Precision Workholding Design for Delicate Sleeve Component Grinding
This paper examines the challenges associated with clamping and positioning workpieces during external cylindrical grinding, as well as the tendency for deformation after grinding. This is particularly relevant for thin-walled sleeve-type components that require high precision, such as diesel engine tappets. Based on the structural characteristics of these parts, we propose a well-designed fixture solution to address these issues. This approach ensures compliance with product processing requirements and serves as a reference for the machining of similar components.
1. Introduction
In daily production, a large number of thin-walled sleeve-type parts are encountered. These parts require high machining precision and are susceptible to deformation. Due to structural limitations, many of these parts cannot be machined with a center hole, which complicates the machining process.
The tappet is a moving part in a diesel engine and is a core component of the valve lift mechanism. This mechanism converts the rotation of the camshaft into the vertical motion of the valve stem, controlling the opening and closing of the intake and exhaust valves to ensure the engine operates normally.
One end of the tappet is equipped with a roller and roller pin, while the other end has a spherical surface that mates with the tappet head’s spherical surface. The tappet is mounted within a guide cylinder, and during diesel engine operation, the tappet and guide cylinder slide relative to each other.
Precise positioning of the roller and the tappet head is crucial to ensure a perfect fit between the roller and the camshaft cam profile, as well as between the tappet and the tappet head seat sphere. To maintain smooth movement of the roller along the camshaft cam profile during high-speed operations, the contact ratio between the roller and the cam profile must be above 95%. This helps prevent eccentric wear of both the tappet and the roller.
The structure of the tappet is complex, imposing stringent dimensional and geometric tolerances on the outer diameter, roller groove, and roller pin hole. External cylindrical grinding is a critical step in finishing this component. However, this process faces significant challenges due to factors such as part deformation and difficulties in clamping and positioning, often leading to production bottlenecks. Therefore, research into precision grinding technology for these components is essential to improve the machining quality and efficiency of diesel engine tappets.
2. Technical Requirements
The structure of a diesel engine tappet is illustrated in Figure 1. It is made from low-carbon alloy steel and undergoes carburizing and quenching processes, resulting in a hardness of 60-63 HRC. The outer diameter of the tappet, denoted as dimension d, requires high precision with a tolerance of 0.03 mm, a surface roughness of Ra = 0.5 µm, and an outer cylindricity of φ0.03 mm. Achieving these specifications necessitates fine grinding on an external cylindrical grinder. Additionally, both the roller pin hole and roller groove must maintain high geometric tolerances on the outer diameter, which demands precise machining of the outer surface.
3. Difficulty Analysis
Figure 1 illustrates that this component is a complex, thin-walled precision part. One end has a roller groove and a machined roller pin hole. The ratio of the roller groove depth (H) to its width (L) is 1.8:1, resembling a cantilever structure. This design results in poor rigidity, making it susceptible to deformation under clamping forces. The other end contains a blind hole with a thin wall thickness, which complicates clamping and positioning. This thin-walled component can also deform under high forces. Additionally, the part’s outer diameter requires full grinding; however, due to its limited structure, adding a process chuck is not feasible. Therefore, it is necessary to resolve how to rotate the part using the machine tool during the grinding process.
4. Fixture Design
Considering the structural characteristics and technical requirements, two process solutions were developed for the cylindrical grinding process.
4.1 Solution 1
Grinding was conducted on a MG1432 cylindrical grinder using a specialized fixture designed to utilize two centering points for the grinding process. The stop size D1 of the part is finely machined to the final dimensions, with the stop tolerance automatically adjusted to H7.
An expandable mandrel is employed to locate and clamp the part at its stop. The locking nut (1) is tightened to expand the outer circle of the expansion sleeve (2) of the expandable mandrel, creating an interference fit with the stop of the part, ensuring that the workpiece is both centered and secured. Simultaneously, the outer circle of the expandable mandrel is flattened to accommodate the heart-shaped chuck, which drives the part to rotate during the grinding operation.
The other end of the part features a roller pin hole that is fine-machined to D2, where D2 equals D – 0.5, with D representing the final size of the roller pin hole. The D2 pin hole is utilized for positioning. The tooling is designed to insert a locating pin (3) into the roller pin hole. The locating pin (3) and the D2 hole are paired with a small clearance of H7/g6. Additionally, the locating pin (3) is inserted into the locating hole of the pull rod (5), which is also matched with H7/g6.
The clamping nut (4) is then tightened to secure the parts in place. A B2.5/8 type center hole is drilled at one end of the pull rod; the diameter of the guide hole for the type B center hole is 2.5mm, while the diameter of the large end is 8mm.
Before proceeding with the machining, the outer circle of the part should be aligned to achieve a radial runout of ≤0.05mm. After this alignment, the clamping nut can be tightened, and processing can continue.
4.2 Option 2
As illustrated in Figure 4, two centers are utilized for positioning, and grinding is performed on the MG1432 cylindrical grinder. The dimension of the part’s stopper, D1, is precisely CNC machined to its final size, with the stopper tolerance increased to H7. A positioning fixture is designed using the part’s stopper as the reference point. The direction and application point of the clamping force are carefully chosen to prevent deformation of the part during the clamping process. The positioning fixture and stopper D1 are fitted with a tight H7/g6 clearance for accurate positioning.
A hook pressure plate 2 is designed to apply pressure against the raised section of the part’s inner hole. The fixture is secured with nut 3, which fastens both the fixture and the part. To help operators identify the orientation of the hook pressure plate, a marking line is engraved on its end to indicate the direction. Additionally, the outer diameter of the positioning shaft is flattened to facilitate the installation of the heart-shaped chuck.
The other end of the metal stamping parts is processed with a 1mm × 60° chamfer at the oil hole D3 (see Figure 1). Prior to processing, the alignment of the outer diameter runout is set to ≤0.03 mm, and the coaxiality between the centerline of the processed chamfer and the outer circle axis is ensured to be ≤φ0.05 mm. The extended center 4 is used to support the 1mm × 60° chamfer of the oil hole D3, allowing for precise grinding of the tappet’s outer circle d, as shown in the actual grinding fixture for the outer circle in Figure 5.
5. Results Verification
The results of the machining verification for the two solutions are as follows:
Scheme 1 requires aligning the outer diameter of the part before machining. Once the part is clamped using the roller groove end, repeated alignment is necessary due to deformation caused by the clamping process. After precision machining is completed and the fixture is removed, the part shows signs of deformation. Testing indicates a pass rate of 68%, with a machining time of 30 minutes per part.
Scheme 2 allows for quick and easy installation of the fixture, requires low clamping force, and does not necessitate any alignment. After machining, there is no deformation due to clamping. Testing yields a pass rate of 95%, with only 5% of the parts being out of tolerance. The machining time per part is reduced to 15 minutes.
In comparing the machining results of the two fixture solutions, it is evident that Scheme 2 produces minimal deformation, a high pass rate, and greater machining efficiency, making it more suitable for mass production.
6. Conclusion
This paper examines the design of a fixture specifically for fine grinding the outer diameters of thin-walled precision components, such as diesel engine tappets. It addresses the challenges of deformation and difficult clamping that arise from conventional machining methods used for these parts. The proposed solutions enhance both machining quality and efficiency. This research offers valuable insights for the design of fixtures for similar components and shows potential for widespread adoption in the industry.
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