Fig.1 Structure of a comb comb cutter
Fig.2 Schematic diagram of thread formation when milling comb cutters for internal thread
1 Introduction Cyclone Milling Threads are an advanced, high-efficiency machining method that uses a comb-type milling cutter with a taper shank on a special thread milling machine to machine high-speed internal and external threads or taper threads with short pitches. The structure of the comb cutter can be seen as a combination of several disk-shaped thread milling cutters. The cutter teeth are ring-shaped, no cutting cones, the load of each cutter is even (see Figure 1), the milling cutter and the workpiece axis are processed. In parallel, the workpiece only needs to rotate one revolution, and at the same time, the milling cutter moves one pitch along the axis of the workpiece, and all the threads can be milled. It has been widely used in the processing of cartridge thread. The cartridge thread is generally a special British serrated internal thread, which functions as a base fire and a sealed primer chamber. Its processing accuracy and processing quality will directly determine the assembly quality of the primer chamber. Once there is a quality problem, the serious consequences of smoke leakage in the primer chamber and fall-off of the primer can occur at the high temperatures and pressures generated when the cartridge is used. Therefore, the design and processing of the comb cutter used for the cyclone milling cartridge thread is very important, especially the design of the outer diameter and the top height of the comb cutter. To this end, we did some research and discussion, and verified the results of the research through production practices. 2 Problems in the design of the comb milling cutter When the cyclone milling cutter thread, the comb milling cutter axis is parallel to the axis of the cartridge, and the annular tooth is perpendicular to its axis, so the rotating edge formed by the rotation of the cutting edge and the cartridge The helix of the thread forms an angle equal to the helix angle. In the cross section, the shape of the comb cutter is always a circle, and the profile of the cartridge thread profile is an Archimedean spiral with an axis as the pole (see Figure 2). In this way, the intersection of the profile line of the cutter tooth and the outline of the thread profile is not on the combliner of the comb cutter and thread, but is offset to one side (as shown in points a, b, and c in Fig. 2) and varies with the profile. Its position is also changing. Because these intersections are on the edge of the comb cutter, the cutting edge formed by these intersections in each section is not a straight line but a curved line; but in practice, in order to facilitate the manufacture, the cutting edge is often made into a straight line so that the milled thread There will always be a certain amount of distortion. This error is allowed for the cartridge thread and is negligible. The barrel thread is the internal thread of a special profile, so the outside diameter of the comb cutter is not only related to the outside diameter and pitch of the cartridge thread, but also closely related to the thread profile and allowable distortion values. If it is too large, it can easily bump the cartridge and damage the accuracy of the machine tool; if it is too small, it can easily cause the tool to fluctuate radially, making the surface of the processed thread appear fishy ripples, and even uneven strips appear at the threaded joint. Therefore, it is very important to reasonably determine the outer diameter of the comb cutter. The outer diameter design of the comb cutter has historically used the former Soviet Union's cutting tool expert Xie Mingqinke, T.A. Alekseikov’s related works and technical data provided by T.A. Aleksei也ov. It is believed that the outside diameter D of the comb-type milling cutter used for milling internal threads cannot be greater than 0.9 times the inside diameter d of the processed thread; XIEMING CHINKO believes that the outside diameter D of the comb-type milling cutter should be 0.85 to 0.95 of the inside diameter d of the processed thread. Times; and some technical data experience the comb cutter diameter D is taken as the thread diameter d of the processing of 0.643 ~ 0.65 times. These values ​​are of different quality problems in the design application. Based on the theoretical research and design practice, we have put forward a calculation of the outer diameter of the comb-type milling cutter for threading of the cartridge through a large number of production practices. Formula: D=(0.7~0.8)dD Where: D——Diameter tolerance of comb milling cutter, usually D=0.2~0.6mm This formula solves the design problem of the comb cutter's outer diameter. , To meet the quality requirements of cartridge threading.
(a) Effect of wear
(b) Effect of comb profile
Figure 3: Influence of wear volume and comb profile on the height of the teeth The determination of the height Hd of the comb-type milling cutter should take full account of the effect of the allowable wear amount dhd (see Figure 3). The purpose is to ensure that the required height of the comb cutter after the flank wears and the threads are still processed, so that the tooth height of the comb cutter must be designed to be larger than the thread height by a permissible amount of wear dhd. We have obtained through a lot of design and processing practices that the relationship between the pitch s of the thread to be machined is: dhd=0.03S0.75mm is more appropriate; the manufacturing tolerance of the comb cutter is dhd=0.0163S1/3. The tooth tip height dd can be calculated as follows: Hd=hg+dhd+Dh+d Where: hg—root height of the thread; Dh—allowed displacement of the comb cutter: Dh=(b-b' )/[2tg(e/2)] where b—thread groove width at bottom; e—thread groove angle; b′—cutter tooth width at top b′=(0.75 to 0.8)b
Fig. 4 rake face tooth profile of comb cutter
3 Analysis of the problems in comb milling cutters If the comb comb cutter has a rake angle, its tooth profile in the shaft section and the tooth profile on the rake face are different from those in the workpiece profile. Compared with g=0°, a certain processing error occurs due to distortion, but this error is not significant, especially when g<5°, the error value is smaller. Therefore, it is generally only necessary to perform correction calculations or table correction calculations on tooth profiles in the profile of the comb-type milling cutter (g>5°) in the machining of the steel cartridge (g>5°), and it can also be used as a map method. Find the tooth profile. The tooth profile correction of the rake face of the comb cutter is corrected by the forming cutter to ensure that the axial tooth profile angle of the rotary surface is not changed. From the geometric relationship in Figure 4, we can see: sinb1=Rsingp/(R-h10) sinb2 =Rsingp/[R-(h10+h20)] h10g=[(R-h10)sin(b1-gp)]/singph20g=[(R-(h10+h20)]sin(b2-b1)]/sinb1 : tg(ag/2)=tg(a/2)=(h10+h20)singp/{[R-(h10+h20]sin(b2-gp)} where: R—cutting cutter radius; gp - comb cutter top edge rake angle
a - thread profile angle; ag - cutter face rake angle
H10,h20——The tooth profile height and tooth root height
H10g, h20g—— tooth height and root height
The comb cutter of the cyclone milling cartridge thread has a small tooth profile angle. In order to achieve the normal working angle of the side blade relief angle a0, the oblique cutter teeth are used, so that the comb cutter teeth form two teeth and the teeth The top must be machined in a single adjustment to ensure that the tooth profile remains the same after regrinding the comb cutter. When the straight-groove comb milling cutter whirls on the milling thread, the cutting is not smooth and the vibration is large, resulting in the surface of the milled thread with uneven ripples, especially when the cutting length L of the comb-type milling cutter is large, the impact is most serious. In order to improve the cutting condition, the spiral groove structure can be used. The spiral angle is usually 5° to 8°. However, the straight groove type is easy to manufacture, sharpening and inspection are convenient, and the cartridge thread length is shorter (l<20mm). Therefore, we believe that comb-milling cutters for milling cartridge barrels can use straight-groove type to achieve processing requirements and are economically viable. 4 Conclusion
In this paper, two important design parameters of the outer diameter and the top height of the spiral comb milling cutter of the cyclone milling cartridge were studied and discussed. The calculation formulas and empirical values ​​were proposed, and the main problems involved in the processing were analyzed. We apply these research results to the design and manufacture of comb cutters. The production practice from the cartridge thread (brass cartridge thread diameter d=13~30mm; steel cartridge thread diameter d=13~32mm) shows that the tool design parameters can meet the processing quality requirements and can be used in production. use.
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