Pratt & Whitney turbine blade grinder and Woods metal fixturing

Pratt & Whitney's manufacturing plant 20 years ago held turbine blades to one-ten-thousandth (~2.5 micron) tolerances using a $10M grinder room-sized machine. Blades cast in Wood's metal (50% Bi alloy that expands on freezing) so there's zero looseness during grinding. Plant restructuring moved from a football-field-sized layout with separated machine types to cellular manufacturing — travel distance cut from miles to ~100 yards.

Remember I talked about linear friction welding and the blisk, where they were trying to attach the turbine blades directly to the disc, to get rid of this big heavy mechanical structure, which has extremely tight machining tolerances. Some of the tightest tolerances in commercial manufacturing in general are how these little curved Christmas-tree inserts, they call them, fit together.

$10M grinding machine, Woods-metal low-melting-point fixturing because mechanical clamping cannot achieve the required tolerance on the blade root.

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.