Abstract:
A magnetic lock for a positionable power tool of the type comprising a workpiece support table moveable relative to a tool base. The magnetic lock comprises a braking surface configured to contact an outer edge of the tool base, and a magnetic device mountable to the workpiece support table adjacent an outer edge of the tool base. The magnetic device is configured to provide a selectively variable force to push the braking surface against the outer edge of the tool base. When the braking surface is pushed against he outer edge of the tool base, the workpiece support table is substantially prevented from moving relative to the tool base.

Description:
TECHNICAL FIELD 
       [0001]    This present invention relates to a locking mechanism for a positionable power tool, and in particular, a magnetic lock for a miter saw. 
       BACKGROUND 
       [0002]    Positionable power tools are well known. In the context of miter saws, such tools generally have a workpiece support table, as well as a saw carriage and blade, which can be rotated through a range of angles relative to a tool base. Typically, a mechanical locking mechanism is provided to lock the position of the workpiece support table at a desired angle. For example, one type of locking mechanism consists of a locking pin connected to the workpiece support table that is engagable in one of a series of corresponding detents or slots located on the tool base. However, with this type of locking mechanism, the angles at which the workpiece support table is capable of locking are limited to the fixed locations of the detents or slots. 
         [0003]    In an alternative type of locking mechanism, once a desired angle is obtained, the position of the workpiece support table may be locked relative to the tool base by a cam or threaded device that pushes against and frictionally engages the tool base, thereby preventing further rotation. However, this type of locking mechanism is disadvantageous in that the cam or threaded device necessarily requires large loading forces applied against the workpiece support table to generate sufficient friction to lock the position of the workpiece support table. Such loading forces can cause deflection of the workpiece support table, resulting in inaccurate cuts. 
       BRIEF SUMMARY 
       [0004]    A magnetic lock for a positionable power tool of the type comprising a workpiece support table moveable relative to a tool base. The magnetic lock comprises a braking surface configured to contact an outer edge of the tool base, and a magnetic device mountable to the workpiece support table adjacent an outer edge of the tool base. The magnetic device is configured to provide a selectively variable force to push the braking surface against the outer edge of the tool base. When the braking surface is pushed against the outer edge of the tool base, the workpiece support table is substantially prevent from moving relative to the tool base. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0005]      FIG. 1  is a perspective view of a miter saw; 
           [0006]      FIG. 2  is a side view of a magnetic lock having a switchable magnet mechanism incorporated into the miter saw of  FIG. 1 ; 
           [0007]      FIG. 2   a  is a side view of the internal magnets housed in the switchable magnet mechanism of  FIG. 2 , oriented in a position where the switchable magnet mechanism is turned off; 
           [0008]      FIG. 2   b  is a side view of the internal magnets housed in the switchable magnet mechanism of  FIG. 2 , oriented in a position where the switchable magnet mechanism is turned on; 
           [0009]      FIG. 2   c  is a phantom side view of the magnetic lock of  FIG. 2 ; 
           [0010]      FIG. 2   d  is a cutaway side view of the magnetic lock of  FIG. 2  further incorporating a conventional locking mechanism; 
           [0011]      FIG. 3  is an alternative embodiment of a magnetic lock having a switchable magnet mechanism incorporated into the miter saw of  FIG. 1 ; 
           [0012]      FIG. 4  is a side view of a magnetic lock having an electromagnet incorporated into the miter saw of  FIG. 1 ; and, 
           [0013]      FIG. 5  is a side view of a magnetic lock having a permanent magnet incorporated into the miter saw of  FIG. 1 . 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0014]    The present invention relates to a magnetic lock for a positionable power tool. Although the description of the magnetic lock is explained below in the context of a miter saw, it should be appreciated that the invention is not limited to miter saws. Rather, the invention may be useful in any positionable power tool, and in particular, those where accuracy is desired. 
