Patent Publication Number: US-11040440-B2

Title: Holding force detection for magnetic drill press

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of co-pending U.S. patent application Ser. No. 15/420,485 filed on Jan. 31, 2017, now U.S. Pat. No. 10,406,672, which claims priority to U.S. Provisional Patent Application No. 62/289,417 filed on Feb. 1, 2016, the entire contents of both of which are incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to power tools, and more particularly to drill presses. 
     BACKGROUND OF THE INVENTION 
     Portable magnetic drill presses are typically used to drill holes in a workpiece to which the drill press is magnetically latched. Magnetic drill presses use magnets (i.e., permanent magnets or electromagnets) to magnetically latch the drill press to the workpiece. The surface of the workpiece may have a variety of characteristics (e.g., rust, metal shavings, dust, holes in the material, etc.) that could reduce the holding force otherwise capable of being developed by the drill press and the workpiece. 
     SUMMARY OF THE INVENTION 
     The invention provides, in one aspect, a drill press including a main housing, a drill unit supported by the main housing for relative movement therewith, a base coupled to the main housing, the base having a bore formed in a top surface thereof and a magnet to create a magnetic field for magnetically latching the base to a workpiece, and a holding force detection assembly having a plug, a sensor positioned outside the plug to detect the magnetic field within the base, the sensor emitting a variable output voltage signal in response to the detected magnetic field, and a printed circuit board received within a slot in the plug. The holding force detection assembly is received within the bore formed in the top surface of the base. 
     The invention provides, in another aspect, a drill press including a main housing, a drill unit supported by the main housing for relative movement therewith, a base coupled to the main housing, the base having a bore formed in a top surface thereof and a magnet to create a magnetic field for magnetically latching the base to a workpiece, and a holding force detection assembly having a plug, a sensor positioned outside the plug to detect the magnetic field within the base, the sensor emitting a variable output voltage signal in response to the detected magnetic field, and a printed circuit board received within a slot in the plug. The holding force detection assembly is received within the bore formed in the top surface of the base. The printed circuit board includes a first end and a second end opposite the first end, the sensor is coupled to the first end, and a wire is coupled to the second end. The first end of the printed circuit board is proximate a bottom of the bore, and the wire extends from the second end of the printed circuit board to a control unit positioned in the main housing. The plug defines a plug axis extending between a first end of the plug and a second end of the plug, and the first end of the printed circuit board extends from the first end of the plug toward the bottom of the bore. The sensor is in contact with the first end of the plug and in facing relationship with the bottom of the bore. 
     Other features and aspects of the invention will become apparent by consideration of the following detailed description and accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a magnetic drill press in accordance with an embodiment of the invention. 
         FIG. 2  is a perspective view of a holding force detection assembly for the magnetic drill press of  FIG. 1 . 
         FIG. 3  is a cross-sectional view of the holding force detection assembly of  FIG. 2  positioned within a base of the magnetic drill press of  FIG. 1 . 
         FIG. 4  is a perspective view of a magnetic drill press in accordance with another embodiment of the invention. 
         FIG. 5A  is a schematic cross-sectional view of a holding force detection assembly for the magnetic drill press of  FIG. 4 , with a strong holding force present and no magnetic leakage. 
         FIG. 5B  is a schematic cross-sectional view of the holding force detection assembly for the magnetic drill press of  FIG. 4 , with no holding force present and detectable magnetic leakage. 
     
    
    
     Before any embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the following drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. 
     DETAILED DESCRIPTION 
       FIG. 1  illustrates a magnetic drill press  10  including a main housing  14  and a drill unit  18  that is supported by and movable relative to the main housing  14 , along a drilling axis  22 , for drilling holes into a workpiece. The drill press  10  also includes a base  26  coupled to the main housing  14  (e.g., using fasteners) for supporting the main housing  14  and the drill unit  18  on the workpiece. In the illustrated embodiment of the drill press  10 , the base  26  includes one or more electromagnets therein for magnetically latching the base  26  to a ferromagnetic workpiece. In alternative embodiments, the base includes one or more permanent magnets therein for magnetically latching the base to a ferromagnetic workpiece. In other words, the base includes one or more magnets (i.e., electromagnets or permanent magnets) for magnetically latching the base to a ferromagnetic workpiece. In the illustrated embodiment, the drill press  10  includes a power cord  28  for electrical connection to an AC power source (e.g., AC electrical outlet). Alternatively, the drill press  10  may include an on-board power source (e.g., a battery) for powering the drill unit  18  and the one or more electromagnets in the base  26 . A main power switch (not shown) is wired in series between the one or more electromagnets and the power source to selectively energize the electromagnet(s). 
