Abstract:
Methods, systems and tools are provided for controllable adjustment of a removal rate of manually-guided material- and tissue-separating tools and effectors which allow control of the removal rate of rapidly rotating tools, without the need to significantly change the rotation speed of the tool or to significantly change the position, orientation or the geometry of the tool, and in particular, for precisely maintaining work space boundaries during material and tissue removal with tools in rapidly rotating machine tools in surgery and freehand manufacturing for producing free-form surfaces. The manually-guided material-separating and tissue-separating tools and effectors allow controllable adjustment of the removal rate and the material-separating cutting edges of the effector and/or at least one cutting edge cover can be mechanically adjusted in their position (position and orientation) at a constant rotation speed in at least two positions relative to tool effector, or retracted and deployed, without appreciable change of the removal movement.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims foreign priority to German Application No. DE 102014105311.7 filed on Apr. 14, 2014 and to German Application No. DE 102013226197.7 filed on Dec. 17, 2013, the entire contents of which are hereby incorporated by reference. 
       FIELD OF THE INVENTION 
       [0002]    The disclosure relates to a method and a system for controllable adjustment of a removal rate of manually-guided material- and tissue-sectioning tools with effectors, and to a corresponding effector. 
       BACKGROUND 
       [0003]    Methods, systems and tools with effectors are useful in particular for the precise adherence to work space boundaries during material and tissue removal with tools of rapidly rotating machine tools in surgery and in hand-crafting of free-form surfaces. 
         [0004]    When hand-crafting in repair shops and in interventional medical treatment of humans and animals, manually-guided and manually-guided tools in form of driven machine tools (for example drill press with a drill or a milling tool, electrical planer) are frequently used for separating and removing material and tissue of an object. This is typically used, in addition to free-form shaping, for the spatial adaptation the object surface to a precisely fitting counterpart with defined boundaries. At the same time, certain structures or surfaces of the object must not be harmed with the tool, for example to protect an already finish-machined surface or structures inside the object, which are hidden from the view of the user. 
         [0005]    To date, the following methods are known in the related art as technical aids for manually-guided and manually-positioned tools: 
         [0006]    a) Navigation methods which measure the position and orientation of tools relative to object and graphically and acoustically indicate to the user the position, orientation and distance of the manually-guided tool to the allowed work space boundaries relative to object (Surgical Navigation, U.S. Pat. No. 5,389,101, DE 19960020) as well as graphically indicate the areas on the object that still need to be machined. 
         [0007]    b) Template attached to the object for limiting the free movement of the manually-guided tool within the allowed work space boundaries relative to the object (Ancient Times, Middle Ages, Surgical Template U.S. Pat. No. 5,141,513). 
         [0008]    c) Compliant mechanisms and robots, to which the manually-guided tool is attached. These mechanisms and robots change, relative to the allowed work space boundaries, the freedom, friction, stiffness and elasticity of the manually-guided movement on the object and apply even forces and torques in addition to the manual guidance (Lueth, T. C. et al., “A surgical robot system for maxillofacial surgery,” Industrial Electronics Society, 1998th IECON &#39;98. Proceedings of the 24th Annual Conference of the IEEE, vol. 4, no., pp. 2470-2475). 
         [0009]    d) Limiting the position, orientation, distance and signal-based removal rate relative to the object or structures of the object (Navigated Control, DE 000010117403C2). 
         [0010]    e) Mechanisms for linearly deploying and retracting a rotating effector relative to a protective sleeve surrounding the effector (Blue Belt Technologies, US 2012/0123418A1) for limiting the removal rate. 
         [0011]    Methods for adjusting of tool cutting edges are known from the published documents (DE 4401 496 A1), however not for handheld or manually-guided machine tools, whose movements of the tool position and the tool orientation cannot be definitely specified in advance by a program. No method is known that allows adjustment of the tool cutting edges without coupling to a programmable motion control. Likewise, no method is known that allows a computer-controlled or position-based adjustment of a rotating tool of a handheld or manually-guided machine tool. 