         [0015]      FIG. 1  shows a conventional miter saw  110 , which could be a standard chopsaw, a compound miter saw, a sliding compound miter saw, a dual compound miter saw, etc. In general, the miter saw  110  includes a tool base  112 , a workpiece support table  114 , a fence  116 , a saw carriage  118 , and a blade  120 . The workpiece support table  114 , the saw carriage  118 , and the blade  120  are rotatable about a central axis  122  relative to the tool base  112  and the fence  116 . The workpiece support table  114 , the saw carriage  118 , and the blade  120  are also interconnected such that rotation of the saw carriage  118  and the blade  120  causes rotation of the workpiece support table  114 , and vice-versa. Thus, the miter saw  110  permits a user to adjust a miter cutting angle  124 , which can be defined as the angle formed between the fence  116  and the blade  120 . The miter saw  110  may also have a conventional micro adjustment lever (not shown), the rotation of which provides for making precise adjustments to the position of the workpiece support table  114  and the miter cutting angle  124 . 
         [0016]    In a first embodiment, a magnetic lock  226  having a switchable magnet mechanism  228  is provided.  FIG. 2  shows a side view of the magnetic lock  226  incorporated into the miter saw  110  of  FIG. 1 . In general, the magnetic lock  226  includes a switchable magnet mechanism  228  slidably mounted in a track  221  ( FIG. 2   c ), and a rotatable knob  232  to selectively activate the magnetic force of the switchable magnet mechanism  228 . A biasing means  225  may also be provided to bias the switchable magnet mechanism  228  away from the tool base  212 . After the user has selected a desired miter cutting angle, the magnetic lock  226  operates to frictionally resist movement of the workpiece support table  214  relative to a tool base  212 . Therefore, the magnetic lock  226  allows a user to lock the position of the workpiece support table  214  at a precise miter cutting angle (not shown) selected by the user. 
         [0017]    The switchable magnet mechanism  228  may comprise any commercially available switchable magnet mechanism, such as the Mag-Jig or the MagSqaure, sold under the name MAGSWITCH, available from Magswitch Technology, Inc., of Littletown, Colo., 80120 USA. Descriptions of suitable switchable magnet mechanisms may be found in U.S. Pat. Nos. 6,707,360 and 7,012,495, the entireties of which are hereby incorporated by reference herein. Although the switchable magnet mechanism  228  may be of any size, shape, or strength, a mechanism having cylindrical magnets 20 millimeters in diameter and a holding force of at least 20 pounds is preferable. 
         [0018]    Briefly, switchable magnet mechanisms are commercially available devices that allow a magnetic field to essentially be mechanically turned on and off, similar to an electromagnet, but without the need for an electric power supply. Switchable magnet mechanisms generally comprise a pair of magnets contained within a housing. Such magnets are typically of the rare-earth type, such as Neodymium. Typically, one of the magnets is fixed to the housing while the other magnet is rotatable with respect to the housing. A rotatable knob operatively attached to the rotatable magnet may extend from the housing of the switchable magnet mechanism to provide a means of rotating the magnet. Rotation of the knob rotates the rotatable magnet. 
         [0019]    A side view of the internal magnets housed within the switchable magnet mechanism is shown in  FIG. 2   a  and  FIG. 2   b . As shown in  FIG. 2   a , when the rotatable magnet  231  is rotated to a position where the north pole of the rotatable magnet  231  is aligned with the south pole of the fixed magnet  229 , the pair of magnets act as a magnetic shunt, thereby limiting magnetic attraction outside of the housing of the switchable magnet mechanism. In this configuration, the switchable magnet mechanism is turned off. As shown in  FIG. 2   b , when the knob is rotated an additional 180 degrees, the rotatable magnet  231  is rotated to a position where the north pole of the rotatable magnet  231  is aligned with the north pole of the fixed magnet  229 . In this configuration, the switchable magnet mechanism is turned on, the magnetic attraction outside the housing is quite strong, and the switchable magnet mechanism may be firmly affixed to a surface. 
         [0020]    The switchable magnet mechanism  228  shown in  FIG. 2  selectively provides a magnetic force. A user may turn on or turn off the magnetic force of the switchable magnet mechanism  228  by rotating a knob  232 . The knob  232  is operatively connected to the rotatable magnet contained within the housing of the switchable magnet mechanism  228 . As shown in  FIG. 2 , the knob  232  and the switchable magnet mechanism  228  may be axially offset to provide the user with additional clearance between the knob  232  and a table top (not shown). In this configuration, a series of conventional gears (not shown) may be provided to translate the rotational motion of the knob  232  to the rotatable magnet contained within the switchable magnet mechanism  228 , thereby allowing a user to rotate the knob  232  to turn on or turn off the switchable magnet mechanism  228 . In addition, the gears could be configured to increase or decrease the rotational motion translated from the knob  232  to the rotatable magnet contained within the switchable magnet mechanism  228 . Alternatively, the knob  232  may be axially aligned with and directly connected to the switchable magnet mechanism  228 . Furthermore, the knob  232  may be provided with a means of providing tactile feedback to the user, such as a series of clicks, or stops, to let the user know when the rotatable magnet within the switchable magnet mechanism  228  has been rotated 180 degrees in a given direction. 
