Abstract:
An apparatus for drilling a hole in a workpiece such as, for example, a printed circuit board wherein a pressure foot, associated with a drill, is moved into a position to exert a pressure on the workpiece. A drill bit associated with the drill is moved with respect to the workpiece, and an arrangement is provided for controlling the operations of the pressure foot and the drill bit in such a manner so as to move the drill bit from a drill starting position into the workpiece, then toward the drill starting position and again into the workpiece while maintaining the pressure exerted on the workpiece by the pressure foot.

Description:
This application is a continuation of application Ser. No. 106,439, filed Oct. 9, 1987, now U.S. Pat. No. 4,872,787. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an apparatus for drilling a printed circuit board using a drill. 
     2. Description of the Related Art 
     In the drilling of printed circuit boards to form through-holes, printed circuit boards 1 are generally stacked between an entry board 2 and a back up board 3 as shown in FIG. 5. A drill 4 rotated at a predetermined speed is disposed above this stack and fed until the bit thereof reaches the back up board 3. 
     With this drilling method, if three 1.6 mm thick printed circuit boards 1 are stacked on top of each other, if the entry and back-up boards 2, 3 placed above and below this stack have thicknesses of 1 mm and 3 mm, respectively, and if the drilled hole is to drill the back up board 3 by 1 mm, the depth Lh of the hole is 6.8 mm. 
     If the diameter of the hole is 1.2 mm, the ratio (Lh/D) of the hole depth Lh to the hole diameter D is 5.8. 
     When the aspect ratio of the hole depth Lh to the hole diameter D is relatively small (6 or less) as in the above example, it is possible to drill a good-quality, accurately-located hole by feeding the drill 4 from a drill resting position or drill start position A to a hole bottom position or drill end position B at a predetermined drilling feed rate then feeding it back from the hole bottom position B to the drill start position A at a predetermined rapid feed rate. 
     On the other hand, if the hole diameter D is 1 mm or less, e.g., 0.8 mm, the ratio of the hole depth Lh to the hole diameter D is 8.5. 
     If a hole of such a large ratio of Lh to D is drilled by feeding the drill 4 from the drill start position A to the hole bottom position B is one stage process, hole is often choked with debris. This increases the surface roughness of the inner wall of the drilled hole, and may also cause smears. Further, the thrust load on the drill 4 increases during drilling, rises drill temperature. This in turn increases the wear of the drill bit. 
     Accordingly, a proposal has been made to make a hole by drilling it in several stages, as shown in FIG. 6. 
     For example, the distance between the drill start position A and the hole bottom position B is divided into three stages with intermediate positions M 1  and M 2  set partway down the hole. The drill is fed from the drill start position A to the intermediate position M 1  at a drilling feed rate V F , then back to the drill start position A at a rapid feed rate V R . Subsequently, the drill is fed from the drill start position A at the rapid feed rate V R  to the intermediate position M 1 , at which the drill feed rate is switched over to the drilling feed rate V F , is fed further down at the drilling feed rate V F  to the intermediate position M 2 , and is then fed back at the rapid feed rate V R  to the drill start position A from the intermediate position M 2 . Thereafter, the drill is fed from the drill start position A at the rapid feed rate V R  to the intermediate position M 2 , at which the drill feed rate is switched over to the drilling feed rate V F , is fed further at the drilling feed rate V F  to the hole bottom position B, and is fed back at the rapid feed rate V R  to the drill start position A. 
     In the above-described method, since the drill is fed retracted between the drill start position A and the intermediate positions M 1  and M 2  to make the hole, chips filling the drill flute are shaken off and discharged while the drill is retracting to the drill start position A after coming out of the entry board. Therefore, the problems caused by chip clogging are eliminated, and the drilling of a good quality hole is enabled. In addition, drill bit wear can be reduced. 
     If the diameter of a hole to be drilled is even smaller and so the diameter of the drill to be used is even smaller, the rigidity of the drill is reduced. In such a case, the above-described method has a disadvantage in that, when the drill bit makes contact with the entry board or the printed circuit board, it may not enter the desired part of the entry board or printed circuit board; the top end of the drill bit may slip sideways from a desired location where the board is to be drilled. This results in a reduction in hole positioning accuracy, or breakage of the drill. 
