I like to build little steam engines with one-piece crankshafts. I have built a total of 1.5 of them.
My trials and tribulations are well documented in several threads on this site. ONE of my issues concerns holding the small diameter crankshaft work pieces in my 8" Chinese 4-jaw chuck. I have to shim the jaws to reduce the run out acceptably. (Doing that was a good learning experience.) The small diameter of the work piece, 3/4", makes it difficult to hold reliably; the work piece slips and that's the end of that. If I tighten it so it to the point it will not slip, the jaws mark the work piece (unless it is protected by soft shims.)How do you get around these problems?
BadDog suggested a 5C collet fixture. I bought one. (Thanks, BadDog!) Before it arrived, I knew I would have to build a plate to mount it across my 11" lathe face plate. That got me to thinking about building a precision mounting fixture for the 5c collet fixture. My efforts and some issues are shown below.
I used 1018 throughout. The "back plate" (got a better name for it?) is from 3/4" x 6" stock. I milled a 3/8" x 4.5" slot and used a t-slot cutter to create t-slots on both sides of its width. (I'm not sure if I wrote that last part correctly.)
The slide mount plate is a piece of 1/2" x 4.5" stock. I milled shoulders (?) on both sides to fit in the wide t-slot created in the back plate. For "precision" positioning, I used a captured 5/16-24 SHCS. The central holes you see in both the back plate and the slide mount plate are unnecessary and I will not include them on the NEXT one, should I decide to build another to fix all my mistakes.
I messed around trying to align this thing for a bit. And then some more. Finally, I stuck a short 3/8" pointy thing in my tail stock mounted drill chuck and ran the tail stock up to the loose 5C collet fixture with a 3/8" collet inserted. This got me very close (two or three thousandths) and I put the final alignment on it using the alignment screw for one axis and a little prying using some not-shown bolts mounted through the face plate for the other axis.
This device solves three problems for me . . .
1. The collet fixture will not mar the work piece.
2. The work piece will not slip as it did in the 4-jaw.
3. Shimming is not necessary.
However, although the run out of this device is far less than my 4-jaw, it still exists. At 7/8" from the collet face (about as close as I could easily get), the run out is about 0.0015". At three inches, the run out is 0.0035". This is acceptable for my purposes. But, it bugs me! How can I reduce it?
One thing I did NOT do during machining was pay attention to stress relief while creating the wide slot in the back plate. I'm not sure how I would have done that. (Maybe a little tiny fly cutter?) The slide plate was machined without regard to making the front and back sides parallel . . . I "trusted" they would be parallel since they are SUPPOSED to be parallel. (If I rebuild it, I won't make that mistake again.)
I am thinking about putting three adjustment screws in the face of the slide plate in a tripod arrangement. My guess is that as little as 0.0005", more or less, would make a big difference out at three inches from the collet face. What do you think about that idea? (The picture in the NEXT post shows where I am thinking about putting the adjustment screws. I marked the spots with red dots.)
The back plate is held to the lathe face plate with four automotive 3/8-16/24 studs. The slide mount plate is held to the back plate by the t-slot. When the correct position is dialed in, the slide mount plate is then clamped to the back plate with four 3/8-16/24 automotive studs. The 5C collet fixture is held to the slide mount plate with 1/2-13 studs.
I will balance the entire thing by bolting weights in the appropriate places to the lathe face plate. I would appreciate your thoughts on any aspect of this project.
Run out video 7/8" from collet face.
Run out video 3" from collet face.
--Bill