         [0021]    When the switchable magnet mechanism  228  is turned off, the switchable magnet mechanism  228  may be disengaged from the tool base  212 . The disengaged switchable magnet mechanism  228  may remain within millimeters of the tool base  212 , yet avoid contact with the tool base  212 , to allow a user to easily position the workpiece support table  214 . Alternatively, although the switchable magnet mechanism  228  is turned off, the switchable magnet mechanism  228  may remain in sliding contact with the tool base  212 . The sliding contact may result in friction between the tool base  212  and the switchable magnet mechanism  228 , but such friction is low and can easily be overcome by the user in adjusting the position of the workpiece support table  214 . Regardless of the orientation of the switchable magnet mechanism  228  when turned off, the magnetic lock  226  is unlocked and the workpiece support table  214  is permitted to move. 
         [0022]    In contrast, when the switchable magnet mechanism  228  is turned on, a portion of the switchable magnet mechanism  228  firmly engages the tool base  212  under the force of magnetic attraction. This portion may be considered a braking surface  227 , which could be part of the housing of the switchable magnet mechanism  228 , or alternatively, a separate layer of material connected thereto. The magnetic force operates to greatly increase the frictional resistance between the switchable magnet mechanism  228 , or the braking surface  227 , and the tool base  212 . When the switchable magnet mechanism  228  is turned on, the magnetic lock  226  is locked, and the workpiece support table  214  is substantially prevented from moving relative to the tool base  212 . With the position of the workpiece support table  214  locked relative to the tool base  212 , a user may proceed with making a cut. 
         [0023]    The switchable magnet mechanism  228  may be mounted to the workpiece support table  214  by any suitable means which permits the switchable magnet mechanism  228  to firmly engage the tool base  212  when the magnetic lock  226  is locked, and disengage the tool base  212  when the magnetic lock  226  is unlocked. Importantly, the switchable magnet mechanism  228  should be mounted to the workpiece support table  214  such that, when the magnetic lock  226  is locked, the switchable magnet mechanism  228  does not transfer any significant loading forces to the workpiece support table  214 . For example, the switchable magnet mechanism  228  and/or the mounting block may be movably attached to the workpiece support table  214  by means of a hinge, a track, a slot, etc., to freely permit the switchable magnet mechanism  228  to move radially, both toward and away from the tool base  212 , while substantially preventing lateral movement relative to the workpiece support table  214 . 
         [0024]    As seen in the phantom view shown in  FIG. 2   c , the switchable magnet mechanism  228  is mounted to the workpiece support table  214  by means of a track  221 . The track  221  is configured to receive the switchable magnet mechanism  228  such that the switchable magnet mechanism  228  may slide both toward the tool base  212  and away from the tool base  212  toward a rear track wall  223 . An arrow  233  represents the direction of movement of the switchable magnet mechanism  228 . As the switchable magnet mechanism  228  moves in the track  221 , the knob  232  also moves with the switchable magnet mechanism  228 . Alternatively, a separate spline gear (not shown) may be provided to keep the position of the knob  232  fixed relative to the workpiece support table  214 . The track  221  is also sized to substantially prevent lateral movement of the switchable magnet mechanism  228  relative to the workpiece support table  214 . The track  221  and/or the switchable magnet mechanism  228  may also include other suitable means to prevent lateral movement, such as a series of ribs or protrusions (not shown). A spring  225  is also provided to bias the switchable magnet mechanism  228  away from the tool base  212 . Thus, when the switchable magnet mechanism  228  is turned off, the spring  225  moves the switchable magnet mechanism  228  out of engagement with the tool base  212 , thereby avoiding interference between the switchable magnet mechanism  228  and the tool base  212  while a user is positioning the workpiece support table  214 . 
