Imagine for a moment that your boss invented the herringbone tooth form and tasked you with building machines to cut them. This was the situation Percy C. Day found himself in when he arrived in Milwaukee in 1913. Others were making them using shaping technology, so this well-understood machining method was leveraged with two horizontal-acting shapers timed together so the pinion-shaped cutters converged on the apex and clipped off the shaved chips at a common point. This became the typical machine design.
Once a part nears completion the operator must be able to measure the tooth thickness so part consistency is achieved. The cutting mechanism is behind the workpiece and there is seldom room to measure over pins or balls. The face width of each helix is usually too narrow to permit a span measurement. Cycle times were very short, so reading a tooth vernier meant delays.
Clever engineers, probably at Farrell or Sykes, established a unique system of comparator blocks. Each tooth size, such as 6 TDP, had a representative block and a dial indicator/bridge that could measure the addendum distance from the outside radius to the target tooth thickness. The system required accurate outside diameter control on the blanks, but allowed for a full range of what we now call “rack offset” to adapt a fixed TDP tooth form to various center distances.
This is where herringbones can become quite confusing, with extended or contacted center distances combined with “long and short addendums” to achieve various design goals. And this is before people started “messing with the old family recipe” of a 20 degree transverse pressure angle with a .80/TDP effective addendum. Some later were bold enough to change the helix angle!
I got to experience much of this confusion first hand when I arrived at the office one morning in early 1979 to discover that the last of the old herringbone guys had abruptly retired. It took several readings to absorb the ancient textbooks that he left behind, but eventually I was able to assist others in performing the needed calculations.