[GreenKeys] More 28 reperf hammer head photos

[Mark J. Blair]

This is a fun project! Seeing the pictures and drawings of other hammer variants is really helpful. I wonder if the chamfered head that Nick sent me really is a 28 ASR variant, vs. something really similar that got mis-identified? Well, I'll work on a reproduction of it no matter what. I hope that other projects like the molding attempt continue whether I succeed in making practical 3D printed replacements or not; variety is good!

I haven't CAD-modeled the hammer head quite yet, but I did spend time studying the hammer under a microscope and trying to figure out how the original mold was designed. I think that I see mold draft angle on the two broad sides of the head, but it's too small for me to measure with my current tools and skill. I can't quite tell whether mold draft on the hammer face and back surface are for the same pull direction or the opposite direction, which would shed light on the mold cavity design. I also can't tell yet whether there is mold draft on the center hole. I can see from faint markings on one face that the rectangular center pin was machined from a separate piece of round bar, which may have either been rigidly fixed to one side of the mold, or have been a moving part (possibly serving as an ejector pin?). None of this matters for a 3D printed part, but I'm just curious about some stuff. I have some small experience with designing parts for injection molding. Mold design and fabrication is currently well beyond my skill level, but I find it fascinating.

I give folks on Twitter a chance to guess what the hammer is. It was too obscure for the nerds who follow me there, but I'm educating them in the wonders of old mechanical teletype gear!

https://twitter.com/nf6x/status/1216904565301207041

Pulling out the hammer arm from the back side wasn't easy, but it seemed like it would be less painful than getting access to the end where the hammer goes from the other side. And anyway, I wasn't just interested in replacing the hammer head; I wanted the whole arm out of the machine for eventual fitting, photographing, and so forth.

-- 
Mark J. Blair, NF6X <nf6x at nf6x.net>
http://www.nf6x.net/