Motor spindle for dental milling machine

Abstract

Motor spindle (20) for a dental milling machine for machining a workpiece, ▪ with an outer housing (9), ▪ with a spindle shaft (3), ▪ wherein the spindle shaft (3) has a front end (16) and a rear end (17) and extends along a central longitudinal axis (L), ▪ with an electric drive (1, 2) for the spindle shaft (3), ▪ with a tool holder (5), ▪ wherein a milling tool (6) for machining the workpiece can be clamped in the tool holder (5), and ▪ with a fan wheel (11), characterized in that ▪ the fan wheel (11) is arranged inside the outer housing (9) and is rotationally fixed on the spindle shaft (3), ▪ that the fan wheel (11) is arranged on the rear end (17) of the spindle shaft (3), ▪ that the outer housing (9) in the region of the front end (16) of the spindle shaft (3) and / or has at least one air outlet nozzle (13) in the area of ​​the tool holder (5), and that the fan wheel (11) is designed toduring operation of the motor spindle (20) in a direction of rotation (D) intended for chip removal about the central longitudinal axis (L) and to generate an airflow (12') within the outer housing (9) in the direction of the at least one air outlet nozzle (13).

Description

The invention relates to a motor spindle for a dental milling machine according to the preamble of claim 1, a dental milling machine with a motor spindle according to claim 15 and a method for cleaning and cooling a milling tool and / or a workpiece to be machined according to claim 16. In dental technology, blanks made of various materials are machined to produce dental prostheses such as crowns, bridges, and the like. This machining is carried out in specialized machines, particularly dental milling machines. These machines typically consist of a motor spindle with a milling tool, which is usually clamped to the front end of the motor spindle for machining a workpiece. The manufacturing process consists of several machining operations, such as milling, grinding, and polishing. Machining removes material, specifically chips, cutting dust, and polishing dust, which accumulates on the workpiece and in the motor spindle or dental milling machine. If the workpiece becomes clogged with this material, it can damage the milling tool and the workpiece. To avoid this, the workpiece is cleaned with compressed air or coolants such as cooling water during processing. When machining materials such as ceramics or titanium using wet machining, the workpiece and / or the milling tool is cleaned and cooled with a cooling lubricant during the milling process. When machining materials that are processed dry, such as zirconia, the workpiece and / or the milling tool are cleaned and cooled with compressed air during the milling process. Compressed air compressors can serve as the compressed air source, either integrated into the dental milling machine or provided as external, stationary units. Such compressed air systems are maintenance-intensive and expensive. If the compressed air compressors are integrated into the machine, they also take up installation space and can affect the external dimensions and the use of the motor spindle. If external, stationary units are used, additional floor space is required, and appropriate piping connections must be installed. To address this, EP 4 201 588 A1 proposes a special milling tool for a dental milling machine and an arrangement consisting of a motor spindle and such a milling tool. A mounting section for attaching a fan wheel is arranged directly on the milling tool or on a shank of the tool. The fan wheel is designed such that when the milling tool is driven in a rotational direction, an airflow is generated by the fan wheel in the direction of the milling tool. The mounting section and the fan wheel are arranged on the outside of the tool and are visible from the outside during machining. The fan wheel is mounted and attached to the spindle shaft from the outside and is accordingly larger in diameter and external dimensions, particularly in the radial direction, than the spindle shaft. A disadvantage of this design is that sufficient focusing or alignment of the generated air volume at a sufficiently high pressure is not possible. This significantly reduces energy efficiency and the effectiveness of cleaning and cooling. The fan wheel is fixed directly to the milling tool. It generates an airflow towards the workpiece, but this flow runs almost parallel to the shank of the milling tool. As a result, adequate cleaning and cooling can only be achieved at very high speeds, which are not attainable at the speeds typical for dry machining. Therefore, additional blow-off cycles during machining are absolutely necessary to achieve sufficient airflow. These additional blow-off cycles consume extra energy without any cleaning or cooling effect, further increasing energy losses and reducing efficiency even more. Due to the large size of the blower wheel / fan wheel compared to the entire motor spindle / dental milling machine, the use of the motor spindle is further limited. Because of the blower wheel's relatively large size, only