         [0012]    A method to extend the cutting edges subsequent to wear of the cutting edges is known from EP 0977644B1. 
         [0013]    With reference to the application of manually-guided driven tools with effectors in surgery, extensive experience has been gained over the last 20 years due to the increasing demand for efficiently fitting medical implants (e.g. knee, hip and the like). The disadvantages are similar to those encountered in hand-crafting of materials. The disadvantages of the present art are as follows: 
         [0014]    a) Pure navigation methods with graphic, acoustic and tactile, vibrating feedback do not solve the problem, that the user inadvertently misaligns the material-removing tool due to a slip or lack of concentration and thus violates work space boundaries. The less time is available for learning and executing the applications, the greater is the risk. The risk increases when the material has different densities, the tool is deflected or the user works in an unergonomic posture and is subjected to shocks or forces. 
         [0015]    b) Templates attached to the object require a sufficiently large available surface for attaching the template. In addition, templates exhibit their advantage primarily when the tool is inserted into the template in a preferred direction. It is currently not possible to mill a three-dimensional free-form surface while at the same time enabling three-dimensional mobility and three-dimensional orientation of the tools. Three-dimensional free-form surfaces can only be produced at great expense. Templates that cannot clearly be form-fittingly applied require navigation methods for aligning and affixing the templates to the object. Templates always require additional space on the object. 
         [0016]    c) Compliant (compliant motion, hands-on) robots, on which the manually-guided instrument is attached, have a limited working space and must be aligned relative to the object prior to use in a complex process. During operation, hands-on robots allow a seemingly free movement of the instrument. The high inertia of the robot in relation to the manually-guided tool or object are a limiting factor for the precise adherence to work space boundaries even with optimal dynamic control of a highly dynamic drive. Robots are also expensive to procure and operate, require a dedicated space next the object, and have a large mass. They also must be covered under sterile working conditions. 
         [0017]    d) Although the method for position-, orientation-, distance-, and signal-based control of the removal rate (navigated control) is indeed optimally suitable for adhering to the work space boundaries, a dynamically controllable change of the removal rate is still a prerequisite in order to compete in terms of time with a template or a mechanisms/robotic solution in the boundary area. Likewise, precise adherence to the work space boundaries can only be achieved when the removal rate of the tool has the steepest possible transition. Shutting off the power is currently problematic with pneumatic or electric drives due to the high mass inertia of the tool, because the force applied to the hand changes strongly during the switching operation. 
         [0018]    e) The mechanism for reducing the removal rate by relative movement of a protective sleeve disposed around the effector makes it possible to limit the removal rate while keeping the rotation speed constant. However, because the protective sleeve is larger than the effector itself and can be blocked by the object surface, often only a fast retraction or deflection of the tool is possible in practice. Masses are here also moved. In addition, the relative movement must be compensated earlier or later by hand, which prevents a fast operation with high precision. 
       SUMMARY 
       [0019]    It is an object of an exemplary embodiment to obviate the known disadvantages of the prior art and to develop a method, a system and an effector for an tool, which makes it possible to control the removal rate of rapidly rotating tools, without the need to significantly change the rotation speed of tool or its the position, orientation or the geometry of the tool with the effector. 
         [0020]    The disclosed system has the particular advantage that no jerk or shock is imparted on the hand that guides the tool with the effector, because the material-separating cutting edges of the tool can be mechanically adjusted in their location (position and orientation) relative to tool effector in at least two positions or can be deployed or retracted at a constant or only slightly changed rotation speed of the effector. This makes it possible to work manually quickly and with high precision. It is also possible to mechanically adjust, or deploy or retract, at least one cover of the cutting edge disposed forward of the material-separating cutting edges of the tool in its location (position and orientation) relative to tool effector in at least two positions in order to adjust the removal rate of the material-separating cutting edges of the tool. It is hereby possible to limit the removal rate of the effector of the tool depending on the direction in order to hereby force a preferred direction or a preferred volume during material removal, because the tool can remove material and further penetrate into the object only in the unblocked preferred removal direction. 
