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While many factors contribute to tap failure, one that is often overlooked is the drilled hole size. When a problem arises, generally the drill size being used is checked against a tap/drill chart, and if correct, we look for some other cause for the failure. But are the tap/drill charts giving us the right information?
First, it must be understood that these charts were developed in the 1950s and 60s. Drill diameters for the various thread sizes were selected based upon the probable hole size a standard bright jobber drill with a conventional point would produce. For example, back then a #7 (.201") diameter drill, used for a 1/4-20 thread, would generally produce a hole that was approximately 0.004" larger than the diameter of the drill, or about .205". Comparing that to the .196"-.207" hole size required for that tap, the #7 drill produces a hole near the maximum limit, which is ideal. This results in approximately a 70% thread height. Removing most of the excess material with the drill significantly reduces the load on the tap without reducing the strength of the thread.
Today, things have changed significantly. There are many advanced drill designs, materials, points, coolant holes, and coatings. Also, accuracy of all drills has improved dramatically as a result of improved drill manufacturing equipment and processes. Todays advanced drills, powered by more accurate, high speed computer controlled machine tools, are producing holes much closer to, or even the same as, the actual size of the drill.
Utilizing modern drills, CNCs, and holders with conventional drill charts could result in tap breakage where none existed before. The #7 drill may no longer be the correct drill size for a 1/4-20. It may be producing a hole much too small for successful tapping.
It is recommended that the tap/drill chart be used as a reference, or starting point, and select a drill diameter for the best hole size. The finished hole size is what is relevant! In the case of the 1/4-20, any drill that produces a hole close to .207 could be used. Even a #5 (.2055) or a special diameter may be acceptable based upon the drill design and conditions.
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