Hole preparation is often a forgotten consideration when threading a part. Depending on the parameters of the application, lack of attention to the size and condition of the hole can contribute greatly to thread size and finish, or the potential failure of the tap.

Holes to be tapped are straight or tapered. They can be cored, drilled, bored, punched, and extruded, all with or without reaming to finished size. Each one of these methods for producing a hole have inherent areas of concern. 

In cored, or cast holes, “hard” spots may be present due to certain casting processes. Have you ever cut into a piece of wood, and hit a knot? Imagine the effect on a tap. The surface of the hole may also include abrasive scale, sand, or other foreign matter left by the casting process. And, holes created by casting share the challenge of tapping a slightly tapered hole left by draft angles in the mold. Machining the hole to a uniform diameter is recommended to improve the performance of the tap, and produce uniform threads.

Surfaces in drilled or reamed holes are occasionally work-hardened due to the use of dull or improper tools. Some materials, like Austenitic Stainless, Inconel, and other super alloys are highly susceptible to work-hardening. This condition adds a level of difficulty to the work of cutting or forming threads. Avoid creating this condition.

Holes created by tools that cut off-axis (making holes that are less than straight), will cause extreme stress on the tap. The tap will always tend to follow the hole. Tap breakage is probable. The greater the depth of the hole, the more likely this is to happen.

If a hole is “out-of-round”, the threads will be “out-of-round”. If a hole has irregularities, so shall the thread created from it. Any effort made to create a “clean” hole will pay dividends with the life and performance of the tools to follow. Also, chamfering, or adding a bevel to the hole will always be a benefit to starting the tap.

Although less than perfect hole conditions are common, it will be beneficial to make an effort to correct any imperfections before beginning the process of threading. Tapping with negative influences like those discussed above, is to be avoided, and never recommended. 

The most important decision in creating a hole for tapping is determining size. Most published charts will recommend a hole size to produce 75% of thread depth. This has been a number accepted by the Industry for many years. Higher percentage of thread, derived from a smaller hole, is certainly possible, but rarely necessary as it will likely expose the tap to extreme torque without the benefit of significant increase in thread strength. Most applications are tolerant of a number between 60 and 75%. 

When machinists are having difficulty with the thread tapping process, their first inclination is likely to be increasing hole-size to reduce cutting forces on the tap. As some experiments show the drop from 75 to 60% thread reduces torque required for tapping by half. However, there are additional factors that must be considered when problems occur. Additional components of the job; like proper tool geometry to match the material being tapped, speed and feed rates, sufficient lubrication, condition of the machine and part fixture, and even the skill of the operator, are major influences on the success of the operation.  Please consider one more thing: the tap point diameter and chamfer angle/length are fixed when the tool is manufactured. Therefore, increasing hole size will allow the tap to enter further into the hole before contacting the workpiece. This will reduce effective chamfer length, and require the remaining threads of the chamfer, now engaged in the part, to work harder. The benefit of increasing the diameter of the hole isn’t always what you would normally expect.

North American’s Thread Tap App and Tap Hole Size Calculator are useful tools for choosing a hole size when it’s not called out on the Part Print. Both are easily available as a free download from our website. If you reach wit’s end with an application, let the design experts at North American Tool assist with possible tweaks to the tool design to improve the odds for success.