a small number of tools can be stored in the tool magazine, requiring a correspondingly large grid. The use of standard tools is not possible. DE 198 17 178 A1 discloses a handle for laboratory micromotors with a tool collet driven by a rotatable shaft of a micromotor, wherein the shaft is supported in a front and a rear bearing, and wherein a pressure barrier is generated by means of a turbine downstream of the front bearing. For this purpose, the turbine has an impeller attached to the shaft and a guide plate attached to the stationary part of the handle, wherein air drawn in through an annular intake opening is guided via longitudinally extending channels into a chamber provided downstream of the front bearing, in which the turbine is arranged. The air is expelled forward through an annular gap provided on the outside of the collet. KR 10 2024 029 264 A describes a passive air curtain spindle for a dental drilling device, capable of sealing the device for machining ceramic teeth. The dental drilling device has a housing that supports a drilling unit. The latter has a spindle positioned vertically within the housing, a spindle shaft representing the axis of rotation of the drilling unit, and a spindle shaft fan located within part of the shaft, featuring a fan-shaped blade. A non-rotating mounting bushing is positioned at one end of the spindle to support it. The mounting bushing has a gap in a section connected to the spindle, allowing air to escape from the housing. The aim of the invention is to overcome these and other disadvantages of the prior art and to provide an improved motor spindle for a dental milling machine that is simple in design, cost-effective and efficient. The main features of the invention are specified in claims 1, 15 and 16. Embodiments are the subject of claims 2 to 14. In a motor spindle for a dental milling machine for machining a workpiece, comprising an outer housing, a spindle shaft (the spindle shaft having a front end and a rear end and extending along a central longitudinal axis), an electric drive for the spindle shaft, a tool holder (in which a milling tool for machining the workpiece can be clamped), and a fan wheel, the invention provides that the fan wheel is arranged inside the outer housing and is rotationally fixed to the spindle shaft, the fan wheel being located at the rear end of the spindle shaft, the outer housing having at least one air outlet nozzle in the region of the front end of the spindle shaft and / or in the region of the tool holder, and the fan wheel being designed toduring operation, the motor spindle rotates around the central longitudinal axis in a direction intended for chip removal and generates an airflow within the outer housing in the direction of at least one air outlet nozzle. Due to the inventive design of the motor spindle, in which the fan wheel is arranged inside the outer housing, the housing is inherently smaller in relation to the motor spindle and the housing. This applies particularly to the radial extent of the fan wheel along the radial direction of the motor spindle. The smaller size of the fan wheel significantly improves the handling, usability, and operational capability of the motor spindle. Compared to the overall machine and motor spindle, the fan wheel is relatively small and preferably does not exceed the radial dimensions of the spindle unit. The arrangement of the fan wheel at the rear end of the spindle shaft represents an efficient drive variant for the fan wheel and for generating an air volume for cooling and cleaning the milling tool, because the airflow generated by the fan wheel cools both the spindle unit, in particular the electric drive, and generates the airflow necessary for cleaning and cooling the workpiece. A further advantage is that the milling tool is no longer affected by the fan wheel due to its placement in the outer housing of the motor spindle. This allows for a greater number of possible tools to be used in the tool magazine. This also advantageously ensures that the option of using standard tools is maintained. The inventive design of the fan wheel and the air outlet nozzle results in a significantly improved focusing and alignment of the airflow towards the tool tip or workpiece. The air outlet nozzle advantageously accelerates the airflow due to the nozzle effect, allowing it to be blown out at high pressure onto the desired area during machining. A compressed air connection is not required. One embodiment of the invention provides that the at least one air outlet nozzle of the outer housing is designed to blow the airflow out of the outer housing in a non-parallel orientation to the central longitudinal axis in the direction of the milling tool and / or the workpiece to be machined. This advantageously enables sufficient focusing and alignment of the generated air volume at a sufficiently high pressure, thus significantly improving energy efficiency and effectiveness in cleaning and cooling the milling tool and / or workpiece. Because the airflow is blown out non-parallel to the milling tool, efficient and targeted cleaning and cooling can be