The machine tool spindle is usually presented as one of the elements of the drive mechanism responsible for fixing and shaping the workpiece. Moreover, its interface with the power plant, the bearing part and the working equipment of the unit is so dense that we can talk about the whole infrastructure of this part. One way or another, the spindle unit (SHU) should be considered as the responsible basic mechanism of the machine, providing the function of transmitting torque and the direction of the processing force.
Product Overview
This mechanism is also called a motor spindle and forms one of the key assembly units of modern wood and metal processing machines. The performance and, to a greater extent, the accuracy of the mechanical impact on the workpiece depend on its characteristics. As already noted, we are talking about a whole complex of elements that form the basis of spindle assemblies. The bearings, lubrication system, seals, means for transmitting torque and bearing parts form the basis of this mechanism. Mostly these are components that perform supporting and auxiliary functions to ensure the operation of the nozzle in the form of a cutting tool.
It is generally accepted that the power potential of machine tools primarily depends on the engine. This is true, but only in part. For example, the spindle assemblies of metal-cutting machines have their own frequency range of rotation, causing restrictive conditions for cutting speeds. But it is important to understand that this range has to a greater extent the function of regulating the optimal processing rate with support for sufficiently high accuracy.
Another key function of the spindle is to directly hold the machining tool, and in some cases the workpiece itself. For this type of mount, special clamps and clips are used, such as tool holders and chucks. Therefore, in choosing the tool according to the size of the shank and determining the acceptable parameters of the processing process, it is important to take into account the characteristics of the spindle.
SHU design
During the development of the design solution for the motor spindle, task performers should focus on the maximum reduction of dynamic and vibration loads on the mechanism. Achieving this quality of the working group directly affects the durability of the machine and the quality of processing. For this reason, today the spindle assembly is increasingly being designed as an independent device in a separate housing, which is called the spindle head.
The following data are taken as initial data for the design algorithm:
- Power.
- Rotation accuracy.
- Speed
- Maximum heat for poles.
- Vibration resistance.
- Rigidity.
Based on the initial parameters, a structural scheme, layout details and manufacturing materials are selected. The type of the future machine also influences the selection of various structural solutions. For example, the design of spindle assemblies for high-precision processing equipment is carried out with the expectation of the layout of hydrodynamic bearings that can ensure the accuracy of mechanical stress in the range from 0.5 to 2 microns. For particularly high-speed units with internal grinding heads, special sliding bearings are used, which require air lubrication. Typically, the principles of spindle base construction with an emphasis on supporting high processing speeds from 600 rpm are used for diamond-boring and universal metal-cutting machines. The parameters of components to support low speeds are traditionally calculated for milling, revolving and drilling machines. Here the rule applies, the more delicate the accuracy of the mechanical impact, the higher the spindle torque should be. For complex rough machining and cutting, low-speed configurations are used.
Spindle Assembly Calculation
Rigidity is considered as the main design characteristic. It is expressed by the indicator of elastic displacements in the processing zone under the total effective force from the own elastic deformation of the spindle with its supporting elements. To determine the characteristics of heavily loaded nodes, a strength indicator is also used, and for spindle heads with high revolutions, the minimum resonance index, that is, high vibration resistance, will become a key factor in successful processing.
Almost all spindle assemblies for metal cutting machines are separately calculated according to cutting accuracy. This calculation is performed for bearings based on the coefficient of radial runout of the spindle end. The permissible beat rate depends on the designed accuracy class, in the definition of which the designers proceed from the requirements for the processing process.
The radial runout index on the inner surface of a bearing ring depends on its eccentricity and track errors with rolling bodies. This accuracy parameter is expressed through the effect of the so-called wandering beating. In the process of monitoring the bearings, their compliance with the established standards is determined, after which, if deviations of the product are detected, they can be sent for revision. Among the measures that can further improve the accuracy of bearings for the spindle assembly during assembly, the following can be distinguished:
- The eccentricities of the inner rings and necks of the bearings are located in opposite directions.
- The eccentricities of the outer rings of the bearings and housing holes are also located in opposite directions.