         [0025]    Referring again to  FIG. 2 , it should be appreciated that the switchable magnet mechanism  228  may be mounted to the workpiece support table  214  in locations other than that shown in  FIG. 2 . Preferably, the switchable magnet mechanism  228  should be positioned such that it engages the tool base  212  near the outer edge of the tool base  212 . As referred to herein, the outer edge of the tool base  212  should be understood to include any surface near the perimeter of the tool base  212 . For example, rather than the switchable magnet mechanism  228  engaging the vertical surface  238  of the tool base  212 , as shown in  FIG. 2 , the switchable magnet mechanism  228  could alternatively be positioned such that it engages the horizontal surface  240  of the tool base  212 . Similarly, the switchable magnet mechanism  228  could be configured to engage both the vertical surface  238  and the horizontal surface  240  of the tool base  212 . However, in the context of a miter saw  210 , or any other rotatably positionable tool, it should be apparent that the switchable magnet mechanism  228  would be most effective in preventing rotation of the workpiece support table  214  when the switchable magnet mechanism  228  is positioned furthest from the central axis (see  FIG. 1 ). 
         [0026]    The tool base  212  may also be fitted with a steel plate  242  on which the switchable magnet mechanism  228  may be affixed when the magnetic lock  226  is locked. As shown in the context of a miter saw  210 , the steel plate  242  is contoured according to the radius of the tool base  212 . However, in the context of other positionable tools, the steel plate  242  may be planer. The switchable magnet mechanism  228 , and any additional surfaces extending therefrom, is also precisely contoured according to the contour of the tool base  212  or the steel plate  242 , thereby increasing the surface area of frictional engagement with the tool base  212 , and providing a larger braking surface  227 . Alternatively, the switchable magnet mechanism  228  may be planar. 
         [0027]    Furthermore, one or more of the tool base  212 , the steel plate  242 , or the switchable magnet mechanism  228  may be coated with a highly durable, weather resistant coating, such as Titanium Nitride, to prevent wear. In addition to improving the durability of the coated components, the coating serves to increase the coefficient of friction, and therefore the frictional resistance, between the switchable magnet mechanism  228  and the tool base  212 . Additionally one or more strips or pads of a frictional material, such as rubber or Silicon, may be provided on the braking surface  227  of the switchable magnet mechanism  228  to improve the frictional resistance between the switchable magnet mechanism  228  and the tool base  212 . In order to keep the frictional material level with the surface of the switchable magnet mechanism  228 , one or more depressions may be machined in the switchable magnet mechanism  228  for retaining the frictional material. 
         [0028]    The tool base  212  includes a series of detents  243  to provide the user with an index of cutting angles, or alternatively, as shown in  FIG. 2   d , an additional means of locking the position of the workpiece support table  214  when used in connection with a separate, conventional locking mechanism.  FIG. 2   d  is a cutaway side view of a magnetic lock  226  incorporating a conventional locking mechanism, such as a pin and detent-type lock. The pin and detent-type lock generally includes an engagement pin  245  for engaging one of the series of detents  243  located on the tool base  212 . The engagement pin  245  is retained in a slot  251  in the switchable magnet mechanism  228 , and is urged in a direction toward the tool base  212  by a biasing means  249 , such as a spring. In its natural state, the engagement pin  245  may protrude from the slot  251  in the switchable magnet mechanism  228 . In operation, the pin and detent-type lock may be used as an additional means of locking the workpiece support table  214  if a detent  243  is present at the location on the tool base  212  where the user wishes to lock the position of the workpiece support table  214 . If a detent  243  exists at such a location, when the switchable magnet mechanism  228  is turned on and engages the tool base  212 , the engagement pin  245  extends into the detent  243  to provide an additional means of locking the position of the workpiece support table  214 . If a detent  243  does not exist at the location the workpiece support table  214  is locked, when the switchable magnet mechanism  228  is turned on, the switchable magnet mechanism  228  and the engagement pin  245  engage the tool base  212 , and the engagement pin  245  is pushed into the slot  251 . Alternatively, a conventional locking mechanism may be connected to the workpiece support table  214  separate from the magnetic lock  226 , for example, at a position adjacent the magnetic lock  226 . 
