Patent Publication Number: US-10328559-B2

Title: Drill

Description:
FIELD 
     The present invention relates to a drill and in particular, to a hammer drill. 
     BACKGROUND 
     A hammer drill typically includes a tool holder in which a cutting tool, such as a drill bit, can be supported and driven by the hammer drill. The hammer drill can often drive the cutting tool in three different ways, each being referred to as a mode of operation. The cutting tool can be driven in a hammer only mode, a rotary only mode and a combined hammer and rotary mode. 
     A hammer drill will typically comprise an electric motor and a transmission mechanism by which the rotary output of the electric motor can either (a) rotationally drive the cutting tool to perform the rotary only mode or repetitively strike the end of a cutting tool to impart axial impacts onto the cutting tool to perform the hammer only mode or (b) rotationally drive and repetitively strike the cutting tool to perform the combined hammer and rotary mode. European Patent Application No. EP1674207 describes an example of such a hammer drill. 
     US Publication No. 2005/0173139 describes an impact driver with a tool holder in which a tool, such as a screw driver bit, can be supported and rotationally driven by the impact driver. The impact driver has a tangential impact mechanism which is activated when a large torque is experienced by the tool. The tangential impact mechanism imparts tangential (circumferential or rotational) impacts onto the tool until the torque applied to the tool drops below a predetermined value. 
     It is known to provide hammer drills with an additional tangential impact mechanism so that the hammer drill can impart rotational impacts onto a cutting tool in addition to axial impacts. U.S. Pat. No. 7,861,797, PCT Publication No. WO2012/144500 and German Patent Document No. DE1602006 all disclose such hammer drills. In each of these hammer drills the additional tangential impact mechanism is rotationally driven at a same rate as the rate of rotation of the hammer drills output spindle. 
     The object of the present invention is to provide a drill with an additional tangential impact mechanism which has an improved operational performance. 
     SUMMARY 
     A drill includes a tangential impact mechanism which is activated when a restive torque above a predetermined value is applied to the spindle of the drill. Such arrangement provides the ability to rotatingly drive the output spindle at a first speed during the normal course of drilling while allowing the tangential impact mechanism to be driven at a second different rotational speed when the tangential impact is caused to be activated. This allows both the drilling performance of the drill and impacting performance of tangential impact mechanism to be optimised as they can both run at desired speeds which are different to each other. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       An embodiment of the present invention will now be described with reference to accompanying drawings of which: 
         FIG. 1  shows a side view of a hammer drill with an additional tangential impact mechanism in accordance with the present invention; 
         FIG. 2  shows a vertical cross section of the rotary drive, the hammer mechanism and the tangential impact mechanism of the hammer drill shown in  FIG. 1 ; 
         FIG. 3  shows a horizontal cross section of the rotary drive, the hammer mechanism and the tangential impact mechanism of the hammer drill in the direction of Arrows B in  FIG. 2 ; 
         FIG. 4  shows a vertical cross section of the spindle and the tangential impact mechanism of the hammer drill in the direction of Arrows C in  FIG. 2 ; 
         FIG. 5  shows a horizontal cross section of the rotary drive, the hammer mechanism and the tangential impact mechanism of the hammer drill in the direction of Arrows D in  FIG. 2 ; 
         FIG. 6  shows a vertical cross section of the planetary gear mechanism of the hammer drill in the direction of Arrows E in  FIG. 2 ; and 
         FIG. 7  shows a sketch of the spindle, sleeve with the V shaped grooves, the anvil, the U shaped recesses and the interconnecting ball bearings. 
     
    
    
     DETAILED DESCRIPTION 
     An embodiment of the present invention will now be described with reference to  FIGS. 1 to 7 . 
     Referring to  FIG. 1 , the hammer drill comprises a motor housing  2 . An electric motor  100  is preferably disposed within motor housing  2 . 
     The hammer drill further includes a transmission housing  4 , which preferably houses a hammer mechanism (which is described in more detail below) to impart axial impacts onto a cutting tool, a rotary drive (which is described in more detail below) to rotationally drive a cutting tool and a tangential (rotational) impact mechanism (which is described in more detail below) to impart tangential impacts to a cutting tool. 
