I just got the first 3 jaw chuck for my lathe. I was using a 4 jaw for the last two years. It just got tiring trying to true up the work and not mar it in the process. I never did get too good at it. I put the 3 jaw on the lathe, checked the body for runout, and it did not register any error on the dial indicator. Chucked up a 1.75" dia. piece of steel, and made a light cut on it. It measured .001 runout after the cut. Probably due to rough surface finish. Then turned this piece around and chucked it in the 3 jaw, using the cut surface as the holding location. When I measured the runout (still using the cut surface to measure), I was indicating .020 out. I tried this several times and it varied from .005 to .020. I found some rust on the inside of the jaws I thought might be causing the problem, but removing it didn't seem to help. My lathe is a Reed-Prentice 16X54, the chuck a Skinner 8.5" 3 jaw I just pruchased on Ebay. The chuck has an integral L01 spindle mount, and it looks like it has sat and gotten a little rusty for most of its life. Any constructive suggestions would be appreciated.
The chuck has one hole (for the key) indicated with a zero. Use this one for precise (more or less) work. Also when turning the piece, don't rotate it, but put a mark on it so that the same place goes to the same jaw.
2.9. Why won?t this chuck hold parts exactly on center?
2.9.. No chuck or collet is perfect. Even a new three-jaw chuck is only good to 0.002? or so unless it is adjustable. If your chuck won?t hold parts that closely, something is wrong. Here?s a list of possibilities. Any of these could cause a good lathe to wobble. It could be that the jaws of the chuck are worn. Letting parts spin in the jaws wears the chuck quickly. It could also be that the scroll of the chuck is worn. The scroll is a flat spiral metal plate behind the jaws. Another possibility is that the chuck is mounted on the backplate wrong or that there are dirt or chips between the chuck and the backplate. Take it apart, clean it carefully, oil it, and reassemble it. That may help. Still another possibility is that there is dirt or chips between the backplate and the spindle. Unscrew the backplate from the spindle, clean the threads in the backplate and on the spindle, oil both, and try assembling it again. Screw it together gently. It should not be forced together or spun on fast with a snap. It could be that the spindle is bent. Put a dial indicator on the spindle nose to check it. It could be that the spindle bearings are bad or that the spindle is improperly installed. Good bearings will feel smooth and have no play. Logan spindles are precision machined to give the bearings a precise preload when the spindle is correctly assembled. Preloaded bearings have no play.
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2.10. What can I do to fix a 3-jaw scroll chuck with worn jaws?
2.10.. If the scroll is worn, there?s nothing you can do without a CNC machine and a lot of time. If the scroll is worn and you fix the jaws, the chuck will be true at the diameter that you used for the repair, but nowhere else. If you?re sure that the problem is just the jaws, then you can set up a toolpost grinder or boring bar and regrind or bore the jaws true. However, you must do this with the jaws pressing against something. One way to do this is with a piece of sheet metal having holes for each jaw. Another way to do this is to drill holes in each jaw for a pin or screw, and have these pins or screws tighten down on a piece of pipe. Tighten the chuck on the sheet metal or pipe and the jaws will be locked in the right place.
If I read your post correctly, you have the un-turned end of the stock sticking out of the chuck, and that is the surface that shows the 5-20 thou run out. Who says that the 1.75" diameter stock was round to begin with? I'd turn that outboard end down and then measuure it. I assumed from your original post (my problem, not yours) that the chuck is new. If it used, all bets are off. I hassled with a couple of used 3 jaw chucks for years and finally gave up and bought a set-true chuck. I paid 3 times as much for the chuck as I did for the lathe 25 years ago, but it was worth it.
.005" runout isn't too bad for an 8-1/2" 3-jaw. .003" is about as good as you can expect even from a very expensive new chuck. However, there are some things you can do. The first thing is to completely disassemble your chuck, and clean it using solvent & compressed air. It's amazing how a tiny chip somewhere can bollux things up. When you put it back together, pay close attention to the fit between the scroll and the central boss. Any clearance there translates to runout in the jaws. Older chucks can accumulate wear there. If it is worn then chances are it is worn uniformly, so you can cut a suitable shim that fits all around the central boss, like a wedding ring. Give it a little lubrication in the back where the pinions go, and leave the jaws dry and clean, and reassemble and test. When you test, shoot for a good finish. Pay close attention to how your piece is oriented in the jaws (maybe mark it where the piece touches the no. 1 jaw) and replace it exactly, and tighten it from the same pinion, and I'm guessing a lot of your error will be gone.
Even with .020" error a 3-jaw is completely usable for many things and a highly desirable addition to your lathe tooling. Many things can be turned entirely in one setting.