achieved. Furthermore, due to the air outlet nozzle and the focusing of the airflow, sufficient cleaning and cooling of the workpiece and / or milling tool can always be achieved, even during dry machining at typical speeds in the range of approximately 20,000 to 25,000 revolutions per minute, because the invention consistently generates a sufficiently strong airflow. This eliminates the need for additional "cleaning cycles" and improves energy efficiency. The overall machining time is reduced. In particular, this ensures optimal cleaning of the workpiece and tool, as well as the removal of dust and chips, during machining. For example, dry machining of materials such as zirconia can be easily performed using the motor spindle according to the invention. The compressed air for cleaning is generated exclusively by the drive of the fan wheel. Therefore, the need for an internal or external compressed air source is eliminated. The invention further provides that the at least one air outlet nozzle of the outer housing is designed to guide the airflow at an angle in relation to the central longitudinal axis, wherein the angle is an acute angle and is approximately 1° to 5°, preferably approximately 3°. This ensures that the airflow from the nozzle head, or at least one air outlet nozzle, is sufficiently strong and focused. This significantly improves the desired cleaning and cooling effect. The air volume generated by the fan wheel is blown out at increased speed and at an angle of attack through at least one air outlet nozzle near the milling tool in the area of ​​the tool holder. This achieves optimal cleaning of the workpiece and milling tool, even at low speeds. Overall, the energy efficiency of the motor spindle is significantly improved. This is also aided if at least one air outlet nozzle of the nozzle head has a flowable cross-section that corresponds to approximately 15% to 25%, preferably approximately 20%, of the size of a flowable cross-section of the airflow channel. By reducing the cross-sectional area through which air can flow in the nozzle head or the tapered section of the outer housing, a nozzle effect is created, allowing the airflow to exit the nozzle head towards the tool tip at increased velocity. The flow is compressed, and the air can flow out in a focused and directed manner at the angle of attack relative to the central longitudinal axis. This compression allows the airflow to exit the at least one air outlet nozzle towards the tool tip with a sufficiently high intensity and pressure. From a design perspective, it is advantageous if at least one air outlet nozzle is formed between the outer housing and the tool holder, whereby the at least one air outlet nozzle can form part of a nozzle head. In a further embodiment of the invention, the nozzle head can be formed by the outer housing of the motor spindle, wherein the nozzle head can be arranged as close as possible to the milling tool, and wherein the spindle shaft, the tool holder, and the milling tool project at least partially beyond the nozzle head in the direction of the central longitudinal axis and thus protrude from the outer housing. This ensures that the exiting airflow is focused and flows towards the area of ​​the tool tip. The distance between the tool tip and the workpiece to be machined is advantageously shortened sufficiently by the close proximity to the milling tool. Preferably, the nozzle head can be equipped with several air outlet nozzles arranged in the direction of rotation of the spindle shaft and at uniform intervals from one another. These nozzles can be designed to focus and expel the airflow at the angle of attack towards the tool tip and / or workpiece. By arranging a large number of air outlet nozzles on the nozzle head, the cooling and cleaning effect as well as energy efficiency can be further improved. According to a further embodiment, two air outlet nozzles can be provided, with the two air outlet nozzles being arranged diametrically opposite each other. This arrangement significantly simplifies manufacturing due to the symmetry. Furthermore, it achieves twice the cleaning and cooling effect, or airflow volume, compared to a single air outlet nozzle on the nozzle head. According to a preferred embodiment of the invention, the fan wheel can have several blades, with blades adjacent in the direction of rotation being arranged at uniform angular intervals around the central longitudinal axis of the spindle shaft. This simplifies manufacturing and allows sufficient compressed air or a sufficient volume of air to be generated with relatively good efficiency by driving the fan wheel and the associated rotation of the blades. The fan wheel is positioned within the outer housing along the airflow direction towards the at least one air outlet nozzle located in front of the electric drive. This ensures that the airflow generated by the fan wheel is not only directed to the air outlet nozzles but also cools the electric drive of the motor spindle. This is also aided if the electric drive is located inside the outer casing and along the central longitudinal axis