         [0021]    The system can be readily and inexpensively produced, reliably applied and readily serviced, because the removal direction set on the tool is easily visually discernable on the tool for the user (for example, by coloring the cutting edges) or is acoustically or graphically signaled (for example, via a display on a display screen). 
         [0022]    The change in the removal rate takes place in less than 1/10 second. 
         [0023]    According to an aspect of an exemplary embodiment, the removal rate is controlled based on a continuously measured length of the manually-positioned or manually-guided tool and optionally their mathematical derivatives as well as spatial relations to defined boundaries relative to the material or tissue to be removed or to an object body measuring marker, wherein the removal rate is controlled based on a sensor signal or control signal. 
         [0024]    According to another aspect of an exemplary embodiment, the position and orientation of the effector due to an adjustment of the cutting edges or cutting edge cover does not change at all or only in the order of magnitude of the cutting edge length of the cutting edges and the spatial extent of the effector due to the adjustment of the cutting edges or cutting edge does not change at all or only in the order of magnitude of the cutting edge length or the cutting edge cover. The adjustment of the removal rate hence does not require a correction of the position or orientation of the effector. The modified removal rate is attained with an unchanged position of the effector. 
         [0025]    Furthermore, it is advantageous to obtain the required energy for maintaining the signal connection and to control of the actuator directly from the kinetic energy or from the drive of the tool movement. It is also advantageous if the cutting edges and/or the cutting edge covers are pressed outwardly by an internal stator by way of at least one adjustable actuating lamella so as to uncover only a portion of the cutting edges for material removal and to thereby limit the removal direction relative to the location of the stator (or of the tool measuring marker), wherein the position of the stator relative to the tool measuring marker is known and the removal rate of the effector is directionally controlled by an adjustment. 
         [0026]    The method and system can advantageously be used, in particular, for precise adherence to work space boundaries during material and tissue removal with tools in rapidly rotating machine tools in surgery and in free-hand manufacturing for producing free-form surfaces. The change in the removal rate at constant rotation speed and effector position enable a very rapid manual tool movement, for example cross-hatched back and forth movements, with precise removal boundaries as well as automatic centering and direction correction of the manually-guided effector and hence also of the tool. 
         [0027]    The method and a corresponding system as well as a tool are another elementary step for increasing the precise processing power of a person by way of his motor skills in combination with a signal-, position-, orientation-, and distance-based fast reduction of the removal rate of a tool that is manually guided by this person. 
         [0028]    Several tools with lower removal rate can be emulated by adjusting the cutting edges, thereby reducing the number the tool changes. Instead of using effectors with shorter cutting edge lengths, the effective cutting edge lengths can be reduced for the surface finish. The surface quality improves. 
         [0029]    The range of applications of rotating tools will increase again at the expense of other separating methods, like lasers, because the removal rate can be adjusted with similar speed and significantly faster than until now. This enables the application of manually-guided tools, which for functional reasons have mass inertia or which can be undesirably deflected due to the interaction between tool and material, for precision tasks. 
         [0030]    The range of applications of handheld and manually-guided machine tools will thus significantly increase because a person can provide services similar to a machine tool. Accordingly, more people can perform more demanding activities, which so far has only been accomplished by machine tools. Furthermore, the spectrum of manual activities demanding precise three-dimensional surface machining will increase. 
         [0031]    With respect to applications in bone surgery, the accuracy of tight fits of orthopedic implants will be significantly improved, i.e. smaller interventions can be carried out with higher precision. The speed with which orthopedic implants are fitted (preparation and fitting) will increase significantly. The reduction in time has a positive effect on the healing process of the patients. The growing number of particularly older people with increasing demand for replacement of knee, hip and shoulder joints will be satisfied with a constant or decreasing number of surgeons. 
