In recent years, manufacturing methods using high hardness (HRC50 and above) blank finishing dies have become increasingly popular. It has been difficult to use high-speed steel taps to make threaded holes in high-hardness die steels. For this purpose, Japan's Yamaha Engineering Co., Ltd. has developed UH-CT-type hard alloy taps for machining high-hardness dies.
1. Features of UH-CT Carbide Taps
The main features of UH-CT carbide taps are listed in the table below. In order to adapt the tap material to the processing requirements of high hardness (HRC50-60) materials, a hard alloy material having a sufficient hardness difference with the material to be processed is used; under the premise of anti-collapsibility and chipping resistance, it is adopted. The ultra-fine particle cemented carbide material with reasonable hardness and toughness; In order to adapt to the high processing torque of the high hardness material processing, a tool holder (enhanced tool holder) larger than the conventional handle diameter is used; Tool tip and improved wear resistance, while using special grooves and cutting angles, increases the number of grooves to reduce the amount of cut per tooth; heat resistance and chemistry are used to improve wear resistance and anti-adhesive properties TiAlN surface coating with excellent stability. The UH-CT tap with the optimized design described above has a significantly better cutting performance than ordinary carbide taps.
Table UH-CT Carbide Tap Features
Material: Under the premise of considering wear resistance and impact resistance, adopt ultra-fine particle cemented carbide shape: In order to improve the rigidity, use the enhanced tap taper groove without neck: in order to obtain good chip and improve wear resistance, Matching groove number and groove shape cutting angle: In order to increase rigidity and prevent chipping, special rake angle accuracy is adopted: taking into account the shrinkage of internal thread, adopt moderately enlarged dimension cut length: considering stability and Life, the length of the cutting part is limited to the diameter of the tip of 5 teeth: To ensure the actual cutting length, a large tap diameter is used to process the material hardness: HRC50～HRC63
Recommended bottom hole diameter: JIS 6H (Max. inner diameter recommended)
N-CT-FC Carbide Taps and UH-CT Carbide Taps for Cast Iron Machining SKD11 Steel Die (HRC60) Tapped Holes (Processing Conditions: Tap Size: M8×1.25; Workpiece Material: SKD11(HRC60); Bottom Hole diameter: φ6.9mm; Machining length: 16mm; Machine tool used: High rigidity CNC tapping machine; Cutting speed: 15m/min; cutting fluid: water-insoluble oil (including extreme pressure additive)), Tool life comparison results show: After the N-CT-FC tap processing 8 holes, chipping occurs and the service life is reached; UH-CT taps have not broken or chipped after machining 60 holes, and can continue processing, processing UH-CT taps for 60 holes. No significant damage was found on the cutting edge. Although the UH-CT tap has a high strength, there is still the risk of damage due to accelerated cutting wear. Therefore, it is necessary to set a safe life standard.
2. UH-CT Carbide Tap Usage Notice
Although the UH-CT tap has excellent performance, due to the very severe tapping conditions of the high-hard tool steel, in order to make the tapping process proceed smoothly, the following matters must be noted:
(1) Since UH-CT taps are not suitable for manual tapping, it is best to use a machine with low vibration and a reasonable amount of feed. (In order to realize stable tapping, it is recommended to use a machining center machine with a fully synchronous feed function.) .
(2) The rigid tap chuck and workpiece holder should be used. Especially for tap chucks, it is best to use special feed chucks and milling fixtures.
(3) The diameter of the bottom hole of the threaded hole is as large as possible within the allowable range (6H maximum internal diameter is recommended). UH-CT taps are based on the recommended bottom diameter to design the diameter of the tap tip. If the diameter of the bottom hole is too small, the tip of the tap is concentrated due to the cutting load, which may cause chipping.
(4) It is recommended to use a non-water-soluble lubricant with excellent lubricating properties.
(5) The cutting speed varies depending on the tapping conditions. According to the hardness of the material being processed, the recommended tapping speed is as follows: Material hardness HRC 50~55: tapping speed 2~6m/min; Material hardness HRC 56~60: tapping speed 1~4m/min.
3. Combination of UH-CT Carbide Taps and Precision Tapping Machines
When UH-CT hard alloy taps are used to machine high-hard-steel threaded holes, if the combination of tap and tapping machine is unreasonable, it will cause the tap to break, which will cause the die to be scrapped or need to be repaired, causing losses to the user. The Microtap precision tapping machine sold by Yamaha Company has a special function of preventing the tap from breaking, and it is very effective in combination with the UH-CT carbide tap. Microtap tapping machine has a real-time detection self-identification function. Once the tapping torque exceeds the preset maximum control torque, it will automatically stop and reverse the reverse. When the momentary tapping torque increases due to chip bite, etc., as long as the preset maximum control torque is less than the tapping torque of the tap, the tap break can be avoided.
Machining SKD11 (HRC60) die steel tap holes with M8×1.25 UH-CT carbide taps (machining conditions: tap size: M8×1.25; workpiece material: SKD11 (HRC60); bottom hole diameter: φ6.9 mm; machining Length: 16mm; Machine: Microtap G16 precision tapping machine; cutting speed: 2.5m/min; cutting fluid: water-insoluble oil (including extreme pressure additive)), the measured torque curve shows that the maximum cutting torque is 1020Ncm The average torque is about 700 Ncm. At this time, the discharged chips are finely crushed and the tapping is smooth without abnormal noise. In the event of a chip bite, tapping torque will increase significantly. The maximum tapping torque of a UH-CT tap is generally 1000 Ncm, and the breaking torque is about 4500 Ncm. If the maximum control torque is set to about 2000 Ncm, the tap can be effectively prevented from being broken. The combination of UH-CT carbide taps and Microtap precision tapping machines enables fast and reliable tapping of high-hardness steel dies.