between the fan wheel and the at least one air outlet nozzle. The spindle shaft and the electric drive are preferably part of a spindle unit, wherein the spindle unit is arranged inside the outer housing in an inner housing and wherein the fan wheel is formed outside the inner housing. The inner housing protects the spindle unit and its components from the airflow generated by the fan. The spindle unit itself can be designed as a pre-assembled module, which further reduces the manufacturing costs for the motor spindle. According to another important embodiment of the invention, the airflow generated by the fan wheel is guided between the inner housing of the spindle unit and the outer housing of the motor spindle, wherein the outer housing of the motor spindle and the inner housing of the spindle unit form an airflow channel at least partially. This simple and cost-effective measure ensures that the airflow generated by the fan wheel is directed efficiently and efficiently to the air outlets. The design incorporates an inner housing of the spindle unit with a smaller diameter than the outer housing, creating an approximately cylindrical flow channel. The invention further provides that the outer housing has at least one air intake channel for drawing in ambient air during operation of the fan wheel and the spindle shaft. This measure ensures that the fan wheel constantly draws in clean and fresh ambient air and directs it to the air outlet nozzles via the airflow channel. Another important embodiment of the invention provides that the outer housing of the motor spindle has a tapered section in the area of ​​the front end of the spindle shaft and / or in the area of ​​the tool holder, which reduces the cross-sectional area through which the airflow channel can pass. The tapered section of the outer housing enhances the nozzle effect and ensures that the airflow is expelled from the nozzle head as close as possible to the milling tool. Furthermore, the design and manufacture of this tapered section via the outer housing is simple and cost-effective. The tapered section is advantageously part of the nozzle head. The invention further provides that the inner housing of the spindle unit has and / or forms air guide elements at least in sections, wherein an air guide element may preferably be formed at the rear end of the spindle shaft, directly between the spindle unit and the fan wheel. It is also advantageous that the air guide element closes off the inner housing of the spindle unit and seals against the airflow generated by the fan wheel in the direction of the at least one air outlet nozzle. The air guide element ensures optimal flow conditions within the preferably cylindrical airflow channel. It also protects the spindle unit along its central longitudinal axis from the generated compressed air or airflow and advantageously guides the airflow towards the airflow channel. The air guide element may have a slope that can be curved towards the airflow channel. This promotes efficient flow guidance. Another embodiment of the invention provides that the spindle shaft is rotatably mounted in bearings, the bearings being arranged within the inner housing. The bearings can preferably be designed as rolling bearings or ball bearings. The airflow channel between the inner and outer housings is positioned as far outwards as possible in the radial direction due to its defined design, directing the airflow generated by the fan wheel around the spindle unit and thus around the ball bearings. This cools the electric drive. Furthermore, the inner housing prevents the bearings of the spindle unit and the spindle shaft from coming into contact with the airflow, effectively preventing the ball bearings from drying out. A further embodiment of the invention provides that the outer diameter of the fan wheel is smaller than the outer diameter of the inner housing of the spindle unit. It is also advantageous if the entire radial extent of the fan wheel blades is smaller than the outer diameter of the inner housing of the spindle unit. This makes the motor spindle extremely compact, especially in its external dimensions, which reduces the space required in a milling machine and improves handling. Advantageously, the electric drive for the spindle shaft is an electric motor comprising a stator and a rotor, wherein the rotor and the spindle shaft are non-rotatably connected to each other. The electric drive is preferably a synchronous motor or an asynchronous motor. The tool holder is preferably formed at the front end of the spindle shaft, wherein the tool holder is rotationally fixed to the spindle shaft and wherein the tool holder can be driven to rotate about the central longitudinal axis by means of the electric drive during operation of the motor spindle. Preferably, a milling tool with a tool tip for machining the workpiece can be clamped in the tool holder of the spindle shaft, wherein the tool holder is or forms a collet. Particularly preferred is the tool holder being part of an automatic clamping system, wherein the automatic clamping system comprises a drawbar, at least one