         [0032]    The costs for the acquisition of durable goods (medical robots) for achieving high implantation rates will decrease substantially. Presently, automated implantation processes are an essential prerequisite to ensure that the necessary joint replacement surgery due to the aging society can be performed with a constant or decreasing number of surgeons. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0033]    Exemplary embodiments will now be described with reference to the drawings. 
           [0034]      FIG. 1  is a schematic view illustrating a handheld tool with a rotating material-separating effector according to an exemplary embodiment, 
           [0035]      FIG. 2  is a schematic view illustrating a dimensionally stable and positionally accurate cavity to be milled in an object with a sensitive structure hidden underneath according to an exemplary embodiment, 
           [0036]      FIG. 3  is a block diagram of a material-separating effector with tool edges that can be retracted and deployed at a high rotation speed according to an exemplary embodiment, 
           [0037]      FIG. 4  is a block diagram view of a cross section through a material-separating effector, illustrating how the cutting edges can be adjusted via articulated joints according to an exemplary embodiment, 
           [0038]      FIG. 5  is a block diagram illustrating a transfer of the actuating forces for adjustment of the cutting edges while the tool rotates with the removal rate according to an exemplary embodiment, 
           [0039]      FIG. 6  is a schematic view illustrating a signal-, position-, orientation- and distance-dependent control of the tool cutting edges in relation to the machined material or tissue according to an exemplary embodiment, and 
           [0040]      FIG. 7  is a block diagram of a cross section through a material-separating effector illustrating how the removal rate of the cutting edges can be reduced by cutting edge covers according to an exemplary embodiment. 
       
    
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS 
       [0041]      FIG. 1  shows a manually-guided power tool  1  of a type used for material separation and/or chip-removing machining such as drilling, milling, sawing, separating and so on. Pneumatic or electric drives are frequently used when high rotation speeds must be achieved for attaining the necessary removal rate according to an exemplary embodiment. The tool  1  transfers the torque for material separation typically by way of a rigid or flexible shaft  3  by which the material separating tool  1  is connected to the actual tool effector  4  by way of a chuck  2 . The rotating effector  4  which is matched to the cutting speed is placed on the material  5  or the tissue  5 . The cutting edges  6  on the effector  4  hereby cut the material  5  open and remove part of the material  5 . 
         [0042]      FIG. 2  shows a surface structure into which a precisely fitting cavity  7  is to be milled, without inadvertently damaging a sensitive structure  8  located in the material  5  with the effector  4  embodied as a milling effector according to an exemplary embodiment. The removal rate of the tool  1  and of the effector  4  may be reduced at the planned boundary of the cavity  7  and proximate to the structure  8  to be protected by adjusting the cutting edges  6 , without the need to reduce the rotation speed of the tool  1 . At most, the rotation speed is slightly reduced. In this way, the change in the rotation speed of the rotating masses of motor and tool  1  does not impart a jerk or shock on the hand holding the tool  1 . 
         [0043]      FIG. 3  is a side view and a view from below of the material-separating tool  1  with the shaft  3 , the effector head of the effector  4  and the adjustable cutting edges  6  according to an exemplary embodiment. The figure shows four cutting edges  6 , which can be pushed out and retracted through openings  9  of the effector head of the effector  4 . The openings  9  for deploying the cutting edges  6  are designed, so that the cutting edges  6 —when retracted—are unable to remove any material or only an insignificant amount of material even at high rotation speed and with direct contact between the effector  4  and the surface of the material or tissue  5  to be removed.  FIG. 3  shows the openings  9  with rounded edges. A retraction mechanism  10  ensures that the cutting edges  6  themselves do not unintentionally cause a material separating effect even at high rotation speed and upon contact, but retract behind the openings  9  for protection. The retraction mechanism  10  in  FIG. 3  is embodied by spring elements connecting the cutting edges. 
