So I'm pretty sure that Precision Plastics was using two different hardness scales but they only declared the one: Rockwell (and gave a value of 73R as the hardness of the Cellulose Acetate). The other scale (I think... totally not sure but it's a somewhat informed guess) is the Durometer and they used it to describe the hardness of the Nitrile rubber balls (as 70A).
Now check this out: the Cherry O-ring silencers that WASDKeyboards sells? They come in two flavors: Red and Blue both have a hardness rating of 40A (and I believe that this is on the durometer scale as well because these O-rings are also a type of rubber: EPDM vs Nitrile above).
WASD keyboards used to carry another flavor: black, and they were much harder (70A if I remember). But I digress... I mentioned the above because these silencer rings are pretty common and if you have them you have a perfect example of what 40A feels like. If you have an old sampler pack that includes the Black Rings then you know what 70A feels like.
But the question is, what do those cellulose acetate balls (73R) feel like? Based on a quick visual inspection of the diagram at the bottom of the link below... I'm thinking that they would be (if it even makes sense because I don't know what the range is of the durometer "function") around 105-100A.
http://www.calce.umd.edu/TSFA/Hardness_ad_.htmNote this is the only link I found that converted Durometer A to Rockwell R: most of them only go from Durometer D to Rockwell R. YMMV.
I had a hard time getting my head around an example of what 73R feels like until I noticed that 73R is the hardness of polypropylene and we all know what that feels like: that's what the lid of a pack of Tic-Tacs is made out of (I didn't know that until I looked up PP). Not super hard but not incredibly pliable either. And the concern I have about this material is this: if you drop it in the barrel of the switch will it snug itself into the bottom, snugly so that it doesn't pop out? Sounds like an interesting theory to test though.
http://en.wikipedia.org/wiki/PolypropyleneHere's another idea: use that belt-hole punch on one of those nitrile balls... If you use too small a ball in the punch maybe the punch would just push the ball right through... maybe you need to go with a bigger ball and then just slice them up.
One thing I noticed with the blue, red and black O-rings when I tried them on my board: they all made my board feel too sluggish (even the 70A black ones). It's tempting to think that 105A is too hard... but sometimes I think that might be just about perfect. In any event, I wouldn't be too eager to dismiss any material with a hardness <= 110A until its "feel" was tested. And I think the only way that hardnesses around 110A are going to work properly is when they get punched to precisely the right diameter (so that they really seat themselves in there).
Cheers,
I never would have thought that this thread be running as long as it has. Sorry I don't have the time to contribute anything more material to the conversation but I hope this helps!
-K