         [0029]    To use the magnetic lock  226 , a user may begin with the switchable magnet mechanism  228  turned off. Advantageously, the user is capable of selecting arbitrary positions at which the workpiece support table  214  is lockable. After rotating the workpiece support table  214  to the desired position, the user could then use a micro adjustment lever (not shown), if provided, to make any precise adjustments. Once the workpiece support table  214  has been moved to the desired position, the user may lock the position of the workpiece support table  214  by rotating the knob  232  one-half a revolution, thereby also rotating the rotatable magnet in the switchable magnet mechanism  228 . With rotation of the rotatable magnet, the switchable magnet mechanism  228  turns on and provides a magnetic attraction directly toward the tool base  212 . If the switchable magnet mechanism  228  is fully disengaged with the tool base  212  in the unlocked state, depending on the means by which it is attached to the workpiece support table  214 , the switchable magnet mechanism  228  will either move into engagement with the tool base  212  under the force of magnetic attraction, or the user will move the switchable magnet mechanism  228  into engagement. 
         [0030]    As shown in  FIG. 2 , the switchable magnet mechanism  228  may be mounted to the workpiece support table  214  in a position such that a portion of the breaking surface  227  is engagable with a detent  243 , while the remaining portion of the breaking surface  227  is engagable with the tool base  212 . Preferably, the switchable magnet mechanism  228  is positioned so that only a small portion of the switchable magnet mechanism  228  is engagable with a detent  243 . Alternatively, the switchable magnet mechanism  228  and the detent  243  may be sized or positioned such that the portion of the breaking surface  227  engagable with the detent  243  is minimal in comparison to the portion engagable with the tool base  212 . 
         [0031]    The magnetic force provided by the switchable magnet mechanism  228  operates to increase the normal force the tool base  212  applies against an engaged switchable magnet mechanism  228 , thus increasing the frictional resistance between the tool base  212  and the breaking surface  227  of the switchable magnet mechanism  228 . The increased friction between the tool base  212  and the switchable magnet mechanism  228  operates to resist movement of the switchable magnet mechanism  228  relative to the tool base  212 . In turn, movement of the workpiece support table  214  is resisted because the switchable magnet mechanism  228  is mounted to the workpiece support table  214  by a means that substantially prevents lateral movement of the switchable magnet mechanism  228  relative to the workpiece support table  214 . Furthermore, because the magnetic force acts to pull the switchable magnet mechanism  228  directly toward the adjacent tool base  212  without transferring any significant forces to the workpiece support table  214 , deflection of the workpiece support table  214  is substantially avoided. Once the switchable magnet mechanism  228  is frictionally engaged under the force of the magnetic attraction, the user can perform the various cutting and other operations provided by the tool. A user may unlock the locking mechanism  226  by rotating the knob  232  one-half of a revolution to also rotate the rotatable magnet within the switchable magnet mechanism  228 . 
         [0032]    An alternative embodiment of a magnetic lock  326  is shown in  FIG. 3 . The magnetic lock  326  includes a switchable magnet mechanism  328 , a mounting block  330 , a lever  332 , a shaft  333 , and a grip  335 . The switchable magnet mechanism  328  is fixedly mounted to the mounting block  330 . As shown in  FIG. 3 , the mounting block  330  is hinged to the workpiece support table  314  about a pin  334 . A hollow lever  332  extends from the mounting block  330  and encloses a shaft  333  operatively mounted to the rotatable magnet contained within the housing of the switchable magnet mechanism  328 . A grip  335  is attached to the end of the shaft  333  to aid a user in rotating the shaft  331  and turning on or turning off the switchable magnet mechanism  328 . 
