With the prop drive collet finished, we turn now to the prop driver itself. While this part may be made of aluminum, I chose steel, specifically 303 stainless, for durability. The prop selected, a Graupner 3-bladed prop, has a base circle of 32.5mm, and a 4-hole bolt pattern of 26mm in diameter. This base will be drilled and tapped 4-40 for the prop drive bolts.
|Once again, my big slug of 303 is chucked. While
"free machining" for a stainless, it is still
not too much fun to work with. Make your cuts aggressive,
or the tool may rub and work-harden the steel.
The OD of the blank is turned to 32.5mm, and the blank is also bored deeply a diameter equal to the small end of the collet.
|With the top slide still in its 5 degree position from the collet job, the driver is bored to this taper, and the collet used as a guage for depth. 2.5mm of the collet must protrude so that the stainless driver clears the cowl and any bolts which protrude from the prop shaft housing nose. Here we see the correct depth of bore.|
|With the tough work done, the driver is profiled to a
pleasing shape. The inner diameter is turned down far
enough so that the 4 securing screws will clear when the
disk is drilled and taped 4-40 on a 26mm circle.
Parting off, keep the feed aggressive, the speed slow, and get the job done in one continuous cut. Be liberal with cutting oil or coolant. I use Tapmatic Gold for all my machining operations.
|Installed on the shaft, with the bearing in place. A tube is placed over the prop shaft, and a nut is used to force the driver home. The prop driver is secured effectively to the shaft.|
|Back in the lathe, the parted surface is cleaned up and trued to the rotation of the shaft itself. The white stuff is tissue wrapped around the bearing to help keep it clean.|
|Given that the prop hub has four bolt holes, I probably didn't need this step, but elected to apply a knurl to the face of the disk to help grip the prop hub rear. Again, the shape of my setup forced me to run the lathe backwards with the knurl on the far side of the shaft.|
|The almost finished prop driver... all that remains
is to drill and tap the 4 holes in the disk itself. This
was done in the vertical mill.
Tapping hints: I have found over the years that tapping a fine thread like 4-40 in stainless is like a root canal at the dentist, unpleasant in the best of circumstances. Here's how to do stainless... first, as much as possible, execute the job for a through-hole tapping. Blind holes collect swarf, and it is the work-hardened swarf which snaps taps like brittle sticks. A through hole allows much of the swarf to flush forward and out. Second, have 2 new taps on hand, one a taper, and the other a plug profile. Start with the taper tap, and cut as much as you can before you decide the forces are getting a bit high. Back out the taper tap, and use the plug tap, again tapping as much as you feel safe. Reinsert the taper tap, and continue this alternating process until you are through. The two profiles complement each other and will get you through the stainless with minimum fuss.
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