         [0044]    To push the cutting edges  6  from the protective position, a deployment mechanism  11  is required. This deployment mechanism  11  is designed in  FIG. 3  as a wedge mechanism, which when depressed exerts a radially acting force on the cutting edges  6 , pressing them outwardly into engagement with the material. All elements shown in  FIG. 3  rotate as part of the effector head of the effector  4 . 
         [0045]      FIG. 4  shows another exemplary embodiment of the tool  1  in which the cutting edges  6  are not retracted, but are instead mounted on the outer surface of the effector  4  with swivel joints  12 . They can be deployed by applying pressure from the inside to the outside. Preferably, a stop  13  suitably prevents the cutting edges  6  from breaking out. The swivel joints  12  are preferably implemented as flexure joints which can be generated by suitable weakening and/or shaping of the material. The mechanism of the cutting edges  6  is shown here in cross section; however, all elements may extend around a cylindrical core in form of a stretched spiral (helix) similar to a drill bit. 
         [0046]    The deployment mechanism  11  in  FIG. 4  is not implemented as a wedge, but instead as an articulated joint, and twisting of the deployment mechanism  11  with respect to the shaft  3  causes the cutting edges  6  to be pulled out beyond the cutting edge protection  14 . The deployment mechanism may also be twisted by a helical linear movement. 
         [0047]      FIG. 5  shows an exemplary embodiment in which actuating forces are transferred for adjusting the cutting edges  6  by way of a cutting edge control attachment  15 . The cutting edge control attachment  15  is connected for this purpose either directly with the hand piece of the tool  1  or it is clamped together with the shaft  3  and the cutting edge deployment mechanism  11  with the chuck  2  during the clamping process. A bearing or support  17  is advantageously arranged to decouple the movement of the cutting edge control attachment  15  from the movement of the tool. A bearing or support  18  is also advantageous for decoupling the cutting edge actuating movement from the tool movement and the cutting edge deployment mechanism  11 . The actual adjusting movement is achieved with an actuator  16  which is designed in  FIG. 5  as a linear motor, for example a piezoelectric motor, but which may also be designed for pneumatic, hydraulic or electrodynamic operation. 
         [0048]    The cutting edge deployment mechanism  11  could also be pulled by another bearing, which is not illustrated in  FIG. 5 . The actuator  16  would then also be able to adjust the cutting edge position in both directions without requiring a separately constructed passive retraction mechanism  10 . 
         [0049]      FIG. 6  shows machine drive  20 , which can be controlled via a power amplifier and/or a motor controller  21  by signals from a control computer  22  according to an exemplary embodiment. The speed settings of the control computer  22  are received via an external signal line  23  or a signal radio link  24 , in order to be able to control for example the rotation speed of the machine with a foot pedal (not shown). Likewise, a cutting edge adjusting control  25  of the actuator serves to adjust the cutting edge via the control signal line  19 . It is also possible to signal via the signal line  23  or the signal radio link  24  the immediate adjustment of the cutting edges  6 , for example when the effector  4  leaves the boundaries of the work space. For measuring the position and orientation of the tool  1 , it is known to affix relative to the tool  1  or the machine a measuring marker  27  for a coordinate measuring system (not shown). A second measuring marker  28  is affixed on the material or tissue  5  to be removed of the object to be processed. The resulting information about the relative movement and the boundary of the work space is either processed externally or transmitted to the control computer  22 , from where the rotation speed or the position of the cutting edge is then controlled. The measuring markers  27 ,  28 ,  30  are in  FIG. 6  designed as reflectors for an optical coordinate measuring systems. However, these could also be measuring markers for an electromagnetic coordinate measuring system. In this case, a rotation speed sensor  26  is preferably mounted on the tool  1  in order to receive the frequency of the electromagnetic interference fields at the place of origin and to forward these to the control computer  22  and the coordinate measuring system, where they are filtered out by a band stop filter. Instead of a position-based adjustment of the cutting edges  6  relative to work space boundaries, direct image signal processing of a signaling or imaging system for generating the cutting edge adjustment is also possible. For this purpose, a signaling or imaging system  29 , for example a nerve monitor or a multidimensional imaging device is used to capture the signals. It can then be directly recognized in the signal or image whether the cutting edges  6  need to be switched. 
