A process analysis was conducted on the irregular eccentric structure of the Z-shaped balance elbow, as well as the difficulties in machining due to its large size, high precision, and inability to be clamped. A standardized machining scheme was proposed. A special lathe fixture was designed, which is suitable for machining multiple models and large batches of products, solving the problems of high machining difficulty, unstable quality, and low machining efficiency of the Z-shaped balance elbow.
01
Introduction
The balance elbow is the core component of the suspension system of tracked special vehicles. It works in conjunction with elastic elements such as the torsion shaft and shock absorber to provide elastic support for the vehicle body and the road wheel [1]. The balance elbow can transfer a large amount of impact energy generated by the up-and-down movement of the road wheel to the torsion shaft, buffer and absorb vibration energy, reduce the impact force on the vehicle body, improve passenger comfort, reduce component damage, and ensure the stability and maneuverability of the vehicle when driving on rough roads [2]. The general balance elbow assembly includes a spline shaft, balance elbow, and road wheel shaft. The Z-shaped balance elbow is an integrated balance elbow that combines all three components. The integrated balance elbow has the characteristics of high maneuverability, high reliability, and lightweight, and is widely used in modern special vehicles [3]. 02
Analysis of the Structure and Machining Challenges of the Z-shaped Balance Elbow
The Z-shaped balance elbow structure, as shown in Figure 1, is an irregularly shaped eccentric structure. It is large in size and heavy in weight, requiring a high material removal rate, high dimensional and positional accuracy, and a long machining cycle with numerous processes. It involves various equipment and trades, including horizontal boring machines, CNC lathes, machining centers, broaching machines, and wire EDM machines. In previous production, numerous problems were exposed in each process step, such as out-of-tolerance dimensions of the shaft outer circle, misalignment between the spline hole and the outer circle, non-parallelism between the axes of the large shaft (spline shaft) and the small shaft (load wheel shaft), substandard surface roughness, unbalanced process cycle time, and low machining efficiency, seriously affecting product quality and delivery schedule.
03
Process Scheme
Z-shaped balance elbow machining process: Rough boring of end face and inner hole → Rough turning of large shaft → Rough turning of small shaft → Finish boring of end face and inner hole → Finish turning of large shaft → Finish turning of small shaft → Milling of outer shape and drilling → Broaching (wire EDM spline). The product blank is a die forging. When roughing and finishing the large and small shafts, a special lathe fixture with counterweights is required. This fixture balances the centrifugal force generated during workpiece rotation, thereby reducing vibration and increasing spindle speed, effectively improving product machining accuracy and cutting speed.
04
Machining Process
(1) Rough boring of end faces and inner holes: A CNC horizontal boring machine is used. A 2mm allowance is left on each side for both the end faces and inner holes. The main function of this process is to quickly remove a large amount of material and to create process center holes for the rough-machined outer diameters of the large and small shafts. The CNC horizontal boring machine has a 360° rotating worktable, allowing for universal machining in the XOY plane in one setup. It can machine four end faces and inner holes at once, ensuring that the process center holes at both ends of the large and small shafts are coaxial and that the center lines of the large and small shafts are parallel. The rough boring of the end faces and inner holes is shown in Figure 2, where the thick solid line represents the machined surface.
Figure 2. Rough boring of end face and inner hole
(2) Rough turning of the large shaft: Machining is performed on a CNC lathe, with a 1.5mm allowance on each side of the outer diameter. The main function of this process is to quickly remove a large amount of material and to create a process reference for the finish boring of the end face and inner hole. Because the center of gravity of the Z-shaped balance elbow shifts from the center of rotation during turning, a special lathe fixture with counterweights is required to balance centrifugal force, reduce vibration, and increase spindle speed. The rough turning of the large shaft is shown in Figure 3.
Figure 3. Rough turning of the large shaft
(3) Rough turning of the small shaft: Machining is performed on a CNC lathe, with a 1.5mm allowance on each side of the outer diameter and end face. The main function of this process is to quickly remove a large amount of material and release machining stress. The rough turning of the small shaft is shown in Figure 4. After rough turning, the large and small shafts have regular outer diameters. A V-shaped clamp is used for more stable finishing during finish boring.
Figure 4. Rough turning of the small shaft
(4) Finish boring of the end face and inner hole: This is done using a CNC horizontal boring machine, with the rough turning of the outer diameter of the large shaft as the process datum and clamping datum. The main function of this process is to machine the four end faces and inner hole of the product to the finished size, ensuring dimensional accuracy and surface roughness, while also creating a chamfer for the finish turning of the outer diameters of the large and small shafts. The finish boring of the end face and inner hole is shown in Figure 5, where the thick solid line represents the surface machined in this process.
Figure 5. Finish boring of the end face and inner hole
(5) Finish turning of the large shaft: This is done using a CNC lathe, machining the outer diameter to the finished size, ensuring dimensional accuracy, geometric accuracy, and surface roughness. This process uses the chamfer created during the finish boring of the inner hole as the clamping datum, ensuring the coaxiality of the outer diameter and inner hole of the large shaft. The finish turning of the large shaft is shown in Figure 6.
