Most interesting idea indeed !
What it is used for is a shear used to cut blue steel, in making of accorion reeds. It has a L-shaped stationary RC58 bottom with moving straight upper blade - this way one can cut both the length of the reed (.4-3") AND the shoulder (typically
1/32-1/16 wide), in one cut, outta strip of blue steel
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Now. to drive the upper blade, some kind of (hand) power transfer system is used. Typically a long handle drives either a gear or an eccentric. One needs some power to cut .032 blue steel - especially the shoulder.
When a gear is used, it drives a curved rack (attached to the upper blade) - thus the original question. Typically the distance from pivot point to the driving gear's outer edge is 8", so the rack would need to be curved with 16" OD.
Use of eccentric makes the whole thing much more compact - at the cost of friction-induced tear/wear on the apparatus :)