Turbine Shaft Part II


Work continues on part 146, the power turbine shaft.

  The crude homemade toolpost grinder shown in Part 1 was a failure. The bearings in the Foredom handle were simply not precise enough, and the finish was poor. So I scrapped the grinder concept and went forward with the traditional home-shop method of producing the shaft.

The special ground HSS tool can be seen here taking a miniscule, shaving cut of perhaps 0.0002" / 0.005mm on the major OD of the shaft, to nearly 12mm. Note the rather extreme rake and tiny point of this tool.

With secondary roughing complete, the shaft is hung between centers and adjusted/indicated as close to truth as possible. On hand are the pinion, the bored turbine wheel, and the two bearings so that we may test fits as we go.

When polishing to size, it is important to do so uniformly. I use silicon carbide paper and a uniform steel backing rather than emery cloth, which is thick and a bit coarse. Start with 320 grit, and as you approach the correct diameters, begin to use progressively finer grits, say 400, 600, 1200, and finally 2000. These latter grits will remove perhaps 0.001mm in 30 seconds of polishing, and are very useful for final fits.

I am about to attack the forward bearing journal. Visible too is the pinion journal, and the blank portion on the end for the threads.

With silicon carbide paper, there is no fear in wrapping around the shaft, as the paper will tear long before finger damage will occur. The coarser grits are best backed with steel; finer polishing can be done as shown here, as the tiny strip can be leaned to one portion or the other to make the journal truly parallel. Use a bit of oil to help the cut and keep the paper from clogging.
As you polish, the shaft is mounted and remounted as needed to test the fits. I made the pinion and forward bearing journals a diameter which will deliver a force fit... in other words, the parts will not slide on by hand, but may be started perhaps 20% of the way only. When installed, the pinion nut will be the motive force to seat the bearing, oil thrower, and pinion. Mike suggests a firm sliding fit. If you have any doubt about your ability to calculate and execute an apropriate force fits, do what Mike says!
The shaft is finished except for the screwcutting at either end! I did not show the trial fittings of all the components, as they are all approached in the same fashion.

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