A method for controlling the machining accuracy of threaded holes is introduced. Through systematic process analysis of each link in the production process, methods such as improving the accuracy level in the part state, controlling thread accuracy and increasing compensation amount by reverse tapping, and designing special protective screws have overcome the technical problems and have been successfully applied. for mass production.
1 Preamble
The combustion chamber shell of a certain type of engine is composed of front connecting piece, thin-walled spinning cylinder, rear connecting piece and support through argon arc welding, heat treatment and sand blasting. The outer surface of the thin-walled shell of the combustion chamber is welded with 2 rows of axial supports with a total of 20 pieces. The design pattern of the supports requires a thread accuracy of M4-6H. The thread of the support is used to install the missile cable cover, and the quality and reliability of the thread connection are required to be high. Due to the limitations of the support structure, material and the space structure of the welding part with the combustion chamber shell, the traditional process is used to process the thread, and the qualified rate of the product is low. In this paper, process analysis and research are carried out on each link of product processing, and a reasonable and effective thread precision control method is obtained through test verification, comparison and analysis.
2 Product structure characteristics and processing difficulties
2.1 Structural features
The outer dimensions of the combustion chamber shell are relatively large, with an outer diameter of 500mm and a length of 4500mm. The support is manually welded on the outer surface of the combustion chamber shell, and its radial span is (114±0.2) mm. Combustion chamber shell and support materials are made of D406A ultra-high-strength steel. The structure of the combustion chamber housing support is shown in Figure 1. The shape of the support is an oblong structure, the outer diameter is 14mm, the width is mm, and the center has an internal thread M4-6H with a pitch of 0.7mm. There is only a 0.7mm gap between the threaded bottom groove and the thin-walled housing.
Picture Fig. 1 Support structure of combustion chamber shell
2.2 Difficulties in processing
The process flow of the support processing is shown in Figure 2. If the threaded holes of the support are processed after welding and heat treatment, there are the following difficulties [1].
1) The gap between the bottom of the threaded hole of the support and the shell is only 0.7mm, and it is easy to damage the surface of the thin-walled shell during machining, posing a quality risk.
2) The gap between the bottom groove of the threaded hole of the support and the shell is small, the tap guide is short during thread processing, the positioning is unstable, the tapping is difficult, and it is easy to process deviation, and the verticality of 0.04mm cannot be guaranteed.
3) The hardness of the material after heat treatment is 48-52HRC, and it is easy to cause the tap to break during thread processing, and the shell will be scrapped due to thread problems, resulting in high manufacturing cost and quality risks.
Based on the above analysis, it can be concluded that the thread of the support needs to be processed before welding, and after welding, it is annealed, sand blasted, quenched and tempered together with the combustion chamber shell. After the quenching treatment, the surface of the thread of the support is oxidized, and there are excess residues attached to the surface of the thread profile. If the thread of the support is processed in place before welding, after the combination of the combustion chamber shell is processed, use the M4-6H tap to clean the excess attached to the surface of the thread profile of the support, and at the same time, the oxide layer on the surface of the internal thread profile of the support will fall off. When using the M4-6H thread stop gauge to detect, the qualified rate is only 67%. Statistics are made on the processing of M4-6H internal threads of 17 combustion chamber housing supports, and the data are shown in Table 1. How to improve the machining accuracy of the thread of the support has become an urgent technical problem that needs to be solved in the production and delivery of products.
Picture Figure 2 Processing process
Table 1 Statistics of M4-6H internal thread processing of 17 combustion chamber housing supports
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3 Technical scheme and process test
3.1 Technical solution
After re-examination, testing, analysis and investigation of various processes in the combustion chamber shell and support processing, it is believed that the main reason for the excessive tolerance of the internal thread size accuracy of the support M4-6H is: after quenching treatment, the surface of the support thread is oxidized, and the thread Tooth surface is attached with excess. While cleaning the excess on the surface of the thread, the oxide layer on the surface of the internal thread of the part of the support will fall off, causing the precision of the internal thread of the support M4-6H to be out of tolerance.
According to the process analysis, two process schemes were developed.
Option 1: Customize special hand taps, which are divided into nose cones and second cones, and control the middle diameter of the nose cones. Use the nose cone to tap the thread in the state of the support part and reserve a machining allowance. After the heat treatment of the combustion chamber shell, tap the thread of the support with a second taper to ensure the final accuracy of the thread.
Solution 2: Improve the thread accuracy M4-6H by one level in the support part state, and process according to M4-5H, effectively compensate the difference between M4-6H and M4-5H, and meet the thread accuracy requirements [2].
