Analysis of production process of automobile engine connecting rod

Connecting rods are extremely critical parts on gasoline and diesel engines, with a wide variety and huge demand in quantity, among which automotive engines are in the greatest demand. Here we take you to learn more about connecting rod manufacturing.

The structure and role of connecting rod
 The connecting rod is a slender variable-section non-circular rod, its rod body cross-section from the large head to the small head gradually become smaller, in order to adapt to the rapid changes in dynamic load in the work. It is composed of three parts: connecting rod big head, rod body and connecting rod small head. The connecting rod big head is separated, half of which is integrated with the rod body and half of which is the connecting rod cover, which is assembled together with the crankshaft main journal by bolts and nuts.

Workpiece materials and blanks
 The material of the connecting rod is mostly made of high-strength selected 45 steel, 40Dr steel, etc., and tempered to improve cutting performance and improve impact resistance, hardness requirements for 45 steel HB217-293, 40Dr HB223-280. ductile iron and powder metallurgy technology is also used to reduce the cost of the blank.
 Steel connecting rod blanks are generally forged production, the blank form has two: one is the body, cover separate forging; the other is the body, cover forging into one, and then cut during processing or use the expansion process to expand the break. In addition, to avoid defects in the blank, 100% hardness measurement and flaw detection are required.

Connecting rod machining process

1. Positioning and clamping 1) The correct choice of rough datum and the reasonable design of the initial positioning fixture is a crucial issue in the machining process. When pulling the connecting rod size head side positioning surface, the benchmark end face of the connecting rod and the small head blank outer circle three points and large head blank outer circle two points coarse benchmark positioning method. This ensures uniform machining allowance for each machined surface on the large and small head holes and covers, ensures uniform weighing and de-weighting of the large head of the connecting rod, and ensures the final shape and position of the part assembly.
   2) In the processing of the connecting rod rod and assembly, the rod end face, the top and side of the small head, and the side of the large head are machined and positioned in the same way. In the bolt hole to the stop oblique joint surface processing process of the connecting rod cover processing, using its end face, bolt two seating surface, a bolt seating surface of the side of the processing and positioning method. This positioning and clamping method with high repeatability and stability can make the part less deformed and easy to operate, and can be used in all processes from roughing to finishing. Due to the uniformity of the positioning reference, the size and position of the positioning points in each process are also kept the same. All these provide good conditions for stabilizing the process and ensuring machining accuracy.

2、The arrangement of the processing sequence and the division of processing stages
 The size accuracy, shape accuracy and position accuracy of the connecting rod are very high, but the stiffness is poor, and it is easy to produce deformation. The main machining surface of the connecting rod is the size of the head hole, the two end surfaces, the joint surface of the connecting rod cover and the connecting rod body, and the bolt. The secondary surfaces are oil holes, lock grooves, etc. There are also processes such as weighing and de-weighting, inspection, cleaning and deburring. The connecting rod is a drop-forged part, the machining allowance of the hole is large, and the residual stress is easy to produce when cutting and machining. Therefore, when arranging the process, the roughing and finishing processes of each major surface should be separated. In this way, the deformation produced by rough machining can be corrected in semi-finishing machining. The deformation generated in the semi-finishing process can be corrected in the finishing process, and finally achieve the technical requirements of the part while the process is arranged to first process the positioning datum.

The connecting rod process can be divided into the following stages.
 1) Roughing stage Roughing stage is also the processing stage before the merger of the connecting rod body and cover: mainly the processing of the datum surface, including auxiliary datum surface processing, preparation of the connecting rod body and cover merged by the processing, such as the milling and grinding of the two counterparts.
 2) Semi-finishing stage Semi-finishing stage is also the processing after the merger of the connecting rod body and cover, such as fine grinding of the two planes, semi-finishing building large head hole and hole chamfering, etc.. In short, is the stage for finishing large and small head holes for preparation.
 3) Finishing stage The finishing stage is mainly to finally ensure that the main surface of the connecting rod on the large and small holes all meet the requirements of the drawings, such as honing large head holes, precision boring small head bearing holes, etc.

• Process flow chart of connecting rod machining
  What kind of connecting rod is a good connecting rod?
   The small head of the connecting rod is connected to the piston through the piston pin, and the large head is connected to the journal of the crankshaft. the size of the large and small heads depends on the pressure-bearing area. The working temperature of the connecting rod is 90~100℃ and the running speed is 3000~5000r/min. In order to ensure the smooth entry of the connecting rod forgings into the automatic precision machining production line and the accuracy of the finished parts in the engine, at the same time, in order to maintain the high speed operation, it can withstand the alternating stresses of tension and pressure with very high frequency, so that the crankshaft is always in balance, the connecting rod forgings are required to have high strength and fatigue resistance life. 

Connecting rod forgings in the drawings to meet the premise of dimensional accuracy should also meet the following technical and quality requirements.
 1. The uninjected die forging slope is between 3° and 5°, and the uninjected fillet radius R is between 2 and 5 mm.

2、The non-machined surface should be bright and clean, no cracks, folds, scars, oxidation skin (depth > 1mm crater) and other defects are allowed.

3、The width of residual flying edge on the parting surface is ≤0.8mm.

4, longitudinal section metal fiber direction should be along the centerline direction and consistent with the shape, there should be no disorder and discontinuity, no pores, cracks, folding and non-metallic inclusions and other defects are allowed.

5、The hardness of the tempering treatment is between 220 and 270HB.

6, should be forging for flaw detection inspection.

7、The defects on the forgings are not allowed to be welded.

8、The quality deviation of each batch of forgings ≤ 3%.