This would result in a drilled hole size of 0.2048", approximately 70% of thread height. 7 (0.2010"-dia.) drill will, on average, produce a hole that is 0.0038" larger than the drill’s diameter. This became known as the "probable hole size," which is often listed on tap/drill charts next to the decimal equivalents of the drill sizes (Table 1). Advanced drillpoint geometries, new flute shapes, coolant holes, improved HSS and carbide grades, and coatings like TiN and TiCN have done much to improve the quality and size of the drilled hole.Īs drill recommendations were being developed for standard-size coarse (UNC) and fine (UNF) series threads, drill sizes were selected based on tests showing that a standard, general-purpose, jobber-length drill - having a standard 118° point angle - will produce a hole size larger than the measured drill diameter. Most importantly, the drills used to make the holes to be tapped have improved. Many changes have taken place in the years since tap/drill charts were first developed - programmable machinery, rigid tapping, synchronous tapping, better toolholders, specialized tapping fluids, and premium high-grade taps. The problem is, that factor can’t be calculated accurately using traditional tap/drill charts. While many factors contribute to tap failure or breakage, one basic, obvious factor is often overlooked - the size of the drilled hole. The key to successful tapping is knowing the true size of the drilled hole.
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