         [0050]      FIG. 7  shows another exemplary embodiment in which—in the context of material removal—not the cutting edge  6 , but a cutting edge protection  14  with a cutting edge cover region  14   a  is adjusted by way of an articulated cutting edge cover joint  31  constructed as a swivel joint or flexure joint is adjusted with respect to the cutting edge  6 . If needed, the opening between the cutting edge protection  14  and the cutting edge  6  closed. When using other—unillustrated—flexure joints between the cutting edge protection  14  and the cutting edge  6 , the surface of the effector  4  may be designed to be completely closed in order to prevent particles from entering the effector  4 . In  FIG. 7  the cutting edges  6  are fixedly and non-adjustably attached on a hollow shaft  32  which rotates with the cutting speed about a centered stator  33  and is preferably supported at two locations in an annular manner. The orientation of the stator  33  relative to the tool  1  or to the tool measuring marker  27  is known. The cutting edge protection  14  is attached directly to the rear of the cutting edge  6  by way of its cutting edge cover joint  31 . In the deployed state, the cutting edge cover region  14   a  covers the cutting edge  6  and also presses during the deployment movement material residues disposed in front of the cutting edge  6  outward. Without inside pressure, the radius-following shaping causes the cover to be pressed into the effector  4 , thereby exposing the cutting edges  6 . At least one adjustable actuating lamella  34  is disposed in a recess in the stator  33 , which can be deployed and retracted from the rotationally symmetric shape of the stator  33  in at least one direction. Other variants for changing the radius of the stator  33  in at least one location are feasible. By deploying the actuating lamellae  34  on the stator  33 , the tool  1  loses its removal effect in this direction. When the lamellae  34  are adjusted in all directions, the entire effector  4  will lose its removal rate. When the lamellae  34  are retracted in a deliberate manner, then the effector  4  assumes relative to the tool  1  or the tool measuring marker  27  an adjustable removal rate limited by the orientation. The removal rate of the effector  4  can then be limited to specific angular segments by suitable control with the control computer  22 . The actuating lamellae  34  can not only be moved electrically, but also hydraulically or pneumatically. The latter is particularly advantageous for pneumatic drives. 
         [0051]    The method allows reducing the removal rate of at least one of the material-removing cutting edges  6  of a manually-guided tool  1  by an adjustment movement of the cutting edges  6  or of a cutting edge protection  14 . In this manner, the removal rate of the material-removing effector  4  on the object can be adjusted and reduced with a uniform tool movement. By maintaining the uniform tool movement and a constant rotation speed, only minimal forces or torques are imparted on the hand guiding the tool. The removal rates can be changed very quickly due to the short adjustment paths. 
         [0052]    A distinction is made, on the one hand, between signal-based control and, on the other hand, control of the removal rate in dependence of the tool pose (position and orientation). 
         [0053]    The following is an exemplary embodiment of the signal-based control: when drilling a hole with an electric drill in a wall of a room, it may be detected by a signaling sensor that an electrical line or a water pipe is located in the direction of the drilling channel which should not be damaged by the drill. In this situation, the removal rate of the drilling tool is immediately reduced, so that the lines are not damaged. 
         [0054]    According to another aspect of an exemplary embodiment: when drilling a cavity in the mastoid region of the cranial bone, it is detected by a signaling sensor that a neural pathway or a blood vessel extends through the bone in the direction or in close proximity of the drilling channel which should not be damaged by the drill. In this situation, the removal rate of the drilling tool is immediately reduced, so that the pathway and blood vessel, respectively, are not damaged. 
