Precision Alignment Methods for Multi-Machine Through-Grinding Applications

To enhance the efficiency of centerless grinding, reduce the labor intensity for operators, and improve product quality, through-type grinding is employed for roughing, semi-finishing, and finishing processes. This technique takes advantage of its extremely high efficiency and machining accuracy, which meet technical requirements.

Through-type grinding facilitates the connection of multiple machines, allowing for adjustments to the center height of the workpiece, the displacement of the guide wheel dresser, the dressing angle of the guide wheel, and the inclination angle of the guide wheel in centerless grinding. Additionally, connection equipment is utilized to achieve efficient processing across multiple machines.

 

01 Preface

Centerless grinding is becoming increasingly popular for machining slender shafts, especially in mass-production scenarios. The efficiency of production improves significantly with larger batch sizes. This grinding method is capable of processing slender shafts with an aspect ratio of up to 25:1 and can achieve high precision levels. The cylindricality can reach as precise as 0.01 mm, straightness can be within 0.005 mm per 100 mm, and surface roughness values can be as low as Ra ≤ 0.65 μm. Consequently, centerless grinding finds extensive applications in industries such as automotive, machine tools, agricultural machinery, and shipbuilding.

There are two primary methods of centerless grinding: direct entry grinding and through-type grinding. Direct entry grinding allows for the processing of multi-step shaft parts through dressing and profiling of the grinding wheel. On the other hand, through-type grinding is achieved by adjusting various parameters, such as the center height of the workpiece, the displacement of the guide wheel dresser, the guide wheel dressing angle, and the guide wheel inclination angle, to meet technical specifications. Each method has its own advantages and disadvantages. Direct entry grinding offers higher processing accuracy and can accommodate special-shaped workpieces; however, its efficiency is lower, and the length of the workpiece is constrained by the width of the grinding wheel. Conversely, through-type grinding has higher efficiency and can handle longer workpieces that exceed the grinding wheel’s width, but it provides lower accuracy and is limited to straight, long workpieces. For this reason, various industries utilize through-type grinding for semi-finishing and finishing processes, benefiting from its high efficiency and sufficient accuracy to meet technical requirements. In the automotive industry and similar sectors, long shaft processing often employs automatic loading and unloading mechanisms, facilitating multi-machine connections that enable automated production from rough grinding through semi-finishing to finishing in a single workflow.

Despite the clear advantages of through-type grinding and multi-machine connections, achieving them remains a significant challenge. This difficulty primarily arises from the need to adjust machine tools and connect multiple machines effectively, which requires a solid theoretical foundation and practical experience.

 

02 Single machine adjustment

2.1 Machine tool inspection

- Machine Tool Feet: The machine tool feet should be leveled and secured with concrete and bolts to minimize vibration.

- Grinding Wheel and Guide Wheel Balance: Use a specialized balance frame, dynamic balancer, or automatic balancer to ensure proper balance of the grinding wheel and guide wheel.

- Grinding Wheel and Guide Wheel Spindle Clearance Inspection: The bearings for the grinding wheel and guide wheel are of the bushing type, with clearance specified between 0.015 to 0.02 mm. If using angular contact precision bearings, the clearance should range between 0.002 to 0.005 mm.

- Grinding Wheel and Guide Wheel Slide Static Pressure Guide Rail Inspection: After the machine tool is started hydraulically, check the floating of the grinding wheel and guide wheel slide static pressure guide rail to determine the floating value. Place a micrometer at each of the four corners of the grinding wheel and guide wheel slide. First, record the value before starting the hydraulic pressure. After hydraulic pressure is applied, observe the floating values (the oil inlet pressure for the hydrostatic guide rail should be adjusted to 0.3 to 0.4 MPa, while the system pressure should be set to 0.9 to 1 MPa). If the hydraulic pressure is adjusted correctly, the floating values at the four corners of the hydrostatic guide rail should be between 0.01 to 0.015 mm. If the floating values are abnormal or inconsistent, check the hydraulic system and ensure the oil distributor is unobstructed and functioning correctly.