Figure 6. Precision turning of the main shaft
(6) Precision turning of the small shaft: Using a CNC lathe, the outer diameter is machined to the finished size, ensuring dimensional accuracy, geometric accuracy, and surface roughness. This process uses the chamfer made during precision boring of the inner hole as the clamping and positioning reference, ensuring the coaxiality of the outer diameter of the small shaft and the inner hole, as well as the parallelism of the center lines of the small shaft and the main shaft. The precision turning of the small shaft is shown in Figure 7.
Figure 7. Precision turning of the small shaft
(7) Milling the outer shape and drilling: Using a vertical machining center, the product's outer shape is machined to the desired position, and dowel holes are drilled. The milling of the outer shape and drilling is shown in Figure 8, where the thick solid lines represent the machined surfaces in this process.
Figure 8. Milling the outer shape and drilling
(8) Broaching the spline (wire EDM spline): When the product batch is large, this process is generally carried out using a broaching machine with a broaching tool, ensuring production efficiency and consistency of spline hole dimensions. When a broach is unavailable and the batch size is small, this process can be performed using a wire EDM machine. The outer diameter of the main shaft serves as the clamping and positioning reference, ensuring the coaxiality of the spline hole with the outer diameter of the main shaft. The broached spline (wire-EDM spline) is shown in Figure 9, where the thick solid line represents the surface machined in this process.
Figure 9: Broached Spline (Wire-EDM Spline)
At this point, the Z-shaped balance elbow product has completed all machining processes. Subsequent processes include flaw detection and surface treatment.
05
Dedicated Lathe Fixture
The dedicated lathe fixture includes components such as a flange, chassis, support body, center, counterweight, and fastening bolts [4, 5].
The flange serves as the connecting component between the lathe and the fixture. A standard flange is generally used. One end connects to the CNC lathe spindle through a tapered hole, and the other end connects to the chassis through a locating boss, ensuring that the rotation center of the lathe fixture is aligned with the lathe spindle.
The chassis, serving as the base of the lathe fixture, features an elongated circular groove. A support body and two counterweights are attached to it, and this three-point mass distribution ensures smoother workpiece rotation during machining, reducing vibration and improving the product's external cylindrical accuracy and surface quality.
The support body, welded to the chassis, also features an elongated circular groove with dimensions matching those of the chassis's groove. This groove serves two purposes: reducing the overall weight of the lathe fixture and preventing interference between the non-machining axis of the Z-shaped balance elbow and the lathe fixture. Several sets of fastening bolts are located at both ends of the groove to secure the non-machining axis of the Z-shaped balance elbow. The elongated circular groove design allows for the machining of Z-shaped balance elbows of different sizes and models using this lathe fixture, achieving multi-purpose functionality.
The center and the support body's positioning stop are fitted together and welded to the support body. During machining, the center and the lathe tailstock center respectively support the two ends of the Z-shaped balance elbow's machining axis, achieving a double-center clamping configuration. To ensure the center's positioning cone surface is coaxial with the lathe spindle, the center's positioning cone surface must be precision-machined on a lathe after the lathe fixture is welded together. The counterweight consists of multiple fan-shaped counterweight plates. The number of counterweight plates can be adjusted to balance the centrifugal force generated during machining of different models of Z-shaped balance elbows. The two counterweights are evenly distributed at 120° to the center of gravity of the support, thus better ensuring dynamic balance during product machining.
To accommodate the machining of Z-shaped balance elbows of different sizes, multiple sets of fastening bolts are installed on both sides of the long oval groove of the support body. The clamping positions of the fastening bolts are shown in Figure 10. The centerline of each set of fixing bolts is higher than the distance H between the outer circle center of the non-machined axis of the Z-shaped balance elbow. This clamping method ensures that the clamping force of the fixing bolts on the Z-shaped balance elbow is opposite to the centrifugal force, effectively reducing the centrifugal force generated when the workpiece rotates. The clamping states for machining small shafts and large shafts are shown in Figures 11 and 12, respectively.
Figure 10. Schematic diagram of fastening bolt clamping position
Figure 11. Clamping state when machining the small shaft
Figure 12. Clamping state when machining the large shaft
06
Verification of Machining Results
Currently, this Z-shaped balance elbow machining process and dedicated lathe fixture have been applied to the production line for over a year. Multiple models and large batches of Z-shaped balance elbow products are machined using this process, resulting in stable and reliable product quality and significantly improved machining efficiency. This fully verifies the feasibility and effectiveness of the process and dedicated lathe fixture. A photograph of some machining steps of the Z-shaped balance elbow is shown in Figure 13.
a) Rough boring of the end face and inner hole
b) Rough turning of the main shaft
c) Finish boring of the end face and inner hole
Figure 13: Actual photos of the machining process of the Z-shaped balance elbow
07
Conclusion
The machining process and special lathe fixture for the Z-shaped balance elbow proposed in this paper are applicable to the machining of various models of Z-shaped balance elbow products, regardless of product material or blank type. It provides a complete process and clamping approach for this type of product, solving the problems of high machining difficulty, inability to clamp, unstable product quality, and low machining efficiency caused by the irregular eccentric structure, large size, heavy weight, and high precision requirements of the Z-shaped balance elbow. Field verification shows that this machining process can ensure that the Z-shaped balance elbow products meet the design accuracy requirements, reduce machining difficulty, and improve product quality consistency and machining efficiency.
