DE4030175C2 - Method for calibrating a motor-driven tool in relation to a workpiece to be machined with it, and device for carrying out the method - Google Patents

Description

The invention relates to a method for calibrating a motor-driven and with the help of a feed device towards a workpiece to be machined and away from it movable tool with respect to the workpiece or a Workpiece-holding holder, and device for performing procedure.

A particularly advantageous application of such a method and a device operating according to this method is disclosed in seen in dentistry, ready-to-use, form giving elaboration of a tooth restoration fitting body.

Such a method is described, for example, in EP-A 0 182 098 described. The tooth restoration fitting body is there in one Operation from a blank made of a suitable material, e.g. B. made of dental ceramics, individual and ready to use worked. For this purpose, a processing machine is provided which is a separator rotatably mounted in a tool carrier disc sheet contains. In addition, the processing machine contain a second, finger-shaped removal tool, with the more complicated one, with the cutting disc blade not work out editable shapes from the workpiece as well as the Have the surfaces of the fitting machined better.

The workpiece is placed on a holder that is tight is on the one hand and one or more stop surfaces, that have an exact positioning in relation to him Ensure the clamping device and on the other hand one or  several reference surfaces that allow adjustment of the position of the cutting blade with respect to the workpiece axis possible, contains.

To produce the desired shape of the fitting body are made Tool carrier and workpiece holder adjustable, where individually controllable adjustment motors for the tool thrust and the tool holder are present.

The fitting body is based on predetermined design data from the previously prepared tooth using a surveying camera and created a computer with appropriate software are worked out from the workpiece blank.

To pre-orientate yourself about the position of the tool with respect to the workpiece and thus the feed To give the system a starting and reference point contains the known device in the field of the feed system for Tool a fork photoelectric switch, with which the output position of the tool can be roughly defined.

To especially wear the work surfaces of the grinding To be able to take tools into account before starting a Machining a calibration of the tools regarding the Axes of the workpiece holder are made. This includes the known device a non-contact rotation number measuring device with a permanent magnet and an in ductive transducer. The frequency signals from the transducer go via a pulse shaper into a counter. In a logical Unit which, like the counter, will belong to a computer discrimination against certain, set frequency values, according to the speed of the tool. Under If the frequency exceeds a certain value, tax will be charged signals triggered.  

The calibration takes place in that the tool first with the normal idle speed to the narrowly tolerated Re reference surface of the workpiece holder opens. Because of this The drive of the tool will touch to a low level braked speed value. Based on the braking curve, the after the frequency value analysis described above can be determined calculated back to the point of contact or this arithmetically be determined.

In a second attempt, the reference point of the in the meantime, the workpiece has been turned again by 90 ° discriminating against a higher speed value. This before gear corresponds to a smaller braking than the first Attempt.

Based on the positions determined using the braking curves and the starting position detected via the fork light barrier becomes the starting point for the start of editing or a into the machine control of the feed device Correction value determined. After calibration, the Be work of the blank in the not to be explained here Start wise.

Although with the known method and the one after that tendency device a calibration of the tool itself is quite possible, but this method is with some spe specific disadvantages. So the braking curve is on who orientates the measurement by several parameters flows:

• - Speed or speed of the cutting disc
• - Shape of the cutting disc
• - Condition of the coolant and lubricant
• - Type of material that is "touched" on
• - Efficiency of the entire drive system

By these parameters which influence the measuring accuracy with regard to the achievable accuracy in determining the Starting position of the tool set certain limits.

The invention is based on the object reasons, in contrast an improved method and one after specify working device, in particular with the aim a calibration of tool and workpiece even more precisely to be able to make, and thus the starting position of the plant or a correction quantity for the tool feed to be able to specify more precisely.

Advantageous refinements and developments of the invention result from the subclaims. Based on the drawing an embodiment of the invention described in more detail. Show it:

Fig. 1 is a dental apparatus with the inventive device in a diagrammatic representation,

Fig. 2 shows part of the apparatus of Fig. 1 seen from above, in section,

Fig. 3 is a block diagram for the control of the drives.

Fig. 1 shows a diagrammatic representation of the overall arrangement of a device working according to the invention in one embodiment for the production of a tooth restoration fitting body, for. B. a tooth crown. The fitting body to be created can be constructed using an optical impression process with the help of a computer on a screen and then constructed in a processing machine from a suitable workpiece, e.g. B. made of dental ceramics. The individual components of this device are an optical camera 1 , a screen 2 , a keyboard 3 , a drawing ball 4 (trackball), a processing machine, generally designated 5 , and further mechanical, electrical and electronic components, not shown, contained in the housing 6 work with the above components to create the fitting body. The basic structure and the mode of operation of such a device are described in the aforementioned European patent.

