Diameter of tapping bottom hole of screw hole
The bottom hole diameter for tapping a screw is a crucial parameter in machining, directly impacting thread quality, connection strength, and assembly precision. The bottom hole refers to the hole drilled before tapping. Its diameter must be precisely calculated and selected based on factors such as the thread specifications, thread profile, and material properties. If the bottom hole diameter is too small, the tool will experience excessive resistance during tapping, potentially breaking the tap or damaging the thread profile. If the bottom hole diameter is too large, the thread profile will be insufficient, reducing the strength and reliability of the threaded connection. Therefore, properly determining the bottom hole diameter for tapping a screw is crucial for ensuring thread processing quality.
The basic principle for determining the tapping bottom hole diameter is based on the thread profile parameters and the material’s plastic deformation characteristics. Different thread types (such as metric, imperial, and pipe threads) have different parameters such as the thread angle and pitch, and the corresponding bottom hole diameter calculation methods also vary. For the most commonly used metric standard thread, the bottom hole diameter is generally calculated as follows: Bottom hole diameter = Major thread diameter – Pitch. This is because during the tapping process, the tap squeezes the material to form the thread profile. This plastic deformation of the material slightly increases the actual thread size. Therefore, the bottom hole diameter needs to be slightly smaller than the major thread diameter to ensure sufficient thread profile height. For example, for an M10×1.5 metric thread with a major diameter of 10mm and a pitch of 1.5mm, the bottom hole diameter should be 8.5mm.
However, in actual machining, the determination of the bottom hole diameter also needs to consider the characteristics of the material being machined. For materials with high plasticity (such as mild steel, copper, and aluminum), due to the significant plastic deformation during tapping, the bottom hole diameter can be appropriately increased, typically 0.1-0.3mm larger than the theoretically calculated value, to avoid deformation or burrs caused by excessive extrusion of the thread profile. For brittle materials (such as cast iron and cast steel), due to the less plastic deformation, tapping is primarily a cutting process, so the bottom hole diameter should be closer to the theoretically calculated value to ensure thread strength. For example, when machining an M10×1.5 thread, when tapping mild steel, the bottom hole diameter can be 8.6mm, while when tapping cast iron, a bottom hole diameter of 8.5mm is more appropriate.
In addition, the thread’s precision grade and intended use also affect the bottom hole diameter. For threaded connections requiring high precision (such as transmission threads in precision machinery), the deviation in the bottom hole diameter must be strictly controlled to ensure that the thread clearance meets the design requirements. For threaded connections subject to less stress or temporary conditions, the tolerance range for the bottom hole diameter can be appropriately relaxed. Furthermore, the type and quality of the tapping tool will also influence the selection of the bottom hole diameter. For example, tapping with a high-speed steel tap and tapping with a carbide tap will require slightly different bottom hole diameter requirements due to the different cutting performance of the tool. In mass production, trial tapping is often required to determine the optimal bottom hole diameter to ensure the stability of thread processing quality.
With the continuous development of machining technology, the method for determining the bottom hole diameter for tapping screw holes is also being continuously optimized. Today, through computer-aided manufacturing (CAM) software, the optimal bottom hole diameter can be automatically calculated based on the thread parameters, material properties and processing technology, and the corresponding processing program can be generated, which greatly improves processing efficiency and precision. At the same time, the application of some new tapping tools (such as spiral groove taps, extrusion taps, etc.) has also put forward new requirements for the selection of bottom hole diameter. For example, extrusion taps form threads by cold extrusion of materials and do not require material removal. Therefore, the calculation method of their bottom hole diameter is different from that of cutting taps. Usually, a larger bottom hole diameter is required to adapt to the plastic deformation requirements of the material. In the future, with the development of intelligent processing technology, the determination of the bottom hole diameter will be more accurate and automated, further improving the quality and efficiency of thread processing.
