The Motor Spindle
Classic motor spindle
Motor spindles are usually mounted on roller bearings and are electrically driven. The direct coupling of the drive with the precisely mounted shaft allows very precise machining of the workpiece with high rotation speed is possible. For machining a workpiece by means of a tool through rotation, it is irrelevant which one rotates. Therefore, motor spindles as well as spindles in general are divided into tool carrying and workpiece carrying spindles. Typically, the tool bearing motor spindles are used in drilling, grinding and milling machines, the workpiece bearing motor spindles are more likely to be found in lathes. In addition to the drive and the storage of the shaft, the tool interface is one of the most important components of a modern motor spindle. The tool is automatically changed and fixed by a clamping system. This is why the motor spindle is nowadays the central component of a machine tool and is largely responsible for its performance and accuracy. Due to the complex structure and interaction of the individual components, motor spindles are not standard products but are developed and produced according to individual requirements. Motor spindles are mainly used in the following areas: machining centers and CNC machine tools from the areas of lathes, grinding and milling machines, HSC, HPC, tool and mold making as well as automotive production and aviation.
The spindle (also called main or working spindle) is a historically evolved term which, in relation to machine tools, refers to a shaft with an integrated interface for holding a workpiece or a tool. This shaft performs a rotational movement to machine the workpiece through the tool rotating either the tool (e.g. milling machine) or the workpiece (e.g. lathe) depending on the application. The increased demands placed on machine tools, primarily in the area of high-speed chipping in the 80’s, the tool carrying spindles had to be directly driven to enable more precise machining at higher rotational speeds. The rapid developments in the areas of tool geometries and materials that began at that time allowed higher cutting speeds in machining which resulted in higher rotational speeds. With conventional drive technologies, which usually coupled the drive and the working spindle by means of a gear and other transmission elements, these high speeds could hardly be achieved at all or only with considerably greater effort. At the same time, the first speed variables electric motors which, together with the progress achieved with contact ball bearings and frequency converters, led to the development of the motor spindle. In order to reach the necessary speeds, a gearbox was no longer required and instead, the spindle was directly coupled with the drive. In the 90s, many machine tool manufacturers started to outsource entire departments. The motor spindle was very well suitable as an independent assembly group for outsourcing which resulted in suppliers specializing in the development of motor spindles. The compact design, simple and safe maintenance, low noise emission and readiness to supply from specialist companies are additional advantages of motor spindles, as a result of which their range of applications has expanded and is no longer limited to the field of high-speed machining.
The basic structure of motor spindles is often the same regardless of the manufacturer. Serious differences can be found depending on the application which can be divided into the functional classes of milling spindles, workpiece spindles and internal grinding spindles. Particularly workpiece carrying motor spindles often have different requirements.
Housing and cooling
The external shape of the spindle is determined by the installation dimensions in the machine tool. An essential feature of motor spindles is their compact design which has a positive effect on the space required in the machine construction area. As a consequence, both air and water need to be used for sufficient cooling of the motor. The most common type, which is used, is a water cooling of Stators integrated in the housing.
The key element of the motor spindle is the work spindle which is a shaft with integrated tool interface. The shaft must be stiff enough to be able to withstand the impact of more radial forces so it does not bend. The aim is to achieve the highest possible stiffness which basically depends on the diameter of the shaft and the material. However, a larger diameter will in turn result in a higher moment of inertia which increases the energy required for acceleration. The dynamic behavior of the shaft also plays an important role. The rotating shaft with drive and bearings represents a vibratory system, which can lead to serious damages when reaching it resonance frequency. In addition, an internal coolant supply is required with more and more machine tools. The coolant is then routed to the tool via a rotary transmission leadthrough into an axial bore hole. The tool itself must contain small bore holes through which the coolant discharges and can thereby cool the tool. For additional lubrication, cooling lubricants can also be used during machining. In addition, a supply of cleaning air is increasingly required, which can be used to blow away any machining residues, e.g. chips. This is either achieved through a separate bore in the shaft or by using the coolant bore requiring the remaining coolant to be blown out beforehand.
Tool or workpiece interface
A tool-bearing work spindle on a machine tool only makes sense if the tool can also be changed. Modern machine tools should work as automatically as possible and therefore also be able to change the tool automatically. The requirement is therefore a so-called tool interface which allows a very high repeat accuracy, i.e. the same tool, which is clamped twice in a row, should run with exactly the same accuracy. This running accuracy has a direct effect on the machining accuracy. On the other hand, an inaccuracy leads to an unbalance which influences the entire process and can have serious consequences at high rotational speeds.
