Centering work on a 4-jaw chuck
I use a 4 jaw independent chuck all the time, and I really enjoy the accuracy and adjustability it allows. I made a video centering a shaft up to install a ferrule a couple months ago when this came up last time, but I never posted it. I will try to get to it sometime soon (you all know how that goes).
Anyway, you definitely need to practice. It is not easy at first, but with time, you should be able to center work in less than a minute to .001 or better TIR (total indicator reading).
A few helpful suggestions:
1. Very important, number your jaws!! If they are not numbered 1, 2, 3, 4, you will go bananas keeping track of what you are doing. I use a sharpie marker on my Taig 4 jaw chuck to mark the jaws. It wears off after a few days, so I would recommend engraving your chuck body with the numbers if it does not already have numbers stamped into it.
2. Only work with one "pair" of jaws at a time. Jaw 1 and 3 is one pair. Jaw 2 and 4 is the other. What this means is center the work on one pair before the other. When the work is way off, you should get both pairs close, and then focus on just one pair.
3. First EYEBALL the work to get it close. Do not use the indicator. That is what the circular lines are on the face of the chuck. Use those lines to get each of the jaws in the same position. Your eyes are very good, and you can get the work probably within .050 TIR.
4. You can now switch to an indicator. I like to use a "last word" type indicator because it is more slimline than a plunge type dial indicator. I also think it is VERY beneficial to mount it on a base that has a fine adjustment. Anyway, on a last word indicator, the travel is only .030. So what I do, is again eyeball instead of measure. What I mean is place the contact point of the indicator ABOVE the work (not touching it), by about .030. Then rotate the work and look at the gap change between the indicator point and the work. Adjust the jaws accordingly to make the gap the same on both sets of jaws.
5. Now, you can move your fine adjustment on the base so that the indicator touches the work. Learn which way the needle moves on the indicator. On mine, it moves clockwise when it is pushed upward. You should write this down because you will probably forget in the beginning. Then go to jaw one and note what the reading was. Then go to jaw 3. You want both numbers to be the same. To make them the same, you have to first loosen the low jaw and then tighten the high jaw. Don't try to make the correction in one shot, because you will almost certainly overshoot it and then have to go the other way. Creep up on it. When you finish with one pair of jaws (1 and 3), repeat the process for the second pair of jaws (2 and 4).
To explain that again, on my indicator, I touch the top of the work. I look at the needle as I rotate from jaw 1 to jaw 3. Whichever jaw is counterclockwise on the needle is the low jaw. So I loosen that jaw. As I loosen the jaw, the needle will move clockwise slightly. Then I rotate the chuck 180 degrees to the high jaw. I tighten that jaw and the needles will move counterclockwise as I tighten it.
6. Some notes about tightening and loosening jaws:
With METAL work, you can really bear down on the jaws. When you are only off .001 or .002, you can usually just tighten the high jaw WIHTOUT loosening the low jaw. It will move the work enough to center it.
The same is true with CUE work, but you must be careful not to damage the work. Shafts can be easily damaged with ANY chuck or collet setup. Just like with a 3 jaw chuck, you have to develop a feel for how tight is tight enough. You will know after some practice when you can just tighten the high jaw without loosening the low jaw.
Now, lets say on metal work where you are making the jaws TIGHT. You tightened them, and you are off center by .005 TIR. You conclude that is too much to take out with just one jaw because you are already TIGHT. So you have to loosen the low jaw first. You loosen it just enough to “break” the bond, then tighten the high jaw. BUT, when you check you indicator, you OVERSHOT your goal, and are off in the other direction. What happened is you loosened the low jaw TOO much, even though you thought you moved it as little as possible, in order to break the bond. What you have to do in these cases is loosen the jaw to break the bond, but then tighten the SAME jaw back up, however, not super tight. Now you can tighten the high jaw because it will have some room to move.
I find this doesn’t really happen in cue work because the jaws are not super tight to begin with. So when loosening the low jaw, you can easily turn it as small an amount as you want. Where as when holding metal work, you have to turn it hard in order to break the bond. That bond is not there with cue work because you don’t tighten them as much.
7. Another point specific to cues…
I made 10 shaft collets for tip and ferrule work. They are from .450 to .550 inside diameter, by .010 increments. The outside on all of these collets is EXACLTY .625. What this means is that my chuck always sees .625 +/- .005 no matter what shaft size I am holding. This means I can put a tip on a .470 shaft, and then put a tip on a .520 shaft. When I put the second shaft in, it is usually only off by about .005 TIR or less. So centering is MUCH easier than if I had to move the jaws drastically for each size. Another thing I do to make things faster and easier is to always loosen and tighten the same 2 jaws when INITIALLY (not during the centering process) installing and removing work. For example, when I finish the tip on the .470 shaft, I will loosen jaws 1 and 2 ONLY. Then I will install the .520 shaft with its corresponding collet, and I will tighten ONLY jaws 1 and 2. This should get me very close to center, before I even indicate anything.
8. How good is good enough? I like to get tip jobs within .002 TIR. I try my best to get ferrule replacements to .001 TIR. If you can do this on a ferrule, you often do not even have to touch the shaft wood to match it to the ferrule. It makes the job so much better. You will find this is not always easy on cue work, because the shafts are not always truly round. You can determine this by using a MICROMETER (calipers are not nearly as accurate as a micrometer) to measure the shaft diameter at several locations. When you have a shaft that is not round, do your best to center it in each set of jaws respectively. Some cues will be off round by .005 or even more. I usually see this on cheap junk cues that probably weren’t dried properly before turned down.
Another thing that can cause your needle to bounce around is junk or scratches on the indicating surface (a scratched ferrule for instance). Just be aware of this and know if you are reading the true surface or a flaw in the surface when you are centering.
On this same point, while you are practicing with metal, take a skim cut off of the diameter of your practice bar for a few inches so that you are certain it is round. That will make your measurements go easier.
9. Conclusion: This may seem like a long process, but it really only takes about a minute or less, especially if you have a collet setup like I described on step 7. This is really a basic machinist skill that everyone should know. If you ever have to center work on a mill, or pick up an existing hole on a mill, the same process is used. I hope I helped.