     A tool holder  6  may be attached to the front of the transmission housing  4  which is capable of supporting a cutting tool to be driven by the hammer drill. 
     A handle  8  may be attached at one end to the motor housing  2  and at the other end to the transmission housing  4 . A trigger button  10  is preferably mounted within the handle  8  and is used by the operator to activate the electric motor  100 . A battery pack  12  may be attached to the base of the handle  8  for providing electrical power to the motor  100 . 
     A mode change knob  14  may be mounted on the side of the transmission housing  2 . The knob  14  can be rotated to three different positions to change the mode of operation of the hammer drill between hammer only mode, rotary only mode and combined rotary and hammer mode. 
     Referring to  FIG. 2 , the motor  100  has a drive spindle  16  with teeth  18  which mesh with two gears  20 ,  22 . The first gear  20  is capable of being drivingly connected to a first shaft  24  (which is rotationally mounted within the transmission housing  2  by bearings  40 ) via a first sleeve  26 . The first sleeve  26  can axially slide in the direction of Arrow Y along the first shaft  24  and is preferably rotationally fixed to the first shaft  24 . The first gear  20  can freely rotate on the first shaft  24 . The side of the first sleeve  26  comprises teeth (not shown) which can engage with teeth (not shown) formed on the side of the first gear  20  when the first sleeve  26  is moved into engagement with the first gear  24  to drivingly connect the first sleeve  26  with the first gear  20 . When the first sleeve  26  is drivingly engaged with the first gear  20 , the rotational movement of the first gear  20  is transferred to the first shaft  24 . 
     The second gear  22  is capable of being drivingly connected to a second shaft  28  (which is preferably rotationally mounted within the transmission housing  2  by bearings  42 ) via a second sleeve  30 . The second sleeve  30  can axially slide in the direction of Arrow Z along the second shaft  28  and is preferably rotationally fixed to the second shaft  28 . The second gear  22  can freely rotate on the second shaft  28 . The side of the second sleeve  30  comprises teeth (not shown) which can engage with teeth (not shown) formed on the side of the second gear  22  when the second sleeve  30  is moved into engagement with the second gear  22  to drivingly connect the second sleeve  30  with the second gear  22 . When the second sleeve  30  is drivingly engaged with the second gear  22 , the rotational movement of the second gear  22  is transferred to the second shaft  28 . 
     The movement of the two sleeves  26 ,  30  is controlled by a mode change mechanism, designs of which are well known in art. For example, the sleeves  26 ,  30  can be moved by a see-saw arrangement similar to that described in U.S. Pat. No. 8,430,182, which is wholly incorporated herein by reference. By moving the first sleeve  26  only into engagement with the first gear  20 , the second sleeve  30  only into engagement with the second gear  22 , or both sleeves  26 ,  30  into engagement with their respective gears  20 ,  22 , the mode of operation of the hammer drill can be changed between hammer only mode, rotary only mode and combined rotary and hammer mode respectively. The mode change mechanism is preferably controlled by rotation of the mode change knob  14 . 
     Crank plate  44  may be rigidly attached to the top of the first shaft  24 . A recess  46  may be formed within the crank plate  44  in which a part spherical ball  48  is disposed therewithin. The part spherical ball  48  can pivot over a range of angles within the recess  46 . The part spherical ball  48  is preferably prevented from exiting the recess  46  by a shoulder  50  engaging with a lip  52  formed on the crank plate  44 . 
     A drive shaft  54  may be rigidly connected to and extend from the part spherical ball  48 . The shaft  54  preferably passes through and is capable of axially sliding within a tubular passage  56  formed in the rear of a hollow piston  58  which is mounted within the rear end of a hollow output spindle  60 . Rotation of the crank plate  44  results in a reciprocating movement of the hollow piston  58  within the hollow output spindle  60 . 
