The idea is to remove any imperfections between the two surfaces so they mate together better and actually get on the shaft to the proper depth. Often times shafts come from one shop and props from another and they can fit together poorly, despite industry standards. If you get a prop and shaft from reputable shop they will have already lap fitted it for you using this same technique.Interesting discussion and thanks to Maine Sail for the effort and expertise.
I'm puzzled though. I can see the benefit of lapping when the machine parts stay matched when torqued. But when a prop is lapped to a tapered shaft with hand pressure, the place on the shaft that is nicely matched to that particular prop is quite different from the fully torqued final resting place of the prop on the shaft. The prop could easily move forward on the shaft 10 to 20 thousandths.
Early
Props are soft and shafts hard. Manganese Bronze vs. AQ17-22 or Nitronic 50. Even in the old days when you could still get Tobin Bronze shafting, which you' can't anymore, I never once witnessed someone developing a ridge unless the prop was loose. As the title implied you are lap fitting the "prop" to the shaft taper and it is the prop that gives up the most. As I mentioned this is standard industry practice and my shafting/prop shop does it to every combo that goes out the door using the same procedure.First, a disclaimer: I’m not a machinist, and can’t afford the book I reference below. I read the stuff I mention from the preview on Google books. My sole professional experience is kludging fixes to large machinery in the field in order to get it home to the real professionals.
After a bit of research I think that if you hand-lap for a good fit each time you mount a prop, (-perhaps even the first time if you’re big, strong and aggressive) it could create a ridge at the forward limit of the lap. When you torque the prop nut, the leading edge will sit high on the ridge, defeating the purpose of the lap fit.
The Shaft Alignment Handbook, 3rd, Piotrowski, p.172 describes a procedure that first identifies the high spots using Prussian Blue paste, laps the high spots only if contact is less than 80%, and cautions that the machine shop will have to re-turn the shaft if you develop a ridge at the edge of the lap.
Early
I have read that and I agree with it but when you are doing this lightly by hand the prop is what tends to smooth out and the shaft just seems to change finish a bit. Never, ever put a groove in a shaft. In fact I called my prop shop today and asked him about it and he's never seen it happen either, a groove in the shaft from lapping a prop and he does a lot more of them than I do...Maine, I agree with you method of lapping the prop to the shaft, but one thing you are removing more metal from the shaft than the prop. When you lap the lap is always softer than the part being laped. the grit sticks in the soft and cuts the hard. Machinery's Handbook has a good section on laping, (28 edition pg 1100). I'm not nick picking, but I had to look it up to make sure that I remembered right, because you almost always have your ducks in a row and I value your posts and advice.
Fred Villiard
Well OK... but my inner 10 yr old and the shade-tree mechanic side of me both still say "whatsa diff?"
(Ok that was mostly the 10 yr old)
Would it really make that much of a difference? Under what situations would leaving the big nut torqued and locking it with the thin one fail sooner?
(Thanks for your patience. It's messy but this is how I learn.)