I like to build little steam engines with one-piece crankshafts. I have built a total of 1.5 of them.
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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