Patent Publication Number: US-2021170503-A1

Title: Dental Milling Machine

Description:
The invention concerns a dental milling machine, according to the preamble of claim  1 . 
     It has been known for a long time to provide dental milling machines, which are also meant to include here other dental processing machines for the machining of workpieces, with means for removing chips produced from the machining area. It has turned out to be comparatively efficient to produce, for this purpose, an aspirational air stream which passes through the machining area and sucks out air together with the chips caught from the milling area. Typically, fresh air—or even circulating air—is introduced into the milling chamber above the machining area for this purpose, and an aspirator connection is provided underneath the machining area. 
     Since with this solution, the cleaning effect is often not satisfactory, there have been numerous attempts of improving the removal of chips. For instance, it has become known from DE 40 27 107 A1 to make the milling chamber and therefore—from an aerodynamic point of view—the aspiration area smaller. However, this results in the path of movement which is available for the workpiece and tool being restricted. Although this problem may at least be minimized by a skilful selection of the arrangement of axes, the provision of particularly designed movement arms for restricted physical conditions is rather complex, and sometimes the milling precision will suffer then as well. 
     It has also been suggested to provide the machining area with blowing nozzles in order to blow up the chips produced there and prevent them from depositing there. Typically, however, the place of actual machining is permanently changing during the milling process, such that a multitude of blowing nozzle must be provided. The blowing nozzles which are not pointing towards the respective current working place are virtually blowing blankly, so that such a solution has always been inefficient. 
     Also as far as the blowing nozzles are concerned, numerous improvements have been suggested. For example, it has become known from EP 3 012 065 A1 to provide the nozzles with electrodes which provide an electrical field for deionizing the chips. This solution is particularly advantageous when dental plastic materials are used since these no longer tend to adhesively deposit at the surfaces of the milling chamber due to electrostatic forces then. 
     With other dental materials, such as ceramics, the tendency of milling dust to become electrostatically charged is lower anyway. 
     Further, different attempts have become known for fluidically optimising the removal of milling dust or of milling chips, respectively. As an example for this, reference should be made to DE 20 2015 100 312 U1. With this solution, a uniform air stream should be produced which should also capture the exposed shanks of tools of a tool magazine. 
     It has further already been suggested as well to arrange air inlet nozzles at two upper corners of the room, which are spaced apart from each other, and provide a central aspirator opening underneath the machining area. 
     Herein, it is, however, disadvantageous that when the finished workpiece is taken out, it may fall directly into the aspirator connection if handled carelessly, such that the dental work will then be lost. In addition, the dental technician will have a feeling of insecurity during the operation, due to this “toilet bowl effect”, because he fears that the restorational result will get lost, which considerably restricts the acceptance of such a solution. 
     In contrast to this, the invention is based on the task of creating a dental milling machine according to the preamble of claim  1  which has a better acceptance, but yet allows a clean work without any disturbance. 
     This task is solved, in accordance with the invention, by claim  1 . Advantageous further embodiments result from the subordinate claims. 
     In accordance with the invention, a dental milling machine should be have a milling chamber closed in itself and particularly designed. Typically, the milling chambers of milling machines are made out of sheets which are screwed to each other or attached in a different fashion, wherein other materials are partially employed as well. In contrast to this, a single-piece design made out of a suitable material, such as a plastic material, is preferred in accordance with the invention and is used in accordance with the invention. The milling chamber will then preferably not have any interior corners, at least not in the lower region, such that no chips and no milling dust can deposit there. 
     The transitional areas between the base and the side walls are each rounded off with radii which amount to several millimetres at least, but preferably several centimetres in the relevant positions, for example 10 cm. 
     The base of the milling chamber is then preferably shaped in the fashion of a hollow fillet which extends at a radius of, for instance, also 10 cm underneath the machining area and across the milling chamber. The inclination of the base is, in addition, preferably at an angle, which is, in such a fashion that the lowest position of the base joins at the aspirator connection. Compared with this, the base underneath the machining area is distinctly higher, such that milling dust or milling chips produced there can already easily be supplied to the aspirator connection with the help of gravity. 