The housing 6 forms a processing chamber 7 into which some components of the processing machine, which will be explained in more detail later, protrude. These are essentially a holding and holding device for a workpiece 8 , from which the fitting body to be produced is manufactured, and two mutually opposite spindle arrangements 9 and 10 , which each have two tools (in FIG. 2 with 11 , 11 'and 12 , 12 'designated) wear for machining the workpiece 8 . The processing chamber 7 can be closed by means of a preferably transparent cover 13 during the processing. The housing 6 contains below the processing machine 5 a water tank 14 designed as a drawer and a chamber 15 in which a container for receiving a care product is held.

The processing machine 5 will he explained in more detail with reference to FIG. 2. Fig. 2 shows the machine in the region of the spindle assemblies 9, 10, sectioned in a plan view. First, the holder and storage of the work piece 8 from which the fitting body is to be created is described in more detail. The workpiece 8 consists of a ceramic block 8 a, which is attached to a metallic pin-shaped holder 8 b by gluing or the like. The workpiece is inserted at the end of a spindle 16 designed as a hollow shaft and is held in a rotationally and tensile manner by means of a clamping device designated 17 .

The spindle 16 designed as a hollow shaft has at its end facing away from the workpiece 8 a spindle nut 25 which interacts with a threaded spindle 26 which is coupled directly to a drive motor 27 fastened to the housing 6 .

With the spindle 16 rotatably connected, a gear 28 , which is in engagement with a toothed shaft 29 , the inter mediate two walls 6 a, 6 b of the housing 6 and just if directly coupled to a second drive motor 30 . The axes of the toothed shaft 29 and spindle 16 run parallel. The drive motor 30 with the toothed shaft 29 serves to set the spin del 16 in a rotational movement, while the drive motor 27 with the threaded spindle 26 ensures a longitudinal feed of the spindle 16 . For this purpose, the bearing 31 facing the workpiece is designed as a longitudinal and rotary bearing. The toothed wheel 28 is designed as a backlash-free double gear; it consists of two coaxially arranged toothed wheel halves 28 a, 28 b, which are supported against each other by a (not shown) spring element, so that regardless of the direction of rotation of the drive, one tooth flank from one or the other gear half with the toothed shaft 29 in A handle stands. Such a backlash-free double gear forms on the one hand a low-noise precision gear, on the other hand it serves for the safe and exact transmission of movement with a longitudinal feed of the spindle. The engagement connection spindle nut 25 , threaded spindle 26 and hollow shaft 16 requires only little space in the axial direction. The drive motor 27 also forms a fixed bearing for the unit and is directly connected to the threaded spindle 26 , that is to say without a coupling. Threaded nut 25 and threaded spindle 26 can be built as a play-free ball screw drive or as a play-free slide screw drive. With a slide screw drive, a backlash-free nut can be combined with the hollow shaft.

The structure of the spindle arrangements 9 and 10 is explained in more detail below.

The spindle assembly 9 contains a ausgebil Dete as a hollow shaft spindle 32 , one end of which projects into the machining chamber 7 and carries a tool carrier head 33 there . An electric motor 35 is arranged in the center of a housing which is fixedly connected to the spindle 32 , the rotary movement of which is deflected by 90 ° via a bevel gear 36 onto the tool drive shafts provided on both sides. The axes of these drive shafts therefore run parallel to the spindle axis of the Spin del 32 . At the free end of the drive shafts, the machining tools 11 , 11 'are held by means of suitable clamping devices.

The oppositely arranged tool carrier head 43 for the spindle arrangement 10 has a similar structure; however, it contains no bevel gear teeth. A motor 40 arranged in the tool carrier head 43 drives the tool drive shafts directly here. The tool axes 45 therefore run parallel to the axis 47 of the workpiece feed spindle 26 . The tool carrier head 43 is in contrast to the tool carrier head 33 , which carries finger-shaped tools, with disk-shaped or pot-shaped tools 12 , 12 'occupied. One side is preferably provided with a disk-shaped grinding or milling disk ( 12 ) and the other with a cup-shaped polishing disk ( 12 '), which can consist of diamond-coated soft material or can also be designed as an exchangeable brush. Because the tool carrier head 43 can be rotated through 180 ° about the spindle axis 46 , grinding and subsequent polishing of the fitting body can be carried out practically without interrupting work.