spring element and a release unit for automatic actuation of the tool holder during a tool change. The automatic clamping system significantly simplifies the operation of the motor spindle during workpiece machining, and especially during tool changes, effectively eliminating potential sources of error. Preferably, the drawbar is arranged inside the spindle shaft, extending along its central longitudinal axis. The clamping system securely holds the milling tool in the tool holder. Tool changes are also automatic, thanks to the axially movable release unit. Compression of the spring element, which can be a spring assembly, moves the drawbar axially, opening the tool holder (preferably a collet chuck) or expanding it radially so that the tool can be removed and replaced. Another important feature of the invention is that the motor spindle can be operated without compressed air, preferably with a sealing air-free seal for the motor spindle. It is advantageous if the outer housing of the motor spindle is sealed by means of at least one labyrinth seal. Likewise, the inner housing of the spindle unit can be sealed by means of at least one labyrinth seal. A pneumatic-free motor spindle offers the advantage that automatic tool changes can be performed electrically instead of pneumatically, and that the sealing is free of air leakage. The combination of a pneumatically operated motor spindle with an integrated fan on the spindle shaft results in a particularly advantageous design overall, one that, in particular, requires no internal or external pressure source such as an air compressor. Another important embodiment of the invention divides the motor spindle into a machining area and a machine room area, wherein the machining area comprises the outer housing, the inner housing, the spindle shaft, the electric drive, the tool holder and the fan wheel, and wherein the machine room area comprises a further housing, the air intake duct and the release unit. The outer casing and the inner casing are preferably permanently connected to each other. However, they can also be permanently and detachably connected to each other. The motor spindle is preferably inserted into a milling machine, preferably a dental milling machine, with the machine room area, while a workpiece is machined with the machining area. Dust and dirt are generated in the machining area, and the outer housing of the motor spindle advantageously protects all components, including the fan and air guide element, effectively from these influences. The additional housing largely separates the machine room area from the machining area and also protects it from external influences. The air intake is advantageously located in a separate machine room area, and therefore outside the machining area, resulting in virtually dust- and dirt-free air. This, in turn, extends the overall service life of the motor spindle. In a dental milling machine according to the invention, it is provided that it comprises at least one motor spindle with at least one of the aforementioned technical features. The dental milling machine also has a control system and an electrical supply for the electric drive, as well as a control system and an electrical supply for the automatic clamping system. The dental milling machine also has a housing that defines an interior space in which the motor spindle is positioned. An inventive method for cleaning and cooling a milling tool and / or a workpiece in a dental milling machine provides for the use of a motor spindle with at least one of the aforementioned technical features. While the motor spindles known in the prior art do not allow sufficient focusing of the airflow, the invention provides that the motor spindle not only generates the airflow necessary for machining and cleaning the workpiece itself, but also focuses the airflow in a targeted and optimal manner onto the workpiece via the air outlet nozzles. This ensures consistently adequate cleaning of the milling tool and the workpiece, even at relatively low motor spindle speeds, such as those used in dry machining. Previously necessary additional cleaning cycles during machining are no longer required with the motor spindle according to the invention. Due to its small size, the motor spindle according to the invention enables versatile use in milling machines. In particular, a large number of motor spindles can be accommodated in a single tool magazine. Moreover, the invention allows the use of standard tools with a small grid on the tool magazine. Consequently, even standard tool magazines can continue to be used. In one embodiment of the invention, the invention relates to a milling spindle with an automatic clamping system. The fan is mounted on the rear end of the spindle shaft, so that airflow is generated during machining. The airflow is also generated within the outer housing and outside an inner housing that contains the spindle shaft and the electric drive. The generated compressed air is thus guided around the spindle shaft and its ball bearings, which very effectively prevents air from passing through the ball bearings and thus