3.2 Test process and results
The first process scheme is carried out in 3 steps. ① Customized special taps (head cone and second tap), the reserved margins for the middle diameter of the head tap are 0.30mm, 0.20mm and 0.10mm respectively. ② Use the nose cone to tap the thread when processing the support parts. ③ After heat treatment, use a second taper to tap the thread. Due to the high hardness (48-52HRC) of the material after heat treatment, and the influence of the large diameter of the combustion chamber shell, the operator is more difficult to tap the thread, the force is unbalanced, and the cutting force is easy to deviate from the axis. During the test, when the middle diameter allowance was 0.30mm, the threaded hole could not be cut when tapped with two cones; when the middle diameter allowance was 0.20mm and 0.10mm respectively, the threaded hole was deflected or the tap was broken, and the product quality Difficult to guarantee [3].
According to the second process plan, the thread precision of the support is improved by one level of processing, and the statistics are made on the processing of M4-6H internal thread of 10 combustion chamber shell supports. The data are shown in Table 2. The thread accuracy has been greatly improved, and the product qualification rate has increased from 67% to 95%.
Table 2 Statistics of internal thread processing of support in scheme 2
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3.3 Analysis of test results
By summarizing and analyzing the test results of Scheme 1 and Scheme 2, according to the processing method of Scheme 2, the qualification rate of the thread of the support is greatly improved. The out-of-tolerance thread is inspected with M4-7H thread gauge, and all are qualified. Compare the thread precision dimensions of M4-6H with M4-5H and M4-7H, see Table 3 for details.
Table 3 M4×0.7mm internal thread precision dimensions (unit: mm)
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It can be seen that the middle diameter of the thread M4-5H is in mm in the picture, the middle diameter of M4-6H is in mm in the picture, and the middle diameter of M4-7H is in mm in the picture. The difference between the maximum limit size deviation of 7H and 6H is 0.032mm, and the difference between the maximum limit size deviation of 6H and 5H is 0.023mm, that is, the deviation of unqualified support thread accuracy does not exceed 0.032mm. In order to compensate the over-tolerance, the thread accuracy in actual processing is increased to 5H, and the compensation amount is 0.023mm, which can basically meet the thread compensation requirements. For individual thread precision out-of-tolerance situations, it can be considered that the amount of out-of-tolerance is very small, and the accuracy is between 6H and 7H [4].
4 Improvement measures and process verification
The processing process is sorted out, and the process method is reasonable and feasible under the condition that the product qualification rate has been greatly improved. Through the analysis of the out-of-tolerance item, it is considered that the thread precision out-of-tolerance is caused by the details of the processing process. In order to completely solve the problem of the thread accuracy of the support, the process improvement is carried out in the following links of the support processing process.
1) When the thread is tapped on the tapping machine, the spindle will vibrate slightly. With the change of the processing depth, the cutting time at the mouth of the thread is relatively long, and there will be a slight difference in the size of the mouth and the root. The method of tapping from the back of the support thread is adopted to compensate for the slight changes in the mouth and root during processing [5].
2) Improve the detection accuracy of thread stop gauge. The thread of the support is still processed according to the precision of M4-5H. It is required that when the thread plug gauge is used for inspection, the through gauge is completely screwed and passed, and the number of screwed turns of the stop gauge is not more than 1.
3) The thread of the support needs to be protected in the sandblasting process before the heat treatment of the combustion chamber shell. The previous method of protection with M4 screws is changed, and the special protection screws are redesigned with an accuracy of M4-6f, and the screw-in length of the threads is controlled within 1 turn to avoid repeated screwing wear.
4) Change the cleaning method. After the combined machining of the combustion chamber shell, use compressed air to blow off the excess in the threaded hole of the support, and then inspect it with the threaded plug gauge M4-6H general gauge. If it fails to pass, first clean it with M4 screw, then clean it with tap M4-5H, and check it with thread plug gauge M4-6H after cleaning.
After several process tests and verifications, the thread accuracy of the support fully meets the product accuracy requirements, and the product qualification rate has increased to 100%, which completely solved the problem of thread accuracy of the support.
5 Conclusion
In order to ensure the high reliability of the support thread after welding and heat treatment, the thread accuracy is controlled by the following measures.
1) In the part state, the thread accuracy is improved by one level of processing, and the thread accuracy of the support is adjusted from M4-6H to M4-5H.
2) Process the threaded support from the welding surface (back side), and detect the front side after heat treatment and quenching to compensate for the size difference between the mouth and the root during processing.
3) Special protective screws are designed for the sandblasting process to reduce the extrusion of threaded holes.
Through the adoption of various technological measures, the precision of thread processing is controlled, the reliability of thread connection has passed the missile flight test assessment, and the product quality is stable and reliable.
