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2 The main structural parameters of the tool are designed and calculated. The micro-miniature gear milling cutter module (0.1mm) is only 1/3 of the national and mechanical industry standard lower limit. Therefore, the smaller-sized hard alloys produced by the existing stereotypes are selected. The milling cutter blank (see Table 1) was manufactured. Since the diameter of the milling cutter is smaller Deg=8mm, considering the processability of the manufacturing process is mainly the problem of retracting the grinding wheel during the shovel grinding process, and also facilitating the grinding of the front blade surface, the number of teeth is unlikely to be too much, so the number of teeth is selected zg=8 Finally, the micro-miniature gear milling cutter has an outer diameter of dao=8mm, an inner hole dio=4mm, a width B=1mm, and a number of teeth zk=8.
2.1 Selection of the rake angle of the tipping blade Since the selected machining method is forming and milling, it is necessary to process the part from the blank to the finished size by a tool during the machining process.
If the micro-small gear milling cutter adopts a 0° rake angle, the profile of the rake face and the axial section is exactly the same as the groove shape of the part in the axial direction, so that the design, processing, inspection and re-grinding of the micro-shaped milling cutter The process is greatly simplified, and the 0° rake angle can meet the requirements of the cutting edge of the milling cutter during the milling process and the requirements of the tooth profile accuracy and surface precision in the finishing process, so it is widely used in the finishing process. The micro-miniature gear milling cutter targets a small amount of machining allowance of only 0.22mm, which is in line with the finishing conditions, and determines that the top edge of the tooth is 0°.
2.2 The selection of the back angle of the cutting edge and the calculation of the backlash amount are to ensure that the cutting edge of the micro-small gear milling cutter has a reasonable side relief angle (αc≥1.33°) during the machining process, and the back angle αa of the tooth top cutting edge should be taken. Larger value. However, if αa is too large, when the back of the tooth is shoveled, the grinding wheel may interfere with the next tooth of the milling cutter, which may cause the milling cutter to be scrapped.
By appropriately reducing the grinding wheel diameter Dw, the interference phenomenon can be avoided, and the back angling angle θ1 of the milling cutter can be ensured, but the grinding wheel diameter Dw can not be too small, otherwise the cutting speed of the shovel is too low, which affects the grinding ability of the grinding wheel. Reduces the efficiency of the shovel, shortens the life of the grinding wheel, and deteriorates the smoothness of the milling cutter, thus affecting the machining accuracy. In order to ensure that the milling cutter has a certain number of regrinds, according to the number of teeth of the milling cutter zk=8, the back angle of the shovel teeth is selected as θ1=14°. Take the diameter of the grinding wheel Dw=20mm (this value is the minimum diameter of the current grinding wheel). Under the AutoCAD2004 environment, draw the grinding wheel and milling cutter in the following three cases: the back angle αa of the tooth top blade is 10°, 10.5° and 11°. The extreme positional relationship of the teeth is shown.
When the back angle αa=10° of the tooth tip blade, the distance between the outer circle of the grinding wheel and the milling cutter is larger than the subsequent tooth, which is 0.074 mm, which is close to the module modulus of the milling cutter and is about 1/3 full tooth height. When the back angle αa=11° of the tooth tip blade, the distance between the outer circle of the grinding wheel and the subsequent tooth of the milling cutter being shoveled is too small, which is 0.007 mm, and there is no guarantee that the two will not interfere during the actual shovel grinding process. According to the domestic cemented carbide shovel equipment and processing level, the back angle αa=10.5° of the tipping edge is determined.
The calculation of the backing amount of the milling cutter mainly considers the outer diameter of the milling cutter, the number of teeth of the milling cutter, the back angle of the cutting edge of the cutter blade, etc. The calculation formula is K=πdaozktanαa(1), dao=8mm, zk=8, αa=10.5° substituting (1) The amount of backing of the micro-forming milling cutter is K=0.58mm.
2.3 The determination of each parameter of the chip flute is based on the design specification of the forming cutter of the forming cutter with the modulus of 0.3mm~1.5mm, and the appropriate adjustment is made to give the principle of determining the depth of the chip flute.
H≈h K (0.3 to 0.6) mm (2) where: h is the tooth height.
The micro-miniature gear milling cutter has a tooth height h=0.22mm, and the depth of the chip pocket is H=1.25mm.
Due to the shovel grinding back angle θ1=14°, the bottom of the chip flute groove and the end of the milling cutter tooth shovel can be obtained. Using Auto-CAD2004, the chip groove angle θ=76.0° was measured, and the measurement accuracy was 0.1°.
The type of chip bottom is divided into two types: linear type and broken line type. The linear groove bottom is easy to process and manufacture, and the broken line groove bottom is beneficial to improve the strength of the milling cutter. Due to the relatively small tooth profile of the micro-machining milling cutter, the cutting force is very limited during the milling process, and the strength of the cutter is sufficient to ensure the selection of the linear groove bottom.
The calculation of the radius of the bottom of the chip is mainly considering the factors such as the outer diameter of the milling cutter, the number of teeth of the milling cutter, the depth of the chip pocket, etc., and the values ​​of dao, zk, H, etc. are substituted by the equation (3), and the bottom arc of the chip pocket is calculated. The radius rg=0.21 mm, rounded to 0.20 mm.rg=π(dao-2H)10zk(3) In summary, the structure of the disk gear milling cutter is shown.