         [0033]    To use the magnetic lock  326 , a user would begin with the switchable magnet mechanism  328  tuned off. When the switchable magnet mechanism  328  is turned off, the switchable magnet mechanism  328  and the hinged mounting block  330  are freely rotatable about the pin  334 . By lifting upward on the lever  331  and/or the grip  335 , a user may fully disengage the switchable magnet mechanism  328  from the tool base  312  as it rotates about the pin  334 , away from the tool base  312 . A releasable means for retaining the lever  331  in an upward position may also be provided. Alternatively, when turned off, the switchable magnet mechanism  328  may remain engaged in sliding contact with the tool base  312 . At this point, a user can move the workpiece support table  314  to the desired position. To lock the position of the workpiece support table  314 , a user would first rotate the switchable magnet mechanism  328  back into engagement with the tool base  312 , if previously disengaged. The user would then turn on the switchable magnet mechanism  328  by rotating the shaft  333  using the grip  335 . As the switchable magnet mechanism  328  is turned on, the switchable magnet mechanism  328  is attracted directly toward and firmly engages the tool base  312 , thereby locking the position of the workpiece support table  314 , without transferring to the workpiece support table  314  any significant loading forces. To unlock the magnetic lock  326 , the user would use the grip  335  to rotate the shaft  333 , and therefore the rotatable magnet within the switchable magnet mechanism  328 , one-half of a revolution. 
         [0034]    In a third embodiment, a magnetic lock  426  having an electromagnet  428  is provided.  FIG. 4  shows a side view of the magnetic lock  426  incorporated into the miter saw  110  of  FIG. 1 . In general, the magnetic lock  426  includes an electromagnet  428  slidably mounted in a track, such as track  221  shown in  FIG. 2   c , and an electric switch  432  to control the delivery of an electric current to the electromagnet  428 . 
         [0035]    Briefly, electromagnets are devices that that allow a magnetic force to be generated and controlled by delivery of an electric current. Electromagnets generally comprise a segment of wire in the shape of a coil wrapped around a core of ferromagnetic or paramagnetic material. When an electric current flows through the wire, a magnetic field is generated around the wire and a magnetic force concentrates in the core. The strength of the magnetic force is dependent on the amount of current flowing through the wire and the number of turns in the coil. The magnetic force generated by the electromagnet permits the electromagnet to be affixed to a surface. The electromagnet  428  shown in  FIG. 4  may be any commercially available electromagnet capable of providing a sufficient holding force and of suitable shape and size. 
         [0036]    The primary difference between the structure, operation, and use of the magnetic lock  426  shown in  FIG. 4  and the magnetic lock  226  shown in  FIG. 2  is that the electromagnet  428  is turned on an off by operation of the electric switch  432 , rather than rotation of a magnet, such as the one enclosed in the switchable magnet mechanism  228 . Thus, when the magnetic lock  426  is unlocked, the electromagnet  428  is turned off, and the electromagnet  428  either remains in sliding contact with the tool base  412 , or it may be disengaged from the tool base  412  under the influence of a biasing means, such as a spring, as explained above. When the magnetic lock  426  is locked, the electromagnet  428  is turned on and the electromagnet  428  firmly engages the tool base  412  under the force of magnetic attraction. 
         [0037]    Like the switchable magnet mechanism  228 , the electromagnet  428  is mounted to the workpiece support table  414  by any means which permit the electromagnet  428  to firmly engage the tool base  412  when the magnetic lock  426  is locked, and disengaged the tool base  412  when the magnetic lock  426  is unlocked. The electromagnet  428  should also be mounted to the workpiece support table  414  such that, when the magnetic lock  426  is locked, the electromagnet  428  does not transfer any significant loading forces to the workpiece support table  414 . Accordingly, the electromagnet  428  may be mounted to the workpiece support table  414  by the same or similar means as those described above with respect to the switchable magnet mechanism  228 . The electromagnet  428  may also be mounted to the workpiece support table  414  in any location that permits the electromagnet  428  to engage the outer edge of the tool base  412 , including the vertical surface  438 , as shown in  FIG. 4 , or alternatively, the horizontal surface  440 , or both the vertical surface  438  and the horizontal surface  440 . 
         [0038]    To position a power tool using the magnetic lock  426 , a user may begin with the electromagnet  428  turned off. As with the switchable magnet mechanism  228 , the electromagnet  428  permits the user to select an arbitrary position at which the workpiece support table  414  may be locked. Once the user has moved the workpiece support table  414  to the desired location, the user may lock the position of the workpiece support table  414  with respect to the tool base  412  by activating the electric switch  432  and turning on the electromagnet  428 . As the electromagnet  428  is turned on, the electromagnet  428  is attracted toward the tool base  412  under the force of magnetic attraction, the breaking surface of the electromagnet  428  firmly engages the tool base  412 , and the frictional resistance between the electromagnet  428  and the tool base  412  is substantially increased. The magnetic lock  426  also avoids deflection because the electromagnet  428  is attracted directly toward and firmly engages the tool base  412  without transferring to the workpiece support table  414  any significant loading forces. 
