The process is simple, and uses the barest amount of addition and subtraction. This post is long only because I went into excruciating detail. There are really only two key steps in the middle of the process. See if you can figure out which ones! /ubbthreads/images/graemlins/smile.gif
The tools? Just a dial indicator and magnetic stand. Harbor Freight has cheap dial indicators and stands on sale just about all the time. Under $15 for both in many cases.
Overview; Make sure the piece is round and straight. Put it in the chuck and adjust it by eyeball till it's close. Set up the dial indicator and adjust opposing jaws, alternately loosening and tightening till it has the same reading first on one pair, then all the way around.
All movement of the chuck is by hand.
detail
1) make sure your stock is round and straight. If it's not, it will be difficult to dial it in, and it won't matter too much anyway since you'll be making it round. Clean off any dirt, paint, etc that will add to the measured diameter.
2) Chuck up the stock and adjust it by eyeball till it's close. You can adjust it by several means;
o adjust till the ends of the jaws stick out the same amount on all sides
o adjust so the jaws are equally spaced where they meet in the middle
o use the rings on the face of the chuck as reference points.
On this picture you can see the jaws just about equal where they stick out of the chuck body and they are about the same using the jaw closest to the camera as a reference point.
I use a small (3 inch long) square piece of aluminum stock instead of the chuck key at this point, since I can spin it with one hand and rapidly move the jaws.
3) I mount the dial indicator (in a magnetic mount) on the cross slide and adjust the cross slide till the indicator's tip is where I want to measure. I move it front to back using the cross slide crank till it has it's highest reading (roughly).
This is the setup. Indicator and magnetic stand from HF.
4) Remember the math? You will use a little in this step. Rotate the chuck until you see the highest reading while one of the 4 jaws is straight up. It will not be the highest reading, that's likely to be between two jaws.
With that jaw straight up, make sure the jaw is snug (not tight) and make a mental note of the dial reading. In the picture it's about 89. We'll call that jaw A.
Rotate the chuck 180 degress so the bottom jaw becomes the top jaw. Take that reading. In the second picture it's about 67. We'll call that jaw B.
The number you should see when it's centered is 1/2 way between the two. You can calculate it by adding the two numbers (89 + 67) and divide by two or, if it's easier, subtract the lower reading from the higher, divide that by two and add to the lower number. In this case, the target is 78.
When rotating the chuck, always bring the jaw to the same position. In my case I use straight up.
5) Loosen the jaw with the lower number by just a touch. I like to back it off (counter clockwise) 1/8 of a turn and then gently tighten till it just makes contact again. You should see the dial change to something higher. If it's above your target, give it just the barest twist tighter.
6) Rotate the chuck 180 degrees and check the reading. If it's higher than your target, tighten it gently (clockwise), going back to step 5 as often as necessary till you get the same reading on both sides.
7) At this point jaws A and B are centered, so you want to rotate the jaws 90 degrees and go back to step 4 to take care of jaws C and D.
SUCESSS
When you are done, you should be able to rotate the chuck 360 degrees and see the same reading all the way around. You should also be able to move the carriage left and right and get the same readings. See?
With a little practice this becomes a very quick operation. It comes out to measure, adjust, rotate, measure, adjust, rotate, measure, rotate, measure, smile, smile, smile.
Gotchas
a) If you have play in your chuck or bearings you will chase that last .001 all over the place.
b) If the stock is crooked, you can center the exact place you measure while the majority of the stock is off center. Moving the indicator left to right will show different readings.
c) If the stock is out of round, you can center it (equal readings on opposing jaws when the appropriate jaw is straight up) but it will cut on only one side at first when you start turning.
d) A gouge, burr or stray metal chip on the end of the stock in the jaws will make your stock slant. This will not slow down your centering, but it will cause the stock to be centered only at that one spot. See (b)
e) When all jaws are even you should make sure that all 4 jaws are snug, but not over tight. Remember, the jaws should not cut through a single layer of masking tape wrapped around your stock.
f) If your readings are jerky, check the dial indicator. I have one that needs lubing once in a while. I have another that I dropped 3 times, so it is not accurate until the plunger is 1/2 way depressed.
g) If the high and low readings go above and below the 0 point on your dial, lower the test indicator till they are on the same side. I find it helpful if the target is on the left hand side of the dial so that as I tighten, the needle goes down, and as I loosen it goes up.
h) Do NOT tighten jaws C and D while adjusting A and B. If C and D are tight AND out of position you will be fighting against them. C and D shuld be finger tight until A and B are done.
I hope this is clear enough. Let me know if I've transposed an up/down, or left and right. I tend to do that.
Daniel