         [0055]    Both embodiments are intended to serve as safety measures which do not require spatial preplanning by the operator/doctor, but where a switching signal is transmitted by an integrated sensor or an external sensor. Under these circumstances, it may in principle be useful to manually increase the removal rate in order to realize a cost-effective tool  1 , while a shutdown is then signal-based. 
         [0056]    In other situations, it is advantageous to allow a startup operation by way of a signal. In this case, the tool  1  would already be located at exactly the correct position with the correct rotation speed when the removal rate is switched in. The known problem associated with torque transmission via static friction and the resulting change in position with insufficient manual holding force is then eliminated. 
         [0057]    An exemplary embodiment of the pose-based control of the tool similar to the performance control from (U.S. Pat. No. 7,346,417) is the following: For introducing an implant support in form of a cavity into a femur for an artificial knee joint, it is necessary to mill a free-form surface into the bone at a specific position and orientation on the bone. Both the access opening and the working angle are hereby severely limited. As soon as the effector  4  of the manually-guided freehand tool  1  is located at the boundary of the allowed work space, the removal rate of the effector  4  is reduced by adjusting the cutting edges  6  or the cutting edge cover  14  without reducing the machine tool power. Because of the largely constant mass inertia due to the constant rotation speed of the machine tool and of the effector  4  of the tool  1 , the removal rate can be abruptly changed at the boundary of the allowed work space, without imparting significant force or torque impulses (jerk) on the guiding or positioning hand. As an alternative to an abrupt change in the removal rate, it may be advantageous to adjust the removal rate relative to the distance to the work space boundary in several steps or continuously, in order to achieve a high surface finish at the boundary surfaces. 
         [0058]    The following is another exemplary embodiment of the tool-position-based control in conjunction with a direction-controlled removal rate: when drilling a hole with an electric drill in a wall of a room, it may be detected by a continuous position measurement (position/orientation) that the drill is located close to the planned drilling position. In this situation, the removal rate is reduced depending on the direction, so that material is removed only in the direction of the planned drilling position. The drill then slides automatically to the planned drilling position. 
         [0059]    The advantages of power control manually-guided freehand machine tools based on position, orientation and distance data has been extensively disclosed in U.S. Pat. No. 7,346,417, so that the applications will not be cited here again. Likewise, coupling of the power control with direct measurement data from nerve monitoring, i.e. the reduction of the removal rate based on effects measured directly on the object with sensors (here a patient&#39;s body), which are based on processing of the object, is known. 
         [0060]    The adjustment of the removal rate, limited to a reduced number of the cutting edges  6 , is advantageous employed when material  5  needs to be removed in narrow working channels in only one direction, for example to produce grooves. The tool  1  then separates the material only in the preset direction. 
         [0061]    It is not always obvious to the operator or the surgeon where and in which direction material  5  must still be removed. It is here advantageous when the effective removal rate as a function of the direction can be visually perceived. This can be achieved, for example, by a clearly recognizable coloring of the cutting surfaces the cutting edges  6 . The user then sees the coloring always in those directions where a removal rate can be achieved, even when the effector  4  of the tool  1  rotates very fast. It may be useful to install a similar mechanism on the shank of the tool  1  in order to render visible blunt or rounded surfaces instead of the cutting edges  6 , which are preferably released by the same mechanism, in order to emphasize the working direction. 
         [0062]    It is also known that the position information can not only be calculated based on the evaluation of markers (U.S. Pat. No. 5,389,101), but also directly based on the evaluation of object geometries (U.S. Pat. No. 7,079,885). 
         [0063]    It can be advantageous, not only with pneumatic drives, to produce the energy for adjusting the removal rate and for controlling the removal rate directly from the drive power of the tool  1  by electrodynamic means.