I'll see your Au contraire!
and raise you two Sacre Bleus!
Once the case is nicely fixtured with custom made expansion dowels screwed into ally blocks and clamped to the mill table, and dialed in to a gnat's patootie.....
A general principle of machining is if you want to make something round, use a lathe. I won't be bolting it to the mill, which means I won't need to make those expansion dowels. It also means the way I'll bolt it to the lathe will have bolt heads projecting above the flange so it won't be possible to attach the timing-side case.
But a question. How are you calibrating your Blake indicator for the variation in arm length and arm angle?
With those Blake indicators I found that any tiny error in the mill collet is magnified significantly at the end of the feeler finger
OK, guys, I only
used that Blake to confirm to my satisfaction that the bearing was originally machined Concentric
with the flange. Having accomplished that, it's the last the case will see of the mill's bed, since I'll install it on the lathe to do the machining. Once on the lathe I'll dial in the outer flange using the most sensitive test indicator that the surface quality of the Al allows.
in the reasonably unlikely event that you might want to move the bushing bore or the bearing bore because of a line bore issue.
Once I have the bearing recess refreshed and ready for the bearing -- see below -- I'll install a faux bearing along with a faux crankshaft to check the timing-side bush. If the bush isn't properly coaxial, I'll make a new underside bush and use the appropriate Sunnen mandrel to line hone it into perfect alignment.
Mon dieu, with that out of the way, it's time to turn to the task at hand. The problem is the walls of the bearing cavity have been hammered oversize so wouldn't work even with an original 2.5"/63.5mm ball bearing, let alone the 62mm roller bearing I intend to use.
Some ways to deal with this problem, each with advantages and disadvantages, are:
-- machine a steel sleeve with the necessary 1.5mm/2 = 0.75mm/0.029" wall thickness (ignoring interference fits for the moment)
-- machine a 6061-T6 Al sleeve with the above specifications
-- build the walls of the case up with weld and machine to size
Although I bought a length of 4140 Cr-Mo hollow tubing for this purpose, I changed my mind and am going with an Al sleeve. Unless I change my mind again, that is. Briefly, a quick search confirmed the TIG filler would be softer than the 6061-T6, and a sleeve also has the advantage of being easily removable and replaceable if the machinist screws up. And welding assumes there wouldn't be any issues with whatever is in the alloy that was cast 92 years ago. As for steel, the wall of the sleeve will be so thin I can see no advantage to it over Al. If the bearing ends up moving again you can all tell me "I told you so," even though there's no way to know that a steel sleeve wouldn't have moved as well.
Britbike doesn't have nearly enough machining photographs, so I'll show all the steps. The next photograph shows I started with a length of 3" OD Al.
I bored it with a series of ever-larger bits up to the maximum 1½" drill bit I have.
I then used a boring bar to increase the ID to ~2mm less than it will need to be for an interference fit for the 62mm bearing.
I left the ID deliberately small to give added stiffness to the sleeve for when I press it into place. Also, any resulting distortion, tilt, etc. will be removed after it's pressed in the case and back in the lathe, where I'll do the final machining for the interference fit of the bearing in it.
I then reduced the OD to 0.003" greater than the largest ID of the present battered bore.
This means it will have 0.003" interference fit over much of the length, but ~0.005" for a band in the center. Yes, I could machine the walls of the bearing cavity smooth, but that would remove Al that I would much rather leave in place on the case. Anyway, after final machining the wall thickness of this sleeve will be just 45% of that in the photograph above.
This is where things sit at the end of the day.