Convex Knurls Speed Rotor-Shaft Production for Power-System Manufacturer 

Convex knurls speed rotor-shaft production for power-system manufacturer

by Ronald M. Klaucke
Engineering Manager
Accu Trak Tool Corp
Cherry Valley, MA 

The knurling operation used at generator manufacturer Generac Power Systems, Waukesha, WI, had presented excessive production time and restricted tool-life problems. The operation helps secure laminations to rotor shafts in generators.

Accu Trak convex knurls' design has sharp, round leading edges which "spread out" loading and ease contact with workpieces.

These rotor shafts turn on bearing mounts deep within the many variations of generator enclosures producing electric back-up power ranging from 8000 to 2 million W. Dies roll serrations on the shafts where laminations are permanently secured. A heavy press muscles the shaft, laminations, and wires into a power-producing rotor shaft. The laminations are reformed on the knurled serrations, locking them together on the rotor shaft with an "interference" fit.

Running the job on CNC turning lathes, Generac's machine shop initially used conventional straight-edge knurls to produce the serrations on stress-proof steel comparable with 1140 steel in hardness. The knurled serrations vary from 13Ž4" to 14" long and are rolled on four mainline shafts measuring from 10" to 17" long. Variations of these rotor shafts used with older power systems and special-order generators extend to 22.28" long. Shaft diameters run from 15Ž8" to 113Ž16". Rotor-shaft production covers 2 1Ž2 days per week based on a 2-shift workday.

Improved productivity came about three years ago when Michael Fogl, Tooling Procurement, brought in convex knurls from Accu Trak Tool. Manufactured from premium-grade hi-cobalt steel, convex knurls are further hardened by the addition of a TiN coating.

The convex design has sharp, round leading edges which "spread out" loading and ease the initial contact with workpieces. According to William Hamman, machine shop supervisor, " you start off with very limited engagement and as you proceed with the axial feed, the more you move toward final engagement." This knurl configuration makes the operation easier on the machines and the tooling dies, as less pressure is required.

With the rotor shaft knurling, Generac has been able to double the rpm while reducing knurling production time by 35% to 40%. Workpieces are now rolled in 3 minutes instead of the previous time cycle of 5 1Ž2 minutes. Knurls are held to a tolerance of 0.002" of the diameters. With the move to convex knurls, Generac turns out 600 to 700 units at any one time. An all-synthetic coolant for flushing and cooling minimizes heat buildup during knurling. The shop might even move to a higher rpm and increase production further.