     With reference to  FIGS. 2 and 3 , the drill press  10  further includes a holding force detection assembly  30  for measuring the strength of the magnetic field permeating the base  26 , which can be used to interpolate the holding force between the drill press  10  and the workpiece. In particular, the holding force detection assembly  30  includes a plug  34 , a printed circuit board  38 , and at least one Hall-effect sensor  42 . Specifically, the Hall-effect sensor  42  emits a variable output voltage signal in response to the magnetic field (i.e., magnetic flux density, B, measured in Tesla) in which the Hall-effect sensor  42  is positioned. The stronger the magnetic field measured within the base  26 , the stronger the holding force present between the base  26  and the workpiece. In other words, the electromagnet, the base  26 , and the workpiece create a magnetic circuit. And, changes in the magnetic field within the base  26  are correlated to the magnitude of the holding force developed. In addition to measuring the strength of the holding force, the Hall-effect sensor  42  can also detect whether or not the base  26  is attached to a workpiece. 
     With continued reference to  FIG. 2 , the plug  34  is a steel slug in the illustrated embodiment, with a slot  46  formed along the length of the plug  34 . In alternative embodiments, the plug  34  may be any type of ferromagnetic material. The printed circuit board  38  is at least partially received within the slot  46  formed in the plug  34 . The printed circuit board  38  includes a first end  50  and a second end  54 , opposite the first end  50 . The Hall-effect sensor  42  is coupled to the printed circuit board  38  at the first end  50 , and the wires  58  are coupled to the second end  54  of the printed circuit board  38 . The printed circuit board  38  further includes additional electrical circuit components  62  (e.g., resisters, capacitors, diodes, transistors, etc.) coupled to the printed circuit board  38  and positioned within the slot  46  of the plug  34 . The plug  34  defines a plug axis  35  that extends between a first end  36  and a second, opposite end  37  of the plug  34 . With reference to  FIGS. 1 and 3 , the plug axis  35  is parallel to the drilling axis  22  of the drill unit  18 . 
     The plug  34  is ferromagnetic in order to direct the magnetic field through the plug  34 , and the Hall-effect sensor  42  is positioned directly beneath the plug  34  such that the magnetic field passes through the Hall-effect sensor  42  to ensure an accurate measurement is achieved. In particular, the Hall-effect sensor  42  is mounted immediately below a flat surface  66  at the second end  37  of the plug  34  to minimize or otherwise eliminate any air gap between the Hall-effect sensor  42  and the plug  34 . 
     With reference to  FIG. 3 , the holding force detection assembly  30  is at least partially received within a bore  70  formed within the base  26 . In the illustrated embodiment, the holding force detection assembly  30  is entirely received within the bore  70 . The bore  70  is formed in an upper-most (i.e., top) surface  74  of the base  26 . The wires  58  ( FIG. 2 ) extend from the holding force detection assembly  30  to a processing control unit provided in the drill press main housing  14 . In particular, the wires  58  extend from the printed circuit board  38 , through the top opening of the bore  70 , and toward the main housing  14  for connection to the processing control unit. In other words, the first end  50  of the printed circuit board  38  is proximate a bottom  71  of the bore  70 , and the wires  58  extend from the second end  54  of the printed circuit board  38  to the control unit. The wires  58  provide electric power to the holding force detection assembly  30  and also transport an output signal from the Hall-effect sensor  42 . With the bore  70  formed in the top surface  74  of the base  26 , the holding force detection assembly  30  can be encased within the base  26  and the main housing  14  without any externally exposed components and without any complex routing of wires. 
     In operation of the drill press  10 , the drill press  10  may be placed and supported upon a workpiece. The user can align the drilling axis  22  with a desired hole location on the workpiece. Then, the main power switch can be actuated to electrically connect the power source with the electromagnets, thereby energizing the electromagnets. Once energized, the electromagnets magnetically latch the base  26  to the ferromagnetic workpiece to stabilize the drill press  10 . Concurrently with energization of the electromagnets, the holding force detection assembly  30  measures the strength of the magnetic field within the base  26 . In particular, the magnetic field created by the electromagnets passes through the base  26 , the metal plug  34 , and the Hall-effect sensor  42  where the measured magnetic field results in an output voltage signal from the Hall-effect sensor  42 . The output voltage signal from the Hall-effect sensor  42  is then processed by the processing control unit to calculate the holding force present between the base  26  and the workpiece. If there is not sufficient holding force present between the base  26  and the workpiece, the user is notified of the low holding force and the motor in the drill unit  18  remains deactivated and is not allowed to start. If there is sufficient holding force present, the motor in the drill unit  18  is allowed to activate, permitting the user to drill the hole with the drill unit  18 . Should the holding force (which again is interpolated from the strength of the magnetic field measured by the holding force detection assembly  30 ) drop below a predetermined threshold during operation, the drill unit  18  is deactivated. In other words, the drill unit  18  is deactivated in response to detection of a holding force below a predetermined threshold. 
     In some embodiments, the strength of the holding force determined by the Hall-effect sensor  42  and the processing control unit can be indicated to the user through the use of at least one indicator, such as a visual indicator (e.g., at least one LED) or an audible indicator (e.g., a sound buzzer or alarm, etc.). For example, four LEDs can be utilized to indicate the strength of the holding force to a user, with all four LEDs being illuminated to indicate an optimum or strong holding force and none of the LEDs being illuminated to indicate a weak or nonexistent holding force. Alternatively, a single multi-colored LED can indicated to a user the strength of the holding force using different colors (e.g., green equals a strong holding force, yellow equals an average holding force, and red equals a weak holding force). In further alternatives, a single LED is illuminated only when the holding force falls below a predetermined threshold. 
