Intro to High-Precision Spindle and How to Improve Its Accuracy

I. Types and Characteristics of High-Precision Spindles

High-precision spindles mainly refer to the spindle system structures such as sliding bearing spindle system, rolling friction spindle system, gas static pressure bearing spindle system, liquid static pressure bearing spindle system, and dynamic pressure bearing spindle system. The subcategories and characteristics of each type of spindle system are explained below.

1. Sliding Bearing Spindle System

(1) Semi-dynamic cylindrical bearing spindle system

There is a ring of precision balls between the conical surface of the bearing sleeve and the cylindrical surface and shoulder end surface of the spindle, forming a conical rolling support, which bears the axial force and has an automatic centering function.

Characteristics: Compared with the cylindrical spindle system, the rotation accuracy has been improved by more than twice, reaching 1, and the movement is flexible, not very sensitive to temperature, and has a longer life. It is often used as a structure for theodolites.

(2) Conical sliding bearing spindle system

The spindle and the bearing sleeve have the same taper, separated by a thin film of lubricating oil.

Characteristics: Easy to repair after wear, high mating accuracy, adjustable working clearance, and rotation accuracy can reach 0.2um. However, continuous lubricating oil will form oil clumps, causing bi-weekly swing errors. It is commonly used as a spindle structure for circular engraving machines.

(3) V-shaped arc sliding spindle system

Each V-shaped sliding spindle system has 3 cylindrical surfaces immersed in oil, and the oil clumps formed by bi-weekly swings cannot be destroyed, and the oil film thickness is uniform.

Characteristics: It can eliminate bi-weekly swing errors, and compared with the conical sliding spindle system, it has greatly improved in rotation accuracy. It is commonly used on instruments such as dial gauges and photoelectric inspection instruments for circular indexing.

2. Rolling Friction Spindle System

(1) Standard rolling bearing spindle system

(2) Single-row rolling bearing spindle system

Characteristics: The balls of the upper and lower bearings are arranged in a single row, with a simple structure, easy manufacture, installation, and maintenance, and the rotation accuracy can reach 0.1um. It is often used in small, low-speed light-load instruments.

(3) Dense ball bearing spindle system

The balls are densely distributed between the spindle and the bearing sleeve, and the matching form is interference fit, thereby reducing the impact of manufacturing errors of various components on the spindle system accuracy, thereby improving the spindle rotation accuracy. The interference of the balls is equivalent to preloading, and it plays a role in eliminating gaps and reducing the impact of geometric shape errors through a small amount of elastic deformation. The dense balls and interference fit work together to equalize the size and shape errors of the bearing sleeve, spindle, and balls, improving the spindle system rotation accuracy.

Characteristics: Low cost, easy to use, long life, rotation accuracy can reach 0.1um, but the bearing capacity is not large. It is often used in light-load, low-speed small instruments.

3. Gas Static Pressure Bearing Spindle System

A bearing system composed of purified air at a certain constant pressure filling the space between the bearing sleeve and the shaft, using it as a lubricating medium.

Characteristics: Because the viscosity of air is extremely small, there is almost no friction torque, there is no wear of parts during normal operation, and there is no need for maintenance and adjustment during work. It can be used in special environments. It has a mean effect, and the rotation accuracy is extremely high, reaching 0.01pm. It is often used as a bearing for high-precision instruments.

Disadvantages: High cost, requires an oil-free, water-free, dust-free air source, requires complex auxiliary equipment, and is not highly rigid.

4. Liquid Static Pressure Bearing Spindle System

Similar to the principle of gas static pressure, the substance between the bearing sleeve and the shaft is replaced with pressure oil, and the spindle system is floated for work.

Characteristics:

1) The friction form is liquid friction, the friction force is extremely small, there is almost no wear, the life is long, the rotation is flexible, and the power consumption is small;

2) Higher rigidity and larger bearing capacity than the gas static pressure spindle system. It is often used on large or heavy-duty instruments and is widely used on machine tools;

3) Under average action, the rotation accuracy can reach 0.05pm;

4) Good vibration resistance;

5) High cost. An additional set of high-quality oil supply system is needed. After the oil temperature changes, it will cause thermal drift of the rotation center, so an oil temperature control system needs to be used in conjunction.