     In particular, however, the inclination of the base supports the current from the air inlet connection towards the aspirator connection across the base, all the more because no obstacles, such as undercuts, are formed in the course of the base. 
     In an advantageous embodiment, the air inlet connection is diametrically opposite of the aspirator connection, which is, at an upper corner of the milling chamber. At least one nozzle, preferably a number of nozzles, are directed towards the machining area, such that said is blown over. 
     In another advantageous embodiment, a base washing nozzle is put into practice. This blows air vertically downwards, such that the air stream of said nozzle will slide along the adjacent side wall and then be led over the expressly large deflection radius into the hollow fillet, and will there blow possible depositions towards the aspirator connection as a laminar current. 
     It is particularly favourable if the inclination of the base, starting from the deflection radius underneath the air inlet is essentially constant and at least is not getting smaller. As a result, the tendency of forming depositions there will be clearly reduced compared with the state of the art. 
     In a modified embodiment, the inclination of the base towards the aspirator connection should even increase slightly, for example from 18 to 23 degrees. Herewith, it is possible to take into account the current velocity&#39;s reduction over the course of the base due to the air inlet nozzle, since the overall flow area becomes larger, such that the tendency of the chips to adhere increases in the direction towards the aspirator connection as a result of the lower current velocity, which phenomenon, however, is compensated for by the larger inclinational angle of the base. 
     Whereas it is preferred to put into practice the milling chamber in accordance with the invention as a single-piece moulding, this does not mean that no door opening might be provided for the milling chamber. The door is preferably distinctly spaced apart from the base, such that the important removal of chips is not blocked by the door. 
     This holds true analogously to the same extent for openings which are necessary for the operation of the milling chamber otherwise, for instance, the drive spindle, operational arms, such as a robot arm for the workpiece, or also openings for changing the workpiece or for a tool change. 
     Preferably, the machining area—relative to the centre of the milling chamber—is neighbouring the air inlet more closely than the aspirator connection. As a result, a comparatively strong air current may be provided at a comparatively small flow area, which makes possible a good blowing-out performance for the machining area. 
     In accordance with the invention, it is particularly favourable that the base may be shaped basically in the fashion of a hollow fillet. Viewed in a side cut, the milling chamber therefore has essentially the shape of a “U” in its lower region. As a result, dust particles or chips are supplied to the middle centre of the hollow fillet, where they may stack up and insofar form some resistance to the current, which promotes their removal. 
     It is to be understood that the precise shape of the cross section of the hollow fillet is adaptable to the requirements to a great extent. So, for instance, the cross section may be selected to be slightly more pointed, i.e. slightly more “V”-shaped, wherein, however, attention must be paid to it that the current velocity is not reduced owing to too small a radius. 
     In accordance with the invention, it is particularly favourable as well that underneath the machining area, a uniform and closed area in the shape of the base is present. Even if the machining area moves, which is the case regularly in case of a four-axes milling machine or a five-axes milling machine, the machining area is always blown over within the area in front of the nozzles of the air inlet, and always above the closed and smooth base. A restorational part which may fall down by accident would remain lying on it and could be used by the operator without any problems. 
     The dental milling machine in accordance with the invention is suitable both for wet operation and for dry operation. For wet operations, the deionizing electrodes may be switched off, and in an advantageous embodiment of the invention, it should be possible to switch between a wet outlet and an aspirator lead destined for dry operation with the help of a switching hatch. The switching hatch may preferably have a joint at the upper side of the connection. In the closed state, the air will then be lead off in an upward direction, and, if necessary, depositions remaining at the hatch in the lower region may be washed off when switching to wet operation and thus opening the outlet. 
     In an advantageous embodiment, the switching hatch, which may also be shaped in the form of a slider, should alternatively either cover the air nozzles and release the deionizing electrodes, as is favourable for milling PMMA or other plastic materials, or cover the deionizing electrodes and release the air inlets, as is desired for milling ceramic materials or for wet milling. 
     It is also possible to use two separate locking elements instead of one switching element, which locking elements may be employed depending on the case of use. Locking the deionizing nozzles, for instance during wet milling, has the advantage that they will then not be polluted by milling dust. 
