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Old 07-02-07, 11:23 AM   #1
seaneee
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Tapping Steel Cranks

Has anyone ever done this? Will it ruin the cutting tool? Just curious.
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Old 07-02-07, 11:56 PM   #2
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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.
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Old 07-03-07, 11:53 AM   #3
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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.
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