Pratt & Whitney turbine blade

The physical blade Tom holds up as exemplar of precision-ground "tree" mechanical attachment, Wood's-metal fixturing for the $10M grinding machine. Returns in §8 as the artifact Tom distributed to high-school visitors with the corner-cut / notch quality-control markings to prevent reuse.

I wanted to mention three other types of friction welding. One is linear friction welding. This is circular friction welding — it's nice and easy to build a machine that goes in circles, but they would love to do linear friction welding. If you want to do some friction welding, you can do it with your hands — you feel your hand warming up. They would love to be able to make turbine discs for jet engines, where they weld the turbine blade to the substrate. Here's a regular turbine blade. The jointing is a mechanical joint, and they call this the tree — turn it upside down, it looks like a tree. This is ground very precisely. This is the most expensive part of the turbine, I think — $10 million grinding machine to grind these flats. You mount the whole thing in Wood's metal, a low-melting metal, to fixture it, because you can't clamp this precisely enough to get the tolerances they want. It has to fit into the mechanical sleeve on the turbine disc within a few tenths of a thousandth of an inch, otherwise at the speed it's going you'll get vibration and fretting wear. You have to be careful what temperature you assemble these at — a few degrees of temperature, you basically have to assemble these in a controlled temperature environment.