- When installing the eccentricities of the inner rings of the bearings of the rear and front parts, their location should be maintained on the same plane.
Operational properties of
The rigidity and accuracy of the set of important technical and physical parameters of the spindle is not limited. Among other significant properties of this mechanism, it is worth highlighting:
- Vibration resistance. The ability of the SHU to ensure stable rotation without hesitation. It is impossible to completely eliminate the vibration effect, but thanks to careful structural calculations it can be minimized, reducing the effect of sources of transverse and torsional vibrations like pulsating forces in the processing zone and torque in the machine drive.
- Speed The characteristic of the speed of the spindle unit, reflecting the number of revolutions per minute acceptable for the optimal operating state. In other words, the maximum permissible speed, which is determined by the structural and technological qualities of the product.
- Bearing heating. Intense heat generation is a natural derivative in machining at high speeds. Since heating can lead to deformation of the elemental base, this indicator should be calculated during the design process. The most sensitive component of the assembly to the thermal effect is the bearing, the change of shape of which may disrupt the function of the spindle. In order to reduce thermal deformation processes, manufacturers must adhere to the norms of permissible heating of the outer bearing rings.
- Load bearing capacity. It is determined through the coefficient of performance of spindle bearings under conditions of maximum permissible static loads.
- Durability. A temporary indicator indicating the number of hours a product has been in service before major repairs. Provided that the axial and radial stiffness of the spindle assembly are balanced, the durability can reach 20 thousand hours. The minimum operating time to the first failure is two and five thousand hours, which is typical for grinding and intra-grinding machines, respectively.
Materials for the manufacture of
The selection of materials for the element base of the spindle is also a factor in ensuring certain technical and operational properties of the equipment. In lapping, thread-cutting and drilling units, the emphasis is on protection against the effects of torque, and the spindle unit of the milling machine, for example, is assembled based on the action of bending moments. In each case, the material must have sufficient resistance to wear of the actuating surface, as well as the bearing journal. Stability of shape and size is the main condition for the proper operation of the product, largely depending on the characteristics of the applied brand of material.
In machines with accuracy classes N and P, spindles made of steel alloys of grades 40X, 45, 50 are used. In some cases, design decisions may require special refinement of the metal by quenching with induction thermal exposure. Typically, hardening of products by quenching is applied to the executive surfaces and bearing journals as the most critical parts of the part.
For elements of complex shape with conical openings, grooves, flanges and step transitions, steel that has passed volume hardening is used. Such processing technology is permissible only in relation to blanks from which it is planned to make the front parts of the machine spindle assemblies with subsequent cementation. In this case, steel 40HGR and 50X are used.
Equipment with accuracy classes A and B is equipped with spindles made of steel grades 18KhGT and 40KhFA, which have undergone nitriding. The nitrogen treatment process is required to increase the hardness of the part, as well as to maintain the primary shape and size. The increase in strength and structural stability is a prerequisite for spindles used in systems with liquid friction.
In the simplified layout scheme of requirements for materials are not so high. Elements with simple shapes can be made of steel grades 20X, 12XHZA and 18HGT, but even in this case, the preforms are previously subjected to hardening, cementation and tempering operations.
ShU constructional models
The main share of spindle mechanisms used in modern machines has a two-support device. This configuration is optimal in terms of equipment optimization and the convenience of the technical organization of the production process. However, large enterprises also use models with additional support from the third pillar.
Bearing placement configurations are also ambiguous in implementation methods. Today, there are trends towards the transfer of critical regulatory functions to the headstock area, which reduces the effect of thermal influences. In simple models of the spindle assembly, roller bearings are used, which also minimizes the risks of deformation from heat generation and increases the efficiency of adjustment. At the same time, along with an increase in rigidity and an increase in the accuracy of rotation, such mechanisms have a drawback in the form of a decrease in speed. Therefore, this configuration is optimally suited for low speed lathes.
Low-speed grinding units are also equipped with roller bearings in the front support part, and the rear side is provided with a duplex of angular contact elements. In particular, this is how the spindle assemblies are realized in the designs of circular and internal grinding machines. Tapered roller bearings also simplify the functional system of the unit. Such a solution as applied to milling units eliminates the need to include an axial bearing group. As a result, the optimum stiffness margin is preserved, but with it the problems of heat generation with limited torque do not go anywhere.
