Through the analysis of the sealing shell of 4J29 Kovar alloy and 022Cr17Ni12Mo2 stainless steel materials, a method of using high-speed milling and reaming technology to process difficult-to-machine materials is proposed, which not only improves the machining accuracy and machining efficiency of the shape and inner hole of the parts, but also saves energy. cutting tool costs.
1 preamble
In order to improve the performance and service life of spacecraft in various deep space environments, aerospace parts mostly choose materials with good heat resistance such as titanium alloys and high-temperature alloys. Such alloy materials have poor processing performance and are difficult to process. The selection of cutting tools High requirements and high processing costs. According to the characteristics of such difficult-to-machine materials, carrying out research on the processing technology of difficult-to-machine materials and prolonging the tool life will help to improve the precision of spacecraft supporting parts and improve processing efficiency. At the same time, it can expand the company's market potential and create greater economic benefits. .
2 Problem overview
The rectangular series sealing shell is a product part newly developed by the company in recent years, as shown in Figure 1, the material is mainly 4J29 Kovar alloy and stainless steel. Since the product design structure requires the use of glass sealing technology, higher requirements are put forward for the surface roughness of the surface and inner hole of this type of sealed shell parts, resulting in increased processing difficulty, reduced tool life, increased tool cost, and reduced processing efficiency. The pass rate is low.
3 Problem Analysis
Taking 4J29 Kovar alloy and 022Cr17Ni12Mo2 stainless steel as an example to analyze a certain type of sealing shell, the structure of the sealing shell parts is similar, and it is necessary to process the row of holes in the inner cavity. The row of holes is used for glass sealing pins, and the glass sealing The connection technology requires that the inner surface roughness value of the row hole is Ra=0.8μm. In the glass sealing process, unqualified products are produced many times, and the yield is low. According to the analysis of the design and craftsmen, the surface roughness of the inner surface of the sealing shell row hole has an important impact on the yield of glass sealing. The burrs at the hole row and the shape and groove processing of the inner cavity are not easy to remove, which also affects the sealing effect of the parts.
3.1 Analysis of the causes affecting the quality of the inner wall of the part hole
The original hole row processing technology used in the production line is drilling → reaming. Since the 4J29 Kovar alloy material has good plasticity, it is easy to stick to the knife during processing; due to the high temperature hardness of stainless steel (022Cr17Ni12Mo2) and poor heat dissipation, it is different from other metal materials. Strong affinity [1], so the drill bit wears quickly, mainly in the following aspects.
The main cutting edge of the drill bit wears too fast, and even chipping occurs. When drilling difficult-to-machine materials, the temperature is high, cutting deformation and chilling are serious, and the tool is easy to stick to produce built-up edge, resulting in inconsistent surface roughness of different inner holes of the same part, and the wear condition of the drill bit cannot be detected and controlled during processing. Try to improve the surface quality and processing efficiency of the inner hole by using tungsten-cobalt cemented carbide drills (YG, YT and YW), which are more suitable for processing difficult-to-machine materials. According to the principle of tool wear [2], it is found that the YG tool is still dominated by adhesive wear during low-speed cutting, but the YT tool is accompanied by a certain amount of oxidative wear and diffusion wear at the same time as the bond wear; the YW tool has three types of wear. The wear mechanism occupies the same position, so YG carbide drills can be preferred for low-speed cutting, and YW or YG carbide drills can be used for high-speed cutting. According to this wear principle, the surface quality of the inner hole is improved after selecting the appropriate drill bit to process the hole row. However, due to the high price of the small-diameter tungsten-cobalt carbide drill bit, the cost of the tool increases, and the efficiency of mass production and processing is not high.
3.2 Analysis of the reasons affecting the shape of the part and the surface quality of the inner cavity
When processing 4J29 Kovar alloy material and stainless steel material (022Cr17Ni12Mo2), the cemented carbide tool with ordinary grain size is used for processing. The bottom edge and side edge of the milling cutter wear quickly, and the tool life is short, so the cutting speed can only be lower than 50m/ If the range of min is selected, the processing efficiency is low. Compared with processing aluminum-based alloys, the service life of milling cutters is only 1/5 of that of processing aluminum-based alloys; compared with processing 314 stainless steel, the service life of milling cutters is only 1/3 of that of processing 314 stainless steel.
In the process of cutting such difficult-to-machine materials, it is easy to generate a large amount of cutting heat in the cutting area, which seriously damages the dimensional accuracy and performance of the processed parts. The dissipation of cutting heat can only be conducted by cutting fluid and internal cooling tools. For the sealed shell of this type of structure, due to the small size of the inner hole and the inner cavity, small-diameter tools or shaped tools are mostly used. A large amount of cutting heat is difficult to dissipate quickly, and the tool wears too fast, resulting in an increase in the surface roughness of the part. If it is too high and fails to meet the technical requirements, it will be judged as unqualified. If the hole spacing is small, the chamfering of the orifice will destroy the size of the adjacent aperture; if the chamfering is too small, the burr will still have flanging, which will affect the sealing quality.
