The influence of automobile steering knuckle forging method on processing technology

The steering knuckle is one of the main parts on the steering axle of the car, which can make the car run stably and transmit the driving direction sensitively. One function is to effectively transmit the angle value of the steering wheel rotation to the front wheel of the car, timely control the route of the car in motion, and thus ensure the safety of the car; the other function is to bear the load of the front of the car, support and drive the front wheel to rotate around the kingpin, and bear variable impact loads when the car is driving. Therefore, the steering knuckle not only requires reliable strength, but also must ensure its high processing accuracy. Its geometric shape is relatively complex, and there are many geometric shapes that need to be processed. The position accuracy between each geometric surface is required to be high. The level of its processing accuracy will affect the steering accuracy of the car in operation. This paper analyzes the forgings produced by two different forging processes, explores the influence of the parting form, allowance distribution and forging error of the steering knuckle forging on its processing technology, and puts forward reference for the fixture design and the selection of positioning surfaces in the processing process.

Structural characteristics of the steering knuckle

The shape of the steering knuckle is relatively complex, and it concentrates the structural characteristics of four types of parts such as shaft, hole, disc ring, and fork frame. It is mainly composed of three parts: supporting shaft, flange, and fork frame. The structural shape of the supporting shaft is a stepped shaft, and its structural features are a rotating body composed of coaxial outer cylindrical surface, conical surface, threaded surface, and shaft shoulder, transition fillet and end surface perpendicular to the axis; the flange part includes flange surface, connecting bolt through hole and threaded hole for steering limit; the fork frame is a fork frame structure composed of the upper and lower ears and flange surface of the steering knuckle.
From the perspective of forging technology, the characteristics of the steering knuckle forging are: the supporting shaft is slender, the flange is large and sometimes has a special-shaped surface, the fork frame is deflected by an angle α from the center line of the supporting shaft and has a complex shape. According to "GB12362-2003 Tolerances and Machining Allowances for Steel Die Forgings", the forging is a typical complex fork-shaped part.

Steering knuckle processing process

The main process flow of steering knuckle processing is: milling the end face of the journal, drilling the center holes at both ends → rough turning the end face of the flange and the journal of the support shaft → semi-finishing and finishing the support journal, fillet, finishing the flange, turning the rear end thread → drilling and tapping the flange surface thread → rough and fine milling the inner and outer end faces of the upper and lower earrings → rough drilling and finishing boring the kingpin hole → surface quenching (as needed) → finishing grinding the large and small bearing journals and fillets → engraving marks → inspection and storage.

The impact of forging methods on processing technology

1. Forging methods

There are two forging forming processes for the production of steering knuckle forgings: horizontal parting (plane parting) and vertical parting (vertical parting). Horizontal parting is a forging method with the center plane of the forging as the parting surface. Because the cross-section of the support shaft part is quite different from that of the flange and fork frame part, the forging process is very complicated to reasonably distribute the blank. Even so, there will still be a large flash at the connection between the support shaft and the flange, and it will gradually decrease along the axial direction until it reaches the normal width at the tail. The material utilization rate of this forging method is low. The vertical parting method is based on the center plane of the flange and takes into account the fork cavities on both sides. This forging method can use closed forging technology during pre-forging, positively extrude the shaft and reversely extrude the forks on both sides, and then final forge to form and discharge excess metal.
Due to different forging production methods, the parting surface layout, machining allowance allocation, and forging error and thickness tolerance during forging design will have different effects on the processing of the steering knuckle. Especially in the process of drilling the center hole on the end face of the milling journal, turning and grinding the support journal (see the processing of parts A and B in Figure 1) and the end face of the flange, processing the threaded holes on the flange that connect the steering knuckle arm and the brake, and processing the fork end face and the kingpin hole of the fork frame (see the processing of parts C and D in Figure 1), the impact is particularly obvious. Therefore, when designing the processing technology and selecting the fixture positioning surface, corresponding countermeasures must be taken according to the different production methods of the forgings.
Schematic diagram of steering knuckle processing parts
Figure 1 Schematic diagram of steering knuckle processing parts

2. Forging tolerance and machining allowance arrangement
When the horizontal parting die is used to forge the steering knuckle, the parting surface is usually selected at the largest section. As shown in Figure 2, A-A is the parting surface of the forging, and the forging direction is perpendicular to the plane where the parting surface is located, that is, along the direction shown by B-B. In this way, the forging is forged by the upper and lower dies. The machining allowance of the forging processing parts is evenly distributed on the support shaft, the end face of the flange, and the upper and lower fork end faces. The draft angle of the forging is along the forging direction, that is, the direction of B-B, which is generally 5°~7°; during the forging process, due to the fluctuation of factors such as the forging temperature and forging force, the upper and lower dies cannot be completely pressed against each other, so the thickness dimension fluctuation along the forging direction will be formed on the forging, usually with a tolerance of ±1mm; and the error caused by the misalignment of the upper and lower dies is generally ±1.5mm.
The parting surface of the vertical parting forging is selected to be perpendicular to the support axis and pass through the center of the flange, but the shape of the forging determines that its parting surface is a curved surface. As can be seen from Figure 2, the curved surface shown in C-C is its parting surface, and the forging direction is perpendicular to the parting surface, that is, along the direction shown in D-D. The forgings produced in this way are different from those produced by horizontal forging in terms of machining allowance distribution. Due to the need for drafting of the upper die forging, no reverse draft is formed, so an allowance needs to be added on the inward side of the upper and lower forks to form a positive draft. The main parts are shown in Figure 2 as E and F. In addition to the normal addition of allowance for the support shaft, in order to facilitate the demoulding of the support shaft, an additional draft angle of 1° to 1.5° is added along the axial direction. Assuming that the support shaft is 200mm long, due to the addition of the draft angle, the additional addition of the allowance from the small shaft end to the outer neck of the support shaft at the root of the flange will increase from 0 to 0.35~0.5mm, and the additional allowance d=200tan (1°~1.5°). The thickness tolerance of the forging is usually ±1.5mm, which is generated along the D-D direction, and the error is generally ±1.5mm, which is generated perpendicular to the D-D direction.
Schematic diagram of horizontal and vertical split die forgings Figure 2 Schematic diagram of horizontal and vertical split die forgings