     Insofar, the hatch insulates against a lower, planar sliding surface which is formed without any undercuts and provides a smooth transition between the region of the outlet in front of the switching hatch and the region of the outlet behind the switching hatch when the hatch is open. 
     The plastic part used for the production of the milling chamber may be created in any suitable fashion, wherein a smooth interior surface is essential. For example, it may be produced by means of centrifugal casting. 
     In accordance with the invention, it is particularly favourable if the outlet extends laterally away in extension of the hollow-fillet base. In particular in the case of wet working, this will lead to an unhindered flow of water which will carry away chips brought along without there being any mentionable re-direction of the current at the outlet of the milling chamber. 
     In an advantageous embodiment of the invention, the cleaning performance of the dental milling machine in accordance with the invention should be further improved with the help of a particular arrangement of the blowing-air nozzles and with the help of a particular control device. 
     For this purpose, the blowing-air nozzles should be switched on purposefully for few seconds at high air pressure and therefore a high velocity of the air with the help of a control device. Due to the pulse-like supply of air, a particularly good swirling and therefore the possibility of particularly good aspiration may be provided. 
     The air nozzles are preferably designed as air amplifying nozzles. With such nozzles, an annular channel is provided at the course of the nozzle. Said channel is then supplied with compressed air. Via a narrow drain nozzle, the air enters into the interior of the nozzle at high speed. 
     Due to a combination of Coanda effect and Venturi effect, the air will adhere to the curved interior wall of the nozzle near the annular gap, whereby the air stream is lead towards the nozzle outlet. In the central inlet area of the nozzle, a vacuum is created as a result, due to which a large volume of air is provided for passing through the nozzle. This results in a large air volume with such air amplifying nozzles, together with a high current velocity. 
     A nozzle in which the combination of Coanda effect and Venturi effect is made use of as well, however at a position lying at the outside, is already known from DE 25 37 953 A1. 
     Due to the channels of the air amplifying nozzles which are open at the inside, air may be aspired in a particularly favourable fashion in accordance with the invention with the help of the air aspirational air stream even when the compressed air is switched off,. For this purpose, it is preferred to operate the aspirational device at reduced power during a milling step, which is done continuously. 
     During the specific pulse-cleaning process after the milling step, the aspirational device is preferably switched on at full aspirational power, in accordance with the invention. 
     In an advantageous embodiment, the added-up flow areas of the air amplifying nozzles should correspond to the flow area of the outlet. 
     The pressures and flow velocities provided in accordance with the invention as well as air volumes are adaptable to the requirements to a great extent. If the inflational air pressure exceeds a given value, it is favourable to use a pressure reducing device which reduces the air pressure to, for example, 5 bar or 2 bar. 
     It is particularly favourable that the pulse-like blowing power provided in accordance with the invention reduces the necessary compressor power for the blowing-air stream. Typically, compressors are provided with a compressed-air depot of, for example, 50 litres. If the compressor, as is usual with economically priced compressors, has a continuous power of, or instance, 30 l/min, the compressed-air depot may be made use of in accordance with the invention. During the pulse-blowing, it is then possible to blow at a volume stream of 240 l/min, for example. 
     In accordance with the invention, it is favourable if the blowing pulse essentially corresponds to the interior space volume of the milling chamber as far as the air volume is concerned, such that during the blowing pulse, the entire milling chamber is provided with fresh air. 
     It is to be understood that the aspirational device must be able to absorb the occurring peak of compressed air without any problems. This, however, is typically unproblematic because such aspirational devices are typically provided with an aspirational air volume of, for instance, 500 l/min, i.e. distinctly more than is envisaged for the pulse blowing in accordance with the invention, as far as the volume stream is concerned. 
     In a particularly favourable fashion, after the blowing pulse, both the aspirational air stream and the blowing air stream are switched off. As a result of the fresh air supply through the air aplifying nozzles, there is an ambient air pressure inside the milling chamber, and, for example, a tool change may be made through a back door in the milling chamber for carrying out the next milling step, wherein no noteworthy exchange of air takes place with the surroundings. 
