Special taps for aluminum are prone to breakage during tapping due to the low melting point, high plasticity, and tendency to stick to the tap. Effective prevention requires a comprehensive approach, addressing material compatibility, process optimization, equipment adjustment, operational procedures, cooling and lubrication, tool maintenance, and emergency handling.
Material selection for special taps for aluminum is fundamental to preventing breakage. Because aluminum is relatively soft but highly adhesive, ordinary taps are prone to sticking due to the fusion of the cutting edge with the aluminum, leading to tap breakage. Therefore, taps specifically designed for aluminum must be used. These taps typically utilize high-hardness, high-wear-resistance materials such as cobalt-containing high-speed steel or cemented carbide, and undergo special coating treatments, such as titanium nitride coatings, to enhance their anti-sticking ability and surface hardness, reducing built-up edge formation and thus lowering the risk of breakage.
Optimization of process parameters is crucial for preventing tap breakage. During tapping, the cutting speed, feed rate, and spindle speed must be adjusted according to the specific aluminum material and tap specifications. Excessive cutting speed leads to heat buildup, softening the aluminum and exacerbating tap sticking; insufficient cutting speed increases cutting force due to poor cutting, potentially causing tap breakage. Excessive feed rate also increases cutting force, exceeding the tap's load-bearing limit; insufficient feed rate may result in thin chips that are difficult to remove, clogging the chip flutes. Therefore, it is necessary to determine the optimal combination of process parameters through experimentation to ensure a smooth tapping process.
The accuracy and stability of the equipment are also crucial factors affecting tap life. The radial runout and axial movement of the machine spindle must be controlled within a minimal range to ensure coaxiality between the tap and the bore, preventing tap breakage due to uneven loading. Simultaneously, the positioning accuracy and rigidity of the fixture must meet requirements to prevent tap skewing or vibration during tapping. For deep hole tapping, it is recommended to use guide sleeves or floating chucks to reduce tap skewing and additional bending moments.
Following operating procedures is equally essential to preventing tap breakage. Operators must receive professional training and be familiar with the usage and precautions of special taps for aluminum. Before tapping, ensure the pilot hole diameter meets requirements to avoid excessive cutting load due to an undersized pilot hole. During tapping, maintain a uniform cutting force, avoiding excessive or fluctuating force. Simultaneously, reverse the tap promptly to remove chips and prevent them from clogging the chip flutes. For blind hole tapping, pay special attention to controlling the tapping depth, avoiding continued turning when the tap reaches the bottom or chips are pressing against it.
The selection and use of cooling lubricants are crucial in preventing tap breakage. When tapping aluminum, use dedicated aluminum tapping oil or coolant. These lubricants have excellent cooling, penetrating, and extreme pressure properties, effectively reducing cutting temperature, decreasing friction, and preventing the aluminum from melting into the tap. Ensure sufficient and even supply of cooling lubricant to the cutting area to maximize its effectiveness.
Tool maintenance and replacement are also important aspects of preventing tap breakage. Special taps for aluminum will gradually wear down over time, leading to increased cutting forces and poor chip removal. Therefore, it is necessary to regularly check the wear condition of the taps and replace severely worn taps promptly. Simultaneously, a tap life tracking system should be established to record the number of machining operations and usage time of each tap, enabling timely replacement and preventing overuse.
In the event of a tap breakage, immediate emergency measures must be taken to minimize damage. First, press the emergency stop button and disconnect the machine tool power to prevent injury from flying debris. Then, using protective equipment such as gloves and goggles, select the appropriate removal method based on the breakage condition, such as mechanical, electrical discharge machining, or chemical dissolution. After removing the broken tap, the workpiece must be quality assessed to ensure that the internal thread accuracy meets requirements.
