The Antikythera Mechanism Episode 3 - The Plates And Main Bearing. 

The #Antikythera Mechanism Episode 3 - The Plates And Main Bearing, by Clickspring.
In this episode the basic structure of the mechanism comes together, and I put forward a theory on a simple method for achieving the extremely close clearances observed in the original device.
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Abbreviated Transcript:
00:23 So what exactly is the structure of the mechanism? Well if you strip everything away, it comes down to this. A single main plate, with a single main bearing located at its center. The front planetary display is speculated to have been positioned just above the main plate, and the rear display is known to have been located on the other side. It's a 3 tiered structure providing support and alignment for the entire mechanism, and you can see that it has features spread across all 3 levels, that are all somewhat related.
01:41 Now its impossible to know for sure if the original maker used the same technique, but certainly some sort of systematic approach would have been essential.
04:07 Using one of the pins to gauge the progress of the taper. The pins were then hammered firmly into place, and the opposing holes on the other plate given a light countersink. Now for the plates to all sit flush when stacked, the pins must sit beneath the surface of each matching plate.
06:41 Registering each plate with the one beneath it. Again taking care to align the plates carefully before drilling, and placing the pins in regions of the mechanism known to be available space. I can now work with the plates aligned in a complete stack like this, or in various combinations as required, and be confident that the relative positioning of features will be accurate across all 4 plates.
07:49 The sub plate was then returned to the stack to pick up the correct position for the pillars, and finally the front dial plate was returned to the stack to drill out the pilot holes for the pillars. Now without register pins, this would have been quite a challenging task to manage.
10:44 And this is significant, because the presence of a square hole in the spacer leads to an unavoidable conclusion: That the part was in fact fabricated from 2 separate pieces. A spacer with a square hole, and a main bearing body, that were then pressed together to form the full bearing profile.
11:29 All throughout the device, virtually every sub assembly incorporates this idea of spacers and wheels, Either riveted together as a sort of layered sandwich, or threaded onto a shouldered arbor, or both. Certainly the wheels and spacers could have been formed from stock selected to achieve a specific clearance, much like this bearing. But its easy to imagine the maker taking it a step further.
12:13 With not much more than an flat abrasive surface and good eyesight, the maker could have achieved exceptional dimension control without any need for absolute measurement.
References:
Gears From The Greeks - Derek de Solla Price: amzn.to/2pii4ZD
The CT and PTM data that the AMRP have made publicly available can be found here: www.antikythera...
Gear schematic can be found in Tony Freeth's 2012 research paper:
dlib.nyu.edu/aw...
Credits:
Clips from "The 2000 Year Old Computer" courtesy Images First Ltd.
The Antikythera Mechanism Episode 3 - The Plates And Main Bearing, by Clickspring.