Tapping Steel Cranks
Has anyone ever done this? Will it ruin the cutting tool? Just curious.
Stop reading my posts!
no, taps are harder than the steel they are designed to cut. If you use a generous dose of cutting oil or tap fluid, and go slow (1/4 turn forward, then backward to clear the chips), there will be no damage to the taps.
What exactly is it that you are trying to do?
Generally, in steel, you check the tables (like this one) and pre-drill to the required size. The drill sizes in the table will yield a typical 75% thread, which, for all practical purposes, is almost as strong as a full depth thread, yet much easier to cut. Easier to cut means you are less likely to break the tap off in the hole.
Just recently, I retapped a French threaded (15mm x 1.25?) crank thread to the standard 9/16-20. They are pretty close, yet just far enough apart that modern pedals will not fit. A few drops of cutting oil, five minutes light effort, and the job was done.
Be careful to start the tap square to the work. Back tap (1/4-1/2 turn) every revolution to break the chip.