     Furthermore, since the drill is fed and retracted between the drill start position and the intermediate positions to drill, the time required for drilling is extended, thereby reducing the output. More specifically, a type of printed circuit board may require several thousands or several tens of thousands of holes. Therefore, if the time required to drill one hole increases by 0.1 second, it means that it will take between several minutes to several hours longer to complete the drilling of the printed circuit board. 
     SUMMARY OF THE INVENTION 
     In view of the above described problems of the prior art, an object of the present invention is to provide an apparatus for drilling a printed circuit board which enables a hole to be drilled highly accurately and highly efficiently. 
     In accordance with the present invention, an apparatus for drilling a printed circuit board is provided which enables a setting in the depthwise direction of a hole of a drill start position in which a pressure foot does not contact the printed circuit board, a pulled out position in which the pressure of the pressure foot acts on the printed circuit board and at which the drill bit is located just above the printed circuit board, and a plurality of intermediate positions which are successively lower in the depthwise direction of the hole, ultimately reaching a hole bottom position at which the drilling is completed. Drill feed rates are set such as an entry feed rate at which the drill is fed into the printed circuit board or an entry board, a rate at which is suitable for drilling, and a rapid feed rate which is faster than the drilling feed rate. 
     In accordance with another aspect of the present invention, an apparatus for a printed circuit board is provided which enables a setting of switchover positions above the individual intermediate positions. 
     With the apparatus of the present invention, it is possible to feed the drill from the drill start position of a first intermediate position at the initial feed speed or entry drilling feed rate, move the drill from the first intermediate position to a pulled out position at the rapid feed rate, feed the drill from the pulled out position to the next intermediate position at the drilling feed rate, and repeat the above steps ending with a returning of the drill to the drill start position at the rapid feed rate after the drill has reached the hole bottom position. 
     Moreover, by virtue of the present invention, the hole drilling may be accomplished by feeding the drill from the drill start position to the pulled out position at a rapid feed rate, feeding the drill from the pulled out position to a first intermediate position and an initial feed rate or entry drilling feed rate, feeding the drill from the first intermediate position to the pulled out position and then feeding to the corresponding switchover position at a rapid feed rate, feeding the drill from the switchover position to the next intermediate position at the drilling feed rate, and repeating the feeding of the drill from the first intermediate position and from the switchover position ending with a returning of the drill to the drill start position at the rapid feed rate after the drill has reached the hole bottom position. By virtue of the features of the present invention, it is possible to enhance the quality of the hole to be drilled and the accuracy of locating the hole. It is also possible to reduce the time required to drill a hole, and thereby increase output, by feeding the drill repeatedly between the pulled-out position C and the intermediate positions, i.e., by omitting the repeated feeding of the drill between the pulled-out position C and the drill start position. According to the present invention, it is possible to further increase the output by repeatedly feeding the drill at the rapid feed rate between the pulled-out position C and switchover positions H. 
     In accordance with the apparatus of the present invention, a pressure foot moving means is provided for moving a pressure foot associated with a drill into position to exert pressure on the workpiece, with a drill bit moving means being provided for moving the drill bit associated with the drill with respect to the workpiece. Means are provided for controlling the operations of the pressure foot moving means and the drill bit moving means in such a manner so as to move the drill bit from the drill-starting position into the workpiece, then toward the drill starting position and again into the workpiece while maintaining the pressure exerted on the workpiece by the pressure foot. 
     The above and other objects, features and advantages of the present invention will become more apparent from the description set forth hereinafter, when considered in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a graph illustrating the relationship between the position of a drill and the time required for the drill to be fed according to a first aspect of the present invention; 
     FIG. 2 is a cross-sectional view of a state in which a hole is being drilled through printed circuit boards; 
     FIG. 3 is a graph illustrating the relationship between the position of the drill and the time required for the drill to be fed according to a second aspect of the present invention; 
     FIG. 4 is a cross-sectional view of a state in which a hole is being drilled through printed circuit boards; 
     FIG. 5 is a cross-sectional view showing a state in which a hole is being drilled through printed circuit boards in the prior art; 
     FIG. 6 is a graph illustrating the relationship between the position of the drill and the time required for the drill to be fed in the prior art; and 
     FIG. 7 is a cross-sectional view of an example of a drill and a pressure foot employed in a printed circuit board drilling machine employing the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Prior to giving a description of the preferred embodiments of the present invention, the structure of a drill and a pressure foot of a printed circuit board drilling machine as employed in the present invention will be described by way of example with reference to FIG. 7 (for further detail, refer to commonly assigned Japanese Pat. application Laid-Open No. 205209/1984). 
