Unknown micrometer

[Tonyg]

Not sure that this is the right place to post this, my apologies in advance if I got it wrong.
I have this micrometer which was part of my dad's tools when he died, it must be at least 80 years old and I have no clue what it would be used for.
It was manufactured by Ambrose Shardlow in Sheffield. I have downloaded one of their tool catalogues (company closed many years ago) and can not find it, also done Google searches and spoken to many machinists with no joy.
The company also manufactured aircraft engine parts during WW2.
Hoping that some of our erudite members will know what it is for.

[mklotz]

I can offer two guesses...

Used to measure projection of some feature from the curved surface of a structure

or

The radius of curvature can be calculated from the height (sagitta) of an arc above a known chord; see...

How to solve circular segments

Purely guesses. I've never seen or used one as shown in the photo.

[Tonyg]

Thanks Marv, I had similar guesses and wondered if it was more of an in-house unit to Ambrose Shardlow for measurement on crankshaft journals.

[clavius]

I have never seen one like that either, but I would second the thought that is was some custom measuring tool used for a some task very specific to the place that your dad worked.

There was an old gent around here that had a small used tool shop as his sort of semi-retirement job. He previously had been an instrument maker that sold and repaired measuring tools. He told me that at one time a pretty significant part of his business was making custom measuring tools for the GE jet engine plant in Lynn Massachusetts when they were in their heyday. He had some pictures of various fixtures and jigs he had made for them, some of them quite complex and interesting. Most of that business went away when CMM's and the like came into play. Great old guy, full of stories. I miss him.

[Isambard]

A key-way or recess depth measuring tool for a specific diameter shaft? Looking at the size of knurled knob on the attachment it would appear to have been made interchangeable, rather than a single purpose made tool.
A late thought! A specific boring bar tool setter. I knew I had seen one before! :-)

[robhc88]

I recon your right on the money as a boring bar depth setting tool,the only trouble is you would have to read it from the back of the bar due to the constant radius

[metric_taper]

If you zero the micrometer, where does the anvil sit on the hole? Is the knurled screw (large one) to adjust the height of that adapter piece that is of the working end? How about that small knurled screw, does that allow the reading line to move? I probably am using the wrong nomenclature to describe the parts of a micrometer.

[Tonyg]

The thumbscrew on the shoe allows you to set the depth anywhere along the rod that goes into it so you can zero it at any point. The large knurled screw is for course adjustment and the small one is a ratchet/light torque slip type adjuster so that when the tip touches it then turns without advancing anymore.

[rbutlerr]

The retired machine shop teacher named Lyle Peterson (Tubalcain on utube) would probably know. Send him a picture. His email address is pete22@mchsi.com

[Tonyg]

I have sent the pic through to Lyle. I will post his reply when I receive it.