Over the past few days I was able to get more machine time in on the swingarm pivot plate. I made a lot of progress but didn't quite finish up. For the first time in a while I had a couple of broken end mills slow progress down. The order of some roughing operations was incorrect and when you have a 1/2 end mill sticking out of the collet 2 1/2" it doesn't take much of an unmachined area to increase the chip load make the end mill snap like a twig.
This clean fracture is an indication of high quality material. There is no bending or elongation with carbide. It's all or nothing, and in this case, nothing. The good thing about mill tool breakages is that if you catch it before a toolchange then usually it's nothing more than resetting the milling chuck with a new end mill. that's what happened this time, no part damage, just a slight snap and then silence.
To rewind a bit to where we were at in the previous post, I had the part blank in its usual place on a subplate on the trunnion:
The pivot bearing arrangement is one that I thought was designed quite cleverly, then upon further research seemed to be the way most modern Japanese sportbikes are done! One major design constraint of the swingarm pivot is to keep the bearing diameter as small as possible in order to keep the sprocket to pivot distance as small as possible. This is desirable because it both reduces the variation in chain tension and allows for a longer swingarm for a given wheelbase, a rare win-win situation.
Hopefully these features will be machined this weekend.