Defect analysis and optimization measures of the h

2022-10-24
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Broaching defect analysis and optimization measures

in batch production, the "drawing instead of inserting" process has been widely used in the processing of involute internal splines, but the broach is expensive, so the quality of broaching, the efficiency and the life of broach all directly affect the product quality and production cost

among transmission parts, most of the parts with internal splines are synchronizer parts, which are made of low-carbon alloy steel and are characterized by low hardness and high toughness. When isothermal normalizing process is adopted for blank pretreatment, the actual hardness is between 126 and 145hb

common defects and influencing factors

broaching defects that often appear in production (as shown in Figure 1) are divided into two types: one is that the processing quality cannot meet the requirements, that is, there are scratches on the machined surface locally, and gnawing occurs in serious cases; The other is abnormal wear of broach. It is analyzed in detail below

Figure 1 common broaching defects

1 The main causes and analysis of local scratches or gnawing on the product surface during broaching

(1) excessive lifting of cutting teeth

when the lifting of broaching teeth is too large, the deformation range is large due to the thicker chips, and deep cracks will occur at the parts where the chips are stripped. 2. After unpacking. Because thick chips are not easy to curl, there are almost no tiles stuck in the chip holding groove before liberation, which is very difficult to remove. When pulling down a product, it will scratch the machined surface. If the workpiece material is 20CrMnTi and the tooth lift of the broach is 0.06, the broaching condition is normal. For low-carbon alloy steel with carbon content less than 0.2%, when the design tooth rise is 0.06, the iron filings will not curl and get stuck in the chip holding groove, which makes cleaning difficult, affects the production efficiency and improves the processing cost

(2) uneven tooth lift

when the broach blade is not well ground, resulting in uneven tooth lift, some chips will be too thick and some too thin. If the chip is too thick, it is easy to crack on the side of the spline teeth, leaving deep grooves (as shown in Figure 1-A), and even gnawing in serious cases

(3) the shape of the chip holding groove is incorrect

the chip holding groove is formed by the smooth connection of a straight line and an arc R. When the current angle is constant with R, the length of its straight line segment is also determined. When the shape of the chip holding groove is incorrect, the straight section is too short and the chip is not easy to curl. When the straight section is too long or the connection is not smooth enough, the chip cannot curl smoothly, resulting in abnormal extrusion in the chip holding groove, which affects the roughness of the broaching surface

(4) workpiece material and blank hardness

machining surface roughness is closely related to the material of the workpiece itself. The finish of hard and brittle materials is better than that of tough and soft materials. The surface of workpieces made of tough and soft materials is relatively rough, and sometimes there are pits gnawed off by the cutter teeth, or there are many burrs and flash at the outlet with chips not falling off, and the most serious is tearing (as shown in Figure 1-B)

alloy steel with carbon content less than 0.2% and hardness less than 160hb has great toughness, and chips are easy to adhere to the blade, causing great friction during cutting, affecting the quality of the machined surface

workpiece with the same material has different processing quality due to different metallographic structure. Those with large grains have poor surface roughness. Therefore, for workpieces with large toughness, heat treatment should be used to improve the hardness and metallographic structure before broaching, so as to meet the broaching conditions as much as possible and improve the economy of broaching

2. Causes and analysis of abnormal wear of cutting tools

abnormal wear of broach refers to abnormal rapid wear of cutter teeth in production, which has a great impact on production efficiency and processing cost. Normally, the regulator can be adjusted and rotated to the best position between the two grinding of broach to broach thousands of parts. However, sometimes in production, when only dozens or fewer workpieces can be pulled after grinding, the cutting edge of the broach has been seriously worn. The main causes are the insufficient hardness of the cutter teeth, the annealing of the cutter teeth during grinding, the uneven lifting of the teeth, the influence of the geometric angle of the cutter teeth, and the collision and missing of the blade caused by careless operation

(1) influence of insufficient hardness of cutter teeth

after heat treatment of broach, the hardness of cutter teeth is too low (≤ 60HRC), or there are soft spots on the cutter teeth, which will cause rapid wear of cutter teeth

(2) the influence of annealing after grinding of the cutter teeth

when sharpening the broach, the cutting edge of the cutter teeth is too large or the grinding speed is too fast, which reduces the hardness due to overheating annealing, and will wear quickly in the broaching process

(3) influence of uneven tooth rise

when the tooth rise is uneven, the tool teeth with large tooth rise will also wear faster due to heavy load during cutting

(4) influence of geometric angle of cutter teeth

when broaching tough and soft materials, large rake angle should be used to make the cutting edge sharp. When broaching the knurled key, due to the high tooth height, the metal material needs to restore elastic deformation after cutting. If the side rear angle is too small, the machined surface will be pressed tightly on the side of the tool tooth, resulting in great friction, that is, both sides of the tool tooth will squeeze the machined surface on both sides of the spline, or scratch the groove mark, and the broach tool tooth is also easy to wear. In actual work, you should carefully observe the friction marks on the side of the cutter teeth to judge whether the side rear angle is appropriate. If the back angle is too small, serious friction will occur between the machining surface and the back cutter surface, which will make the cutter teeth wear rapidly. If the back angle is too large, the strength of the cutter teeth will be reduced, so the wear will be accelerated instead

(5) careless operation, missing the blade

due to careless operation in the process of loading and unloading and handling, the broach teeth collide with the machine tool or other hard objects, making the blade blunt after missing, which is particularly easy to wear during broaching. In view of the above reasons, taking corresponding measures in Broach Design, manufacturing, process preparation and operation can make the blade wear normal

causes of broaching defects

when broaching defects occur in production, check the workpiece, chips and broach. Observe whether the expanded production range of 9 enterprises in the chip roll is more than 10 million square meters, whether the curve is normal, whether the thickness is uniform, and whether the back of the chip is smooth; Check whether the hardness of the workpiece is too low and whether the metallographic structure is normal; As broaching is a forming process, the broach teeth have a corresponding relationship with the workpiece. When the workpiece has a scratch on the tooth surface, you can roughly judge the position relative to the upper teeth of the broach according to its position. After that, check whether the blade is damaged and whether the top edge has a large white edge. Analyze and judge according to the specific situation, and then take corresponding solutions

broach optimization design and improvement effect

broaching belongs to forming processing. Broaching quality and broach life are closely related to the design parameters of broach. For different workpiece materials and blank States, matching parameters should be designed. Because the design parameters of broach restrict each other, comprehensive consideration should be taken in the design

Figure 2 broaching effect has been significantly improved

according to the characteristics of broaching parts with high toughness and low hardness, after analyzing the problems often occurred in our company's field, we reduced the tooth lift of the broach and increased the rake angle of the broach. Under the condition of normal grinding, the chip curling during cutting is smooth, and the problem of chip sticking and tool sticking is basically solved. In addition, by increasing the hardness of broach, the cutting effect can also be significantly improved (see Figure 2), and the service life of broach can be greatly improved. (end)

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