         [0039]    In a fourth embodiment, a magnetic lock  526  having a permanent magnet  528  is provided.  FIG. 5  shows a side view of the magnetic lock  526  incorporated into the miter saw  110  of  FIG. 1 . In general, the magnetic lock  526  includes a permanent magnet  528 , a mounting block  530  hinged about a pin  534 , a lever  532 , and a grip  535 . The permanent magnet  528  may be any commercially available magnet providing a holding force sufficient to lock the position of a workpiece support table  514  with respect to a tool base  512 . Preferably, the permanent magnet  528  is a type of rare earth magnet, such as Neodymium 
         [0040]    The magnetic lock  526  differs from the embodiments shown in  FIGS. 2 through 4  because the magnetic lock  536  utilizes a permanent magnet  528  producing a magnetic field continuous in time. As such, to unlock the position of the workpiece support table  514 , the permanent magnet  528  must be fully disengaged with the tool base  512  in order to avoid the frictional resistance between the magnet  528  and the tool base  512 . The permanent magnet  528  cannot remain in sliding contact with the tool base  512 . When the magnetic lock  526  is locked, the permanent magnet  528  is firmly engaged with the tool base  512  under the force of magnetic attraction. 
         [0041]    The permanent magnet  528  is attached to the workpiece support table  514  by any suitable means which permit the permanent magnet  528  to firmly engage the tool base  512  when the magnetic lock  526  is locked, and fully disengage the tool base  512  when the magnetic lock  526  is unlocked. In order for the magnetic lock  526  to remain selectively lockable, a user must be able to overcome the magnetic attraction between the permanent magnet  528  and the tool base  512 . Accordingly, as shown in  FIG. 5 , a lever  532  is provided extending from the mounting block  530 . The mounting block  530  is hinged to the workpiece support table  514  about a pin  534 . The combination of the lever  532  and the hinged mounting block  536  aid the user with a mechanical advantage to overcome the magnetic attraction between the permanent magnet  528  and the tool base  512 . 
         [0042]    It should be appreciated that the permanent magnet  528 , like the switchable magnet mechanism  228  and the electromagnet  428 , may be mounted to the workpiece support table  514  such that the permanent magnet  528  engages the outer edge of the tool base  512  on the vertical surface  538 , as shown in  FIG. 5 , or alternatively, the horizontal surface  540  of the tool base  512 , or both the vertical surface  538  and the horizontal surface  540 . Furthermore, the permanent magnet  528  may be attached to the workpiece support table by alternative means, such as those described above with respect to the switchable magnet mechanism  228 . 
         [0043]    To position a power tool using the magnetic lock  526 , a user begins with the permanent magnet  528  engaged with the tool base  512 . The user may unlock the magnetic lock  526  by pulling upward on the grip  535  and/or the lever  532  to overcome the magnetic attraction between the tool base  512  and the permanent magnet  528 . As the user pulls the lever  532  upward, the mounting block  530  and the permanent magnet  528  rotate about the pin  534 , the permanent magnet  528  disengages the tool base  512 , and the workpiece support table  514  is permitted to move with respect to the tool base  512 . A releasable means of retaining the lever  532  in an upward position may also be provided. Once the workpiece support table  514  is moved to the desired position, a user may lower the lever  532  until the permanent magnet  528  is firmly engaged with the tool base  512  under the force of magnetic attraction. Like the switchable magnet mechanism  228  and the electromagnet  428 , the permanent magnet  528  substantially avoids deflection during the locking of the workpiece support table  514  because the permanent magnet  528  is attracted directly toward and firmly engages the tool base  512  without transferring to the workpiece support table  514  any significant loading forces. 
         [0044]    Thus, the present invention provides a positionable power tool having an improved magnetic lock. The foregoing description of the invention has been presented for purposes of illustration and description, and is not intended to be exhaustive or to limit the invention to the precise form disclosed. It will be apparent to those skilled in the art that the present invention is susceptible of many variations and modifications coming within the scope of the following claims.