Product quality control
After assembling the headstock, the clearance-tightness of the bearing group is checked. This operation is necessary to assess the readiness of the mechanism for full-fledged workloads. Verification is carried out by loading the device with a jack and dynamometer. Direct measurements are carried out by indicator devices, including measuring heads, sensors, micro-sensors, etc. The measuring device is installed on the headstock as close as possible to the front bearing. When fixing a stepwise change in load, a spindle end displacement schedule is built.
The rigidity of the spindle turning unit with supporting elements is controlled by the method of point-to-point measurement. First, two control points are set on the linear portion of the load curve. Then, strain data are recorded along each line, after which a comparison is performed. As standard indicators, both design values and figures from the general technical requirements for the machine can be used. Moreover, complex data for comparison obtained as a result of testing should be presented in the form of arithmetic mean values. In the same way, measurements of axial and radial loads are carried out with the fixing of the resulting clearances between the bearings.
If deviations from the standard values are detected, the interference clearance is set. When servicing the spindle nodes of a lathe for such tasks, the support heating technique is used. Under the thermal effects of thermometers and thermocouples in a certain range, tightening and adjusting the nuts is performed.
Seals for the mechanism
The headstock also includes special seals that increase the insulating and sealing properties of the mechanism. What is it for? Since the working process of a lathe is associated with the release of large volumes of small waste in a lubricated environment, clogging of functional parts is common. Accordingly, when assembling the spindle assembly, devices must be provided that protect the working elements from dust, dirt and moisture. It is for this that the sealant is used. As a rule, this is a consumable in the form of a ring, which is mounted on the spindle using a centering belt. In the process of operation of the mechanism requires periodic replacement or adjustment of the position. In conditions of increased external pollution, a protective cover ring can be additionally used. If the machine operates at medium or low revs, then a lip seal must also be fixed.
ShU maintenance
The main task of staff during the operation of the headstock is to monitor the lubrication of its parts. Usually, the spraying method on the surfaces of rotating gears, impellers and disk components is used for this. The optimal composition for this type of lubricant should have a viscosity index of 20 when heated to 50 ° C. The design of the milling spindle unit provides for the possibility of directing oil into the bearing through the collector or directly to the working group. Moreover, part of the oil should remain after the end of the working session. Old contaminated fluid is replaced with a new one. To simplify the filling process in modern machines, a circulating supply of oil is organized simultaneously to the gearbox and spindle in automatic mode as the waste mass is drained.
In addition to updating the oil, it is necessary to maintain the technical condition of the mechanism. Problems of a technical and constructional property may arise due to overheating, excessive deformation, high vibrations, or interturn circuit. Typical repair of spindle assemblies as part of the production process may consist in replacing damaged parts, consumables, or restoring seats. For example, when deforming or installing new elements, sometimes additional correction of sockets or parts themselves is required through sharpening, grinding, grinding or building operations.
production in Russia
Domestic manufacturers produce part of the spindle components required for the assembly of machine tools at their own machine-building facilities, relying on the development and experience of Soviet industry. There are practically no problems with the manufacture of conventional drive spindle assemblies for a milling machine or turning units that do not focus on high-precision machining.However, modern high-tech electrospindles are produced in Russia only in parts and on the basis of imported components. These limitations are associated not only with the lack of advanced technologies in this area, but also with a shortage of qualified personnel who must solve engineering, technical and production problems.
Conclusion
The spindle is one of the central functional components of various types of processing machines. The accuracy of work operations, the ergonomics of equipment control and the efficiency of regulation of the power potential of the drive mechanism depend on the quality of its basic functions. Therefore, it is so important to pay attention to the characteristics of the spindle unit in the lathe when choosing it. Moreover, this applies not only to the industrial segment, where in-line machining operations are performed. The basic knowledge of the spindle head should also have an ordinary home foreman performing simple operations in a garage or country house. Skills of handling the spindle mechanism will make the operating process more reliable, and machine maintenance - economical.