In order to meet the special requirements of IT (mainly titanium alloy nuts for smartphones) and aviation companies, the key to solving the above processing problems is to improve the processing performance of taps for titanium alloy internal thread tapping. The parameters are optimized for design and appropriate processing conditions are adopted. It is a very important measure to improve the structure of the tap and the tool parameters. At the same time, the appropriate tap is designed and selected according to many different conditions such as the form of the workpiece screw hole, the accuracy of the screw hole, and the conditions of the machine tool.
1. Selection of tap material
Cemented carbide is used as a tool to improve productivity. A fine grain tungsten-cobalt cemented carbide with small affinity for titanium alloy, good thermal conductivity and high strength should be selected. At the same time, the tap can also use high-speed steel with good high-temperature performance: W2Mo9Cr4VCo8 (M42), W6Mo5Cr4V2AI (501) and powder metallurgy high-speed steel. Secondly, in order to improve the performance of the tap, the surface can also be strengthened by coating. TiAlCN, TiCN, TiAIN, TiN and other coatings can be selected. The coated tool has a hard layer, good wear resistance, and a small coefficient of friction with the processed material, but does not reduce the toughness of the matrix material and the durability of the tool Significantly increased.
Second, the design of the tap structure
In terms of tap structure, first of all, the rigidity of the tool material should be improved, and good materials combined with appropriate cutting tool forms can exert good performance. A variety of forms such as straight flute taps, jump tooth taps, and modified tooth angle taps can be selected as required.
1. Straight fluted tap
(1) The influence of taper rake angle and rake angle on its workability.
According to the observation of the production site, when processing titanium alloy, the titanium alloy is prone to elastic deformation and elastic recovery when being cut, which increases friction and cutting force, accelerates tool wear, and reduces the accuracy of the cut thread. The rake face of the wire attack is severely broken, and most of them occur near the guide part on the cutting cone. At first, a single screw tooth collapses, and then the rake face of several screw teeth continues to collapse. This is due to the fact that the cutting edge at this location is close to the outer diameter of the thread, the cutting teeth become sharp, and the strength is weakened. When the front angle is large, the strength is weakened more obviously, and the force of the blade is concentrated on the part between the teeth, 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, and a smaller rake angle should be selected. For this reason, the rake angle is about 5 °.
Improper selection of the relief angle also has a certain effect on the collapse of the tap. If the clearance angle of the cutting cone is too large, the chips that have not fallen off and the small chips that have fallen off during the tapping of the tap are easily squeezed into the back of the blade instead of being cut , Strong friction and squeezing with 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 tap sticking, and improve thread surface quality, the back angle of taps for processing titanium alloys was increased to 4 ° ~ 8 °. At the same time, the entire tooth profile surface of the tap should be fully toothed back to ensure that the back angle of the tooth side is 30 ′ ~ 40 ′.
(2) When tapping through holes in common materials, the cutting tap length L5 = 2P, cutting taper Kr = 17 °, cutting thickness per tooth ac = P · sinKr / N (N is the number of teeth), N = 3, When P = 1, ac = 0.09745mm. When tapping through titanium alloy through holes, in order to reduce the tapping cutting force, select the length of the cutting cone L5 = (10 ~ 20) P. In order to ensure the smooth introduction of the threaded bottom hole, select the cutting taper according to tanKr = (0.7 ~ 0.8) P / L 5, that is, Kr = 4 ° ~ 2 °, the cutting thickness per tooth: N = 3, when P = 1, ac = 0.023 ~ 0.012mm; the cutting thickness of each tooth is greatly reduced. c. In order to reduce the friction and the expansion of the threaded hole, increase the back angle of the cutter teeth of the calibration part, and increase the reverse taper of the tap for processing titanium alloy to (0.04 ~ 0.08) mm / 25mm.
(3) The fine cone is a precision CNC tool for cutting the final complete tooth shape during tapping. In order to improve the dimensional accuracy and surface quality of the titanium alloy tapping thread, the dimensional accuracy and surface quality of the fine cone must be improved to reduce cutting Load.
(4) The influence of tap groove shape on processing. The shape of the chip flute has a great influence on the shape of the chip. The chip flute should ensure the smooth curling of the chip and have a large chip space. This is especially important for blind hole tapping.
