Crankshaft hole precision machining solution

2022-03-30 by No Comments

The machining quality of the crankshaft hole has a significant impact on the engine performance. Based on the actual machining situation, the Jishui Engine Division has continuously researched and improved the finishing plan for the crankshaft hole of the cast iron engine cylinder, and has achieved remarkable results. Effectively improve the processing quality and efficiency.

The cylinder block is an important basic component of the engine, and its main function is to assemble various mechanisms and systems into a whole. The most important moving parts of an engine, the crankshaft, pistons and connecting rods, are closely related to the cylinder. At present, the maximum speed of a typical gasoline engine exceeds 6000r/min. The crankshaft forms an oil film between the crankshaft hole and the bearing bush of the engine block, and the sliding bearing supports and lubricates the high-speed crankshaft. This puts forward high technological requirements for the precision machining of the crankshaft hole.

The process requirements for engine crankshaft hole machining are usually very strict, because the quality of crankshaft hole machining has a significant impact on engine performance. Including diameter, position, roundness, straightness of each crankshaft hole center and surface roughness. In order to meet these demanding process specifications, precision machining usually uses precision boring or hinge machining.

Precision boring analysis

Compared with the hinge method, the fine boring method for machining the crankshaft hole has a relatively low initial investment cost. It can be placed in the same process with other parts of the finishing machining center or special equipment.

Due to special process requirements, the current excellent solution is to combine the following two types of precision boring tools:

The first fine boring tool guides the crankshaft holes in the first and second gears.

The second fine boring tool is finely supported by the remaining crankshaft holes below the first and second gear crankshaft hole supports.

The hole on the crankshaft must maintain a high degree of coaxiality between the gears, and must be fed in the same direction during finishing. However, if such a long crankshaft boring tool is used to drill a 1st gear crankshaft, it may lack support and cause excessive jumps. Therefore, a long boring tool and a short boring tool are combined to finish the hole on the crankshaft.

At present, fine hole drilling tools for crankshafts generally use cemented carbide or CBN. Cemented carbide blades are cheaper, but the surface roughness of the processed workpiece is poor, and the blade durability is poor. To solve this problem, we cooperated with tool suppliers to collect the most common blade coating materials in the industry. Using technologies with different cutting angles to conduct cutting experiments on 16 cemented carbide inserts. Then I tried to optimize various suitable processing parameters. Through many verifications, the best experimental result of cemented carbide inserts that meets the roughness and other process requirements is 80 times at a time. Still can not meet the requirements of mass production.

Then, we verified another solution for fine boring tool insert materials: replacing the existing carbide insert materials with CBN insert materials. We rely on tool manufacturers to develop more than 10 CBN inserts for checking the cutting of the inner diameter of the crankshaft.

The test results are shown below. In terms of roughness control, CBN inserts are significantly better than carbide inserts, with a machining life of more than 200 pieces. However, some new problems have also emerged at the same time. When processing about 150 pieces with a CBN blade, slight burrs will appear on the side of the hole, and flanging will appear in severe cases. The burrs and flange processing here will bring hidden quality hazards that affect the performance of the engine. In severe cases, it will cause quality accidents such as bushing scratches and crankshaft lockup. When dealing with crankshaft hole burrs, we increased the number of crankshaft hole brushes, which reduced the existence of burrs to a certain extent, but there was still a greater quality risk. In addition, compared with hinges, the quality stability of the crankshaft hole size scheme is lower, which greatly increases the risk of quality control due to frequent tool replacement. As the tools become less durable, the change in the surface roughness of the workpiece produced by the fine boring tool decreases rapidly.

Hinge processing analysis

We have done a lot of research on the crankshaft hole plan for boring precision machining. We have developed a variety of new inserts for processing verification, but have not yet achieved the expected processing results, so we will re-examine the crankshaft reamer method.

From the cost point of view, the one-time investment of the crankshaft hole hinge is relatively large.

The interim quotation from a well-known German honing board manufacturer is 400,000 Euros (price depends on configuration requirements). However, the post-processing cost of the hinge is much lower than the cost of fine boring, and the processing quality is stable.

The crankshaft hole hinge is processed by a horizontal honing machine. Due to the long working stroke of the horizontal honing machine, it is suitable for honing the crankshaft hole and other deep holes, and the depth can reach 3000mm. During processing, the honing head grindstone is fed radially under the action of the telescopic mechanism, and the workpiece is gradually processed to the required size. The outer circumference of the honing head is inlaid with 2-10 whetstones, which is about 1/3 to 3/4 of the hole length, and it rotates and reciprocates during boring. At the same time, the honing head’s spring or hydraulic control is evenly lifted, and the contact area with the hole surface is large, and the processing efficiency is high.

The honing tolerance is usually 0.2 mm or less. When processing cast iron or non-ferrous metals, the honing circumferential speed can exceed 50m/min, and the honing reciprocating speed does not exceed 15-20m/min. The pressure of oil stone in the hole wall is generally 0.3-0.5 MPa, rough boring reaches about 1 MPa, and fine boring is less than 0.1 MPa. Since the grinding stone is in contact with the surface of the workpiece during honing, the vertical pressure of each abrasive particle on the surface of the workpiece is only 1/50 to 1/100 of the vertical pressure during grinding. In addition, the slower honing speed can keep the temperature of the cutting zone in the range of 50°C to 150°C. This helps to reduce the residual stress on the machined surface and improve the surface quality. The cutting fluid used for honing has a constant working pressure and must be filtered to remove chips, avoid blockage of oilstone, and reduce the temperature and surface roughness of the cutting zone. Most cutting fluids use kerosene or kerosene and spindle oil, and extreme pressure emulsions can also be used.

The processing stability of the hinge is very high, and the durability of the polishing head can process more than 10,000 at a time, which greatly improves the production efficiency. The precision machining method of CBN inserts needs to be replaced every 200 pieces, and the average tool change and initial part inspection require 42 minutes. If you convert the time of frequent tool changes to each workpiece, you will lose 12.6s/piece. Based on the existing 120 pieces/piece processing line, the output of parts is 160,000 pieces/year, and the annual processing loss is 2.16 million s. At this time, it can process more than 16,800 pieces per year.

This table shows the tool loss cost loss caused by different finishing methods of crankshaft holes, from which it can be analyzed that the tool cost difference of a single part is more obvious. According to analysis and calculations, when 1.03 million pieces are produced, the tool loss of the hinge solution is about 3.81 million yuan compared with the CBN precision solution, which is slightly the same as the price of the hinge device. The quality control of hinged solutions is less difficult and less risky.

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