Once Upon a Time There Was a Magical Form Tap
Once upon a Time, not so far away, a tap manufacturer with a long history in the industry made an error processing a manufacturing order. We all do from time to time. Where it went from there is the story.
A cutting tap left the manufacturer missing a feature. The customer complained that they had received a tap without any flutes. Being desperate, they used it anyway, and reported that it “kind of” worked. The manufacturer quickly got the customer the finished tap as originally intended, but decided to research the performance of the “unfinished” tap.
They saw the obvious benefits of a “chip-less“ tap, but identified multiple downsides. As the material is not being cut, but pushed out of the way, friction would likely be an issue. On top of that, threading a blind hole would create hydraulic pressure in the hole (with 100% thread engagement, there would be no escape route for air or liquid beneath the tap).
These features would facilitate lubrication, by allowing some clearance between the surface of the tap and the material being threaded. They would also allow an escape route for any building pressure at the bottom of the hole.
They realized that the material being formed “flows” in multiple directions, both into and away from the thread-form of the tap. One very important detail was quickly recognized. The material being formed must be malleable. No flow, no go!
Ferrous and glass-filled materials are not a candidate for forming. A different hole size would be recommended to reduce the percentage of thread produced to a more acceptable level, and allow the same clearance for the minor diameter of the mating part allowed by a “cutting” tap.
Additional lube grooves were employed in applications that benefited. Recommended speeds were increased. Coatings were added to reduce friction and improve wear resistance. Coolant-holes became an option for those with the machine capability. Base materials were developed and improved. Like anything else, improvements continue as application demands evolve.
According to the trusted source, this is the way it happened. The real point of the story remains the same whether or not it is completely true. The “error” and solutions that followed illustrate most of the differences between the two styles of producing thread, and the taps that do the job.