     A ram  62  may be mounted within the hollow piston  58  which is preferably reciprocatingly driven by the hollow piston  58  via an air spring  64 . The ram  62  may repetitively strike a beat piece  66  mounted within a beat piece support structure  68  inside of the hollow spindle  60 , which in turn may repetitively strikes an end of a cutting tool held by the tool holder  6  inside the front end of the hollow spindle  60 . 
     A cup shaped gear  70  is preferably mounted on the rear part of the hollow output spindle  60  in a rigid manner. Teeth  72  may be formed on an inner wall of the cup shaped gear  70  facing inwardly towards the hollow spindle  60  as best seen in  FIG. 6 . Rotation of the hollow spindle  60  about its longitudinal axis  102  preferably results in rotation of the cup shaped gear  70  and vice versa. 
     A sleeve  74  may be rotationally mounted on the hollow spindle  60  via bearings  76 . The sleeve  74  is preferably axially fixed relative to the hollow spindle  60 . The rear end of the sleeve  74  preferably extends inside of the cup shaped gear  70 . An annular shaped gear  78  may be rigidly mounted on the rear end of the sleeve  74  inside of the cup shaped gear  70  which has teeth  80  which face away radially outwardly from the hollow spindle  60  towards the teeth  72  of the cup shaped gear  70 . Rotation of the sleeve  74  preferably results in rotation of the annular shaped gear  78  and vice versa. 
     A sliding bearing  82  is preferably mounted on the sleeve  74 . A ring shaped first bevel gear  84  in turn may be mounted on the sliding bearing  82 . The first bevel gear  84  is preferably capable of freely rotating around the sleeve  74  on the slide bearing  82  but is axially fixed relative to the sleeve  74 . The first bevel gear  84  preferably comprises teeth  86  which mesh with teeth  88  of a second bevel gear  90  rigidly attached to the second shaft  28 . Rotation of the second shaft  28  preferably results in rotation of the second bevel gear  90  which in turn rotates the first bevel gear  84  on the slide bearing  82  around the sleeve  74 . 
     Three pins  92  may be attached to the side of the first bevel gear  84  in angular positions of 120 degrees relative to each other. The pins  92  may extend rearwardly in parallel to the longitudinal axis  102  of the hollow spindle  60  and to each other into the inside of the cup shape gear  70 . 
     A circular gear  94  with teeth  96  may be mounted on each pin  92  in a freely rotatable manner. The teeth  96  of all three circular gears  94  preferably mesh with both the teeth  72  of the cup shaped gear  70  and the teeth  80  of the annular shaped gear  78 . The three circular gears  94 , the cup shaped gear  70 , the annular shaped gear  78  and the first bevel gear  84  form a planetary gear system with the three circular gears  94  forming the planetary gears, the cup shaped gear  70  forming a ring gear, the annular shaped gear  78  forming the sun gear and the first bevel gear  84  forming the carrier for the planetary gears  94 . 
     A clutch sleeve  104  may be rigidly attached to the rear of the sleeve  74 . A ring shaped ball bearing cage  106  is preferably mounted on the clutch sleeve  104 . Ball bearing cage  106  preferably holds a number of ball bearings  108  in preset positions within the ball bearing cage  106  but in a freely rotatable manner. The ball bearing cage  106  can axially slide on the clutch sleeve  104  but may be rotationally fixed to the clutch sleeve  104 . 
     Four bevel washers  110  may be sandwiched between the clutch sleeve  104  and ball bearing cage  106 . The bevel washers  110  preferably act as a spring, urging the ball baring cage  106  rearwardly towards a side wall  112  of the cup shaped gear  70 . 
     A groove (not shown) is preferably formed within the side wall  112  around the axis  102  of the hollow spindle  60 . This groove may act as a path for the ball bearings  108 . Indentations  114  are preferably formed along the path. The number of indentations  114  preferably corresponds to the number and relative positions of the ball bearings  108 . The ball bearings  108  are held within the path and indentations by the ball bearing cage  106  which presses them against the wall  112  due to the biasing force of the bevel washers  110 . Persons skilled in the art shall recognize that the clutch sleeve  104 , the bevel washers  110 , the ball bearing cage  106 , the ball bearings  108  and the path with the indentations  114  within the wall  112  of the cup shaped gear  70  effectively form a torque clutch. 
