Classic & Vintage - Tapping Steel Cranks
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07-02-07, 11:23 AM
Has anyone ever done this? Will it ruin the cutting tool? Just curious.
07-02-07, 11:56 PM
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.
07-03-07, 11:53 AM
What exactly is it that you are trying to do?
Generally, in steel, you check the tables (like this one (http://www.newmantools.com/tapdrill.htm)) 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.
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