     It is particularly favourable, in accordance with the invention, that cleaning may be pre-set with the help of the pulse blowing in accordance with the invention, corresponding to the basic milling programme, and be carried out automatically. If, for instance, a milling step takes 10 min, the pulse blowing may be carried out for between 5 seconds and 15 seconds, with an aspirational air stream being at the same time increased, after the end of the milling step in a pre-set fashion. The necessity for an intervention by the user may be avoided in accordance with the invention. If, for example, a bridge made up of a plurality of individual components is to be milled, a number of milling steps may be made one after another without any problem, each with little interruption by the pulse blowing, and, if required, by a necessary tool change. 
     The dental milling machine in accordance with the invention is not restricted to one particular material. For instance, the process described as preferred herein may be employed for milling ceramics out of zirconium dioxide. When milling composite or plastic materials, respectively, the air amplifying nozzles should be used with deionizing electrodes in front of them, which, when not used, are protected by a cover. The deionizing electrodes are operated at an alternating voltage of between 5,000 volt and 10,000 volt and are designed for deionizing the plastic chips accumulating during the milling of plastics so that they cannot adhere to the tool or the wall of the milling chamber and can thus be carried away better. 
     During the milling of silicate or feldspar ceramics, such as lithium disilicate, for example, wet milling is possible as well. Here too, the air amplifying nozzles in accordance with the invention may be employed for cleaning. 
     If a change from wet to dry milling is to be made, a particularly good drying and cleaning is necessary; this can be carried out by means of pulse blowing interlaced several times, in according to the invention 
     It is to be understood that in accordance with the invention, the milling or robot arm which holds the workpiece is cleaned as well. Its most probable position is in the area arranged to the right in the preferred embodiment of the dental milling machine, such that two nozzles are provided there. The milling arm is rotated in any suitable fashion in order to produce as little wake flow as possible on the one hand, and make sure, on the other hand, that all surfaces of the milling arm can be cleaned all around. 
     Preferably, the dental milling machine in accordance with the invention is designed as a 5/0 milling machine, i.e. with 5 axes of movement of the robot arm carrying the workpiece and no axis of movement of the milling spindle. 
    
    
     
       Further advantages, details and features result from the subsequent description of several exemplary embodiments of the invention with the help of the drawings, which show: 
         FIG. 1  a schematic view of one embodiment of a dental milling machine in accordance with the invention; 
         FIG. 2  a side view of the milling chamber of the machine according to  FIG. 1 ; 
         FIG. 3  a horizontal cut through the embodiment according to  FIGS. 1 and 2 ; 
         FIG. 4  a schematic view of one embodiment of a milling chamber in accordance with the invention of a dental milling machine; 
         FIG. 5  a detailed view about one embodiment of a dental milling machine in accordance with the invention; 
         FIG. 6  a schematic depiction of another embodiment of the invention; 
         FIG. 6 a    a cut through an air amplifying nozzle to be preferably employed in accordance with the invention; 
         FIG. 7  a cut through the embodiment according to  FIG. 6 , however from a position rotated by 90 degrees; and 
         FIG. 8  a time diagram for the control in a dental milling machine in accordance with the invention. 
     
    
    
     The dental milling machine  10  which is depicted in  FIG. 1  has a milling chamber  12  which is designed in a particular fashion. Milling chamber  12  is a closed element and is made of some material with a smooth and non-adhesive surface, such as a plastic material, for example. In a basically known fashion, it accommodates a spindle  14  which serves as a tool and is driven by a spindle motor  16 . Further, it accommodates workpiece  19  which is mounted in a workpiece holder  22 . Via a number of movement axes, the relative position between workpiece  18  and spindle  14  may be modified, for example in five axes. 
     For the change of the workpiece, a rear wall of milling chamber  12  is provided with a door, and behind the milling chamber a magazine with further workpieces is arranged, out of which another workpiece  24  is visible in  FIG. 1 . 
     Further, the front wall of milling chamber  12  is provided with an access door in its upper region. 
     The left upper side, in the depiction according to  FIG. 1 , of milling chamber  12  accommodates an air inlet opening  26  which is provided with a number of nozzles each. The nozzles are mainly directed to a machining area  30  in which the spindle machines the workpiece. Nozzles  32  are attached to a nozzle carrier  34  which in turn is attached to milling chamber  12  from the outside. 
     Within the area of nozzle carrier  34 , electrodes  36  are arranged in addition which are supplied with electric voltage for deionizing the air let in. The supply is done during dry operation, while the electrodes are closed by a sliding device, which is not depicted, during a wet operation of dental milling machine  10 . Thus, either deionizing electrodes  36  or nozzles  32  may be covered by the sliding device. 
     Starting from the left upper corner, according to  FIG. 1 , the inlet air provided will flow to machining area  30  and beyond it to an outlet  40 . Outlet  40  is positioned diametrically opposite of air inlet opening  26 . Floating particles, such as milling dust, for example, are thus removed from the milling chamber and are carried away with the air stream. 
     In many cases, however, milling chips are created which fall down, into the area of the base  42  of the milling chamber, due to their weight. In accordance with the invention, the base  42  of the milling chamber should now be inclined, which is, in such a fashion that it will drop towards outlet  40 , in particular at an essentially constant slope. The milling chips tend to slide in this direction due to their own weight. 
     In a particularly favourable embodiment, a directed carrier stream is additionally provided for the base, which stream is created by a nozzle  44 . Nozzle  44  causes air to flow vertically downwards along side wall  46 . At the bottom, adjacently to side wall  46 , a stream re-directing device  48  is arranged, which has a very large radius of, for instance, half the diameter of the milling chamber. The stream from nozzle  44  will therefore be essentially laminar over the stream along side wall  46 , further along stream re-directing device  48 , and then across base  42 . There, this carrier stream  50  will take away chips which are present there, and will supply them to outlet  40 . 
     For this purpose, outlet  40  is preferably connected, via an aspiration line  52 , with a source of vacuum which is not depicted. Therefore, a vacuum is created at outlet  40 , which increases the flow velocity there. 
     As a result of the carrier stream  50  created, it is additionally possible to observe that some milling chip falling down will not even reach base  42 , but rather be immediately carried away by carrier stream  50  and supplied to outlet  40 . 
     It can be taken from  FIG. 2  in what fashion milling chamber  12  may be formed in a lateral vertical view. Milling chamber  12  is formed, in the area of base  42 , in the fashion of a hollow fillet and has an essentially “U”- or “V”-shaped appearance. In a similar fashion as, for instance, a rain gutter, milling dust which is flying around is supplied to the central area  54  of base  42  and may thus be supplied to outlet  40  together with carrier stream  50 . 
     It can also be taken from  FIG. 2  that a door  60  is formed in the upper/front region of the milling chamber, via which door the milling chamber may be opened. 
     A back door  62 , which is also positioned rather far upwards, is closed during the milling operation and serves for the purpose of exchanging the workpiece. 
     From  FIG. 3 , a dental milling machine  10  in an embodiment in accordance with the invention is visible in a horizontal cut. Equal reference numbers indicate the same or corresponding parts here as well as in further Figures. As can be seen, milling chamber  12  has rounded corners  64  and  66 , such that there is no danger of any milling chips depositing there. 
     Another embodiment of a dental milling machine in accordance with the invention is visible schematically from  FIG. 4 . The machining area, and together with it, workpiece  18 , are positioned in the left third of milling chamber  12 , i.e. closely neighbouring nozzles  32 . In this position, the air stream has a comparatively high flow velocity, whereas this is typically reduced in the direction towards outlet  40  due to the larger flow area. Base  42  is provided with a continuous inclination in the embodiment according to  FIG. 4  as well. 
     From  FIG. 5 , a switching device for wet and dry operation can be seen. A switching hatch  70  may be swivelled between the upper wet position  72  which is depicted in  FIG. 5 , and the lower dry position  74  extending vertically. The axis of switching hatch  70  is positioned upside at the corresponding lead. 
     In dry position  74 , switching hatch  70  is closed. The air stream through outlet  40  is in the direction towards aspirational lead  52 , which branches off switching hatch  70 . At switching hatch  70 , deposits  80  may form in this embodiment. 
     When switched to wet operation now, switching hatch  70  is swivelled to wet position  72 . In this position, deposits  70  lie freely inside the washing water which passes through outlet  40 , such that the water current will take deposits  70  with it and will supply them to waste water connection  82 . 
     Above switching hatch  70 , there is an additional maintenance duct  84  in the embodiment depicted here, which makes possible an occasional basic cleaning and maintenance in a basically known fashion. 
     In  FIG. 6 , another embodiment of a dental milling machine in accordance with the invention is depicted. Equal reference numbers indicate the same or corresponding parts here as well as in the other Figures. The air nozzles  32  in accordance with the invention are formed as air amplifying nozzles, for which the design can be taken in detail from  FIG. 6   a.    
     Three air nozzles  32  are provided, which are each arranged in an inclined position. The air amplifying nozzle  32  which is arranged to the far left, and which is arranged farthest away from outlet  40 , has an inclination of approximately 20 degrees to the left, i.e. towards the side wall adjacent to it. The air stream is thus carried out following in a particular good fashion the internal radius  86  of base  42 . 
     The air amplifying nozzle  32  which is arranged to the far right is also facing its neighbouring side wall, also at an angle of 20 degrees as well. Deposits at the side walls may be prevented in this fashion. 
     Central air amplifying nozzle  32  is translocated in the direction towards outlet  40  and inclined into the direction pointing away from the outlet, i.e. it predominantly supplies the central region of milling chamber  12 . 
     Further, air amplifying nozzles  32  are translocated into the direction towards door  60 , i.e. towards the front inside milling chamber  12 , as can be taken from  FIG. 7 . 
     It is to be understood that the arrangement of air amplifying nozzles  32  may be adapted to the requirements to a great extent. They are fed together via a compressed air lead  88  with a distributor  90 . Distributor  90  allows the relative adjustment of the current volumes towards the individual air amplifying nozzles  32 . 
     In the exemplary embodiment depicted, a pressure reducer  92  is further provided, and a valve  94  switches the compressed air, based on the signal of a control device  96 . 
     Valve  96  is connected to a compressed air network  98  on the input side, which network is under pressure, for instance 7 bar to 10 bar. 
     A possible embodiment of an air amplifying nozzle  32  can be taken from  FIG. 6 a   . Via supply line  100  and an annular gap  102 , air is introduced at a high current velocity due to the narrowing of the annular gap. The current leans to the curved edge  104  of the nozzle. In the central entrance area  106 , a considerable negative pressure is created which causes the aspirational air to be sucked in at a high current velocity and be blown out through the nozzle, wherein at the same time a high volume of air is achieved. 
     If no inlet air is admitted inside through air inlet channel  100 , the air amplifying nozzle  32  in accordance with the invention may act as an air inlet via its central opening  108 , such that an equalization of pressure between the milling chamber and the ambient air may be carried out as a result. 
     In accordance with  FIG. 7 , air amplifying nozzles  32 , or at least a part of them, should be inclined diagonally in the direction towards door  60 , and in addition be arranged adjacently to it. 
     As a result, the door is blown free, such that the inside of the milling chamber is well visible. 
     From  FIG. 8 , a time diagram for an embodiment of a milling machine in accordance with the invention is visible, which is controlled by control device  96 . 
     The aspirational air is depicted with interrupted lines, and the inlet air with solid lines. During milling step  110 , vacuuming is carried out at an aspirational power of 50% of the maximum power. The milling step may last 10 min, for instance. Immediately subsequent to this, a blowing pulse  112  is output, such that air exits air amplifying nozzles  32  at high pressure and a large air volume. At the same time, or shortly after, the aspirational power is increased to 100%. This takes approximately 10 seconds. Subsequent to this, if necessary, a tool change is made in a tool-changing step  114 , which, for example, takes 30 seconds to 1 minute. At this moment, both the blowing air and the aspirational air are switched off. 
     Subsequent to this, another milling step  116  takes place, again with the aspirational air half switched on. 
     It is to be understood that the exact timings, pressures and air volumes may be controlled depending on the dental restoration part to be milled, such that they may also be considerably shortened or extended.