Tonight, while playing catch-up, I had a breakthrough. I've replaced the "grip" (upper and lower contact surfaces) with a thicker lip and a ring that stops the inner where I want it. This design squeezes the inner tight but has more durability.
I've had instances where somebody left some glue on the inner, then tried to force the inner into the groove, rubbing against the inner walls until they cracked. I've also had instances where those little plastic bumps, deep inside the inner, managed to exert undue pressure. Somebody in a bit of a hurry would just force the issue, only to get an agonizing crack (As with replacing a tire on your car, putting the thing on straight makes a difference). It's an aggravation that needed to be dealt with. Unfortunately, when you get down to tolerance of one-eighth of an inch, there's a fragility that makes the classic design less than user-friendly. On a completed headphone, where all the parts have been assembled in the factory, these fragility issues don't come up but when you hand a shell, or an inner, to someone else, you have to ask yourself whether we're dealing with a "customer serviceable" part.
I, personally, prefer thicker shells, regardless of what they do in Brooklyn. Trying to make my ideas fit their tolerances has been a bit of a headache, but I think I've found a way through the labyrinth. I'll follow up with some pics. In short, I've thickened the outer lip, removed the inner lip and replaced its "grip" with a tighter fit from a thicker lip. I've also created a ring that stops the inner exactly where I want it. I can now check the fit before I've left the drill press. Creating the "cradle" is now one of the earliest steps in the process.
I skipped the last 6 classes.. I'm thinking I should realllly start going.