3. Processing influencing factors
The different influences of factors such as allowance and tolerance in the above two methods of producing forgings must be considered when designing the processing technology, otherwise it will affect the processing quality of the steering knuckle. The processing influences that need to be focused on are:

(1) Processing of the steering knuckle support shaft The main processing procedures for the steering knuckle support shaft are milling the shaft end face, drilling the center hole, and turning and grinding the journals of various parts (see parts A and B in Figure 1). These two processes are interrelated, especially the center hole drilling process. The center hole is not only the positioning reference for the subsequent processing of the support journal, but also the measurement reference for various size and position tolerances on the support shaft. If the connecting line of the two center holes cannot coincide with the axis of the support shaft forging during the processing, it will cause uneven distribution of the forging margin and the appearance of machining journal oxide scale (i.e. residual forging surface). By comparing the journal parts of the forgings produced by the two methods, it can be seen that for the steering knuckle produced by vertical split die forging, since the final forging of the support shaft part is formed in the cylindrical cavity, the roundness of this part is good and the margin is uniform, so it is easier to select the positioning position when processing the center hole. However, due to the influence of factors such as misalignment, thickness tolerance, and trimming residue, the horizontal forging will form an irregular circle. The distribution of the allowance at various locations of the journal will fluctuate significantly under the influence of multiple factors. Based on this shape fluctuation of the forging journal, when selecting the location of the center hole for machining, it should be considered to design it with a V-shaped fixture at a 45° angle to the parting surface. This can avoid the influence of flash residue and misalignment, and make the center hole connection line approach the theoretical center line of the forging journal, so that the subsequent machining allowance is evenly distributed.

(2) Axial dimensions of the support shaft The axial dimension chain of the steering knuckle shown in Figure 1 is related to each other. The wall thickness dimension of the steering knuckle kingpin hole 11mm is particularly important. It is related to the strength of the kingpin hole wall thickness, so it must be guaranteed. From the analysis of the relationship between the axial dimension chain, the fluctuation of wall thickness should be considered from different forging methods when the center hole is drilled in the first processing step of milling the end face. If it is a horizontal forging, the axial dimension of the forging is generated in the upper and lower dies, and its fluctuation is mainly affected by the forging error. If it is a forging produced by vertical forging, the axial dimension of the forging is generated in the upper and lower dies, and the fluctuation of its axial dimension is mainly affected by the thickness tolerance of the forging. Therefore, in this state, it is recommended to select the upper die produced in the same die as the wall thickness of the kingpin hole when selecting the initial axial positioning dimension, that is, select the flange surface near the fork.

(3) Flange processing The flange part of the vertical forging is formed in a complete cavity, so its shape error fluctuation is small. When processing the connecting holes around the flange, as long as the positioning of the center hole of the support shaft is accurate, the wall thickness around the connecting hole will be very uniform. For forgings produced by horizontal forging, the flange part is formed in the upper and lower dies. Due to the influence of forging error and thickness tolerance, and based on the center hole centering problem analyzed above, when processing the connection holes around the flange, there is a risk of uneven wall thickness around the hole or even too small. Attention must be paid. If necessary, it can be required to add appropriate allowance around individual holes to avoid this risk. In addition, the flange thickness of forgings produced by vertical forging is affected by the thickness tolerance of the forging. The machining allowance of the flange end face of forgings from different batches will fluctuate. This also needs to be paid attention to during machining.

(4) Steering knuckle fork processing Because there is an angle α between the support shaft part of the steering knuckle and its fork part, when using vertical forging to produce the steering knuckle, in order to avoid reverse draft and enable the upper die part of the forging to be removed from the cavity, the allowance must be increased. In the E and F positions shown in Figure 2, especially the F position, the allowance is large. Assuming α=7°, the fork depth is 70mm, and the normal draft angle is 3°, the increase in the fork root allowance is: δ=70tan7°+70tan3°=12.2mm. In this way, the large allowance cutting of this part must be considered during the fork processing, especially the rough processing; while the allowance of horizontal forgings in these parts can be arranged according to the conventional method, so the amount of cutting is small, but because there is a draft allowance in the middle of the two forks, in order to ensure the accuracy of the center hole, this part is generally face milled. In addition, when processing the fork, the journal is usually used for positioning. For vertical forgings, due to the influence of the error, the machining allowance of the fork will change, and in severe cases, the machining allowance will be insufficient and oxide scale will be produced.

Conclusion
Both horizontal and vertical forgings are currently widely used in the production of automotive steering knuckle forgings. For steering knuckles produced in different ways, due to the differences in forming methods, machining allowance distribution and forging parting structure, targeted analysis should be carried out during mechanical processing, and measures should be taken according to different situations, so that different positioning and cutting methods can be used in the process of center hole processing of the supporting shaft, flange processing and fork processing, so as to obtain good processing technology.