Precision Challenges: Controlling Deformation in Composite Brake Drum Production

When the steel composite brake drum is clamped, the clamping position experiences elastic deformation. After processing, when the clamping mechanism is released, the position undergoes a change in deformation. By testing the clamping force and the uniformity of the clamp, we can identify the main cause of this deformation. Through targeted improvements, the two parts connected by each rotating shaft of the clamp are enabled to rotate relative to each other. This design ensures that during the clamping process, the clamping force is evenly distributed across all clamping points. As a result, it effectively secures the workpiece while also controlling the processing deformation of the steel composite brake drum within the specified range.

 

1. Preface

As society continues to develop, people’s expectations for the performance and comfort of automobiles are becoming increasingly higher. Consequently, automobile manufacturers are also placing greater demands on automotive parts. A critical component of the automobile braking system is the brake drum, which plays a significant role in determining the reliability and safety of the entire braking system.

 

In addition to requiring high strength, stiffness, and wear resistance, brake drums must also possess precise dimensional accuracy and reasonable geometric tolerances. Since the brake drum is a thin-walled part, it tends to undergo elastic deformation when clamped. This clamping can lead to changes in dimensions after processing, making it challenging to control size and geometric tolerances.

 

If the inner diameter of the brake assembly becomes deformed, the effective size may be reduced. This could result in an assembly gap that is too small when fitted with the wheel hub, potentially leading to assembly failure and scrapping of the component. Similarly, if the brake surface is deformed, the runout of the brake surface may exceed tolerance levels, causing uneven gaps between the brake surface and the brake pad.

 

During braking, poor contact between the brake surface and the brake pad can decrease braking friction, significantly impairing the vehicle’s braking performance and affecting operational stability and driving safety. Additionally, noise may be generated from localized friction and collisions between the brake surface and the brake pad, diminishing driving comfort.

 

Addressing the issue of processing deformation is a complex but urgent challenge that needs to be resolved.

 

2. Current status and analysis of steel composite brake drums

In terms of design, the wall of steel composite brake drums is thinner than that of conventional gray iron brake drums, and steel is more prone to deformation than gray iron. During processing, the inner diameter of the assembly and the circular runout of the braking surface often exceed tolerance limits, which leads to frequent rework, repairs, or scrapping. To address this issue, the tolerance range must be narrowed to control the size of the inner diameter within upper and lower limits, increasing both processing difficulty and costs.

 

The outer layer of the steel composite brake drum consists of a steel shell that is formed by spinning steel plates. The shape and size of the outer circle can vary significantly. Currently, the existing moving parts of the fixture cannot rotate relative to each other or be adjusted based on the shape changes of the blank surface. The machining of the steel composite brake drum is completed in one clamping setup, and all lathe operations are performed in a single sequence (see Figure 1). This approach minimizes cumulative errors resulting from multiple clampings and reduces the circular runout of the braking surface.

 

However, after processing, the clamp was released, and both the assembly inner diameter and braking surface experienced deformation. To determine whether this deformation resulted from excessive clamping force or uneven clamping force, two sets of tests were conducted.

 

3.1 Test plan

Tests were conducted from two perspectives: the clamping force of the fixture and the uniformity of the fixture’s clamping force to determine the primary cause of deformation.

 

3.2 Effect of clamping force on deformation

The test was conducted on a CNC vertical lathe equipped with a hydraulic self-centering chuck. A fixture was mounted on the hydraulic chuck, allowing the chuck’s pressure to be distributed to the clamping points through the base and jaw arm of the fixture. Initially, the chuck pressure was set at 3.2 MPa. Other conditions remained unchanged during the experiment. The chuck pressure was then reduced to 2 MPa. Circular runout and inner diameter roundness measurements of 12 brake surfaces were taken continuously before and after the pressure adjustment for comparison. The dimensional results are presented in Table 1.

 

As shown in Table 1, reducing the chuck pressure, or clamping force, can improve deformation to some extent. However, this approach does not fully meet the requirements outlined in the drawing. When the pressure is decreased beyond a certain point, the clamping force becomes insufficient, leading to potential movement of the CNC rapid prototyping workpiece during processing. In the worst-case scenario, this could result in the workpiece being scrapped, or even flying out of the fixture, which would pose a serious safety risk. Therefore, simply reducing the clamping force cannot adequately control deformation within the required parameters.

 

3.3 The influence of clamping force uniformity on deformation

(1) During the clamping process, the clamp adjusts its position according to the shape of the outer circle of the blank barrel, ensuring that the clamping force is evenly distributed.

 

(2) Clamp Design

The clamp consists of upper and lower layers. Each clamp is equipped with two toothed ball screws that secure the outer circle of the brake drum barrel. Each set of clamps features 24 clamping points. When in use, the two parts connected by each shaft can rotate relative to one another. During the clamping process, the pressure from the self-centering chuck is evenly distributed across all 24 clamping points.

 

(3) Clamp composition and structure

The clamp consists of a base, a clamp arm, a clamping block, a rotating shaft, and upper and lower clamps connected together by bolts (see Figure 2).

