farthest out. That's it. You might have to do it a couple of times to get it right. Works every time. This won't work if your jaw surfaces are out. I do it all the time when flipping a part around to get the concentricity as close as possible.

My speculation as to why this works, the scroll has some play in it, Particularly on older chucks. Giving it a good tap seems to move it just enough to get it right. Ironically, it doesn't move when working the part, and I usually push my speeds and feeds.

Marcel
 
Marcel,

is this something you do once to correct or set the runout, or every time you chuck up a part?

jim
 
The 0 marked key hole on the three jaw chuck is the one marked by the manufacturers as the hole to use for best repeatable concentricity, which begs the question "so why would you want to use the other two holes ever?" and I suppose the answer is "when it really doesn't matter" (whenever that is).
Xalky, I like that idea! I will try it next time I am dialing in for accuracy!

To all, If you look at the specs of a new, good quality 3 jaw chuck, you will see that the concentricity is rarely specified as greater than .003", which is, I assume, the repeatable concentricity. You can get nearer than this with a bit of tapping and dialling in, but if you want better, "the most accurate chuck you posses is an independant 4 jaw"!
Phil
 
I tighten all three when it matters. It always seems I can get a bit more from each. But then again, I have a cheap chuck.
 
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