Causes and prevention methods of scrap when tapping and threading
Tapping (using a tap to produce internal threads) and threading (using a die to produce external threads) are common methods for thread processing. Improper operation can easily result in scrap. Common types of scrap include out-of-tolerance thread dimensions, skewed tooth profile, rotten teeth, and slipped threads. Out-of-tolerance thread dimensions refer to the mean diameter, major diameter, or minor diameter of the thread exceeding the tolerance range. The main reasons include unqualified dimensional accuracy of the tap or die, improper selection of cutting parameters, and uneven feed rate. For example, a worn tap will cause the mean diameter of the internal thread to be smaller, while a worn die will cause the mean diameter of the external thread to be larger. Excessive feed rate during tapping will increase the mean diameter of the thread, while too little feed rate will result in a smaller mean diameter. Skewed tooth profile is often caused by improper workpiece clamping, misalignment of the tap or die axis with the workpiece axis, or uneven force applied to the tap during tapping. This manifests as the thread profile not being perpendicular to the workpiece axis, affecting thread fit.
Tooth decay refers to the destruction of the thread profile, resulting in defects such as missing teeth and broken teeth. The main reasons include blunt tool edges, improper use of cutting fluid, and excessive or insufficient material toughness. After the tap or die cutting edge is worn, the cutting force increases, which can easily cause extrusion deformation and lead to tearing of the tooth profile. When processing plastic materials (such as low carbon steel), if a cutting fluid with good lubricity is not used, built-up edge will occur and scratch the tooth profile. When processing brittle materials (such as cast iron), excessive cutting force can easily lead to tooth fracture. Slippage refers to the thread profile being worn flat or crushed, losing its load-bearing capacity. It is mostly caused by excessive force when tapping or threading, insufficient strength of the workpiece material, or repeated loading and unloading leading to thread fatigue damage. For example, when tapping soft materials such as aluminum alloys, excessive torque can easily cause plastic deformation of the internal thread profile, forming a slippage.
To prevent thread size deviations, first ensure the accuracy of taps and dies, select standard tools, check tool size and wear before use, and replace tools with excessive wear in a timely manner. When tapping and threading, select appropriate cutting parameters based on the workpiece material and thread accuracy. When tapping steel, the cutting speed is 8~15m/min, and when tapping cast iron, the cutting speed is 6~10m/min; the cutting speed for threading is slightly lower than that for tapping, generally 5~10m/min. The feed rate should be uniform. When tapping manually, keep the tap feed vertical to avoid excessive force; when tapping mechanically, ensure that the machine feed rate is consistent with the thread lead to prevent dimensional deviations due to feed errors. During the processing process, regularly check with a thread gauge and adjust the cutting parameters in a timely manner to ensure that the dimensions are within the tolerance range.
Preventing tooth profile skew requires both clamping and operation. When clamping the workpiece, ensure that the axis of the workpiece coincides with the axis of the tap or die. A protractor can be used to correct the perpendicularity of the workpiece end face to the machine tool spindle, or a special fixture can be used for positioning. When tapping, first guide the tap with a guide sleeve to ensure the correct initial position. When tapping manually, use a right-angle ruler to check whether the tap is vertical, and correct it while tapping. When threading, install the die on the die holder, first gently cover the end of the workpiece with the die, and while rotating the die holder, hold the side of the die holder with your hand to ensure that the die is perpendicular to the workpiece. For workpieces with a relatively large length-to-diameter ratio, auxiliary support should be added when tapping or threading, such as using a center stand, to prevent the workpiece from bending and causing the tooth profile to skew. In addition, the shank of the tap or die should be firmly installed to avoid loosening and deflection during processing.
The key to preventing thread rot and thread slippage lies in the proper selection of tools, cutting fluids, and controlled cutting forces. Select taps and dies with sharp cutting edges, and grind the tools before machining to maintain a smooth cutting edge. When machining plastic materials, use extreme pressure cutting oil or sulfurized cutting fluid to reduce friction and built-up edge. When machining brittle materials, use lower cutting speeds and smaller feed rates to prevent thread breakage. When tapping and threading, control the cutting force. Apply even force during manual operation, avoiding fluctuating speeds or forced feeds. During motorized operation, set appropriate torque protection, automatically shutting down the machine when the torque exceeds a threshold to prevent tool damage and thread slippage. For soft material workpieces, drill a bottom hole (for tapping) or pre-machine the external diameter (for threading), leaving an appropriate machining allowance to avoid thread deformation due to material compression. After machining, avoid repeated assembly and disassembly of threaded fittings to prevent fatigue damage. These preventative measures can significantly reduce scrap rates for tapping and threading, ensuring thread quality.