 

1) The base comes equipped with T-slots, comb teeth, bolt holes, positioning holes, and screw holes. During installation, the base is positioned on the hydraulic self-centering chuck using a combination of T-slots and T-nuts. It is then securely fastened to the hydraulic self-centering chuck with bolts and T-nuts.

 

2) The clamp arm features positioning holes, bolt holes, and screw holes. It is attached to the base by means of rotating shaft 1 and an elastic sleeve. The rotating shaft 1 is secured to the base with bolts. After this, the clamp arm is locked in place on the base using a locking screw (refer to Figure 3).

 

3) The clamping block is equipped with upper and lower positioning holes, as well as left and right positioning holes, bolt holes, and screw holes. It is mounted onto the clamping arm using rotating shaft 2 and an elastic sleeve. After positioning, rotating shaft 2 is fastened to the clamping arm with bolts. Finally, the clamping block is secured to the clamping arm using a locking screw. (see Figure 4).

 

4) Positioning holes for bolts and screws are located on both the upper and lower clamping jaws. The upper and lower clamping jaws are mounted on the clamping block using rotating shaft three and an elastic sleeve. Subsequently, rotating shaft three is secured to the clamping block with bolts. Finally, the upper and lower clamping jaws are locked to the clamping block using locking screws. (see Figure 5).

 

(4) Fixture installation

The fixture is mounted on a hydraulic self-centering chuck. Before use, the upper and lower jaws must be self-machined. Once self-machined, the circle formed by the upper and lower jaws should be concentric with the center of the chuck, and the wall thickness of the machined parts must be uniform.

 

After the fixture is self-machined, loosen the locking screws in various parts. It is important that the two CNC machining components connected by each shaft maintain a gap of 0.03 to 0.05 mm. When the fixture is clamped, if the outer circle of the cylinder is irregular and the clamping force is uneven, the elastic sleeve may be stressed, leading to compression deformation. In this situation, the jaw arm can rotate slightly relative to the base, the clamping block can rotate slightly relative to the jaw arm, and the upper and lower jaws can also rotate slightly relative to the clamping block. This adjustment helps ensure that the clamping force is evenly distributed across all clamping points.

 

(5) Testing and verification of the fixture

Make a set of fixtures according to the above plan, and track and record the use of the fixture for test verification.

 

1) After the new fixture is manufactured, it is installed on the hydraulic self-centering chuck of the CNC vertical lathe (refer to Figure 6). Once the components are secured with locking screws, they cannot rotate relative to one another, and self-turning is then performed. To assess how uniform and uneven clamping forces affect deformation at each clamping point of the fixture, the locking screws are not loosened after self-turning. Consequently, the two parts connected by each shaft remain fixed and cannot rotate relative to each other.

 

The outer circle of the cylinder, due to its irregular shape and size, is clamped, leading to uneven clamping forces at each point. The circular runout and inner diameter roundness of 12 brake surfaces were processed and measured continuously, and the results are summarized in Table 2.

 

 

2) To maintain consistent pressure and other conditions, first loosen the locking screws. This allows the two parts connected by each shaft to rotate relative to one another while securely clamping the outer circle of the cylinder, regardless of its irregular shapes and sizes. The clamping force at each point is evenly distributed. Continuously process and measure the circular runout and inner diameter roundness of 12 brake surfaces. The results are presented in Table 3.

 

The following conclusions can be drawn from the data in Tables 2 and 3.

1) When the two components connected by each rotating shaft cannot rotate relative to one another, the clamping point cannot be adjusted based on the shape changes of the outer circle of the blank cylinder. During the clamping process, the clamping force at each point of the fixture is uneven, resulting in areas of excessive force. This can cause the workpiece to deform once the fixture is loosened after processing. As a result, the roundness of the braking surface and the inner diameter is generally poor. The roundness of the braking surface typically meets only 50% of the requirements, while the inner diameter roundness meets only 33% of the standards.

 

2) When the two parts connected by each rotating shaft can rotate relative to each other,

The clamping point can be adjusted to accommodate changes in the outer circle shape of the blank cylinder. During the clamping process, the clamping force at each point of the fixture is uniform. After processing, when the fixture is released, the workpiece is less likely to deform. The roundness of the braking surface and the inner diameter is generally minimal. The roundness of the braking surface can meet a requirement of 100%, as can the roundness of the inner diameter.

 

3) Five sets of fixtures were manufactured and installed on various CNC vertical lathes. After two weeks of usage, a statistical analysis was conducted on the performance of the fixtures. The results indicated that both the circular runout of the brake surface and the roundness of the inner diameter met the requirements with 100% compliance.

 

4. Conclusion

The results from the two sets of tests on clamping force size and uniformity indicate that the primary reason for the deformation of the brake drum during processing is the uneven distribution of clamping force. When the fixture is designed to allow the two parts connected by the rotating shaft to rotate relative to each other, it ensures that the clamping force is evenly distributed across all clamping points. This design minimizes elastic deformation during clamping, allowing the processing deformation of the brake drum to be maintained within the required range.

 

 

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