The two spindle assemblies 9 and 10 have the same drive and drive transmission elements with respect to their pivoting and advancing movement, which are explained below for the spindle 10 arrangement.

On the spindle 46 designed as a hollow shaft, a spindle nut 48 is fastened, which engages with the threaded spindle 44 driven by a motor 49 . With these output elements, the spindle 46 can be adjusted in the axial direction.

With the spindle 46 rotatably connected, in turn, a double gear 51 , which is constructed similarly to that marked with 28 be. This double gear 51 meshes with a toothed shaft 53 driven by a further drive motor 52 . With this drive element, the entire tool carrier head 43 can be pivoted about the spindle axis by at least 90 °, but preferably by 180 °. A pivoting through 90 ° is indicated in order to pivot the tool carrier head from the working position shown in FIG. 2 into a basic or rest position in which access to the workpiece 8 is facilitated.

After the feed of the individual spindles he very precisely must follow, it is necessary to use the drive provided for this purpose precisely control motors. Therefore, they become stepper motors used, which is per se in very small incremental units be controlled. To compensate for step losses anyway can, it is suggested the stepper motors with a back message. This can be that close the feed spindle, or coupled to this, on contr level, capacitance, inductance, ultrasound, magnet and / or provided optical basis building sensor means are with a via a computer, not shown associated stepper motor are electrically connected. Through a such measuring system, which is both analog and incorrect can work mentally, it is possible to increase the accuracy of the Increase grinding machine.  

On a housing wall 6 c is a fork light barrier 54 is angeord net, which interacts with a ring 55 interrupted by a slot of the double gear 51 so that the rest or starting position of the spindle can be detected precisely, both with respect to their rotary ( Angular) position and in relation to their position in the direction of the feed axis. While the latter is detected by immersing the ring 55 in the fork light barrier, the angular position is detected by a signal when the slot 55 interrupting the slot is passed through the light barrier. With the help of the fork light barrier, the distance a of the tool axis 45 to the workpiece axis or workpiece holder axis 47 can thus be determined.

To fit the fitting body, in the present application case a tooth crown drawn in dashed lines in the workpiece 8 in FIG. 2, on the basis of predetermined construction data, which are created from the prepared tooth stump with the aid of the camera mentioned above, a computer with software, from the workpiece 8 To be able to, it is necessary to calibrate the tool and workpiece or workpiece holder or to determine a precisely defined starting position for the machining tools 11 , 12 , from which the workpiece machining can begin in accordance with the specified design data. It is important to compensate for wear and / or manufacturing tolerances of the tools and any deviations of the workpieces from specified target values, as well as other deviations that occur due to aging of the components.

To calibrate or determine the starting position, construction-related fixed variables are taken into account, e.g. B. the aforementioned distance (a) of the axes 45 , 47 from tool to workpiece or clamping device in the starting position detected by the fork-light barrier 54/55 , the distance (b) between the axis 47 of the workpiece or Holder and the touching surface (reference surface), and the grinding wheel diameter as the specified target size. These known fixed quantities are stored in a memory of a computer which will be explained in more detail below.

Taking these fixed quantities into account, the Above-mentioned deviations from the target sizes determined by the machining tool by the the fork light barrier fixed starting position up to Contact with the reference surface on the workpiece or a reference surface of the tool holder is driven and the actual appropriate size introduced into the computer and with the predetermined target sizes is processed. Determine from this Deviations can be used as a correction quantity in the machines control, i.e. in the control program for the tool feed, be introduced, it being possible to either the output leave position as start position and the correction in the given design data for creating the fitting body to incorporate, i.e. to change the design data, or to leave this unchanged and instead the starting position of the Tool, i.e. its starting position, to correct the correction size change.

The actual value is determined according to the invention as follows: The drive motors 35 and 40 of the machining tools 11 , 12 , whose normal machining speed is approximately 20 to 30 × 10³ U / min, are first down-regulated to the lowest possible speed. This is only so large that the tool chuck when thawing, i.e. when touching the workpiece, and cannot dig into the workpiece surface due to the low kinetic energy. In the present case, this speed, referred to below as the driving speed, is approximately 100 to 400 rpm. It is particularly favorable to use collectorless DC motors as drives, which are equipped with integrated Hall sensors for detecting the speeds. In connection with the described tool feed device, which includes the stepper motor 49 , the tool can be moved towards and away from the workpiece in arbitrary small steps.

