Greenfield along with Cutting Tools Chicago aka General Cutting Tools answers the question of whether tap drill charts have the right information. While many factors contribute to tap failure, drill hole size is one that is often overlooked. When a problem arises, you may check the drill size being used against a tap/drill chart. If it appears correct, you might look for some other cause for the failure. But, are the tap/drill charts giving us the right information?
First, it is important to note that these tap/drill 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, a #7 (.201 “) diameter drill used for a 1/4-20 thread would generally produce a hole that was approximately .004″ larger than the drill’s diameter, 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 an approximate 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. The accuracy of all drills has also improved dramatically due to enhanced drill manufacturing equipment and processes. Today’s advanced drills, powered by more accurate, highspeed, computer-controlled machine tools, are producing holes much closer to, or even the same as, the actual size of the drill.
Using 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 thread. It may be producing a hole much too small for successful tapping. While it is recommended that the tap/drill chart be used as a reference, or starting point, for selecting a drill diameter for the best hole size, the finished hole size is the most important factor. In the case of the 1/4-20 thread, 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.