prevents them from drying out. Air is drawn in from the surroundings, ensuring that clean ambient air is always used for cooling and blowing out the workpiece. The generated compressed air is focused via one or more air outlet nozzles near the cutting edge for a targeted and powerful airflow. This ensures optimal cleaning of the workpiece and tool, even at low speeds. Tool changes can be performed electrically. The motor spindle is sealed without air leakage via at least one labyrinth seal. A permanent magnet synchronous or asynchronous motor with low power loss enables continuous operation without cooling. Further features, details and advantages of the invention will become apparent from the wording of the claims and from the following description of an exemplary embodiment with reference to the drawing. Figure 1 shows a schematic side and sectional view of a motor spindle according to the invention. The motor spindle, generally designated 20 in Fig. 1, is generally suitable for use in machine tools and specifically for use in a dental milling machine (not shown). The dental milling machine, not shown in detail, can have a housing that defines an interior space, in which the motor spindle 20 can be positioned for machining a blank or workpiece. The motor spindle 20 has an outer housing 9 and a spindle unit 20' arranged in the outer housing 9 with a spindle shaft 3, wherein the spindle shaft 3 has a front and a rear end 16, 17 and extends between the two ends 16, 17 along a central longitudinal axis L. The motor spindle 20 further comprises an electric motor 1, 2 for electrically driving the spindle shaft 3. As can be seen, the electric motor can be formed by a stator 1 and a rotor 2. A tool holder 5 is arranged at the front end 16 of the spindle shaft 3 or of the outer housing 9, which can be driven rotating about the central longitudinal axis L by means of the electric motor 1, 2 during operation of the motor spindle 20. As can be seen particularly from the sectional view in Fig. 1, the spindle shaft 3 can have two bearing elements 4 for this purpose. The bearing elements 4 can be rolling bearings. The bearing elements 4 can also be designed as ball bearings. A milling tool 6 with a tool tip 6' can be clamped or is clamped in the tool holder 5 of the spindle shaft 3 for machining a workpiece. Furthermore, the spindle shaft 3 has a fan wheel 11 which is arranged inside the outer housing 9, wherein a nozzle head with at least one air outlet nozzle 13 is provided in the area of ​​the front end 16. The fan wheel 11 of the spindle shaft 3 is designed to rotate around the central longitudinal axis L in a direction of rotation D intended for chip removal during operation of the motor spindle 20 and to generate an airflow 12' in the direction from the fan wheel 11 towards the at least one air outlet nozzle 13. In this case, at least one air outlet nozzle 13 is designed to blow the airflow 12' out of the nozzle head in a non-parallel orientation to the central longitudinal axis L in the direction of the tool tip 6' and the workpiece to be machined. As can be seen in the region of the front end 16, the airflow 12' can be blown out of the nozzle head towards the tool tip 6' at a predefined angle of attack α with respect to the central longitudinal axis L. The angle of attack α with respect to the central longitudinal axis L can be approximately 1° to 5°, preferably approximately 3°. The outer housing 9 of the motor spindle 20 can have a tapered section in the region of the front end 16, which reduces the cross-sectional area through which the airflow channel 12 can flow. The nozzle head with the at least one air outlet nozzle 13 can be formed by the tapered section of the outer housing 9. The at least one air outlet nozzle 13 of the nozzle head can have a flowable cross-section which can correspond to approximately 15% to 25%, preferably approximately 20%, of the size of a flowable cross-section of the airflow channel 12. As further illustrated in Fig. 1, the motor spindle 20 can include an airflow channel 12 for directing the airflow 12' generated by the fan wheel 11 towards the at least one air outlet nozzle 13. The airflow channel 12 can be formed between the outer housing 9 of the motor spindle 20 and an inner housing 9' of the spindle unit 20'. Furthermore, the spindle shaft 3 can have an air guide element 15 adjacent to the fan wheel 11, which directs the airflow 12' generated by the fan wheel 11 towards the airflow channel 12 and simultaneously seals the spindle unit 20' in the direction of the central longitudinal axis L. The air guide element 15 can be arranged at the rear end 17 of the spindle shaft 3 directly between the spindle unit 20' and the fan wheel 11. The outer housing 9 of the motor spindle 20 can have a machining area B comprising the spindle unit 20' and the fan wheel 11, wherein the outer housing 9 of the motor spindle 20 can have a machine space area M adjoining the machining area B. As can be seen, the outer housing 9 can also have at least one air intake duct 14 for drawing in ambient air during operation of the fan wheel 11 and the spindle shaft 