A hand-held micrometer requires the operator to simultaneously hold the part, hold the micrometer, and operate the spindle, while a bench micrometer eliminates at least one, and sometimes two, of these. It's not that hand-held micrometers can't be accurate, it's that bench micrometers allow the operator to concentrate all their attention on the actual measurement itself. So, rather than the hand-held style of micrometer tool Goodson sells I'm making the equivalent of a bench micrometer.
With the above in mind, the 1" micrometer I will be using allows setting the cutter over a 2" range of diameters. So, I designed the instrument to cover the range of diameters from 1" (i.e. a smaller seat pocket than I can imagine ever needing) to 3" (larger than I can imagine ever needing). I want it to be as accurate as possible, but since it only will be rarely used, I didn't want to spend a lot of time on aesthetics. Basing it on a repurposed chuck of Al satisfied these requirements.
For no good reason (I mean it, for no good reason) some time ago I bought a used Black & Decker valve grinding set, and with that set came a number of duplicate pilots. One of those duplicates would do quite nicely, so I picked one that the cutter tooling fit over with no perceptible play (so, probably less than 0.001" clearance, although there was no reason to take the time to measure it).
The first photograph shows the large diameter micrometer head I won't
be using along with the bracket it came with.
I decided to repurpose the bracket rather than make something from scratch, so that only required making a sleeve to match the smaller OD of the 1" micrometer head to the ID of the hole in the bracket. Conveniently, the OD of the head is an Imperial ⅜" so it was an easy matter to ream it for a snug fit, shown in the next photograph.
The sleeve was thick enough that I could have tapped it for a setscrew, but I decided to slit it instead and rely on gentle clamping pressure to hold the micrometer, as shown in the next photograph.
With the micrometer portion done, I turned to making the base from a large Al bracket in the scrap bin. The next photograph shows it after I had roughed it to size with a saw and then faced it to the desired thickness on the mill.
I ran out of time at that point, but the final photograph shows that what remains to do is to tap two holes in the side to hold the micrometer bracket, and drill a hole the depth of the Al block to hold the pilot.
The pilot I've chosen is for a guide that is +0.010" over ⅜", so I can drill the hole for it as deep as possible with nearly-inflexible ⅜" carbide tooling. Although I'll have to finish drilling that hole with an extra-long ⅜" HSS bit, the earlier portion of the hole should keep it quite straight. I'll then remove the remaining ~0.01" with the Sunnen hone for a tight push fit.
Making a 1.000"-dia. standard to fit over the 0.375" top of the pilot will let me zero the micrometer (actually, any accurately-sized standard that is 1" or larger would do).
NYBSAGUY and I were laughing about it earlier today, but this new precision instrument will be yet another inscrutable object in the garage for my daughters to puzzle over when settling my estate sometime in the (far distant, I hope) future, before tossing it in the trash and moving on to the next of the thousands of unidentifiable objects.