Are there any detailed explanations for how bimetals are assembled? It would be super cool if there were some kind of “How It’s Made” style video for yoyos and especially bimetals. I looked on YouTube but didn’t see anything.
In a nutshell, it’s precision machining and a press fit of the parts.
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How do they press fit it on, from which side of the yoyo half? The wide side, or the narrow side? Wouldn’t there be scratches if it was from the narrow side, so I guess it has to be pressed on from the wide side?
Do they ever use glue or adhesive? I’ve seen evidence of that on weight rings in POM yoyos but never metal.
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Generally it would require a jig or fixture. Based on what I’ve seen from operations on non yoyo items, the ring would be face down in one fixture, and the shell would be pressed into the ring from the back side. If the fit is right no adhesive is needed. Never saw evidence of adhesive on any YYJ yoyos. (Obviously not bi-metal, but similar concept.) I worked in a facility that did precision models. Lots of press fit items produced.
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There’s a little clip of the rings on a MagicYoyo Stealth being put on here, at 34 seconds:
From bottom up they have a table, some sort of loose slab with a cloth over it that the operator slides around, the steel ring (this is an outer ring yoyo), then the aluminum cup, then a jig that’s stuck on the bearing seat part of the yoyo body, and then some machinery that presses downwards that the operator controls.
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I’m struck by the degree to which the Streath appears to be hand made/assembled!
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Interesting, I expected the ring to be fitted with a single press rather than working around the perimeter, but maybe that was only one step and they didn’t divulge all their secrets 
Does anyone know how much interference is used for press fitting weight rings in general? I was thinking .003” but that’s a wild guess. I wouldn’t be surprised to hear .005”
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AFAIK most are press-fitted, but not all are. Northern Spin Co.'s Helix, for example, has rings that are adhered with epoxy.
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