I recently purchased a used 4-jaw chuck for my Atlas 12" lathe.
The problem is that none of my chuck keys will fit it. The 3/8 is too large, and the 5/16 marginally too small, so unless it's some bastard metric dimension, an 11/32" inch square chuck key would be perfect. The problem is that I can't find any on eBay or even in the tool catalogs.
The question is: "Does anyone know where a can purchase a 11/32" square chuck key for a lathe 4-jaw chuck?
Hi Harry. Other posters have suggested serviceable solutions to your problem, (grinding), but I just have to giggle a little at your characterisation of metric dimensions as "bastards", when you seem quite happy to work in illegitimate fractions like 11/32"!
Another alternative is to chuck a length (5/8 diam., perhaps) of round stock in your three jaw (use O1, if you want to harden and temper it), and turn an appropriate length down to the width of the diagonal of an 11/32 square. Then file the four flats by hand right in the lathe.
You don't have to mechanically index it. You can get it square with a flat file and a small square to check the right angles; just keep the 11/32 diagonal diameter at the corners. Don't file the flats in sequence, but after the first, file one next to that, then go back to the other side of the first one and file that for the third.The first try may be a goof, but by the second or third, it will come together.
Turn a short concavity or chamfer to blend the key back into the main piece.
Part it at 4-5 inches. For a handle, drill the back end radially for a
1/4-5/15 rod. You can use a crotch center in the tailstock for that (with the drill held by the chuck), or the drill press. There's a number of ways to secure the handle from slipping, if that's wanted (one variation uses a notch around the center of the handle and a ball bearing held by a coil spring).
Who made the chuck? It may well be a metric dimension. My calculations suggest that it may be 8.75 mm (your 11/32" converts to
Do you have a milling machine? If so, get one of the square 5C collet holders (usually comes in a set with a hex one), add a collet to match the diameter of the stock (I used drill rod for mine, because that is what I had handy), turn the end to the diagonal measure of the square socket, clamp it in the milling vise and mill a bit off one side. Rotate it 90 degrees and mill again, repeat until you have done all four sides, then measure the size of the (partial) square you have produced. Subtract the desired size from it, and divide by two to tell how far to advance the qull or crank up the knee, and mill all four sides again. Keeping it in the collet chuck, test the fit in the chuck you are fitting, and if it right, turn the workpiece around in the collet chuck and cross-drill the other end for a press fit of the handle. Drill it to be a slide fit, and knurl the middle of the handle for a press fit.
I don't know where you'd find one that size. You may need to make one, and CRS would probably be sufficient. With an endmill, you could make a suitable key on your lathe, assuming that you can mount the EM in a collet or even a 3-jaw chuck (not the best holder but it'll work as long as it's done cautiously).
Grinding an existing square-ended key or other tool could be a simple solution, as suggested earlier. An old 3/8" socket drive extension would be good material for the key stem. You might not want to try drilling the extension for the handle though if it's high grade steel. Welding a section of round stock for a T handle on the end could work.
I've made a few keys from CRS and they're holding up well (I like a longer stem for lathe chuck keys). For the endmill method, with a boring bar holder mounted on the lathe compound, using a shop square will get the opposing side cuts parallel. The blending radius that someone mentioned makes a good looking finished part.
A tool that I find useful is a 1/4" drive screwdriver-style handle with a
1/4" to 3/8" adapter on it, it's handy for quickly setting the independent jaws on lathe chucks, then followed with the T handle type for jaw tightening.