3 Micro-miniature gear milling cutter tooth shape design The target part is a standard modulus gear, so the milling cutter tooth shape is a standard involute gear milling cutter. The tooth profile of the milling cutter consists of three parts: the involute part of the base circle, the transition curve part and the arc of the tooth top. There are three methods for determining the tooth shape: calculation method, table look-up method and generation arc method. Due to the high precision of micro-manufacturing, the calculation method can freely determine the step size of calculating the coordinates of each point on the tooth profile, and it is easy to control the accuracy. Therefore, the calculation method is used to determine the tooth profile.
3.1 Calculation of involute partial tooth shape As shown in the local tooth profile of the small modulus gear, the center of the gear is the coordinate origin o, and oy is the symmetry line of the tooth groove. The radius of any point M on the tooth profile is Ry, and its coordinate xy, yy is calculated as xy=Rysinωy=Rycosωy (4) where: ωy is the center half angle between the teeth of M point.
Ωy=ωb invαy(5) where αy is the pressure angle of M point; ωb is the central half angle between the base scallops.
Ωb=ωt-invααy=arccos(rb/Ry)(6) where: ωt is the central half angle between the indexing scallops, satisfying the formula ωt=π-4ξtanα2z Δsmz(7) simultaneous (5)~(7) The equation can be obtained ωy=π-4ξtanα2z Δsmz (invαy-invα) (8) For the standard modulus gear milling cutter, the displacement coefficient ξ=0, the indexing circle tooth thickness reduction amount Δs=0, the formula (8) can be Simplified to ωy=π2z (invαy-invα) (9) According to a series of Ry values, the coordinates of a series of involute tooth points can be calculated. Through the secondary development of AutoCAD, the AUTOLISP program is programmed to complete the calculation of each coordinate point.
The coordinates of each point on the tooth profile of the milling cutter can be obtained by using the LIST function of AUTOLISP.
3.2 Calculation of the transition curve The target gear tooth number zg=20, in the range of 19 to 34 teeth (equivalent to 15 is a set of standard modulus gear milling cutter No. 312 to 512), the milling cutter tooth shape has an involute The three parts of EF, straight line FC and arc CA are shown.
3.2.1 Calculation of point F 3.2.2 Calculation of the azimuth angle of the common tangent FC and calculation of the point C. The angle between the common tangent and the y-axis of the point F is φ=ωF αF(11) will be ωF=6.24°, αF=1.75° Substituting the above formula, the azimuth angle φ=7.99° is obtained. Because the coordinates of the F point and the angle φ are known, the angle bisector of the root tangential line and the common tangential line FC is formed. The intersection of the angle bisector and the y axis is the o point, and the arc is made with the o point as the center. Cut, the cut point is C point. The complete milling cutter profile can now be drawn.
3.2.3 Milling cutter side back angle verification The check formula of the side back angle of the micro-miniature gear milling cutter is αc=arctanKzkπdaosinφmin(12) where: φmin is the minimum angle of the tangent to the tooth-shaped symmetry axis of the side edge of the milling cutter The value, substituted into the known value, can be obtained αc=1.47°>1.33°, and the side back angle test is qualified, indicating that the parameters such as the back amount, the back angle and the number of teeth are reasonable.
4 Gear milling cutter manufacturing precision requirements In order to ensure the machining accuracy of small modulus gear milling, combined with the existing manufacturing process level, the manufacturing precision of the milling cutter should meet the following technical indicators.
(1) The non-radiality of the rake face is less than 0.02mm; (2) The radial runout of the cutter is less than 0.012mm; (3) The tooth profile error is less than 0.005mm; (4) The maximum cumulative error of the chip pocket is less than 0.025mm (5) tooth thickness deviation is less than ±0.008mm; (6) tooth end face runout is less than 0.005mm; (7) end face runout is less than 0.004mm.
5 The application uses the designed micro-small gear milling cutter to micro-mill the target part to obtain the experimental sample. After the sample is assembled as a key component, it constitutes the fuze delay mechanism. After the mechanism is used by the relevant departments, the effect is good, and the designed tool meets the trial production requirements.
1 Tool material selection Fine grain cemented carbide is the ideal tool material for making small modulus gear forming milling cutters. With the refinement of cemented carbide grains, the size of the hard phase is reduced, the distribution of the binder phase is more uniform, and the reverse relationship between hardness and flexural strength is broken. It has good comprehensive physical properties: good wear resistance. After a long cutting distance, the tool can still maintain a small cutting edge radius and smooth surface; the cutting edge strength is high, the material removal is smooth, and the stable surface quality can be obtained; the fracture toughness is good, and the tool is cut during the micro cutting process. Uniform wear, good tool reliability; large modulus of elasticity, good rigidity of the prepared tool, which is beneficial to ensure the processing accuracy; fine grain structure can reduce the chipping defects in the forming process, and the prepared cutting edge is sharp. The surface layer has uniform microstructure, stable geometric parameters and good dimensional consistency, which can meet the manufacturing requirements of micro-miniature gear milling cutters.