     An anvil  116  is preferably mounted on the sleeve  74 . The anvil  116  can axially slide along the sleeve  74  or rotate around the sleeve  74 . Formed on the inside of the anvil  116 , on opposite sides of the sleeve  74  in a symmetrical manner, are two U shaped recesses  122  (shown as dashed lines in  FIG. 7 ) having the same dimensions, the entrances  124  of which face forward. The height of the U shaped recess  122  is preferably constant across the length and width of the U shaped recess  122 . 
     Two V shaped grooves  126  may be formed on the outside of the sleeve  74 , on opposite sides of the sleeve  74  in a symmetrical manner. Preferably, the apexes  128  of the two V shaped grooves point forward. Each arm  130  of each of the V shaped grooves  126  preferably extends both around the sleeve  74  and rearwardly (left in  FIG. 2 ) along the sleeve  74  in a spiral manner, the arms  130  of each V shaped groove  126  being preferably symmetrical with the other arm  130  of the same V shaped groove  126 . 
     The anvil  116  is preferably mounted on the sleeve  74  so that each U shaped recess  122  locates above and faces towards a V shaped groove  126 . A ball bearing  132  is preferably located in each V shaped groove  126 . The diameter of these two ball bearings  132  may be equal. Preferably the diameter of the ball bearings  132  is greater than the depth of the V shaped grooves  126 . Therefore the side of the ball bearings  132  preferably project into the U shaped recesses  122 . The diameter of the ball bearings  132  is slightly less than the combined depth of the V shaped grooves and height of the U shaped recesses  122  so that the ball bearings are held within the V shaped grooves  126  by an inner wall of the U shaped recesses  122 . 
     A helical spring  118  may be sandwiched between the anvil  116  and a shoulder  120  formed on the sleeve  74  to urge the anvil  116  in a forward (right in  FIG. 2 ) direction. When the anvil  116  is urged forward, the ball bearings  132  engage with the rear walls of the U shaped recesses  122  and are then urged forward. As the ball bearing  132  are moved forward, they move along an arm  130  of a V shaped groove  126  until they reach the apex  128 . The apex  130  of the V shaped grooves prevents any further forward movement of the ball bearings  132 . The ball bearings  132  in turn prevent any further forward movement of the anvil  116 . The ball bearings  132 , V shaped grooves  126  and U shaped recesses  122  together with the spring  118  form a cam system by which the relative axial position of the anvil  116  on the sleeve  74  is controlled as the anvil  116  rotates relative to the sleeve  74 . 
     Formed on the front of the anvil  116 , on opposite sides of the anvil  116 , in a symmetrical manner are two protrusions  134  which extend in a forward direction (right in  FIG. 2 ) parallel to the longitudinal axis  102  of the spindle  60 . Formed on opposite sides of the spindle  60  in a symmetrical manner are two impact arms  136  which extend perpendicularly to the longitudinal axis  102  of the spindle  60  away from the spindle  60  in opposite directions. When the ball bearings  132  are located at the apex of the V shaped grooves  126 , resulting in the anvil  116  being in its most forward position, the two protrusions  134  extend in a forward direction past the two impact arms  136 . The length of the impact arms  136  is such that if the spindle  60  rotates relative to the sleeve  74  (with the anvil  116  which is mounted on and connected to the sleeve  74  via the cam system) and the anvil  116  is in its most forward position, the side surfaces of the impact arms  136  would engage with the side surfaces of the protrusions  134  and prevent any further rotation of the anvil  116 . 
     The spring  118 , anvil  116 , sleeve  74 , V shaped grooves  126 , the ball bearings  132 , the U shaped recesses  122 , and protrusions  134  form a tangential impact mechanism which imparts tangential strikes onto the side surfaces of the impact arms  136  of the spindle  60 . 
     The operation of the hammer drill will now be described. 