     In drilling a hole through stacked printed circuit boards 1 by a drill 4 of the drilling machine, the printed circuit boards 1 must be clamped firmly so that the position of each does not shift in relation to the others while the drill 4 is being pulled away from them after they have been drilled to a predetermined depth. Accordingly, the drilling machine is constructed such that the drill 4 thereof is mounted via a chuck 101a at the leading end of a spindle 101, and that a pressure foot 5 is disposed close to the top end of the drill 4 in such a way as to be movable along the axis of the drill 4. The pressure foot 5 comprises a piston 104 slidably engaged with a slide portion 103a of a cylinder 103 which is disposed outside the spindle in such a manner as to be coaxial therewith, and a collar 105 mounted on the other end of the piston 104. 
     A flange 103b formed at the lower end of the cylinder 103 engages with a flange 104a formed at the upper end of the piston 104, and the inner wall of the cylinder 103, the outer wall of the spindle 101, and the flange 104a of the piston 104 together form a chamber 115 into which pressurized air under predetermined pressure P is supplied via a fluid conduit 116 by an air source (which is not shown). 
     The cylinder 103 is mounted on an elevating device (not shown) in such a manner that it can move down toward and up away from a stack comprising an entry board 2, a plurality of printed circuit boards 1, and a back up board 3 disposed on a base (not shown) below the cylinder 103. 
     When the printed circuit boards 1 are drilled, the cylinder 103 is lowered together with the spindle 101, so that a protrusion 105a formed at the bottom of the collar 105 contacts the entry board 2 disposed on the printed circuit boards 1, pressing down the printed circuit boards and the entry board 2 against the back up board 3 on the base (not shown) and thereby clamping the printed circuit boards in a fixed position. Thereafter, the spindle 101 is further lowered. During this lowering the flange 103b of the cylinder 103 separates from the flange 104a of the piston 104, and the flange 104a slides along the inner wall 103a of the cylinder 103 so as to decrease the capacity of the chamber 115. A bit 4a of the drill 4 then projects out of the collar 105 so as to provide for drilling of a hole of predetermined depth in the printed circuit boards 1 in accordance with the drilling method of the present invention. During drilling, the space 117 around the drill 4 is evacuated to blow the chips. After the drilling has been completed, the spindle 101 returns to its original position (rises). As the spindle 101 rises, the capacity of the chamber 115 increases. Thereafter, the collar 105 of the pressure foot 5 is pulled up after the drill 4 has been pulled up and out of the holes drilled in the printed circuit board stack 1 and the entry board 2. 
     A first embodiment of the present invention will be hereinunder described with reference to FIGS. 1 and 2. 
     The printed circuit boards 1 to be drilled are stacked between the entry and back up boards 2 and 3, and the printed circuit boards 1 and the entry and back up boards 2 and 3 are firmly pressed together by the pressure foot 5. A drilling machine includes the drill 4 and the pressure foot 5 which is movable along the axis of the drill 4 and is urged downward. 
     The drilling is started from a drill start position A. When the drill 4 is located at this position, the pressure foot 5 is not in contact with the entry board 2. At this time, the top end of the drill 4 is spaced vertically from the bottom of the pressure foot 5 by a gap G. 
     A reference symbol C designates a pulled-out position at which the pressure foot 5 is in contact with the entry board 2 and at which the top end of the drill 4 is located just above the entry board 2. At this time, a gap formed between the top end of the drill 4 and the bottom of the pressure foot 5 has a length g. In other words, the pressure foot 5 is pressed down against the entry board 2, the printed circuit boards 1, and the back up board 3 by a force corresponding to the difference between the gap G and the gap g. 
     Reference symbols M1 and M2 indicate intermediate positions which are successively lower in the depthwise direction of a hole. The M1 corresponds to a position at which the drill 4 drills the uppermost printed circuit board 1. 
     A reference symbol B designates a hole bottom position which corresponds to a position at which the drill 4 drills the back up board 3 by about 1 mm. 
     A reference symbol V E  denotes an entry feed rate. This is set to a value which ensures that the drill 4 can be caused to drill the entry board 2 and the printed circuit boards 1. 
