Thread forming tap drill size manuals#
Thread forming tap drill size series#

Thread forming tap drill size manuals#

To determine the proper hole sizes that maximize strength for various lengths of engagement, consult the ASME standard or other commercial manuals containing this information. In summary, drill close to the minimum minor diameter in thin parts and near the maximum minor diameter in longer assemblies.

Therefore, for thin parts the smallest drill size is recommended for the highest percentage of thread to keep the threads as strong as possible.Īs the length of the thread engagement between the nut and screw, also known as the length of engagement becomes longer, there are a greater number of threads contributing to the strength of the assembled nut and screw therefore, the hole size can be enlarged. If, for instance, the internally threaded part or nut were made of something thin, such as heavy-gage sheet metal, the one or two threads in the nut might not be strong enough to support the load when the screw is tightened. While it is best to use the largest drill size possible, doing so may adversely affect the strength of the threaded assembly in some situations. Table 2: Minor diameters and pitch diameters for 1/4-20 UNC threads. Larger drills are less fragile and have a lower depth-to-diameter ratio, and their larger flutes ease chip evacuation. Along with lowering cutting forces and reducing tap breakage, this strategy will also benefit the drilling operation. Remove as much material with the drill as possible for a given length of engagement to make tapping easier. The ASME industry standard B1.1 is one of the best sources for this information. Dimensions for the minimum and maximum minor diameter of screw threads are available in many publications. Once the hole is drilled, it is important to gage the hole to verify the size using pin or plain plug gages matching the minimum and maximum minor-diameter specifications. For example, for a 1/4-20 UNC-2B thread, any drill that produces a hole between 0.1960' and 0.2070' in diameter could be used. The drill selected should produce a hole size that falls between the minimum and maximum minor diameter for the class of thread. The drilled hole must be within the specification for the class of thread. If tap/drill charts are not adequate for today’s machining practices, then how do you establish proper drill size? For a start, keep in mind these factors: Table 1: Typical tap/drill chart, featuring probable hole sizes. 5 drill to produce a hole that’s 70% of thread height. 7 drill may no longer be the correct drill size for a 1/4-20 UNC-2B thread. Changing to one of these advanced drills could result in tap breakage where none existed before. Producing smaller pretapped holes substantially increases the workload on the tap.

Today’s advanced drills - powered by more accurate, higher speed machine tools - are producing holes much closer to the actual measured size of the drill, or even smaller than the drill diameter. And staying below the maximum minor diameter prevents the production of threads that are too shallow. This makes tapping easier and reduces the possibility of tap breakage. The drill is removing as much material as possible from the hole, reducing the load on the tap. Compared to the minor-diameter specifications for a 1/4-20 UNC-2B tap (0.1960'- to 0.2070'-dia.), the finished hole size is near the high limit, 0.0022' under the maximum minor diameter (Table 2). 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).

Thread forming tap drill size series#

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.