The main tool holder that has become standard is the steep taper and the hollow shank taper. The hollow shank taper has some advantages especially at high speeds but steep taper tools are still widely used by users which is why the steep taper is still used. At high rotational speeds, as is the case, for example, with machine tools for the automotive industry or HSC applications, you will find hollow shank tapers, almost without exception.
In addition to the tool holder, the tool interface of a powerful motor spindle consists of an automatic tool clamp which is supposed to fix the tool to the spindle. The choice is between hydromechanical or mechanical systems, i.e. systems based on spring force. The robust design of the diaphragm spring tensioner is still by far the most frequently used system. The release of the tool is carried out via a hydraulic or pneumatic unclamp unit that presses against the spring force during standstill and thus unclamps the tool. Tool clamps with a gas spring are new to the market but are currently still in the testing stage.
Similar to tool carrying spindles, workpiece carrying spindles also have an interface but which is referred to as a clamping chuck. However, the workpiece is rarely changed automatically, since the workpieces to be machined usually differ in their outer shape and the associated fixing.
Accessible motor spindle for training purposes. Rotor and stator are clearly visible.
Another essential component of the motor spindle is the drive in the form of an electric motor. Here, the essence of the motor spindle as drive emerges as there is no gear between drive (motor) and output (spindle) for the transmission present as it is the case with spindles with external drive. The motor design with respect to rotation speed and torque must therefore directly correspond to the desired requirements of the spindle.
The maximum power of a motor is directly proportional to the stator volume. At the same time, the motor is an integrated component and must therefore be adapted to the spatial, usually very compact, dimensions of the headstock. In addition, there is the problem of waste heat which increases with the continuous output of the motor and must be dissipated by adequate cooling. For these reasons it is very difficult to increase the motor power under the given spatial conditions and quickly reaches its limits. Conversely, an optimum of motor performance must be demanded from the given spatial conditions which represents the essential task in the design of the motor. One approach is to increase the quality of the current signal for which the frequency converters is responsible. By optimizing the frequency converter, the current should become a sinusoidal signal as ideal as possible which increases the power dissipation and increases the continuous power of the motor. Which type of motor is ultimately used entirely depends on the application. Synchronous motors are mainly suitable for spindles that have to convert high torques at low speeds. A significantly higher torque can be provided with the motor volume and the same current. Another application for synchronous motors can be seen with highly dynamic, fast running spindles, which have to deliver low continuous power. The advantages of asynchronous motors especially lie in the area of “standard motor spindles”, i.e. spindles for universal centers with speeds of up to 20,000 rpm where relatively high torques have to be worked with in the lower area and nevertheless sufficient power is also required at high speeds.
The storage of the shaft also has a significant influence on the vibration behavior of the system and must be adapted to the requirements. In spindle construction, angular contact ball bearings, also known as spindle bearings, have almost exclusively been used. In addition to radial forces, angular contact ball bearings can also accommodate unevenly applied axial forces which occur due to the feed. The high rotational speeds of the shaft ensure high centrifugal force loads in the contact ball bearings which is why hybrid contact ball bearings (ceramic ball, steel rings) are frequently used. Due to the use of ceramic (silicon nitride) for the balls the firmness can be increased while the density can be reduced, int turn reducing the centrifugal force loads. The angular contact ball bearings are always amounted in pairs. Depending on speed and mechanical load, the bearings are paired differently, in the simplest case in an O arrangement. Because of the simple handling, the majority of the spindles are still greased for life. Non-toxic synthetic greases are mostly used, the base oils of which are continuously supplied to the bearing over a very long period of time. For higher speeds, however, in the last few years, the oil-air lubrication has proven to be more suitable. An extremely small amount of highly viscous oil is permanently added to an air stream which transports the oil directly into the bearing. This requires an oil supply bore holes in the spindle and an oil-air unit on the machine. Despite the higher costs, oil-air lubrication is currently indispensable at very high speeds.
Since modern motor spindles are used in highly productive machines, any malfunctions that may occur must be detected at an early stage and passed on to the machine control system. In addition to the motor temperature, the position of the tool clamp is also captured. The use of controlled motors makes it necessary to capture the rotor position. In addition to these standard sensors, there are numerous options, ranging from bearing temperature monitoring to recording the vibration condition and capturing the exact tool position.
Source: German Wikipedia