     In order to operate the hammer drill in hammer only mode, the first sleeve  26  is moved into driving engagement with the first gear  20  (downwards in FIG.  2 ) while the second sleeve  30  is moved out of driving engagement with the second gear  22  (upwards in  FIG. 2 ) by the mode change mechanism. As such, the rotation of the first gear  20  results in rotation of the first shaft  24  while the rotation of the second gear  22  is not transferred to the second shaft  28 . Therefore rotation of the drive spindle  16  results in rotation of the first shaft  24  only via the first gear  20  and the first sleeve  26 . 
     Rotation of the first shaft  24  results in rotation of the crank plate  44  which in turn results in the rotation of spherical ball  48  and the drive shaft  54  around the axis  140  of the first shaft  24 . As the drive shaft  54  can only slide within the tubular passage  56  of the hollow piston  58  which passage  56  extends perpendicularly to the axis  102  of the spindle  60 , it will always extend in a direction perpendicular to the axis  102  of the spindle  60  and therefore the whole of the drive shaft  54  moves left and right (as shown in  FIG. 2 ) in a reciprocating manner in a direction parallel to the axis  102  of the spindle  60  while pivoting about the axis  102  of the spindle  60  at the same time. 
     As the drive shaft  54  reciprocatingly moves left and right in a direction parallel to the axis of the spindle  60 , it reciprocatingly moves the hollow piston  54  within the spindle  60 . The reciprocating movement of the hollow piston  58  is transferred to the ram  62  via an air spring  64 . The reciprocating ram  62  repetitively strikes the beat piece  66  which in turn repetitively strikes a cutting tool held within the end of the spindle  60  by the tool holder  6 . 
     In order to operate the hammer drill in rotary only mode, the first sleeve  26  is moved out of driving engagement with the first gear  20  (upwards in  FIG. 2 ) while the second sleeve  30  is moved into driving engagement with the second gear  22  (downwards in  FIG. 2 ) by the mode change mechanism. As such, rotation of the second first gear  22  results in rotation of the second shaft  28  while the rotation of the first gear  20  is not transferred to the first shaft  24 . Therefore, rotation of the drive spindle  16  results in rotation of the second shaft  28  only via the second gear  22  and the second sleeve  30 . 
     Rotation of the second shaft  28  results in rotation of the second bevel gear  90  which in turn results in the rotation of the first bevel gear  84  about the axis of the spindle  60 . This in turn results in the three pins  92  moving sideways, perpendicularly to their longitudinal axes, around the axis  102  of the spindle  60 . This in turn results in the three circular gears  94  rotating around the axis  102  of the spindle  60 . 
     Under normal operating conditions, the amount of restive torque on the hollow spindle  60  is low and therefore is less than that of the threshold of the torque clutch. As such, the ball bearings  108  of the torque clutch remain held within the indentations  114  in path on the side wall  112  of the cup shaped gear  70  due to spring force of the bevel washers  110 . Therefore, the cup shape gear  70  is held rotationally locked to the clutch sleeve  104  which in turn results in the cup shaped gear  70  being rotationally locked to the annular shaped gear  78 . As such there is no relative rotation between the cup shaped gear  70  and the annular shaped gear  78 . This is referred to the torque clutch “not slipping”. 
     The circular gears  94  are drivingly engaged with both the cup shaped gear  70  and the annular shaped gear  78 . Therefore, as the pins  92  rotate around the axis  102  of the spindle  60 , the three circular gears  94  also rotate around the axis  102  causing both the cup shaped gear  70  and the annular shaped gear  78 , which are rotationally locked to each other, also to rotate around the axis  102  in unison. As the cup shaped gear  70  and the annular shaped gear  78  are rotationally locked to each other and move in unison, the three circular gears  94  do not rotate around the pins  92  upon which they are mounted. 