 

2.2 Machine tool adjustment

(1) Calculation and adjustment of workpiece center height (H) The details are as follows.
1) Calculation of Center Height: The center height of the grinding workpiece, which is the distance between the center of the workpiece and the center of the grinding wheel, is a crucial parameter that influences the cylindricality of the workpiece. Depending on the specific grinding conditions, the center height should be adjusted accordingly.

Based on theoretical analysis and practical experience, the following formula is generally used for the calculation:

\[ H = \frac{\pi \gamma (\phi_{sand} + \phi_{work})(\phi_{guide} + \phi_{work})}{360[(\phi_{sand} + \phi_{work}) + (\phi_{guide} + \phi_{work})]} \]

In this formula, \( \gamma = a + \beta \) represents the tangent angle of the contact point between the workpiece and the grinding wheel as well as the guide wheel, which usually ranges from 5° to 8° (see Figure 1).

 

2) Adjustment of center height: The center height is primarily adjusted by the thickness of the pallet pad. To minimize errors and enhance rigidity, it is advisable to use a specially designed pad that corresponds one-to-one with the pallet. This approach reduces the number of pads required (see Figure 1).

(2) The adjustment of the displacement (L) of the guide wheel dressing diamond pen is illustrated in Figure 2. The diamond pen moves a distance L behind the axis of the guide wheel. The displacement (L) of the guide wheel dressing diamond pen is related to the height of the workpiece center (H), the diameter of the guide wheel (φ), and the diameter of the workpiece (φ).

Theoretical analysis indicates that the calculation formula for L is given by:

\[ L = H \left( \frac{\phi + \frac{\phi}{2}}{\phi + \phi} \right) \]

When the diameter of the grinding workpiece is relatively small, the formula can be simplified to:

\[ L = H + \left( \frac{\phi}{\phi + \phi} \right) \]

(3) The selection and adjustment of the guide wheel inclination angle, θ, are based on the desired accuracy of the grinding CNC lathe parts and production efficiency. From practical experience, selecting an inclination angle of θ = 2° typically results in better cylindricity, surface roughness, and dimensional accuracy. The adjustment direction for the guide wheel inclination angle should be higher at the input port and lower at the output port of the machine tool.

 

(4) The adjustment of the guide wheel dressing angle, θ₁, is essential for smoothly achieving through-type grinding of the workpiece. This angle must be set to create an ideal hyperbolic surface for the guide wheel. The dressing angle, θ₁, is related to the guide wheel inclination angle, θ, the diameter of the guide wheel, and the diameter of the workpiece. The formula for calculating the guide wheel dressing angle is:

\[
θ₁ = θ \left( \frac{φ_{\text{guide}} + φ_{\text{work}}}{2} \right) / (φ_{\text{guide}} + φ_{\text{work}})
\]

From this formula, it is evident that θ₁ is always less than θ.

 

(5) The selection and adjustment of the linear speed, v, of the guide wheel directly impacts the passing speed and cylindricity of the workpiece. Typically, v is set between 20 and 80 m/min. A faster speed is chosen for lighter workpieces, while a slower speed is preferred for heavier workpieces. If the cylindricity of the grinding workpiece is poor, the speed of the guide wheel should be increased. The general rotational speed, n, is usually set between 20 and 80 r/min.

 

(6) The linear speeds of both the grinding wheel and the guide wheel should be kept as low as possible. A common linear speed is around 150 mm/min, while the dressing speed of the guide wheel is typically set between 150 and 200 r/min.

 

2.3 Adjustment of tooling and auxiliary tools

(1) Selection of the β Angle of the Support Plate

The β angle of the support plate (refer to Figure 3) is a crucial factor that influences the cylindricality of the grinding workpiece. The appropriate size of this angle primarily depends on the strength and rigidity of both the machine tool and the auxiliary tools used.

If any inconsistencies, such as prisms, are observed in the grinding process, one can improve the situation by reducing the β angle. However, it is important to note that lowering the β angle may increase the positive pressure of the workpiece against the guide wheel. Therefore, it is essential to enhance the strength and rigidity of the guide plate in such cases.

