[R-390] How are your knobs ?

[Charles Steinmetz]

Dave wrote:

>Well the bushing is a bit thicker than the original fingers so the 
>clamp goes on the bushing a bit tight

When I saw the photo, I thought the OD of the fingers could actually 
stand to be turned down a bit, to give them more flexibility and, 
consequently, a stronger clamping force on the shaft for a given 
clamp screw torque.

>The fit of the bushing in the casting was a loose sliding fit, being 
>drilled by hand it was tighter at the bottom than at the top.

Ideally, you would like the ID of the fingers to have a specific 
amount of "bell" so that they become perfectly parallel when it is 
tightened on the shaft (any collet-type joint holds strongest when 
this condition is met).

My only criticism is that the fingers are a bit short (as you noted), 
which reduces the abvailable clamping force.  I think I'd have glued 
the bushing in with the fingers sticking out maybe 0.050" beyond the 
clamp (I don't think there is any advantage to bottoming the bushing 
in the hole).  But I doubt seriously there is any practical problem 
with the repair job.

>I think it's an acceptable fix for the price and done at 
>home.  Right now it feels like it's going to hold up.

I suspect it will last until the radio is scrapped.  Good job!  (You, 
too, Pete.)

Charles