256 form tap drill size full#

The solution was a modified tap from North American Tool. The caused premature wear and stress on the that thread in the flutes and eventually to catastrophic failure.

256 form tap drill size full#

Instead, because the diameter was of the hole was so large the cut tap did not engage the threads along the chamfer length and instead engaged at the first full thread at the end of the chamfer. As was stated above the operator thought that by increasing the drill diameter size would lead to better tool life. Such was the case with the application porblem. When using oversize hole diameters, the tap should also have an oversize chamfer point diameter so all threads in chamfer are cutting and not just dropping in a hole. In higher tensile steels, larger hole sizes (less percentage of thread) may work better to reduce torque in tapping.

To Tap This Size Screw Or Bolt: Use This Drill Bit: (Closest Fractional:) Decimal Inches. Too low of a percentage of thread can lead to cross threading when engaged with the fastener. RECOMMEND TAP DRILL TO USE FOR 75 DEPTH OF THREAD.

They still provide the same pull out torque.

A lower percentage of thread still provides the same pitch diameter, which is the only point the flanks of a fastener contact with the threads that have been tapped.

Excessive torque can lead to catastrophic failure, with broken pieces of tap or galling in the thread form.

The higher the percentage of thread, the greater the stress factors on the tap and the greater the likelihood of failure.

This guideline range to use is a reflection of several factors: On page 115 of the current North American Tool catalog a range for percentage of threads from 50% to 85%, with the recommended range between 65% and 75%. If 75% is good, then a higher number is better, right? WRONG (in many cases). This leaves the good machinist to select a drill that is more or less than 75%.