5. Dynamic Pressure Bearing Spindle System

In the process of relative movement between the spindle and the bearing, the lubricating oil between them forms an oil wedge. The change of the oil wedge during rotation can generate oil dynamic pressure, thereby lifting the spindle to avoid friction between the spindle and the bearing sleeve.

Characteristics:

1) High rotation accuracy, radial error motion can reach 0.025pum.

2) Large bearing capacity, the bearing capacity of this spindle system is generated after the spindle rotates, that is, the spindle has a large bearing capacity when rotating.

3) Good rigidity.

4) No wear under dynamic conditions, long life.

5) Manufacturing, use, and maintenance are all relatively convenient, the structure is simpler than liquid static pressure, and no hydraulic pump station is needed.

6) When starting, the spindle and the bearing are in rigid contact, there is wear, and it is difficult to establish an oil film under low speed and heavy load.

7) The spindle can only rotate in the direction of reducing the oil wedge, and cannot reverse.

II. Methods to Improve Spindle Accuracy

1. Improving the Manufacturing Precision of Spindle Components

(1) Precision machining after assembly. For CNC machine tools, their spindles are more precise, requiring sufficiently high rigidity and rotation accuracy. In machine tool manufacturing, the machining accuracy requirements for spindle system components are high. It is very difficult to achieve certain accuracy indicators for the whole machine by controlling the machining quality of parts one by one. For example, the spindle assembly of a coordinate boring machine, the allowable jump tolerance of the spindle taper hole axis is only 0.001-0.002mm. If it is entirely dependent on the accuracy of the spindle or bearing, it will put forward accuracy requirements that are currently unattainable. In order to ensure the accuracy of the spindle assembly, the process adopts the method of precision machining after assembly, which is assembled first, and then the outer circle of the spindle sleeve is precision ground on a precision cylindrical grinder with the two ends of the spindle taper hole as the reference. Then, with the outer circle of the sleeve as the reference, the spindle taper hole is precision ground. After the precision grinding is completed and the inspection is qualified, disassemble and clean, and then assemble it into a finished product. In the maintenance of such machine tools, if the bearing needs to be disassembled, markings should be made between the outer ring of the bearing and the body or sleeve hole, and between the inner ring of the bearing and the shaft neck before disassembly. Otherwise, if it is misplaced when reassembled, it cannot guarantee the accuracy before disassembly, and it will be twice the result with half the effort.

(2) The same position and direction matching method. Using the same position and direction matching method, before assembly, find the highest point of the eccentric direction of the center of the inner hole of the front and rear bearings to the raceway, make assembly marks, and then find the lowest point of the eccentric direction of the center line of the spindle low degree hole, and also make assembly marks; then install the front and rear bearings according to the same position and direction matching method, and align the assembly marks of the spindle with the assembly marks of the bearing, which can control the offset of the center line of the spindle taper hole and improve the rotation accuracy of the spindle. The method is simple and reliable to operate. Practice has proved that this method is one of the best methods to improve the rotation accuracy of the spindle.

(3) Error matching method. For the spindle with a taper hole, when measuring its spindle rotation accuracy, it is necessary to match the measuring rod in its spindle taper hole. The radial runout of the end of the measuring rod is equal to twice the vector sum of the error a generated by the front bearing, the error b generated by the rear bearing, and the machining error c of the spindle taper hole. a, b, c all have a certain direction, so these three errors can cancel each other out if they are assembled in a certain direction. First, measure the radial runout Δa and Δb of the inner rings of the front and rear bearings and their directions, and then calculate a and b. The directions of a and b are determined by the directions of Δa and Δb respectively. Put the spindle (without bearing) on the V-iron, you can measure the size and direction of the machining error c of the taper hole. Adjust the positions of the inner rings of the front and rear bearings and the spindle, adjust these three errors to be on a straight line, let the two smaller ones (a, b) face one direction, and the largest one (c) face the other direction, so the error can be reduced to d, but it cannot be completely cancelled out.

(4) Use high-precision rolling bearings, or adopt high-precision multi-oil wedge dynamic pressure bearings and static pressure bearings. Secondly, improve the mechanical processing precision of the related surfaces of the box body support hole, spindle neck, and parts that cooperate with the bearing. In addition, before assembly, the radial round runout of the rolling bearing and the spindle taper hole can be measured, and then the position of the radial round runout can be adjusted to compensate or cancel each other to reduce the impact of bearing errors on the spindle rotation accuracy.

