Tapping difficulty of titanium alloy

May 30, 2023

Titanium alloy tapping is the most difficult process in titanium alloy cutting, especially tapping small threads.This difficulty is mainly manifested in the large total torque during tapping, which is about twice that of No. 45 steel; the tap teeth wear out too quickly, chipping, and even being “bitten to death” in the threaded hole and broken.This is due to the fact that the elastic modulus of titanium alloy is too small, and the threaded surface produces a lot of rebound, which increases the area of contact between the tap and the workpiece, causing a lot of friction torque and increased wear; in addition, the chips are small and not easy to bend, and there is a sticky knife phenomenon, which makes chip removal difficult.Therefore, the key to solving the problem of titanium alloy tapping is to reduce the contact area between the tap and the workpiece during tapping.

01
Ordinary tap
Titanium alloy threads must be processed technically before they can be tapped.The measures for the treatment of ordinary taps are: increase the chip space and reduce the number of teeth; after leaving a 0.2~0.3mm knife belt on the calibration teeth, increase the rear angle to 20°~30°, and grind off the middle part of the tooth back along the full length of the tap; after retaining 2~3 buckle calibration teeth, increase the rear inverted cone from 0.05~0.2mm/100mm to 0.16~0.32mm/100mm.When other conditions are exactly the same, if the width of the tooth back is reduced (worn off) by 1/2 to 2/3, the tapping torque will decrease by 1/4 to 1/3.

02
Correction tooth tap

The correction tooth tap is to change the forming method of the standard tap to the gradual forming method of processing the thread. The working principle is shown in Figure 7-6.As can be seen from the figure, the tooth angle α0 of the corrected tooth tap is less than the thread tooth angle α1, so that the tooth side of the tap and the side surface of the cut thread form a side gap angle φ=(α1-α0)/2, and the tap thread is made into a larger inverted cone, which greatly reduces the friction torque and is also conducive to the cooling and lubrication of the cutting fluid.

The inverted cone of the standard tap starts from the calibration tooth, and the amount of inverted cone is (0.05~0.2)mm/100mm; the inverted cone of the corrected tooth tap starts from the first cutting tooth, and the inverted cone value is much larger than the standard tap. For example, the corrected tooth tap with kcr=7°30' can reach 1.437mm/100mm.Due to the increase in the amount of inverted taper, the calibration part of the correction tooth tap cannot play a guiding role. When cutting the front end of the cone, a cylindrical guide part must be made to avoid skew when the tap is just tapped. The nominal size and tolerance of the cylindrical guide part depend on the size of the bottom hole before tapping.

03
Jumping tap
The vibrating tap removes the screw buckle between the cutting tooth and the calibration tooth. Its biggest feature is that the contact area between the tap and the workpiece is effectively reduced, and the tapping torque is significantly reduced.Due to the tapping of the interdental teeth, there is a wide space between the side edges of the adjacent screw buckles, which improves the conditions for chip capacity and cutting fluid to enter the cutting area, and improves the durability of the taps. At the same time, when manufacturing taps, the top of the outer edge of the grinding wheel does not need to be too sharp, which improves the grinding conditions.After testing and comparison under the same cutting conditions, the tapping torque of the vibrating tap is about 30% to 50% of the standard tap and 35% to 60% of the corrected tooth tap. The durability is 1 to 3 times higher than that of the corrected tooth tap, and the tapping effect of the vibrating tap on titanium alloy is the best.

04
Threaded bottom hole
For titanium alloy tapping, the diameter of the bottom hole is generally selected based on the tooth height rate (the ratio of the actual tooth height of the screw hole to the theoretical height) does not exceed 70%, that is, the diameter of the threaded bottom hole d1=d0-0.7578p (d0 is the nominal size of the thread, p is the screw moment).The high thread rate of small diameter or coarse thread thread can be larger. When the strength of the processed material is low or the thread depth is less than the basic diameter of the thread, the high thread rate can be appropriately increased, but the tapping torque can be increased too much, and even the tap can be broken.In order to ensure the tapping accuracy and surface quality, the threaded bottom hole should be the hinged hole.

The tapping speed of titanium alloy should be determined according to the type and hardness of the material.The tapping speed of α-titanium alloy generally takes Vc=7.5~12 m/min, α+β-titanium alloy takes Vc=4.5~6 m/min, and β-titanium alloy takes Vc=2~3.5m/min; when the hardness of titanium alloy is ≤HB350, a higher cutting speed is selected, and a lower cutting speed is selected on the contrary.When tapping titanium alloys, extreme pressure cutting fluids containing Cl and P are generally used to have a better effect, but extreme pressure cutting fluids containing Cl must be cleaned after tapping to prevent intergranular corrosion of parts; mixed oils of 60% castor oil and 40% kerosene can also be used as cutting fluids.

Drilling is semi-enclosed cutting. The cutting temperature is very high during the drilling of titanium alloy, and the rebound is large after drilling. The drill cutters are long and thin, easy to bond and not easy to discharge, which often causes the drill bit to be bitten, twisted and other vicious accidents.Therefore, the drill bit is required to have high strength and good rigidity. The chemical affinity between the drill bit and the titanium alloy should be small. It is best to use cemented carbide drill bits, but the most commonly used is still twist drills. After taking some measures to improve, better results can be achieved.