When motor spindle , a drive mode is in a machine tool referred to, wherein when the main drive motor directly drives the main spindle and is not conventionally driven through a transmission or a V-belt.
The spindles are driven by electric motors, mostly three-phase asynchronous machines are used. Up to a speed of 30,000 min −1 , the motor spindles are usually mounted on roller bearings , with air bearings even significantly higher speeds are possible.
The advantages of motor spindles compared to conventional tool drives lie in the higher possible speeds and the lower play. This is why modern machining techniques such as high-speed machining are only possible through the use of motor spindles due to the required speeds.
In the 1980s, powerful motors made it possible to drive tool-carrying spindles directly in order to enable more precise machining at higher rotational speeds. The rapid developments in the areas of tool geometries and materials that began at this time allowed higher cutting speeds in machining, which resulted in higher rotational speeds. With the conventional drive technology, which usually coupled the drive and the work spindle through a gearbox and other transmission elements, these high speeds could hardly be achieved or only with considerably greater effort. At the same time, frequency converters made variable-speed asynchronous motors possible, which together with advances in ball bearings led to the development of the motor spindle. In the 1990s, many machine tool manufacturers began to outsource entire departments due to the crisis in the industry . As an independent assembly group , the motor spindle was very well suited for outsourcing, which means that suppliers specialized in the development of motor spindles and implemented innovations. Compact design, easy maintenance, low noise emissions and high reliability are features of motor spindles that were previously unattainable.
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. Motor spindles that carry workpieces in particular 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 installation space. As a result, adequate cooling of the motor must be provided, for which both air and water are used. Most often, water cooling of the stator integrated in the housing is used.
The central element of the motor spindle is the work spindle, a shaft with an integrated tool interface. The shaft must be stiff enough to bend as little as possible due to radial forces. The stiffness depends on the diameter, material and length of the free shaft. A larger diameter, however, in turn leads to a higher mass moment of inertia , which increases the energy required for acceleration. The dynamic behavior of the wave also plays an important role. The rotating shaft, with its drive and bearing, represents a system that is capable of vibrating and which can become unstable when its critical speed is reached . In addition, more and more machine tools require an internal coolant supply. The coolant or the cooling lubricant is fed through a rotary feedthrough into an axial bore in the shaft to the tool. The tool itself must contain small bores through which the coolant can escape and thereby also cool the workpiece. In addition, compressed air is often necessary, with the z. B. chips can be blown away. The compressed air passes through a separate bore in the shaft or through the coolant bore from which the remaining coolant is blown out beforehand.
Tool or workpiece interface
A tool-carrying 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 thus also be able to change tools automatically. The tool interface must have a very high repeat accuracy, that is, the same tool that is clamped twice in a row should run in the same position if possible. This accuracy directly affects the accuracy of the machining. Inaccurate clamping can also lead to an imbalance .
The steep taper and the hollow shank taper have essentially established themselves as tool holders . 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 speeds, you will almost always find hollow taper shanks.
A tool clamp is used for clamping, which has the task of fixing the tool after it has been inserted. There are hydromechanical and mechanical, i. H. systems working with spring force. The robust design of the disc spring tensioner is still by far the most frequently used system. The tool is released via a hydraulic or pneumatic release unit, which presses against the spring force when the machine is stationary and thus releases the tool. Tool clamps with a gas pressure spring are still at the testing stage.
As with tool-bearing spindles, work-piece-bearing spindles also have an interface - the chuck .
An electric motor drives the spindle directly. Its speed and torque are therefore the same as those of the spindle. Because of the high power densities required, the motors usually have to be cooled with water. Synchronous motors are mainly used for spindles that have to convert high torques at low speeds. With them, a significantly higher torque can be provided with the same engine volume. Synchronous motors are also used for highly dynamic, high-speed spindles with low continuous power. Asynchronous motors are standard drives for spindles in machining centers with speeds of up to 20,000 min −1 , in which relatively high torques have to be used in the lower range and yet sufficient power is also required at high speeds.
The shaft bearings must absorb axial and radial forces and should not have any play. So far, angular contact ball bearings have been used almost exclusively in spindle construction. In addition to radial forces, angular contact ball bearings can also absorb unidirectional axial forces that arise during feed (e.g. Z-axis of a 3-axis CNC machine). Angular contact ball bearings are therefore always installed in pairs.
The high speeds of the shaft lead to high centrifugal forces in the ball bearings , which is why hybrid ball bearings (ceramic ball, steel rings) are now often used. By using silicon nitride ceramic in the balls, the hardness (compressive strength) can be increased and the density reduced, which reduces the centrifugal force. Because of the simple handling, the majority of the spindles are still permanently lubricated with grease. In most cases, non-toxic synthetic greases are used, the base oils of which are continuously fed into the store over a very long period of time. However, oil-air lubrication has proven to be more suitable for higher speeds . An extremely small amount of highly viscous oil is permanently added to an air flow that transports the oil directly into the bearing. This requires an oil feed hole in the spindle and an oil-air unit on the machine. Despite the greater effort, oil-air lubrication is indispensable in the area of very high speeds.
Since modern motor spindles are used in highly productive machines, any malfunctions that may occur must be recognized early and passed on to the machine control. In addition to the motor temperature, the position of the tool clamp is also recorded. The use of regulated motors makes it necessary to record the rotor position. In addition to these standard sensors, there is a wide range of options, from bearing temperature monitoring to recording the vibration status and recording the exact tool position.
- Joachim Klement: Milling head and motor spindle technology . Expert-Verlag, 1st edition, 2008, ISBN 3-8169-2712-2
- Klaus-Jörg Conrad: Pocket book of machine tools . Fachbuchverlag Leipzig, 1st edition, 2001
- Manfred Weck: Construction of spindle-bearing systems for high-speed material processing . Expert Verlag, 1990, ISBN 3-8169-0376-2
- Uwe Rondé and H. Schulz: Investigation of systems for clamping cylindrical shank tools with special consideration of their suitability for high-speed machining . Carl Hanser Verlag Munich, Vienna 1994, ISBN 3-446-17988-7
- Herbert Schulz: High-speed machining . Carl Hanser Verlag, ISBN 3-446-18796-0
- motor spindle. In: The Spindeldoctor. Accessed October 12, 2019 (German).