     In further alternative embodiments, more than one Hall-effect sensor  42  and/or more than one holding force detection assembly  30  can be positioned within the base  26 . With more than one holding force detection assembly  30  utilized, an average can be calculated to provide a more accurate representation of the holding forced developed between the base  26  and the workpiece. In particular, with measurements from more than one Hall-effect sensor, localized variations in the workpiece surface (e.g., rust) can be accounted for by virtue of multiple measurements in multiple locations. In further alternative embodiments, the Hall-effect sensor  42  is replaced with any suitable sensor for the measurement of a changing magnetic field within the base  26 . 
     In further alternative embodiments, the holding force detection assembly  30  can work in conjunction with a lift-off detection system  78  that detects when the base  26  has lifted off and away (i.e., becomes separated) from the workpiece. Once lift-off is detected, the holding force is lost and the drill unit  18  is deactivated. 
     With reference to  FIGS. 4-5B , a magnetic drill press  110  according to another embodiment of the invention is illustrated. The magnetic drill press  110  includes a main housing  114  and a drill unit  118  that is movable relative to the main housing  114  along a drilling axis  122  for drilling holes into a workpiece  124  ( FIG. 5A ). The drill press  110  also includes a base  126  coupled to the main housing  114  (e.g., using fasteners) for supporting the main housing  114  and the drill unit  118  on the workpiece  124 . In the illustrated embodiment of the drill press  110 , the base  126  includes one or more magnets  127  therein for magnetically latching the base  126  to a ferromagnetic workpiece. In alternative embodiments, the base includes one or more electromagnets therein for magnetically latching the base to a ferromagnetic workpiece. In other words, the base includes one or more magnets (i.e., electromagnets or permanent magnets) for magnetically latching the base to a ferromagnetic workpiece. In the illustrated embodiment, the drill press  110  includes a power cord  128  for electrical connection to an AC power source (e.g., AC electrical outlet). Alternatively, the drill press  110  may include an on-board power source (e.g., a battery) for powering the drill unit  118 . 
     With continued reference to  FIGS. 4-5B , the drill press  110  further includes a holding force detection assembly  130  for measuring the strength of the magnetic field permeating from the base  126 , which can be used to interpolate the holding force between the drill press  110  and the workpiece  124 . In particular, the holding force detection assembly  130  includes a printed circuit board  138  and at least one Hall-effect sensor  142 . Specifically, the Hall-effect sensor  142  emits a variable output voltage signal in response to the magnetic field (i.e., magnetic flux density, B, measured in Tesla) in which the Hall-effect sensor  142  is positioned. In contrast to the holding force detection assembly  30  of  FIGS. 1-3 , the holding force detection assembly  130  of  FIGS. 4-5B  is positioned outside the base  126  and within the main housing  114 . In other words, the holding force detection assembly  130  is encased by the main housing  114 . More specifically, the printed circuit board  138  is positioned within the main housing  114  and the Hall-effect sensor  142  is coupled to the printed circuit board  138 . In the illustrated embodiment, the printed circuit board  138  is a main control board for the drill press  110 . By spacing the holding force detection assembly  30  away from the base  126 , the Hall-effect sensor  142  is configured to detect the magnetic field that is leaving (i.e., leaking) from the base  126 . In other words, the sensor  142  detects the magnetic field leakage. The stronger the magnetic field measured outside (e.g., above) the base  26 , the weaker the holding force is present between the base  126  and the workpiece  124 . In other words, the larger the magnetic flux leakage, the weaker the holding force is present between the base  126  and the workpiece  124 . Said another way, the magnetic field leakage detected by the sensor  142  is inversely correlated to the holding force between the base  126  and the workpiece  124 . 
     With reference to  FIG. 5A , the magnets  127 , the base  126 , and the workpiece  124  create a magnetic circuit  144 . In the configuration shown in  FIG. 5A , a strong holding force is present between the base  126  and the workpiece  124  since the majority of the magnetic field created by the magnets  127  stays within the base  126  and passes through the workpiece  124 . As a result, when there is a strong holding force there is little or no magnetic field leakage detected by the Hall-effect sensor  142 . With reference to  FIG. 5B , the workpiece  124  is no longer positioned underneath the base  126  so there is no holding force present. In this case, the magnets  127 , the base  126 , and the surroundings (e.g., air) create a magnetic circuit  148  that includes a magnetic leakage path  149  that extends beyond the base  126  and into the main housing  114 , passing through the holding force detection assembly  130  in the main housing  114 . The magnetic field along the leakage path  149  is detected by the Hall-effect sensor  142 , which creates a signal that is processed by a control unit, which in turn determines that there is no holding force present due to the large amount of magnetic flux leakage detected. In other words, changes in the magnetic field leakage outside the base  126  are inversely correlated to the magnitude of the holding force developed. 
     Various features of the invention are set forth in the following claims.