4 problem solving
4.1 Improvement of hole inner wall quality
In view of the inconsistent surface roughness of the inner hole of the sealed shell, it is necessary to improve the processing method and select a suitable tool. Through the trial cutting process, the hole row processing technology is firstly changed to drilling → reaming → fine milling of the inner hole, the surface quality of the inner hole is obviously improved, but the number of holes is large, and the tool is still worn when the small diameter milling cutter is used for fine milling the inner hole Fast, and the phenomenon of chip entanglement and tool clearance is generated, the processing efficiency is still not high, and the cost of the tool increases. Secondly, it is changed to drilling → reaming → fine boring. The surface roughness of the inner hole meets the requirements, and the processing efficiency of single hole is improved, but the small diameter overall boring tool needs to be customized, the tool cost is high, the boring tool life is short, and it cannot meet multiple rows of holes. boring.
By referring to the fixed-diameter hole reaming technology, the aperture of the reaming process is generally 3 to 100mm. Due to the long cutting edge of the reamer, each cutting edge participates in cutting at the same time during reaming, so the production efficiency is high, and it is widely used in the finishing of holes. The final processing technology is determined as drilling → reaming → reaming. Because the reaming processing technology of small-diameter holes (<φ2mm) has not been adopted in our company, a suitable domestic small-diameter custom carbide reamer is selected (see Figure 2).
Through calculation and trial cutting, select reasonable cutting parameters. The principle is as follows.
Check the reamer tool information and collected reaming parameters, and process difficult-to-machine materials such as stainless steel. The reamer speed should not be too high [3], and select the reference value: cutting speed vc = (6 ~ 12) m/min, feed rate f = (0.05 ~ 0.1) mm/r. The diameter of the inner cavity of the rectangular sealed shell is (1.7~1.8) mm, so the φ1.8mm reamer is selected to calculate the spindle speed n and feed speed vf during processing, where vc=7m/min, f=0.06mm /r.
Because cutting speed vc=πDn/1000 (D is tool diameter, n is spindle speed), so spindle speed n=1000vc/(πD)=1000×7/(3.14×1.8)≈1238 (r/min).
From this, the feed speed vf=fn=0.06×1238≈74 (mm/min) can be calculated.
According to the calculation results, the actual machining and cutting parameters are selected as n=(1200-1300) r/min, vf=(70-80) mm/min, and the drilling → reaming → reaming process is adopted. Due to the sealing of the shell The hole spacing is compact and the hole diameter is small, so the margin before reaming is controlled to 0.05mm. The final actual processing effect is shown in Figure 3. When the φ1.83mm reamer has more than 1000 reamed holes, the surface roughness Ra of the inner hole can still reach 0.8 μm, which meets the process requirements and improves the processing efficiency.
4.2 Improvement of Surface Processing Quality and Tool Life
In order to improve the processing efficiency and tool life of materials with high temperature hardness and poor heat dissipation, such as high-temperature alloys, titanium alloys, and stainless steels, imported cemented carbide tools are often used for rough and finish machining, and the cost of tool usage is very high. Comparative analysis of the wear difference of different tool materials when cutting titanium alloys at high speed, including uncoated cemented carbide, TiAlN PVD coated cemented carbide and PCBN, etc., it is found that PCBN tool materials are at high cutting speed, low feed rate and low When cutting titanium alloys with back cutting, a relatively stable cutting force and a lower surface roughness value can be obtained [4]. By applying the principle of high-speed milling and using domestic PCBN tools, higher cutting The processing method of high speed and small feed increases the service life of the tool.
Through multiple trial cutting and verification, the analysis shows that when cutting difficult-to-machine materials at high speed, the interaction between the feed per tooth fz and the back engagement ap has a significant effect on the surface roughness within a relatively high confidence probability Influence. This phenomenon shows that the effect of feed per tooth or milling depth on surface roughness is closely related to the selection of milling depth and feed per tooth. In contrast, under the medium and low speed cutting conditions, the interaction between the various cutting parameters is not obvious, or there is no interaction. This means that under a specific cutting condition, simply examining the single-factor effect of the feed per tooth or the back-cutting amount on the surface roughness cannot accurately predict the value of the processed surface roughness. Therefore, in order to obtain the ideal surface roughness, when determining the feed rate per tooth, it needs to be selected in conjunction with the amount of back engagement, and vice versa.
The 4-blade domestic solid carbide milling cutter is selected for high-speed rough machining of the shape and inner cavity. Due to the small back engagement ap and small cutting thickness ae, it can effectively protect the bottom edge and side edge of the tool. The generated cutting heat conducts rapidly, reduces the probability of built-up edge on the tool tip, and correspondingly increases the milling speed vc and the feed rate per tooth fz, which not only ensures the processing quality, but also improves the processing efficiency. To calculate the machining wear time of the rough milling cutter, it is only necessary to cut off the effectively used worn part, and the remaining part of the cutter can still meet the needs of roughing again after sharpening, which greatly improves the utilization rate of the cutter and reduces the cost of the cutter.
