Answer.—“We have repeatedly tried annealing or softening taps that were of long or short pitch caused by tempering, and invariably found them about the same as before the annealing. The second tempering will generally shorten them more than the first. Sometimes, however, a second tempering will bring a long pitch nearer correct.”
“Do you soften your taps after roughing them out in the lathe?”
Answer.—“Never, if we can possibly avoid it. Sometimes it is necessary because of improper annealing at first. The more times steel is annealed the worse the results obtained in making the tool, and the less durable the tool.”
The following are answers to similar questions addressed to the Morse Twist Drill and Machine Co.:—
“The expansion of taps during hardening varies with the diameter. A 1-inch tap would expand in diameter from 1⁄1000 to 3⁄1000 inch.”
“Taps above 1⁄2 inch diameter expand in diameter to stop the gauge every time.”
“The great majority of taps contract in pitch during the hardening, they seldom expand in length.”
“The shortening of the pitch and the expansion in diameter have not much connection necessarily, though steel that did not alter in one direction would be more likely to remain correct in the other.”
“There does not seem to be any change in the diameter or pitch of taps if measured after hardening (and before tempering) and again after tempering them.”
“Taps once out in length seem to get worse at every heating, whether to anneal or to harden.”