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| Burn that 75% thread depth drill chart. Go find a 50% thread depth drill chart. What the traditional 75% chart neglects is that most workable metals extrude threads to make a full thread and then some in soft steel and worse in aluminum and copper and that's what locks up the tap. Backing off every half turn sometimes isn't enough. The 75% chart guarantees breaking taps, good business for the tap makers.
The choice of tap can help too, if it has an angled cutting edge up front to push the chips ahead of the tap and the hole is through the metal that can help clear the thread. Most hand taps don't have that "spiral" point.
Before I found the 50% thread chart, I always went two or three number sizes LARGER than the chart and still had problems. A 50% thread with engagement length about the diameter of the screw outside won't strip anyway.
Gerald J. | |
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Broken drill tap