2. Methods to Improve Spindle Rotation Accuracy during Use

(1) Measures to eliminate spindle rotation errors. If the rotation accuracy of the machine tool spindle itself is low, an appropriate dial can be designed to separate transmission and positioning, eliminating the error caused by the radial runout of the spindle.

(2) Proper pre-tightening of rolling bearings can eliminate clearance, and even produce a small amount of interference. Due to the mutual restraint of the elastic deformation of the inner and outer rings of the bearing and the rolling body, it not only increases the rigidity of the bearing, but also equalizes the errors of the inner and outer ring raceways and the rolling body, thereby improving the rotation accuracy of the spindle.

3. Make the Spindle Rotation Error Not Reflect on the Workpiece

Directly ensuring the rotation accuracy of the workpiece during mechanical processing without relying on the spindle is the simplest and most effective method to ensure the shape accuracy of the workpiece. For example, when grinding the outer cylindrical surface on an external cylindrical grinder, in order to avoid the influence of the rotation error of the workpiece headstock spindle, the workpiece is supported by two fixed centers during loading, and the spindle only plays a transmission role. The rotation accuracy of the workpiece is completely determined by the shape error and coaxiality error of the center and center hole. Improving the accuracy of the center and center hole is easier and more economical than improving the accuracy of the spindle components. Also, when machining holes on the parts of the box on a boring machine, a boring model with front and rear guide sleeves can be used. The tool rod is floatingly connected with the spindle, so the rotation accuracy of the tool rod has nothing to do with the rotation accuracy of the machine tool spindle, and is only determined by the fit quality of the tool rod and the guide sleeve.

4. Use High-Precision Parts for the Spindle System

(1) Improve the accuracy and rigidity of the spindle bearing

Under general circumstances, machine tools should use rolling bearings as much as possible, especially for most vertical spindles, using rolling bearings can use grease lubrication to avoid oil leakage. Only when the machining surface roughness is small and the spindle is horizontal, such as external cylindrical grinders, surface grinders, high-precision lathes, etc., sliding bearings are used.

Although rolling bearings have a series of advantages and have been widely used in mechanical equipment, in high-speed, high-precision, heavy-load, and structural requirements for disassembly, sliding bearings must be used. This is because sliding bearings have some characteristics that cannot be replaced by rolling bearings. Its main advantages are: simple structure, easy to manufacture, assemble and disassemble; good impact resistance and vibration absorption performance, smooth operation, high rotation accuracy, long life. The main disadvantage is: complex maintenance, high requirements for lubrication conditions, when the bearing is in the boundary lubrication state, the friction wear is severe.

Due to the factors of its own structure, ordinary sliding bearings are prone to low oil film stiffness, short accuracy retention period, difficult maintenance, high processing surface roughness, and obvious ripple in production. Practice has proved that when the spindle sliding bearing is selected as the liquid friction sliding bearing, it has higher accuracy and rigidity, and the processing quality and productivity can be improved.

(2) The accuracy of the bearing should be compatible with the accuracy of the shaft neck and the bearing seat hole

Reasonably selecting the type and size (model) of the bearing according to the actual work needs is a necessary condition to ensure the normal work of the bearing, but in the process of using the bearing, the relationship between the bearing and other related parts must also be considered, that is, to solve the axial position of the bearing. Fixed, matching with other parts, adjustment, assembly and disassembly, lubrication and sealing and a series of problems. Including the axial fixation of a single bearing, the axial fixation of a bearing combination and other issues need to be considered.

Conclusion

High-precision spindles are an essential component in many machines and instruments, with their accuracy directly affecting the performance and precision of the whole system. Various types of spindles, such as sliding bearing spindle systems, rolling friction spindle systems, gas static pressure bearing spindle systems, liquid static pressure bearing spindle systems, and dynamic pressure bearing spindle systems, each have their unique characteristics and applications. To improve the accuracy of these spindles, it is crucial to enhance the manufacturing precision of spindle components, apply methods to improve spindle rotation accuracy during use, ensure that spindle rotation errors do not reflect on the workpiece, and use high-precision parts for the spindle system. By carefully considering these factors and implementing appropriate techniques, the accuracy and efficiency of high-precision spindles can be significantly improved, thereby enhancing the performance of the machines and instruments in which they are used.