For reasons I won't go into right now, I spent some time today modeling a Thomson seatpost. It's a fun little project, and one that involves some weird intersecting surfaces. I definitely got some good use out of my radius gage set, and even plunked down to get a new one that goes up to 1" (mine, a Brown & Sharpe set that I got from an old machinist, is 1/32"-1/2").
While I'm excited to check out the new 3D scanners soon (I'm hoping this weekend's Maker Faire will have a couple), manually measuring and modeling something like this is pretty fun. It's interesting to think about the way the geometries work. A Thomson seatpost consists of three main parts, which take a number of manufacturing techniques:
- The post is made from an oblong extrusion. Its OD looks like a circle with a lobe on the front and back, and its ID is an ellipse with its short axis oriented front-back. After being extruded, it's turned down to its finished diameter and milled to accept the saddle cradle and hardware. I'm guessing that both of these operations happen in a cool multiaxis turning center, but they could just as well be done in separate setups.
- The lower cradle is forged. I believe it's also machined, though only on one side and probably just one or two passes.
- The top cradle (not shown - I still need to model it) is also forged. I believe the bolt slots are then machined, and of course the Thomson logo is laser etched after anodization.
The hardware is made by a combination of forging, cold thread forming, turning, and machining. It's also all hardened.
Thomson is a badass manufacturing company. I'm excited to finish this portion of my modeling, which amounts to describing the parts they engineered, and move onto the really cool part - screwing with all of it.