View Full Version : Four Jaw Questions

07-13-2010, 11:59 AM
A while back there was a thread on the accuracy of 5c collets and it was suggested to " use a 4 jaw chuck for a week " to get the same or better accuracy. So I did just that, I made a chuck key clone and a magnetic indicator holder to mount on my rear cut-off block, I then viewed tubalcains U tube vid. It has worked a treat for me, but I have a few questions. How tight do I tighten it to hold a piece properly, I really don't want to crank it too tight, But I also don't want the work flying out. My method now is when the piece is centered to my satisfaction and all the screws are lightly snug I take another small turn equally on each screw. I then take a another swipe with the indicator and readjust if necessary. If it is out a lot .002 to .003 I loosen to adjust, if it is out a thou or less I tighten to bring it in. I'm not really sure if this is okay. Second, even though I really don't need this much accuracy now, the last few tenths are hard to get. In case I do need to get something really close I would like to practice getting it.


07-13-2010, 01:36 PM
I've sometimes wondered the same thing Stu.

In practice however, I've not hesitated to use the tightening option even for .010 or more if I still have "reasonable" slack in the pinion adjust screws.
In this case I define "reasonable" as meaning I don't have to grunt and make a face and stick out my tongue to tighten. (Well maybe not quite that tight yet. :)
In other words, I go by feel, and back off and sort of start over, at the point just before I think I might distort something permanently.
Of course that's an incremental process, since usually torqueing one a little bit will neccessitate further tightening (but less) of another.
Usually tho, by working with opposing pairs, I will be within 4 or 5 thou before the pinions are anywhere near to "too tight."

Not saying that's right, but it's how I do it. Would appreciate the more experienced among us correcting me if that's wrong.

07-13-2010, 01:52 PM
You can get away with the "brute force" method for most things. What you want to keep in mind, though, is that when you really grunt to get the jaws tight (and bring it into concentric alignment), you are deflecting something quite a bit. You can be squeezing the hole out of round, simply taking up slack in the screws or, in many cases, pushing the jaws (and workpiece) out away from the chuck.

Sometimes it is educational to sweep not only the shaft or hole that you are indicating for concentricity, but also the face of the work piece. You might be surprised at how much it moves during tightening.

I think it all comes down to feel and keeping an eye on the indicator. Regarding those last few tenths - well keep in mind that surface finish comes into play as well as heat. It's fun to try and see just how "dead nuts" you can come, but don't take it too seriously. Most professional shops out there never have to worry about tenths, and when they do they are much more likely to go to grinding/lapping etc.


07-13-2010, 03:05 PM
How tight is just right is something you have to learn over time. I don't like to use a cheater bar on a chuck and since I am of average strength I don't think I over tighten the jaws. To indicate at the jaws you can tighten until the indicator don't move and then release the opposite jaw some to move it in and retighten that jaw and most the time it stays put.

As to the end out of the jaws I use a lead hammer or soft hammer to knock it in. I made a roller on a square bar to push up against the work and spin the chuck by hand to center the work and that will usually work.

How tight will come after a few :eek: damn that was close events as the part whizzes past your head.