Typically, the β angle is set between 30° and 70°: a smaller angle is recommended for fine grinding, while a larger angle is preferred for rough grinding. Additionally, the surface roughness of the working surface of the support plate should be approximately Ra = 0.4 μm, and both straightness and flatness must be maintained at a high standard.

 

(2) Adjustment of the guide plate of the bracket The adjustment of the guide plate is an important link that affects the straightness and cylindricity of the grinding workpiece.

1) When selecting the length of the guide plate, it is important to consider a few key factors. The guide plate typically features an alloy surface with a surface roughness value of approximately Ra = 0.4 μm. It should have high flatness, and its length should be carefully calibrated: if the guide plate is too long, it may complicate adjustment; if it is too short, it can negatively impact the stability of the grinding workpiece, reducing both cylindricity and straightness.

The optimal length of the guide plate is determined by the effective grinding distance of the workpiece from the grinding wheel and the guide wheel. Ideally, this length should be greater than 4/7 of the overall length of the custom aluminum parts.

 

2) Adjusting the axial distance between the input and output guide plates and the grinding wheel is crucial. The recommended axial distance from the grinding wheel to the input and output guide plates (refer to Figure 4) should be between 1 and 2 mm. Additionally, the radial distance between the two guide plates on the side of the grinding wheel should be between 0.5 and 2 mm, while the radial distance on the side of the guide wheel should be half of the one-time grinding feed length. It is particularly important to emphasize that the strength and rigidity of both the guide plate bracket and the guide plates themselves should be reinforced.

 

03. Multi-machine connection

Multi-machine connections in centerless grinding typically involve linking two or more machines to create an automated production unit. This setup enables various grinding processes, such as transitioning from rough grinding to semi-finishing grinding, or from semi-finishing grinding to fine grinding.

Connection Mechanism
To establish a multi-machine connection, additional equipment is required. This equipment mainly consists of a feeding mechanism, conveyor belt, and receiving mechanism. All these components are electronically controlled, with the feeding mechanism managed by PLC (Programmable Logic Controller) microcomputer programming, which regulates feeding and the production rhythm.

Margin Allocation for Multi-machine Connection
In a production unit that moves from rough grinding to semi-finishing grinding, the first machine typically grinds a larger amount, ranging from 0.15mm to 0.30mm in one pass, while the second machine grinds between 0.10mm and 0.20mm. Conversely, when transitioning from semi-finishing grinding to fine grinding, the first machine grinds approximately 0.10mm to 0.20mm, and the second machine grinds from 0.05mm to 0.10mm.

Selection of Grinding Wheel
The choice of grinding wheel material should be based on the workpiece’s material and heat treatment state. However, there is a common factor: ceramic grinding wheels are generally used. The particle size for grinding wheels varies; for coarse grinding, a particle size of 70# to 80# is recommended, while for semi-finishing grinding, a size of 100# is suitable, and for fine grinding, a size range of 100# to 120# is ideal.

Cooling System
For semi-finishing and fine-grinding processes in centerless grinding, it is essential to have a cooling system equipped with a filtering device. The effectiveness of the filtration directly impacts the cleanliness of the cooling medium, which enhances the cooling effect, improves the precision of the ground workpiece, and results in a lower surface roughness value.

 

04. Conclusion

The adjustment of multi-machine connections in centerless grinding is a complex technical challenge that requires thorough research and fine-tuning from several angles, including the machine tools, process parameters, and collaboration among multiple machines. By carefully selecting and adjusting the grinding wheel and guide wheel, optimizing process parameters, and precisely coordinating the collaborative efforts of the machines, we can significantly enhance the accuracy, efficiency, and stability of centerless grinding operations. This improvement serves as a strong foundation for the production and manufacturing needs of enterprises.

In practical applications, it is essential to adapt adjustment methods based on various factors, such as different workpiece materials, specific processing requirements, and conditions at the production site. Continuously summarizing experiences and lessons learned while optimizing adjustment strategies is crucial. As manufacturing technology advances—especially with the integration of intelligent control and big data analysis in centerless grinding—future adjustment methods for multi-machine connections will become increasingly intelligent, precise, and efficient. This evolution will contribute significantly to the high-quality development of the manufacturing industry.

 

 

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