Sorry for well and truly raising a thread from the dead - but at least it's the right time of year for such things!
Having need for a depthing tool and liking the simplicity of this design, I decided to have a go myself. In the absence of detailed drawings I've needed to sketch it out myself, hence the reason for necro-posting to add some dimensioned drawings should anyone in the future have a similar need and come across this thread.
[ You are not allowed to view this attachment ]
I went with 4.5mm balls in 4mm reamed holes (no countersinks or chamfers!) with 6mm balls in 5mm holes for the hinges. I also roughly cut out the 'C' shape of the frames first to let any stresses and distortion out of the metal, then machined to shape and again unclamped to let it relax before then drilling and reaming all the holes as the last step so they should all remain true to each other.
The adjustment has a half-round 'nut' which sits in a similar half-round groove on the inside of the one plate, with the end running in a short vertical half-round groove on the opposite side:
[ You are not allowed to view this attachment ]
If I wasn't too careful slackening off the adjusters the flippin' balls would drop off their seats. I fixed this by fully cleaning all the parts then assembling and clamping the runners in position before using a wooden stick to add a small drop of cyanoacrylate glue down the hole behind. The ball is blocking the hole and is held in position by the runner, so the glue is just going to tack them in position without the risk of preventing them sitting down accurately into position.
I haven't tried it in anger yet, but just setting the arbors together for this photo seems to suggest it's going to do the job for me just fine.
[ You are not allowed to view this attachment ]
Ian.