     A reference symbol V F  indicates a drilling feed rate which is set to a value which ensures that the drilled hole is of optimum quality. 
     A reference symbol V R  indicates a rapid feed rate which is set to a value sufficient to ensure that the quality of the drilled hole is not lowered. 
     In drilling the printed circuit boards 1, the drill 4 is first fed at the entry feed rate V E  from the drill start point A to the first intermediate position M 1  past the intermediate resting position or pulled-out position C so as to form a hole which drills the printed circuit board 1 located on top of the stack through the entry board 2. The drill 4 is then retracted from the intermediate positon M 1  to the pulled-out position C at the rapid feed rate V R . During this time, the chips attached to the drill 4 are shaken off and the drill 4 which has heated up during drilling cools down. Subsequently, the drill 4 is fed from the pulled-out position C to the next intermediate position M 2  at the drilling feed rate V F  so as to effect drilling of the printed circuit boards 1. At this time, the previously drilled hole serves as a guide for the drill 4, relocating the drill 4 accurately at the position to be drilled. The drill 4 is then fed from the intermediate position M 2  to the pulled-out position C at the rapid feed rate V R , during which period the chips are sent up out of the hole and the drill 4 cools down. Thereafter, the drill 4 is fed from the pulled-out position C to the hole bottom position B at the drilling feed rate V F  so as to make a hole which reaches the back up board 3. Finally, the drill 4 is retracted from the drill end position or hole bottom position B to the drill start point A at the rapid feed rate V R  to complete the drilling of the hole. 
     The printed circuit boards 1 and the drill 4 are then fed relative to each other parallel to the upper surface of the printed circuit boards 1 for subsequent drilling. 
     A second embodiment of the present invention will now be described with reference to FIGS. 3 and 4, in which the same reference numerals and symbols are used to designate those parts which are the same as those of the first embodiment. 
     Reference symbols H 0 , H 1 , H 2  denote switchover positions. The switchover position H 0  is a position at which the pressure foot 5 makes contact with the entry board 2 or a position which is slightly above that position. The switchover positions H 1  and H 2  are located above the intermediate positions M 1 , M 2 , respectively, with a predetermined distance therebetween. 
     In drilling the printed circuit boards 1, the drill 4 is first moved from the drill start position A to the switchover position H 0  at the rapid feed rate V R .The drill 4 is then moved from the switchover position H 0  to the intermediate position M 1  past the pulled-out position C at the initial or entry feed rate V E  so as to make a hole which drills the printed circuit board 1 located on the top of the pile through the entry board 2. Subsequently, the drill 4 is moved from the intermediate position M 1  past the switchover position H 1  to the pulled-out position C at the rapid feed rate V R . Thereafter, the drill 4 is moved to the switched over position H 1  at the rapid feed rate V R  and then further to the next intermediate position M 2  at the drilling feed rate V F  so as to make a hole which passes through the printed circuit boards 1. The drill 4 is then moved from the intermediate position M 2  past the switchover position H 2  to the pulled-out position C at the rapid feed rate V R . Thereafter, the drill 4 is fed from the switchover position H 2  to the hole bottom position B at the drilling feed rate V F  so as to make a hole which reaches the pick up board 3. Finally, the drill 4 is fed from the hole bottom position B to the drill start position A at the rapid feed rate V R . 
     Thus, the time T required for drilling can be reduced to less than that of the first embodiment by increasing the stages in which the drill 4 is fed at the rapid feed rate V R . If the drill 4 is fed at a rate which is faster than the rapid feed rate V R  between the drill start position A and the switchover position H 0 , the time T can be further reduced. 
     In the above-described embodiments, two intermediate positions M 1  and M 2  were set. However, more than two intermediate positions may be provided. 
     As the depth of the hole increases, the discharge of chips becomes more difficult. Therefore, if the number of printed circuit boards 1 to be stacked is increased so that the hole to be made becomes deeper, it is preferable that the spacing between the intermediate positions M m  and M n  is made narrower. 
     In the above-described first and second embodiments, the printed circuit boards 1 are pressed down by the pressure foot 5 from immediately after the drilling is started to immediately before the drill 4 is returned to the drill start position A. In consequence, the printed circuit boards 1 do not float or shift during the drilling, and this enables highly-accurate drilling of a plurality of stacked printed circuit boards 1.