2. Jumping tooth tap
(1) In order to reduce the friction force between the tap and the thread cut on the workpiece, in the working part of the straight flute or spiral flute tap, the teeth on the same blade are removed every other tooth, and then adjacent to the same spiral line direction 2 The tooth removal parts on the blade flap are staggered by one tooth, and this tap becomes a jump tooth tap. The space left due to the removal of teeth at intervals leaves room for the elastic deformation of the titanium alloy workpiece, reducing the friction between the surface of the tap thread and the surface of the workpiece thread, reducing the tapping torque and reducing the sticking phenomenon, and improving the cutting Conditions; at the same time, the space left by removing the spacing 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. Using this improved tap to process the thread on the titanium alloy improves the thread processing and the effect is very obvious.
(2) When processing the tooth jump tap, the tooth removal form is: keep the thread of the cutting cone unchanged (that is, do not change the cutting pattern and cutting thickness), and start from the second complete tooth after cutting the cone to form the calibration part Each tooth of the helix is ​​removed by one turn, so that the cutting conditions of the tap are improved, and the welding phenomenon in the calibration part can be avoided. When removing teeth, the maximum diameter of the remaining part of the tooth shape is approximately equal to the small diameter of the tap or allowed to be greater than 0.1 mm, preferably not greater than the diameter of the bottom hole before tapping. Care should be taken to avoid the grinding wheel touching the tooth surfaces of the tooth profile on both sides.
(3) At the same time, it should also be noted that the meridian should be measured with a three-needle before the tooth removal of the tap, otherwise it is difficult to measure. Because there is no standard three-needle that is tangent to the tap near the middle meridian and does not touch the residual tooth shape after removing the tooth.
3. Correct tooth profile angle tap
(1) The tooth shape angle of an ordinary tap is equal to the tooth shape angle of the thread profile to be processed. In this way, the clearance angle of the tap side blade is 0. When cutting a titanium alloy workpiece, due to elastic deformation and elastic recovery, the tap side blade and the workpiece have greater friction and the cutting torque is larger. To improve this situation, a modified tooth angle tap can be used.
The tooth profile angle of the modified tooth profile angle tap is smaller than the tooth profile angle of the thread to be processed. That is, when the thread profile angle is 60 °, the profile angle of the modified tooth profile tap is generally 55 °. Because the tap adopts a 55 ° tooth profile angle when machining a 60 ° tooth profile thread, it can ensure that there is sufficient backlash between the tap and the workpiece tooth profile, which is used to improve the elastic deformation and elastic recovery of the workpiece to reduce friction and reduce cutting forces. decline.
(2) Taper of the thread part of the tap.
The tooth shape angle of the modified tooth shape angle tap is smaller than the tooth shape angle of the processed thread. Through theoretical analysis, in order to make the tap to process the correct thread shape of the processed thread, the tip of each cutting tooth of the tap must fall on the processed Thread profile surface. For this reason, when grinding the tooth shape of the tap, the last tooth on the tap cutting cone must be higher than the first tooth, so that the teeth of the cutting cone are raised to the same plane for grinding, so that the thread profile of the taper cutting taper has A gradually increasing larger taper, that is, the required reverse taper, this is the biggest difference between the modified toothed angle tap and the ordinary tap. The thread profile of the cutting tap of the ordinary tap is a gradually decreasing taper. When machining titanium alloy, the cutting cone angle is 6 °, and the reverse cone angle of the modified toothed angle tap can be 39 ′.
(3) The thread diameter of the tap.
After grinding the taper with reverse taper, the median diameter of each tooth is different. Generally, the median diameter of the first calibrated tooth is measured, using a 55 ° angle with three needles. The calculation formula of the middle diameter when processing titanium alloy is: d20 = d2-0.07P.
This tap is to change the thread forming principle from the standard tap forming method to the progressive method. When this type of tap is used to tap threads on a titanium alloy, the cutting torque can be reduced by 30% to 35% compared to ordinary taps Compared with the jumping tooth tap, it can be reduced by 20% to 25%. This kind of tap requires high precision and is more complicated to manufacture.