Hi All,
A month ago I was obsessing over an engine line-boring problem, and Harry was kind enough to redirect my efforts to a more feasible approach. As anyone will see from the attached photos, my setup less-elegantly approximated Harrys excellent CAD drawing, which is in this string. The boring bar solution worked for me because of the relatively small distance between the journal blocks [~2.0], and previously having made a boring bar that was up to the task.
I took some care in locating the journal blocks on the engine base, and used a DI to square the base to the lathe axis. I originally thought to press in the bushings first and then bore to the .312 shaft diameter. In fact that was exactly what I did the first time, which was why I was doing it again. Rethinking the process, by simply boring the much larger holes in the aluminum journal blocks to the approximate OD of the bushings, I could use a stiffer, longer boring bar. Then, turning and boring the bushings on the lathe from one piece of bronze rod allowed me to cut each of them to size and press them in with the reasonable expectation that they would be in alignment. In practice, that was approximately the case: the shaft jammed halfway through the second bushing and I had to lap it to allow the shaft to pass completely through.
Because of the length of the boring bar, I limited my initial cuts to no more than .010. My final cut was of the order of .001 or so at about 1,000rpm. The photos dont show a faceplate counterweight for balancing the load. I had to make one up for this job, sized 2.0 Dia x 1.0, to keep the lathe from jumping off the bench. Not having a capable band saw, I rough cut the blank with a carbide angle cutter from a discarded, cast iron exercise weight. I have read some comments about spotty quality in some weights, but mine was a 20lb rectangle and very decent material to machine, relatively speaking.
A word of interest: I used a small Arbor Press for seating the bushings, which despite my efforts to tighten it up, still had some side-play in the ram. I realize now that I might have ensured far better initial bushing alignment by turning down the first .062 length of each bushing to achieve a tight slip fit into the journal block, then pressing the bushing home. I might also have paid closer attention to the orientation of each bushing to the other during the parting process [as I had originally intended], and pressed them in to the journal blocks in sequence.
Regards,
Brian