     As such, the spindle  60 , which is rigidly connected to the cup shape gear  70 , also rotates around the axis  102 . This in turn rotatingly drives the tool holder  6  which in turn rotatingly drives any cutting tool held the tool holder within the end of the spindle  60 . The sleeve  74 , which is rigidly connected to annular shape gear  78 , also rotates an as the cup shaped gear  70  and the annular shaped gear  78  are rotationally locked to each other. As such, the sleeve  74  will rotate at the same rate and in the same direction as the spindle  60 . As there is no relative rotation between the sleeve  74  and spindle  60 , there is no movement of the anvil  116  and therefore the tangential impact mechanism will not operate. As such, there is a smooth rotary movement applied to the spindle  60 . The driving force is transferred from the first bevel gear  84  to a cutting tool held within the front end of the spindle  60  via the path indicated by solid line  160 . The rate of rotation of the spindle  60  versus the drive spindle  16  is determined by the gear ratios between the drive spindle  16  and the second gear  22  and the gear ratio between the second bevel gear  90  and the first bevel gear  84 . 
     However, when the operating conditions cease to be normal and the amount of restive torque on the spindle  60  is excessive, for example during kick back where a cutting tool is prevented from further rotation within a work piece, the restive torque becomes greater than that of the threshold of the torque clutch. When the amount of restive torque on the spindle  60  is excessive, the rotation of the spindle  60  will be severely hindered or even completely stopped. However, the drive spindle  16  of the motor  10  will continue to rotate, rotationally driving the second gear  22 , second shaft  28 , the second bevel gear  90  and first bevel gear  84  which in turn will continue to rotationally drive the pins  92  and circular gears  94  around the axis  102  of the spindle  60 . However, as rotation spindle  60  is hindered or stopped, the rotation of the cup shaped gear  70  is similarly hindered or stopped. Therefore, the torque clutch slips due to the ball bearings  108  of the torque clutch moving out of the indentations  114  in path on the side wall  112  of the cup shaped gear  70  against the spring force of the bevel washers  110  and travelling along the path, allowing the cup shape gear  70  to rotate in relation to the clutch sleeve  104 . This in turn allows the annular shaped gear  78  to rotate in relation to the cup shaped gear  70 . Therefore the rate of rotation of the cup shaped gear and the annular shaped gear will be different. As the circular gears  94  are meshed with the cup shaped gear  70 , each of the three circular gears  94  will be caused to rotate around the pin  92  upon which they are mounted in addition to rotating around the axis  102  of the spindle  60 . As the circular gears  94  rotate around the pin, they cause the annular gear  84  to rotate as it is meshed with the circular gears  94 . As the cup shaped gear  70  is severely hinder or even completely stopped, there is a relative rotation between the cup shaped gear  70  and annular gear  84  and therefore a relative rotation between the sleeve  74  and spindle  60 . 
     Because the spindle  60  is attached to the cup shaped gear  70 , and the sleeve  74  is attached to the annular shape gear  84  and that the rotary drive from the motor is imparted to the planetary gear system via the circular gears  94 , the direction of rotation of the sleeve  74  and spindle  60  when the torque clutch is not slipping (ie the cup shaped gear  70  and the annular shaped gear  84  are rotationally locked to each other and there is no relative rotational movement between the two) remains the same as the direction of rotation of the sleeve when the torque clutch slips (ie when there is relative rotation between the cup shaped gear  70  and the annular shaped gear  84 ). 