Before determining the exact starting position is further explained, the principle circuit in Fig. 3 drive and control of the drive and adjusting motors for the tools 12 , 12 'and the workpiece 8 will be described. Analogously, the control for the drive motor 35 of the tool 11 and for the unspecified adjusting motor of the feed device for the tools 11 , 11 '.

An arithmetic unit designated 60 contains a speed setting actuator 61 with which the drive motor 40 can be set to the aforementioned starting speed. The arithmetic unit 60 also contains two pulse generators 62 , 63 , with which on the one hand the stepping motor 49 for the advance of the tools 12 , 12 'and on the other hand the stepper motor 27 for the advance of the workpiece 8 are controlled. A memory in the form of a counter 64 is used to be able to store the signals received from the fork light barrier 54 , 55 at the start of the feed movement until the workpiece 8 is touched by the grinding wheel 12 . In a comparator 65 , the actual value (input) of the speed of the drive motor 40 is compared with a target value (+ input) of a reference speed. In the present case, this setpoint reference speed corresponds to the application of the speed zero, ie the stepping motor 49 receives control signals until the grinding wheel 12 touches the workpiece 8 and the actual speed thus becomes zero. If the actual speed is not equal to zero, the downstream pulse generator 62 delivers a pulse to the counter 64 and to the stepper motor 49 . On the one hand, the tool is moved a defined distance in the direction of the workpiece 8 , on the other hand, the distance covered is temporarily stored in the counter 64 . This process is repeated until the actual value and the setpoint match, ie until the starting speed has become zero at the point of contact. The comparator 65 then switches off the pulse generator 62 . The counter 64 transfers the current counter reading to an arithmetic unit 66 , where it is stored. The stored value is a measure of the path actually traveled by the tool tip or work surface (denoted by P in FIG. 2), from the starting position to the reference surface (denoted by R in FIG. 2). A comparison can be made with a comparison with the already mentioned fixed variables, which are stored in the arithmetic logic unit 66 , and the starting position for processing corresponding predetermined construction data stored in a memory 68 of a control unit 67 can be calculated therefrom, in the event of deviations then either the default data for the feed device or the starting position of the feed device can be changed accordingly. With this control data, the actuators 49 , 27 and 30 and the drive motors 40 and 35 are then controlled from the control unit 67 , wherein the control unit 67 can also be an integral part of the computing unit 60 .

It can be advantageous if the stepping motor 30 receives a pulse sequence from the pulse generator 63 , which causes the workpiece to be rotated by 180 ° after the first touch and the same process is carried out again after the rotation. The arithmetic unit 66 are then two values over covered distances, from which the thickness (diameter) of the workpiece (2 × d) in the working plane or the diameter of the grinding wheel 12 can be calculated.

In order to achieve higher accuracy, it is proposed that the tool so far after contact with the workpiece drive back from the workpiece until the starting speed is reached again. In the arithmetic unit, the two Signals corresponding to "zero speed" when touched and "start speed "processed when detached from the touch. With more feel and then drive away again using this method both the point of contact and eccentricities of round touch surfaces much more precisely.

The diameter or the starting position of the end mill 11 in the grinding unit opposite can also be determined according to the same principle.

Claims (9)