3. The air intake duct 14 can be formed essentially perpendicular to the outer housing 9 of the motor spindle 20, and the air intake duct 14 can be arranged in the machine room area M of the outer housing 9 of the motor spindle 20. The fan wheel 11 can be fixed to the rear end 17 of the spindle shaft 3, so that when the motor spindle 20 is driven by the electric motor 1, 2, the fan wheel 11 can rotate together with the spindle shaft 3 around the central longitudinal axis L to generate the airflow 12' for cleaning and cooling. The fan wheel 11 can further comprise several blades, wherein blades adjacent in the direction of rotation D can be arranged at uniform angular intervals around the central longitudinal axis L of the spindle shaft 3. The total radial extent of the blades of the fan wheel 11 in the radial direction R can be less than the diameter of the inner housing 9' of the spindle unit 20'. The motor spindle 20 shown in Fig. 1 can be operated without compressed air, wherein the electric motor 1, 2 can be, for example, a permanent magnet synchronous motor or an asynchronous motor. Furthermore, the sealing of the motor spindle 20 shown can be achieved without air purging, wherein the motor spindle 20 can include a labyrinth seal and the air purging-free seal can be formed by the labyrinth seal. The tool holder 5 at the front end 16 can be designed as a collet and be part of an automatic clamping system for the milling tool 6. As can be seen, the automatic clamping system can also have a drawbar 7, at least one spring element 10 and a release unit 8 for automatic actuation of the tool holder 5 of the clamping system during a tool change. The control and electrical supply of the electric motor 1, 2 and the control and electrical supply of the automatic clamping system with the tool holder 5 can be carried out via one electronic control unit or via several electronic control units. The invention is not limited to the embodiments described above, but can be modified in various ways. It is apparent, however, that the invention relates to a motor spindle 20 for a dental milling machine, comprising an outer housing 9 and a spindle unit 20' arranged in the outer housing 9 with a spindle shaft 3, wherein the spindle shaft 3 has a front and a rear end 16, 17 and extends along a central longitudinal axis L, an electric drive 1, 2 for electrically driving the spindle shaft 3, wherein a tool holder 5 is arranged at the front end 16 of the spindle shaft 3, which can be rotated about the central longitudinal axis L by means of the electric motor 1, 2 during operation of the motor spindle 20, wherein a milling tool 6 with a tool tip 6' can be clamped in the tool holder 5 of the spindle shaft 3 for machining a workpiece.According to the invention, the spindle shaft 3 has a fan wheel 11, which is arranged inside the outer housing 9 and outside an inner housing for the spindle shaft 3 and the electric drive 1, 2, wherein a nozzle head 13' with at least one air outlet nozzle 13 is provided in the region of the front end 16 of the spindle shaft 3, and wherein the fan wheel 11 is designed to rotate about the central longitudinal axis L in a direction D intended for chip removal during operation of the motor spindle 20 and to generate an airflow 12' in the direction from the fan wheel 11 towards the at least one air outlet nozzle 13, wherein the at least one air outlet nozzle 13 is designed to blow the airflow 12' out of the nozzle head 13' in a non-parallel orientation to the central longitudinal axis L in the direction of the tool tip 6' and the workpiece to be machined. The proposed motor spindle can generally be used for machining workpieces. For this purpose, the motor spindle according to the invention can be positioned in a suitable machining center. In particular, the motor spindle can be used in dental milling machines. It can further be seen that the stator 2 drives the rotor 1 on the shaft 3 by means of an electric current, setting the latter into rotation. The shaft 3 is supported in the inner housing 9' by ball bearings 4. A clamping system consisting of a drawbar 7, a spring assembly 10, and a collet 5 is located in the shaft 3. The machining tool is clamped using this clamping system. Tool changes are carried out via an axially movable release unit 8. By compressing the spring assembly, the drawbar 7 is moved, the collet 5 opens, and the tool can be removed. A fan wheel 11 is mounted on the rotating shaft 3. The fan wheel 11 draws in air through the air intake opening 14 and conveys it as compressed air via the airflow channel 12 towards the machining area. The air intake takes place in a machine compartment M, a separate area of ​​the machine outside the machining area B, ensuring that the air is virtually dust- and dirt-free. The special air routing protects the ball bearings 4 from the generated compressed air and directs the air towards the designated air outlet nozzles 13. Very close to the tool 6, the generated airflow escapes from the air outlet nozzles 13. The appropriately designed outlet nozzle 13 allows a targeted airflow for optimal tool / workpiece cooling and cleaning at low speeds. Reference symbol list B Machining area D Direction of rotation M Machine space R Radial direction L Central longitudinal axis (axis of rotation) α Angle of approach 1 Electric drive (stator) 2 Electric drive (rotor) 3 Spindle shaft 4 Bearings (rolling bearings, ball bearings) 5 Tool holder (collet) 6 Milling tool 6' Tool tip 7 Drawbar 8 Release unit 9 Outer housing (motor spindle) 9' Inner housing (spindle unit) 9'' Tapered section 9''' Further housing 10 Spring element 11 Fan wheel 12 Airflow duct 12' Airflow (directed) 13 Air outlet nozzle 13' Nozzle head 14 Air intake duct 15 Air guide element 16 Front end 17 Rear end 20 Motor spindle 20' Spindle unit

