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Hubei Feining Steering Machine Co., Ltd. is a professional manufacturer of automobile steering gears. Its leading products are hydraulic assisted circulation ball steering gear and mechanical circulating ball steering gear, with an annual production capacity of over 200,000 units. “Fei Ning†brand steering gear has great popularity and market share in the industry.
First, the question is raised
The screw nut pair is one of the most important components in the steering gear assembly. The key structure of the steering rack and the screw shaft is the spiral raceway. Spiral raceway processing is one of the more difficult processes recognized in the machining industry, and the machining of the inner spiral raceway is more difficult. The number of products involved in the inner spiral raceway processing is very large, mainly represented by various pitches. A variety of medium diameter. Our company's usual process arrangement is: broaching of the rolling broach----rough grinding raceway to ensure the shape and the medium diameter----fine grinding raceway. The raceway processing accounts for more than 50% of the total process production cycle. Moreover, the use cost of the raceway broach is also high, and the use requirements are also very strict, which are the shortcomings of the capacity increase, and are not suitable for the development strategy requirements of improving productivity and reducing production cost.
The spiral raceway is composed of two eccentric arcs. The smallest bottom hole of the raceway is only φ29, and there are various pitches (pitch t=11, 10, 9.525, ...), and there are intermittent cutting during the raceway processing. The original production process of our company adopts special spiral broach broaching processing, and the single piece consumes 4 minutes of working time. The spiral broach itself is a relatively complicated and expensive special tool. When used, the requirements for the cooling environment, the adjustment accuracy of the tooling, the accuracy of the machine guide rail, and the sharpening of the tool itself are very high, and
As the tool itself wears out, the part size changes. The rough grinding process has to be used to ensure the size and correct the shape of the raceway. These directly lead to very high production costs, which is not conducive to improving production efficiency.
Second, the preliminary plan
In order to promote the company's production capacity, I consider other high efficiency processing. First of all, I thought of using the corn-tooth spiral milling cutter to process the inner spiral raceway. However, after discussion with the relevant tool engineers, combined with the structural characteristics of the parts, the corn-toothed helical milling cutter is not suitable for multi-pitch and high cost. Finally, I decided to develop the machining process plan for the milling thread using the tool. Under the integration of various experience, combined with the characteristics of the product spiral raceway large pitch and complex cross-section shape, and taking advantage of the processing status of the outer spiral raceway, I made a forming blade with YT15 cemented carbide blade and welded the four blades to the blade. Correction of the sharpening is performed on the arbor. When testing, take the cutting speed υ=65m/min, the tool speed ν knife=800n/min, the workpiece speed ν work = 32n/min. Adjust the equipment to start the trial processing. Milling takes a 3.5 minute, which is slightly faster than the broaching process. The surface roughness of the raceway after milling was Ra 6.3 μm. The inner diameter error of the raceway 70mm is 0.08mm, but it is acceptable. After 10 pieces of continuous processing, it was found that the back corner of the top edge of the blade began to wear, and the spiral track of the workpiece appeared to have a vibration pattern. After 40 pieces of continuous machining, the back angle of the top edge of the blade was severely worn, and the squeaking sound was heard. The test was stopped.
Third, the program is further optimized
After analysis, it is found that the following factors affect the durability of the tool: 1 the inner diameter of the spiral raceway is small, only φ30; 2 because of the structure of the part itself, the tool has intermittent cutting; 3 the cutting depth is too large, up to 3.5mm; The selection of rod material is unreasonable and the structural rigidity is not enough. 5 The geometric accuracy of the self-made forming blade has a large error. In response to many unfavorable factors, I decided to change the blade material from YT15 cemented carbide to high-speed steel HSS material, and designed the cutter bar as a monolithic taper structure to improve the rigidity of the cutter body, modify the relevant parameters of the cutter head, and provide tool drawings by professional cutters. Manufacturers outsource production, the cutting tool shape and accuracy are guaranteed by the tool manufacturer. Soon the tool manufacturer sent two qualified tools, the price of the tool is 110 yuan.
In the second round of trials, refer to the tool manufacturer's suggestion to modify the cutting parameters, take the cutting speed υ=37m/min, the tool speed ν knife = 400n/min, the workpiece speed ν work = 20n/min. Cool with 3-5% emulsion . After 30 pieces of continuous machining, the tool began to wear out. Stop the test again. After sharpening the tool, the cutting parameters are adjusted according to the situation: the cutting speed is υ=25m/min, the tool speed is ν-knife=250n/min, and the workpiece rotation speed is = 24n/min. Considering that the depth of the knife is too large, it is decided to divide the feed twice. The first knife has a depth of 2.5 mm and the second knife has a thickness of 1-1.5 mm. Once again, the surface roughness of the raceway can reach Ra3.2-Ra6.3μm, and the mid-diameter error of the raceway is within 0.05mm. The single-piece production man-hour is about 4 minutes, which is the same as the broaching process. After 80 pieces of continuous machining, the top edge back angle wear occurred. The test proved that the improvement effect is very obvious, and the method is very correct.
The actual trial situation was communicated with the tool manufacturer, and it was decided to improve the tool-related parameters again: increase the back angle of the top edge from 6° to 8°, and reduce the front angle to 4°. After the trial again, the effect is very obvious. Only 100-150 pieces of tools need to be sharpened and trimmed. The shift production is slightly higher than the broaching process and can be shaped. After mass-manufacturing tools, the cost per piece of tool was reduced to 80 yuan.
Fourth, the effect comparison
The use of this milling process, after 9 months of normal production verification, the production line reflected better, and was well received by the workshop leaders and workers. According to the survey and statistics, the process innovation for this process has made the workshop get a lot of benefits in terms of output increase, energy saving and consumption reduction. The main advantages are as follows:
1. Self-made milling cutter milling cutter saves tool cost compared with broach broaching, and can reduce the cost by 18% after accounting for single parts;
2, can completely unify the size of the diameter of the part raceway, avoiding the chaos of various sizes due to the broach wear;
3. The same type of milling cutter can be applied to the processing of parts with different pitches and different medium diameters, and the situation that different products require multiple broach processing is abandoned;
4. Due to the guarantee of the quality and size of the milling raceway, the coarse grinding process of the raceway is eliminated, the production cycle is greatly shortened, and resources such as equipment personnel are saved. Increased production capacity more directly.
V. Prospects
Considering that the knives are discarded after the use of the integrated tool, a certain cost is wasted. After further communication with the tool manufacturer, the next step is to change the integral HSS milling cutter to the cutterhead structure shown in Fig. 7, using the positioning key. Positioning, M6 screw clamping, making a component body structure. After the tool wears, only the small cutter head is repaired, and the cutter bar is intact. This can further save tool costs and reduce tool change time. Considering the overall rigidity of the split type cutter, it is solved by making the corresponding rod diameter series of cutter rods with different inner holes (different middle diameters of the raceways).
After the milling process has been successfully used on the raceway products, I am going to extend the process to the processing of other products of the company, such as the processing of the internal thread of the joint and the processing of the labyrinth seal groove. Make greater contributions to Feining's “Process Innovation and Product Quality Improvementâ€!
Abstract: Introduce the design, improvement and use of self-made tools for spiral raceway machining in our company. Discuss the application and promotion of self-made tools for machining of special parts.