This design is an offshoot from the idea I had a while back for an ER32 collet block. This one has been designed primarily to solve a long-standing issue with the length of time it takes to mount my 3-jaw chuck on my rotary table. The difficulty in mounting is because they're roughly the same diameter and the chuck has no through mounting holes. I have a plate that goes between but it takes way too long to set up and I've always avoided using it.
So, attaching the new collet fixture is FAST. It uses 4 bolts and T-nuts in the provided slots. The interesting hole pattern is due to my annoying need to make everything fit everything else. It allows a 4-bolt circle and a 3-bolt circle (top, lower-left, lower-right) and the other two holes (upper-right, upper-left) are for balancing, should I ever want to bolt this to something that rotates quickly.
I wanted to keep it as low-profile as possible, as this was another issue I had with the 3-jaw mounted on the rotary table. It would almost fall on it's nose with all that overhanging weight.
There's no reason this couldn't be bolted directly to the table if I ever saw the need. Though the plan is to leave it attached to the rotary table for most of its life.
The recess in the back was an afterthought. I was thinking about the way 5C collets have integral threads for a collet stop. This led to building in the ability to mount a plate in the recess (no threaded holes in the body for this yet) that would act as, or accept various designs of stops.
From model to product, there are only a few hours of computer and machine time. The turning and threading were all done on "my" CNC lathe at work. It started out as a chunk of 4-1/2" round 4140 HT steel. This is beautiful stuff to work with when you have the right tools. Crisp threads and surface finishes of RSW grade (Madmodder running joke) are almost guaranteed.