Claims

Motor spindle (20) for a dental milling machine for machining a workpiece, ▪ with an outer housing (9), ▪ with a spindle shaft (3), ▪ wherein the spindle shaft (3) has a front end (16) and a rear end (17) and extends along a central longitudinal axis (L), ▪ with an electric drive (1, 2) for the spindle shaft (3), ▪ with a tool holder (5), ▪ wherein a milling tool (6) for machining the workpiece can be clamped in the tool holder (5), and ▪ with a fan wheel (11), characterized in that ▪ the fan wheel (11) is arranged inside the outer housing (9) and is rotationally fixed on the spindle shaft (3), ▪ that the fan wheel (11) is arranged on the rear end (17) of the spindle shaft (3), ▪ that the outer housing (9) in the region of the front end (16) of the spindle shaft (3) and / or has at least one air outlet nozzle (13) in the area of ​​the tool holder (5), and that the fan wheel (11) is designed toduring operation of the motor spindle (20) to rotate about the central longitudinal axis (L) in a direction of rotation (D) intended for chip removal and to generate an airflow (12') within the outer housing (9) in the direction of the at least one air outlet nozzle (13). Motor spindle according to claim 1, characterized in that the at least one air outlet nozzle (13) of the outer housing (9) is designed to blow the airflow (12') out of the outer housing (9) in a non-parallel orientation to the central longitudinal axis (L) in the direction of the milling tool (6) and / or the workpiece to be machined. Motor spindle according to claim 1 or 2, characterized in that the at least one air outlet nozzle (13) is formed between the outer housing (9) and the tool holder (5). Motor spindle according to claim 3, characterized in that the at least one air outlet nozzle (13) is part of a nozzle head (13'). Motor spindle according to one of the preceding claims, characterized in that the fan wheel (11) is arranged inside the outer housing (9) along the airflow (12') in the direction of the at least one air outlet nozzle (13) in front of the electric drive (1, 2). Motor spindle according to one of the preceding claims, characterized in that the spindle shaft (3) and the electric drive (1, 2) are part of a spindle unit (20'), wherein the spindle unit (20') is arranged inside the outer housing (9) in an inner housing (9') and wherein the fan wheel (11) is formed outside the inner housing (9'). Motor spindle according to claim 6, characterized in that the airflow (12') generated by the fan wheel (11) is guided between the inner housing (9') of the spindle unit (20') and the outer housing (9) of the motor spindle (20). Motor spindle according to one of the preceding claims, characterized in that the outer housing (9) has at least one air intake channel (14) for drawing in ambient air during operation of the fan wheel (11) and the spindle shaft (3). Motor spindle according to one of the preceding claims, characterized in that the outer housing (9) of the motor spindle (20) has a tapered section (9'') in the area of ​​the front end (16) of the spindle shaft (3) and / or in the area of ​​the tool holder (5) which reduces a flowable cross-section of the airflow channel (12). Motor spindle according to one of the preceding claims, characterized in that the inner housing (9') of the spindle unit (20') has and / or forms air guide elements (15) at least in sections. Motor spindle according to one of the preceding claims, characterized in that the spindle shaft (3) is rotatably mounted in bearings (4), wherein the bearings (4) are arranged inside the inner housing (9'). Motor spindle according to one of the preceding claims, characterized in that the outer diameter of the fan wheel (11) is smaller than the outer diameter of the inner housing (9') of the spindle unit (20'). Motor spindle according to one of the preceding claims, characterized in that the tool holder (5) is formed at the front end (16) of the spindle shaft (3). Motor spindle according to one of the preceding claims, characterized in that the motor spindle (20) can be operated without compressed air. Dental milling machine with at least one motor spindle (20) according to one of the preceding claims. Method for cleaning and cooling a milling tool and / or a workpiece in a dental milling machine according to claim 15 by means of a motor spindle (20) according to one of claims 1 to 14 .

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