When it comes to the reliability of taps in machining, a crucial but often underestimated factor is the size of the drilled hole. In traditional tap/drill charts, this seemingly straightforward element can be elusive in accuracy.
Over the years, advancements in machinery, tapping techniques, tool holders, specialized fluids, and high-grade taps have transformed the landscape. Notably, the drills, the initial architects of the holes to be tapped, have undergone significant enhancements. From advanced geometries to improved materials and coatings, today’s drills produce greater precision and quality holes.
In the evolution of drill recommendations for standard-size coarse (UNC) and fine (UNF) series threads, the concept of “probable hole size” emerged. Traditionally, charts featured a standard, general-purpose drill producing a hole larger than its measured diameter. For instance, a No. 7 drill (0.2010″ diameter) would create a hole around 0.2048″, approximately 70% of thread height. This practice aimed to reduce tap load and minimize the risk of breakage.
However, the game has changed with the advent of advanced drills powered by high-speed machine tools. These modern marvels produce holes closer to the measured size or smaller. While this enhances precision, it places a higher workload on the tap. The familiar No. 7 drill may no longer be the ideal size for a 1/4-20 UNC-2B thread; a shift to a 13/64, No. 6, or even a No. 5 drill might be necessary to maintain the desired thread height.
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Determining Drill Size:

Fractional Sizes UNC and UNF

Table 1: Typical tap/drill chart featuring probable hole sizes.If tap/drill charts fall short in today’s machining practices, how do we determine the proper drill size? Consider these factors:
Stay Within Thread Specifications:The drilled hole must align with the class of thread specifications. Select a drill that produces a hole size falling within the thread class’s minimum and maximum minor diameter.Gauge the Hole:
After drilling, gauge the hole to verify its size using pin or plain plug gauges matching the minimum and maximum minor-diameter specifications. Industry standards like ASME B1.1 provide these dimensions.Material Removal Strategy:
Remove as much material as possible with the drill for a given length of engagement. This not only eases tapping but also benefits the drilling operation. Larger drills, with lower depth-to-diameter ratios, are less fragile and facilitate chip evacuation.

Length of Engagement:

While using the largest possible drill size is generally advantageous, it may impact the strength of the threaded assembly, especially in thin parts. Opt for the smallest drill size for thin materials to maintain thread strength. As the length of engagement increases, allowing more threads to contribute to the assembly’s strength, the hole size can be enlarged.
For various engagement lengths, consult standards like ASME B1.1 or commercial manuals containing this information. Longer engagements may necessitate a larger drilled hole size, balancing the number of threads contributing to strength.

Diameters for 1/4-20 UNC Threads:

Length of EngagementRecommended Hole Size Limits 

Table 2: Recommended hole-size limits for various engagement lengths for 1/4-20 UNC threads.
In instances where the length of engagement exceeds 1 1/2 times the thread diameter, the hole size can be enlarged further. However, caution is advised, considering the engagement between parts, not just the depth of the tapped hole.

Thread-Forming Taps:

Thread-forming taps employ larger drill diameters to accommodate material displacement. This inward displacement shapes the finished minor diameter of the thread. For a 1/4-20 UNC thread, a No. 1 drill (0.228″ dia.) is typically recommended. However, applying the oversized drilling principle is essential, as high-performance drills can produce smaller holes. Enlarging the drill diameter may be necessary to prevent tap breakage or rough threads due to excessive material displacement. Unlike using taps to create the minor diameter, it is not advisable. Instead, taps can aid in eliminating “spin-down” burrs formed below the minor diameter during tapping when employed as “controlled root” taps. These taps ensure a smooth thread by clipping off burrs with their controlled minor diameter. While various factors impact tap breakage and performance, establishing the correct hole size is often underestimated. The focus should be on the finished hole size rather than just the drill diameter, emphasizing the need for the optimal hole size for the best results.