1. A method for calibrating a motor-driven and with the aid of a feed device to a workpiece to be machined to and from this movable tool with respect to the workpiece or a workpiece receiving holder, characterized in that initially the speed of the drive motor is (35, 40) set at a starting speed for the tool (11, 12) which is so low that upon contact of the workpiece (Ba) or of the workpiece holder (8 b) by the tool, its speed becomes zero that the tool with this starting speed is moved from a starting position to the workpiece or the workpiece holder until it touches it and is then brought into a starting position, the distance of the tool feed from the starting position to the touch of a path measuring device ( 54 , 55 ) is detected by starting with the advance movement until touching by the drop in speed signals are generated which are used to determine the exact starting position of the tool in relation to the workpiece or the workpiece holder or to determine a correction quantity for the tool feed, taking into account the detected starting position as actual values in a computing unit ( 60 ) in which design-related fixed variables and the output variables are stored, whereby deviations are determined in the computing unit by comparison with the actual signals and the stored values and entered as a correction variable in the control program for the tool feed, with either the starting position being defined as the new starting position and the design data given in the computer for machining the workpiece are changed or the starting position is changed by the correction quantity. 2. The method according to claim 1, wherein the workpiece ( 8 a) or the workpiece holder ( 8 b) is held in a clamping device ( 17 ), which allows at least one rotation of the workpiece about the axis ( 47 ) of the clamping device, thereby characterized in that for determining the thickness of the workpiece in the feed plane, the process sequence is carried out several times with a changed rotational position of the workpiece ( 8 a) or the workpiece holder ( 8 b). 3. The method according to claim 2, characterized in that the process sequence with 180 ° rotated workpiece ( 8 a) or workpiece holder ( 8 b) is carried out. 4. The method according to any one of claims 1 to 3, characterized in that to achieve a higher measuring accuracy, the tool ( 11 , 12 ) after contact with the workpiece ( 8 a) or the workpiece holder ( 8 b) again so far from the workpiece or workpiece holder is moved away until the starting speed is reached again, the two signals corresponding to "zero speed" when touched and "starting speed" when detached from the contact are processed in the computing unit ( 60 ). 5. The method according to any one of claims 1 to 4, characterized in that the values determined by the computing unit ( 60 ) on the position of the tool ( 11 , 12 ) and / or the thickness of the workpiece ( 8 a) one in the computing unit Integrated or separate control unit ( 67 ) are supplied which, based on predetermined machining constructions data, taking into account the starting position of the tool and the thickness of the workpiece, the drive motor ( 35 , 40 ) and the feed device ( 32 , 33 ; 43 , 46 , 49 ) for the Control the tool ( 11 , 12 ) and a servomotor ( 27 , 30 ) for the workpiece ( 8 a) in the sense of machining the workpiece according to the specified design data. 6. The method according to any one of claims 1 to 5, characterized characterized that it is used to create a medical, in particular dental prosthetic fitting body, is inserted. 7. Apparatus for carrying out the method according to one of claims 1 to 6, characterized in that a feed device ( 43 , 46 , 49 ) which can be controlled in small steps by a stepper motor ( 49 ) is provided, with which the tool ( 12 ) in at least one level and in this level in at least one direction to the workpiece ( 8 a) and can be moved away from it that as a drive motor ( 40 ) for the tool is provided in its speed adjustable to zero DC motor, the Via integrated sensors, speed-proportional signals provide that the computing unit ( 60 ) also contains a memory ( 64 ) in which quantities corresponding to the feed movements of the tool ( 12 ) are stored, and an actuator ( 61 ) which contains the speed of the drive motor ( 40 ) for the tool ( 12 ) to the lowest possible (starting) speed, and that the computing unit also contains a comparator ( 65 ), in which the actual value of the speed of the drive motor is compared with a predetermined speed setpoint, wherein if the setpoint and actual value match the computing unit ( 60 ), an electrical signal for switching off the feed actuator ( 49 ) is given. 8. The device according to claim 7, characterized in that the computing unit ( 60 ) has a pulse generator ( 62 , 63 ) for controlling on the one hand the actuating motor ( 49 ) for the feed of the tool ( 12 ) and on the other hand the servomotor ( 27 , 30 ) for the workpiece ( 8 a) comprises that a counter ( 64 ) is present as a memory, which covers the path of the tool, starting from the starting position, and that the arithmetic unit contains an arithmetic unit ( 66 ), which consists of the counter sizes the distance traveled by the tool and from this the starting position and possibly a size corresponding to the thickness of the workpiece are determined. 9. The device according to claim 7 for machining a workpiece ( 8 ) for the purpose of producing a tooth restoration fitting body, comprising a first spindle ( 16 ) for receiving the workpiece ( 8 ) to be held in a clamping device ( 17 ), at least one further spindle ( 32 , 46 ) for receiving at least one rotatable machining tool ( 11 , 12 ), the spindles ( 16 , 32 , 46 ) being arranged and mounted in such a way that the machining tool and workpiece in the sense of material removal from the workpiece toward one another and can be moved away from each other, also includes drive motors ( 27 , 30 , 49 , 52 , 35 , 40 ) for adjusting the spindles ( 16 , 32 , 46 ) and for driving the machining tools ( 11 , 12 ) containing furthermore at least one reference surface (R) arranged on the tool piece ( 8 a) on the workpiece holder ( 8 b) or on the clamping device ( 17 ), against which the machining tool ( 11 , 12 ) travels, a signal defining the feed path of the tool being generated when the reference surface is touched, which signal is processed in the computing unit to determine the exact starting position of the machining tool ( 11 , 12 ).