     As the sleeve  74  starts to rotate, the anvil  116 , which is connected to the sleeve  74  via the ball bearings  132  and which is in its most forward position because the ball bearings  132  are urged to the apex  28  of the V shaped grooves  126  of the sleeve and rear walls of the U shaped recesses by the spring  118 , starts to rotate with the sleeve  74 . However, as the anvil  116  rotates, the two protrusions  134  engage with the two impact arms  136  which, as they are attached to the spindle  60 , are either stationary or rotating much more slowly than the sleeve  74 . The anvil  116  is therefore prevented from rotating further with the sleeve  74 . Therefore, as the sleeve  74  continues to rotate, the ball bearings  132  are forced to travel backwards along one of the arms  130  of the V shaped grooves  126  due to the ball bearings  132  and the V shaped grooves  126  acting a cam and cam follower to accommodate the relative rotational movement between the anvil  116  and the sleeve  74 . As the ball bearings  132  move backwards and as they are engaged with the rear walls of the U shaped recesses  122 , they pull the anvil  116  rearwardly (left in  FIG. 2 ) against the biasing force of the spring  118 . As the anvil  116  slides rearwardly, the two protrusions  134  slide rearwardly whilst in sliding engagement with the two impact arms  136 . Once the anvil has been moved rearwardly sufficiently, the two protrusions  134  disengage with the impact arms  136  and slide to the rear of the two impact arms  136 . In this position, the impact arms  136  no longer hinder the rotational movement of the anvil  116 . As such the anvil  116  is free to rotate. Therefore, the rotational movement of the sleeve  74  is imposed onto the anvil  116 . Furthermore, as the anvil  116  is free to rotate, the spring  118  drives the anvil  116  forward, causing it to rotate on the sleeve  74  at a much faster rate than the sleeve  74  due to the ball bearings  132  travelling along the arms  130  of the V shape grooves  126  which act as cam and cam followers. As the anvil  116  moves forward and rotates, the two protrusion  134  move between and head towards the two impact arms  136 . As it continues to move forward and rotate, the protrusions  134  tangentially strike impact surfaces on the sides of the two impact arms  136 . As the protrusions  134  strike the two impact arms  136 , they impart a tangential impact to the spindle  60 . Once in engagement with the impact arms  136 , the anvil  116  is prevented from further rotation relative to the spindle  60 . 
     However, the sleeve  74  continues to rotate forcing the ball bearings  132  rearwadly along the arms  130  of the V shaped slots  126  and causing the whole process to be repeated. In this manner, the tangential impact mechanism tangentially strikes the spindle  60 , which in turn transfers the tangential impacts to a cutting tool held with the front end of the spindle  60 . 
     The size and speed of the tangential impact is determined by the mass of the anvil  116 , the strength of the spring  118  and the shape of V shaped grooves  126 . 
     The tangentially impact driving force is transferred from the first bevel gear  84  to a cutting tool held within the front end of the spindle  60  via the path indicated by solid line  162 . The rate of rotation of the sleeve  74  versus the drive spindle  16  is determined by the gear ratios between the drive spindle  16  and the second gear  22 , the gear ratio between the second bevel gear  90  and the first bevel gear  84  and the gear ratio of the planetary gear system. This is a different ratio to that of the spindle  60  and the drive spindle  16 . This provides the benefit of having the spindle  60  rotate at one optimised rate when the hammer is operating with only a smooth rotation of the hollow spindle  60 , and the sleeve  74  rotate at a second optimised rate when tangential impact mechanism is operating. The sizes of the cup shaped gear  70 , circular gears  94  and annular shaped gear  78  can be determined so that the gear ratios between the drive spindle  16  and the second gear  22  and between the second bevel gear  90  and the first bevel gear  84  can be optimised for driving the spindle  60  while the ratio of the planetary gear system optimises the rate of rotation for the sleeve  74  of the tangential impact mechanism. 
     In order to operate the hammer drill in rotary and hammer mode, the first sleeve  26  is moved into driving engagement with the first gear  20  (downwards in  FIG. 2 ) while the second sleeve  30  is also moved into driving engagement with the second gear  22  (downwards in  FIG. 2 ) by the mode change mechanism. As such, rotation of the second gear  22  results in rotation of the second shaft  28  whilst the rotation of the first gear  20  results in rotation of the first shaft  24 . Therefore rotation of the drive spindle  16  results in rotation of both the first and second shafts  28 . The hammer mechanism and rotary mechanism then each operate as described above. 
     The tangential impact mechanism is described above with the use of V shape grooves  126 . The use of V shaped grooves  126  allows the tangential impact mechanism to operate when the spindle is rotated in either direction as is well known in the art. If it is desired that the tangential impact mechanism should only operate in one direction of rotation, then only a single spiral groove angled in the appropriate direction is required. 
     The description of the invention is merely exemplary in nature and, thus, variations that do not depart from the gist of the invention are intended to be within the scope of the invention. Such variations are not to be regarded as a departure from the scope of the invention.