The aviation field is inseparable from the thread gauges, especially for the thread tools. For example, the materials to be processed are difficult to process materials, the workpiece precision requirements are high, and the cutting speed required for machining is high. At the same time, the large-scale use of CNC machine tools is required. Matching tools. These use characteristics require tool companies to develop unique thread cutters that are uniquely suited to difficult-to-machine materials.
Titanium alloy has become one of the most important engineering materials in the aerospace industry and has gained wider and wider application. However, titanium alloy is a typical hard-to-machine material, and its cutting performance is relatively poor. On titanium alloy materials, it is especially difficult to tap the internal thread with a tap, and an optimized design is needed.
First, the processing characteristics of titanium alloy
1 Titanium alloy material characteristics
Compared with other metal materials, titanium and its alloys have three significant advantages: high specific strength, good medium temperature performance and corrosion resistance. At room temperature, the specific tensile strength of titanium alloy is 1.26 times that of high strength structural steel and 1.38 times that of high strength aluminum alloy. In the temperature range of 400-550 °C, the titanium alloy has better specific strength, specific creep strength and specific fatigue strength than heat-resistant stainless steel.
Titanium alloys can be classified into an α phase titanium alloy, a β phase titanium alloy, and an (α + β) phase titanium alloy from a metal structure. The hardness and strength increase in the order of the α phase, the (α + β) phase, and the β phase, and the machinability decreases in this order.
2 titanium alloy machining characteristics
Titanium alloy hardness is more than 350HBS or 300HBS is difficult to cut, but the difficulty is not in the hardness, but in the mechanical, chemical and physical properties of the titanium alloy itself, the cutting characteristics are: deformation coefficient is small; cutting The temperature is high; the main cutting force is small and the back force is large; the chips are squeezed, and the machined surface is easy to form a hard and brittle metamorphic layer; the sticking knife phenomenon is serious.
3 titanium alloy internal thread tapping characteristics
Due to the many characteristics of titanium alloy, internal thread tapping on titanium alloy material is the most difficult process in titanium alloy cutting process, especially for the processing of small hole thread, and it is inseparable from tap tapping. The main performance is that the total torque of the tapping thread is large, about 2 times of the tapping torque of 45 steel; the tap teeth are worn too fast, chipped, and even broken by "biting". The main reason is that the elastic modulus of the titanium alloy is too small, and the surface of the processed thread will have a large rebound, giving a large normal force behind the side of the tap tooth and behind the tooth tip, thereby causing a large friction torque; In addition, the cutting temperature is high, the chip has a sticky knife phenomenon, and the cutting fluid is not easy to reach the cutting area; the machining surface is prone to stress concentration, and a hard and brittle layer is formed, and the surface quality of the machined thread is poor.
Second, the design of taps for titanium internal threads
In order to meet the specific requirements of aviation enterprises, the key to solving the above-mentioned processing problems is to improve the processing performance of taps for titanium internal thread tapping. It is necessary to optimize the geometric parameters of taps for titanium alloy tapping and adopt appropriate processing conditions. It is a very important measure to improve the structure and tool parameters of the tap. At the same time, according to the workpiece screw hole form, screw hole precision requirements, machine tool conditions and other various conditions, the appropriate tap is designed and selected.
1 choice of tap material
Cemented carbide is used as a tool to increase productivity. Fine-grained tungsten-cobalt type hard alloys with low affinity to titanium alloy, good thermal conductivity and high strength should be used. At the same time, the taps can also be selected from high-speed steel with high temperature performance: W2Mo9Cr4VCo8 (M42), W6Mo5Cr4V2AI (501) and powder metallurgy high-speed steel. Secondly, in order to improve the performance of the tap, it is also possible to carry out a coating strengthening treatment on the surface of the tap. TiAlCN, TiCN, TiAIN, TiN and other coatings can be used. The surface of the coated tool has a hard layer, good wear resistance, and a small coefficient of friction with the material to be processed, but does not reduce the toughness of the substrate, and the durability of the tool. Significantly increased.
2 tap design
In terms of tap structure, the rigidity of the tool material should be improved first, and the good material combined with the appropriate cutting tool form can achieve better performance. Straight groove taps, skip taps and modified tooth angle taps can be used as needed.
(1) Straight groove tap
a. The influence of the taper anterior and posterior angles on its processability.
