Calculation of bottom hole size before tapping
Calculating the bottom hole size before tapping is a critical step in ensuring the quality of threaded connections. An oversized bottom hole will result in insufficient thread profile height, reducing connection strength; an undersized bottom hole will increase cutting resistance during tapping, potentially causing tap breakage or workpiece deformation. Determining the bottom hole size requires comprehensive consideration of the thread type (metric, imperial, pipe, etc.), the plasticity of the workpiece material, the thread accuracy grade, and the tap type. Combining theoretical calculations with practical experience can yield a reasonable bottom hole diameter, laying the foundation for subsequent tapping operations.
The calculation of the bottom hole size for standard metric threads is based on the thread pitch and nominal diameter. The key is to ensure the effective height of the thread profile. For plastic materials such as steel and bronze, tapping causes plastic deformation, resulting in bulging of the thread profile on both sides. Therefore, the bottom hole diameter must be slightly larger than the minor diameter of the thread. The calculation formula is: bottom hole diameter D drill = D – P, where D is the nominal thread diameter and P is the pitch. For example, for machining an M10×1.5 thread (nominal diameter 10mm, pitch 1.5mm), the bottom hole diameter should be 10 – 1.5 = 8.5mm. For brittle materials such as cast iron and brass, where there is no significant plastic deformation after cutting, the bottom hole diameter can be slightly smaller. The formula is: D drill = D – (1.05 – 1.1) P. For an M10×1.5 thread, the bottom hole diameter is approximately 10 – 1.05×1.5, which is approximately 8.43mm. In actual production, adjustments must be made based on thread accuracy. The bottom hole of a 6H-grade thread can be slightly larger than that of a 6g-grade thread by 0.05-0.1mm to ensure the proper fit during assembly.
Calculating the bottom hole size for imperial and pipe threads has its own unique rules. Imperial units must be converted to metric units before calculation. Imperial threads express the pitch in threads per inch: P (mm) = 25.4mm / TPI. The bottom hole diameter calculation formula is similar to that for metric threads, but it’s important to note that the nominal diameter of imperial threads is the major diameter (in inches), which must be converted to millimeters (1 inch = 25.4mm). For example, for a 1/2-inch (12-thread-per-inch) imperial thread, the pitch P = 25.4/12 = 2.117mm, resulting in a nominal diameter D = 1/2 × 25.4 = 12.7mm. The bottom hole diameter for plastic material is 12.7 – 2.117 = 10.58mm. The bottom hole size of pipe threads (such as G1/2) needs to refer to national standards. Due to the special tooth shape and matching method of pipe threads, the bottom hole diameter is usually larger than that of ordinary imperial threads. The bottom hole diameter of G1/2 pipe threads is 18.9mm to ensure the sealing and connection strength of the threads.
The calculation of the bottom hole size for special threads (such as trapezoidal and modular threads) must be adjusted based on their profile characteristics. Trapezoidal threads have a profile angle of 30°, and the bottom hole diameter must ensure the working height of the thread. The calculation formula is: D drill = D – 2h1, where h1 is the thread height (h1 = 0.5P + ac, where ac is the crest clearance, generally 0.25mm). For example, for a Tr30×6 trapezoidal thread (pitch 6mm), h1 = 0.5×6 + 0.25 = 3.25mm, resulting in a bottom hole diameter of 30 – 2×3.25 = 23.5mm. The bottom hole diameter calculation for modular threads (such as worm gears) is based on the module m, with pitch P = πm. The bottom hole diameter = nominal diameter – P. For example, for a worm gear with a module m = 3 (nominal diameter 30mm), the bottom hole diameter = 30 – 3.14×3, which is ≈ 20.58mm. The bottom hole dimensions of these special threads must be calculated strictly according to the standards and verified by trial drilling when necessary to ensure the engagement performance of the threads.
Correcting and verifying the calculated pilot hole dimensions is crucial for ensuring machining quality. Theoretically calculated pilot hole dimensions need to be adjusted based on actual conditions. When the workpiece material is hard, the pilot hole diameter can be increased by 0.05-0.1mm to prevent rapid tap wear during threading. When using coated or high-speed steel taps, the pilot hole diameter can be slightly smaller, as these taps offer improved cutting performance and can withstand greater cutting forces. Before mass production, a trial tapping should be performed using a thread gauge to check the pitch diameter of the thread. If the gauge is too loose, the pilot hole is too large and the diameter should be reduced. If the gauge is too tight or cannot be inserted, the pilot hole is too small and the diameter should be increased. Furthermore, the pilot hole depth should be appropriately calculated, generally equal to the effective thread length + 0.7D (D is the nominal diameter), to ensure that the tap can fully tap to the desired depth and avoid incomplete threads at the bottom. Accurate calculation and trial drilling verification ensure that the pilot hole dimensions meet the requirements, ensuring high-quality thread production.
