I would imagine the problem, in hand, would have been ‘spotting’ the ring of dots before starting to drill. How they determined where the holes should be positioned wrt each other is the answer to the question I would be interested in.

Not really answering Niel's question but 'Clickspring' on YouTube has some videos showing his build of the mechanism using tools and technology that the original maker might have used.

Apart from showings the marking out of the parts he actually made the files and other tools totally from scratch.

The videos and presentation are among the very best of the Youtube

Ian P

Depends on the maker IMO

Was he a dabbler or a Harrison

What an interesting forensic puzzle, Neil

I would will certainly start by getting my best estimate of the radius of that pitch circle.

To inform that exercise, we obviously need to identify the centre point of each hole

… So the obvious question is :

Do you have access to a higher-resolution image of the X-ray, or is that all we have ?

MichaelG.

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P.S. __ Your second image shows a radius … but on what basis was that defined ?

Edited By Michael Gilligan on 05/10/2021 07:14:18

As the Lunar Year is 354 days possibly the mechanism is based on, or predicts, phases of the moon rather than earth years and days. In which case the hole placement and accuracy looks reasonable for hand methods.

I like Ady's method, but would expect the reference holes to be more accurately set than intervening holes and don't see much sign of that – they're all a bit random. If I was making it, I'd try marking out a much larger circle, and using a long rod to scale down to the instrument's circle. The outer end of the rod could be moved a fixed distance from the previous point to step out the angle, or moved between pre-made marks.

Not sure how I'd mark out the outer circle because the lowest factor of 354 is a prime number (59) which can't be made easily by dividing by two. However, as 60 is easily divided by two, maybe it was done by marking out 60 divisions on the big outer ring and then losing one by eye.

A long rod being a little bendy coupled with a 60/59 approximation might explain why some of the holes are noticeably off, see the wide gap between 55 and 56.

The mathematics of the day were fairly advanced, making the theory available to the builder. What's surprising is the use of gears in an instrument this ancient. Most impressive.

Bow driven drill seems likely, but not obvious to me how very a largish number of fine drill heads could be made to the same diameter. They weren't ordered off the internet!

Dave

Do we know whether the maker(s) might've had magnifying lenses? Could make a lot of difference to marking-out precision.

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Not an estimate, Neil but possibly relevant … Have a look at the section about John Bird : **LINK**

https://www.academia.edu/5340304/Dividing_to_Rule_Precision_Mathematical_Instruments_in_mid-18th_Century_England

MichaelG.