When drilling a 6-40 tap, selecting the correct drill size is the foundational step that dictates the success of the thread-forming process. The designation 6-40 refers to an Imperial Unified Screw Thread, where the number six represents the diameter, and 40 indicates the threads per inch. To create a strong, functional thread in materials ranging from soft aluminum to hardened steel, the drill bit must remove the appropriate amount of material, leaving a precisely sized hole for the tap to form the threads without excessive stress or breakage.
Understanding the 6-40 Thread Specification
The 6-40 thread is a fine-threaded fastener commonly used in electronics, machinery, and instrumentation where space is limited or a high thread count is required. Unlike coarse threads, fine threads offer greater tensile strength and a better grip in thin materials. However, this precision demands exacting standards; the drill size must be accurate to within thousandths of an inch. An incorrect hole diameter will result in either a tap that cannot cut the thread or a weak, loose connection that is prone to failure under vibration.
Calculating the Theoretical Drill Size
The theoretical calculation for a 6-40 tap drill size involves subtracting the thread pitch from the basic major diameter. The major diameter is 0.1250 inches (1/8 inch), and the pitch is the reciprocal of 40, which is 0.0250 inches. Performing the subtraction (0.1250 - 0.0250) yields a calculated diameter of 0.1000 inches. This places the ideal drill size squarely in the range of a number 29 drill bit, which is standardized at 0.1045 inches, or via a decimal equivalent often referenced as approximately 21/64 inches for fractional bit selection.
Hole Diameter for Various Materials
While 0.1045 inches is the general standard, the specific material being tapped necessitates slight adjustments to the drill size. The interaction between the drill, the tap, and the workpiece material determines the final thread quality. Below is a reference table outlining the recommended drill sizes for the 6-40 tap across common engineering materials.
The Risks of an Incorrect Drill Size
Deviating from the recommended drill size, even by a few thousandths, can lead to significant workshop frustrations. Using a drill that is too large results in a thread that is too shallow, compromising the holding power and creating a bolt that strips easily. Conversely, a drill that is too small forces the tap to cut through excessive material, generating immense heat and pressure. This often causes the tap to snap, damages the workpiece, and leads to a rough, uneven thread that fails to seal properly.