According to the observation of the production site, when the titanium alloy is processed, the titanium alloy is liable to undergo elastic deformation and elastic recovery when it is cut, so that the friction force and the cutting force are increased, the tool wear is accelerated, and the precision of the thread cutting is lowered. The rake face of the tap is seriously damaged, and it occurs mostly near the guide portion on the cutting cone. At the beginning, a single screw collapses, and then the rake face of several threads collapses. This is because the cutting edge of the portion is close to the outer diameter of the thread, the teeth are sharpened, and the strength is weakened. When the current angle is large, the strength is weakened more obviously, and the blade is concentrated on the inter-tooth portion, which is more likely to collapse. Therefore, the reasonable rake angle of the tap should be selected according to the processing characteristics of the titanium alloy material to be processed. The smaller rake angle should be selected, and the rake angle is selected to be about 5°.
Poor selection of the back angle also has a certain influence on the tapping of the tap. If the back angle of the cutting cone is too large, the chips that have not fallen off when the tap returns and the fine chips that have fallen off are easily squeezed from behind the blade instead of being cut off. Strong friction and squeezing on the back of the tap, sometimes causing the collapse of the rake face of the tap. In order to reduce friction and friction torque, avoid taper sticking, and improve the surface quality of the thread, the post-angle of the tap of the processed titanium alloy is increased to 4°-8°. At the same time, the entire tap profile should be fully shovel back, ensuring that the back angle of the flank is 30'~40'.
b. When tapping the through hole of common material, the length of the cutting cone of the tap is usually selected as L5=2P, the cutting taper is Kr=17°, and the cutting thickness per tooth is ac=P·sinKr/N (N is the number of teeth), N=3,P When =1, ac=0.09745mm. In order to reduce the tapping cutting force during the tapping of titanium alloy through-hole, the length of the cutting cone is selected as L5=(10-20)P. In order to ensure the smooth introduction of the thread bottom hole, the cutting taper is selected according to tanKr=(0.7~0.8)P /L 5 , that is, Kr=4 °～2 °, the cutting thickness per tooth: N=3, P=1, ac=0.023 ~0.012mm; the cutting thickness per tooth is greatly reduced. c. In order to reduce the amount of friction and expansion of the threaded hole, increase the back angle of the calibrated part, and increase the reverse taper of the processed titanium alloy tap to (0.04 to 0.08) mm/25 mm.
d. The precision cone is a finishing tool that cuts out the final complete tooth shape during tapping processing. In order to improve the thread size accuracy and surface quality of the titanium alloy tapping, it is necessary to improve the dimensional accuracy and surface quality of the precision cone and reduce the cutting load.
e. The influence of the groove shape of the tap on the processing. The shape of the tap chip flute has a great influence on the shape of the chip. The chip flute should ensure smooth curling of the chip and a large space for chipping. This is especially important for blind hole tapping.
(2) Jumping tap
a. In order to reduce the friction between the tap and the cut thread on the workpiece, in the working part of the straight groove tap or the spiral groove tap, the teeth on the same blade are removed every other tooth, and then adjacent to the same spiral direction 2 The teeth on the blade are staggered with one tooth, and the tap becomes a tapping tap. The space left by the spacing of the teeth leaves room for the elastic deformation of the titanium alloy workpiece, reduces the friction between the tap thread surface and the thread surface of the workpiece, reduces the tapping torque and reduces the sticking phenomenon, and improves the cutting. Conditions; at the same time, the space left by the spacer teeth can accommodate a part of the cutting fluid, so that the cooling is sufficient, the cutting heat is reduced, and the cutting conditions are further improved. The use of this improved tap to machine the threads on the titanium alloy improves the threading process and the effect is very significant.
b. When machining the jumper tap, the tooth removal form is as follows: the cutting taper thread is kept unchanged (ie, the cutting pattern and the cutting thickness are not changed), and the calibration part is formed from the second complete tooth after the cutting cone The teeth of the line are removed one turn at a time, so that the cutting conditions of the tap are improved, and the welding phenomenon in the calibration part can be avoided. When the tooth is removed, the maximum diameter of the residual portion of the tooth shape is approximately equal to the diameter of the tap or allowed to be larger than the small diameter of 0.1 mm, preferably not larger than the diameter of the bottom hole before tapping. Care should be taken to avoid the grinding wheel hitting the tooth flanks on both sides.
c. At the same time, it should be noted that the middle pass should be measured with three needles before the tap is removed, otherwise it will be difficult to measure. Because it can not find the standard three-pin which is tangent to the tap near the middle meridian and does not touch the residual tooth after the tooth removal.
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