I recently got me a Mamiya/Sekor 135mm f/2.8 lens with M42 mount. Turned out the outmost edge of the aperture ring reached farther back then the M42 threads -> the aperture ring was in a way of properly mounting the lens to the EF adapter and when mounted, the aperture ring would not turn. Thus I had to machine away a bit.
The DSLR I am using is Canon EOD 450D, and I am mounting the M42 lens with passive M42 adapter.
Here’s the lens before the operation:
Taped up the lenses to protect them from the metal dust:
I used Dremel router mount table to keep the cutting disc precisely on the desired level:
The router table worked very fine here. As you can see, the removed part stayed in one piece despite of being just fractions of millimeter thick - sign of the cutting disc staying put as it should:
The lens after the operation:
The image magnifies the deepness of the cut; the shiny, cut section is actually less than 1/20th of millimeter below the inmost surface of the aperture ring. If it was sanded down and painted black, you would have to strain to see it was cut.
Because this comes to my own use and you can’t really see the cut part when the lens is mounted, I did not sand or paint it; I only filed down any sharp edges.
Here’s the lens mounted after cleaning up the dust. It’s just as desired; the gap between mounting adapter and the aperture ring is around 0.25-0.4mm, just enough for the aperture to turn smoothly but not having to cut out any more metal than absolutely necessary: