fig1 shows a system 1 for making a dental restoration . the system 1 has a body 10 in which a machine unit 7 , and a control unit 5 are contained . the machine unit 7 and the control unit 5 are connected by an air channel ( shown in fig2 ). the machine unit 7 has a machining chamber 2 in which dental restorations can be machined . in the example the system 1 is a dental milling machine which is adapted to mill dental restorations from blanks of ceramic material . the dental workpieces are supplied to the system via an input magazine 3 in which multiple workpieces may be held and made available for successive machining in the machining chamber . finished dental workpieces are preferably automatically placed in an output magazine 4 which typically also provides capacity for multiple finished dental workpieces . the dental workpieces are fixed in support frames that are shaped appropriately for automatic handling in the machine and for precise positioning in the machining chamber . a support frame for holding a dental workpiece as it may be used with the present invention is for example disclosed in patent application wo2008 / 097874 . a ventilation device 6 ( shown in fig2 ) is used to evacuate the machining chamber 2 so that chips , dust or other particles or gases originating from machining of the dental workpieces are continuously discharged ( for example sucked away ) from the machining chamber 2 . this allows for example for continuous operation over a relatively long time because interruptions for manually disposing of process - originated waste may be prevented . the control unit 5 is preferably configured to control the operation of the system , particularly for controlling the operation of the machine unit 7 . the control unit 5 in the example has electronic circuitry to drive servo - motors of the machine . the servo - motors drive and position slides and / or pivots of the machine . such slides and pivots are adapted and arranged with respect to one another to position the workpiece and a machining tool ( in this case a milling tool ) three - dimensionally relative to one another during machining of a workpiece . a servo - motor may also be used to drive a spindle for rotating the milling tool . further the control unit 5 may be adapted to run computer software , like an operation system as well as a user specific program . such user specific program may , for example , comprise commands that are interpreted to control the machine operation to achieve a desired shape of the dental workpiece . fig2 is a block diagram of the system 1 illustrating the function of the system 1 in more detail . an air channel 19 provides air connection between the machine unit 7 and the control unit 5 , in particular the air channel 19 may provide an air connection from the machine unit 7 toward the control unit 5 . the air channel is illustrated as a pipe , but may be implemented in a variety of different forms , such as one or more pipes , hoses , openings , and combinations thereof . further the system 1 has a supply opening 11 allowing air to penetrate from outside into the system 1 via the machining chamber 2 , and an exit opening 12 allowing air to escape from the system 1 to the outside via the control unit 5 . thus an overall air flow path 20 is provided in the system between the supply opening 11 and the exit opening 12 . the openings 11 , 12 are indicated as discrete openings for better understanding only . such openings may however be implemented in various different forms , for example in the form of multiple openings in the body of the system . particularly the inlet opening 11 may be dispensable in case the machining chamber is not hermetically or tightly sealed . this may be the case for example if the machining chamber has a door which openably closes the machining chamber , but which does not have seals to hermetically seal the machining chamber . in the illustrated embodiment , the air channel 19 has an inlet 13 and an outlet 15 . the air channel 19 has a ventilation device 16 which forces air to flow along flow path 20 , in particular forces air to flow towards the outlet 15 so that further air is drawn into inlet 13 . the ventilation device 16 may also be arranged at other places , for example adjacent the exit opening 12 , adjacent the supply opening 11 or at any other suitable place that allows air to be forced along flow path 20 . exemplary alternative or additional areas for the ventilation device 16 are indicated by elements 16 ′ and 16 ″. the air channel 19 further has a filter 14 which air flowing from the inlet 13 to the outlet 15 is required to pass . air flowing along flow path 20 , and particularly flowing in the air channel 19 , thus is filtered . therefore process - originated material , such as chips and / or dust , eventually entrained in the air may be retained in the filter . the control unit 5 preferably encapsulates the electronic circuitry 17 within a housing 18 . the housing 18 , the outlet 15 and the exit opening 12 are preferably adapted and arranged with respect to one another such that the air which flows from the outlet 15 toward the exit opening 12 is guided over the electronic circuitry 17 . therefore a thermal exchange between the electronic circuitry 17 and the air may be provided , so that for example heat which is generated in the electronic circuitry 17 may be dissipated from the control unit . in the example the control unit 5 is only ventilated by the air flow 20 . therefore an additional ventilation of the control unit is not necessary as it is typically present in prior art . as an advantage in this way the control unit may only be passed through by filtered air . further a separate filter that may be required for separate ventilation can be saved . advantageously a ventilation device as it may be used with the present invention is adapted to generate an air flow that is strong enough to entrain the process - originated material ( chips or dust ). such a ventilation device further may provide relatively high air flow rates , for example in a range of 600 m 3 / h to 1000 m 3 / h , and preferably about 800 m 3 / h . the flow speed of the air in the vicinity of the inlet 13 or in the inlet 13 may be in a range of about 4 m / s to about 8 m / s , and preferably at about 5 . 5 m / s . the air flow rates may be sufficient to ventilate , for example to cool , powerful components of the electronic circuitry ( for example power amplifiers for controlling the servo - motors ). another advantage results from the fact that typically the filter retaining the process - originated material is frequently serviced for removing the material from the filter . therefore the filter is typically held in a good condition so that the flow resistance is kept low and consequently also the air flow rate can be maintained on a relatively high level . the cross - sectional area of different portions of the flow path 20 may vary between the supply opening 11 and the exit opening 12 ( larger or smaller ) so that the air velocity through those portions is increased or decreased . for example , the velocity in the vicinity of the workpiece may be relatively high to provide for entrainment of the process - originated material . also in the vicinity of the circuitry the velocity may be high to provide for a good heat transfer . however in the area of the supply openings and or exit openings a larger cross - sectional area may provide for a lower velocity , for example to avoid noise . with increasing air flow rates the flow resistance of the filter may cause increasing back pressure in the upstream flow path . therefore also in the area of the filter a lower velocity of the air may be preferred to prevent the filter from causing undesirably high back pressure . this can also be achieved by a widened cross - section of the flow path in the filter area . the filter 14 and the ventilation device 16 may be arranged in an external unit which is connected to the machine unit 7 and the control unit 5 . in one example the filter and the ventilation device may be parts of a commercially available industrial vacuum cleaner . dental materials as they may be used with the present invention are for example dental ceramic materials or a dental glass - ceramic material . such materials may be pre - sintered , or sintered . the raw breaking resistance of the pre - sintered material or the facing precursor as referred to in this specification is preferably in a range of 10 to 15 mpa , more preferably in a range of 11 to 13 mpa , and preferably about 12 mpa according to the “ punch on three ball test ” as specified in iso 6872 . the sintered material referred to in this specification preferably has a material density in a range of 2 g / cm 3 to 2 . 7 g / cm 3 , and the pre - sintered material preferably has a material density in a range of 30 % to 92 % of the material density of the sintered material . preferably the material density of the pre - sintered material is in a range of 40 % to 60 % of the material density of the sintered material , and more preferably in a range of 45 % to 55 %. the raw breaking resistance of the sintered material as referred to in this specification is preferably in a range of 50 to 120 mpa , more preferably in a range of 68 to 74 mpa , and preferably about 72 mpa according to the “ punch on three ball test ” as specified in iso 6872 . a ceramic material as referred to in this specification may be made of a pre - sintered or sintered material , for example a ceramic based on zirconium oxide . in particular the ceramic material may comprise between 90 % and 99 % by weight zirconium oxide , and preferably 91 % to 97 . 25 % by weight zirconium oxide . the ceramic material of the frame may further comprise 0 %- 1 % by weight aluminum oxide . the ceramic material of the frame may also be based on aluminum oxide , meaning the ceramic material may comprise 90 % to 99 % by weight aluminum oxide and 0 % to 1 % by weight zirconium oxide . further the ceramic material of the frame may comprise 0 %- 10 % by weight of at least one of hafnium oxide , yttrium oxide and oxides from gallium , germanium , and indium . the ceramic material of the frame may also comprise 0 . 0005 % to 1 . 5 % by weight of coloring additives , selected from the group consisting of the oxides fe 2 o 3 , er 2 o 3 and / or mno 2 . the ceramic material is preferably selected to be compatible for use in human bodies . the glass - ceramic material as referred to in this specification is preferably selected to be compatible for use in human bodies . typical glass ceramic materials are high - strength oxides of the elements of the main groups ii , iii and iv and the subgroups iii and iv as well as their mixtures , in particular aluminum oxide , zirconium oxide , both partly and also fully stabilized , magnesium oxide , titanium oxide and mixtures of aluminum oxide , zirconium oxide , magnesium oxide and titanium oxide . an exemplary formulation of a glass ceramic as it may be used with the present invention comprises 60 % to 70 % by weight of silica , 9 % to 13 % by weight of alumina , 5 % to 10 % by weight of potassium - oxide , 9 % to 13 % by weight of sodium - oxide , 0 % to 1 % by weight of lithium - oxide , 2 % to 5 % by weight of calcium , 1 % to 2 % by weight of barium - oxide , 0 % to 1 % by weight of zirconium oxide and 0 % to 1 % cerium - oxide or cerium - fluoride . other materials may also be used , such as dental metals or alloys , and / or dental composites , as appropriate to form at least part of a dental restoration . it will be appreciated that the embodiment shown in the figures is just one example of how a system in accordance with the invention can be employed . however other embodiments providing equivalent effects are possible . in particular the arrangement of components relative to each other may be changed , or equivalent components may be used instead or in addition to the components described . further , a single component may be implemented by two or more of the same or equivalent components . for example , it may be possible to implement an air connection between the machine unit and the control unit by two or more air channels which for example extend parallel , or two or more machine units may be connected to a single control unit , for example .
in one embodiment , a system for making a dental restoration is provided . the system includes a machine unit for processing a dental material to form the dental restoration , and a control unit for controlling the operation of the machine unit . an air channel for discharging a process - originated material from the machine unit provides an air connection between the machine unit and the control unit . certain embodiments of invention are advantageous in that they help to increase the reliability of the machining operation .