For the burrs generated by difficult-to-machine materials, manual removal is difficult to meet the existing technical requirements, so CNC machining is used, and TiC-coated high-speed steel materials are selected for chamfering milling cutter processing. After rough milling improves the quality, the shell parts are fine The burrs generated during milling are relatively small, and the chamfer milling cutter only needs to process according to the contour track of the part to ensure a smooth transition of sharp edges. For the flanging and burrs of the holes of the sealing shell, the processing method of milling the chamfering of the holes with a chamfer milling cutter → fine reaming with a reamer is used to ensure that the holes are free of burrs and bonded. The cutting parameters of the tool before and after improvement are shown in Table 1, and the processing effect of the shell is shown in Figure 4 and Figure 5.
Table 1 Tool cutting parameters before and after improvement
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Figure 4 Processing effect of 4J29 Kovar alloy shell
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Figure 5 Processing effect of stainless steel material (022Cr17Ni12Mo2) shell
5 Popularization and application of reaming technology for difficult-to-machine materials
A certain type of push rod parts (see Figure 6) is made of 00Cr17Ni14Mo2 stainless steel, which is a difficult-to-machine material. The φ5mm through hole on the outer circle is processed, the depth is 15mm, and the surface roughness value Ra=1.6μm is required. The original process is: fitter drilling→polishing the hole wall. Since the material is stainless steel, the fitter process uses a drill to drill holes, the drill bit wears quickly, the position of the hole is out of tolerance, and the efficiency of polishing the inner hole is low. Therefore, the improved process is: lathe drilling → Boring. Since the turning process needs to use special tooling to clamp the push rod parts, and the size of the special tooling is too large, it is not easy to install. Therefore, although the actual processing has guaranteed the surface roughness value Ra=1.6μm, the processing efficiency has not been improved. 00Cr17Ni14Mo2 stainless steel caused The boring tool wears quickly and the cost of the tool is high.
Picture Figure 6 Two-dimensional diagram of the push rod
Using the experience gained from reaming small-diameter holes, the processing technology of drilling → reaming → reaming in the machining center is used to solve the problems of low processing efficiency of φ 5mm through holes and difficulty in guaranteeing the surface roughness value Ra=1.6μm. The implementation process is as follows.
Select the reference value: cutting speed vc=(6~12) m/min, feed f=(0.15~0.2) mm/r. Choose the φ5mm reamer to calculate the tool speed and feed rate during processing, take vc=7m/min, f=0.18mm/r.
Because cutting speed vc=πDn/1000 (D is tool diameter, n is spindle speed), so spindle speed n=1000vc/(πD)=1000×7/(3.14×5)≈445 (r/min), feed Quantity vf=fn=0.18×445≈80 (mm/min).
According to the calculation results, the actual machining and cutting parameters are selected as: spindle speed n = (450-500) r/min, vf = (80-90) mm/min, the allowance before reaming is controlled to 0.1mm, and the final actual machining The final object is shown in Figure 7. When the φ5.02mm reamer (see Figure 8) has more than 500 reamed holes, the surface roughness Ra of the inner hole can still reach 1.6 μm, which meets the process requirements and improves the processing efficiency. The manufactured positioning tool (see Figure 9) has a simple structure and is easy to clamp.
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Figure 7 The real object of the push rod after processing
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Figure 8 φ5.02mm reamer
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Figure 9 Effect of positioning tooling for push rod processing
6 The effect achieved
Through this research, we have accumulated technical experience in processing difficult-to-machine materials. Subsequent research and development of parts made of difficult-to-machine materials such as high-temperature alloys and titanium alloys can also be processed with reference to reaming technology, and good results have been achieved. For example, using a φ2.12mm reamer, Complete reaming of superalloy materials, diameter pictures, and deep holes with a depth of more than 40mm. The reaming processing technology not only saves the tool cost, but also improves the processing efficiency. See Table 2-Table 4 for the comparison of parts processing effect before and after improvement.
Table 2 Processing pictures of rectangular sealing shell holes before and after improvement
Table 3 Processing of push rod holes before and after improvement
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Table 4 Tool costs before and after improvement
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From Table 2 to Table 4, it can be concluded that the use of the improved processing method has improved the processing quality, the pass rate of parts has increased to 99%, the production efficiency has increased by 33%, and the tool cost has been greatly reduced.
7 Conclusion
The emerging new materials and difficult-to-machine materials in the aerospace field have put forward higher requirements for cutting processing technology. Only by in-depth research on the cutting characteristics of difficult-to-machine materials and mastering more properties of new materials can we choose matching tools for cutting. The tool cutting status monitoring system is introduced to monitor the usage status of the tool in real time. According to the different service life of different materials, the tool can be judged and selected in time, which can reduce the cost and increase the efficiency while improving the machining accuracy of the supporting parts of the spacecraft. Effect.
