DRILLING MACHINE COMPRISING A PRESSURE FOOT WITH A CONTACT PAD

Abstract: In a drilling machine, a pressure foot is provided with a central bore (62) fractionally larger in diameter than the diameter of a drill bit which passes through the pressure foot to reach a workpiece during a drilling operation. The pressure foot includes a contact pad (54) coaxial with the bore and includes a rim (58) surrounding an exit of the bore to form a contacting surface which abuts the surface of a workpiece being drilled. Thereby, the contacting area is minimized and the outermost dimension of the contacting area is minimized so as to permit the contact pad to establish a reference plane for gauging the depth of a drilled hole substantially without influence of irregularities in the surface of the workpiece, particularly irregularities at a distance from the drilling site. An elongated chamber (64) is formed in a sidewall of the bore to serve as a duct for guiding vacuum drawing air past the contact area to remove debris for further precision in the drilling operation.

Description: This invention relates to a drilling machine employing a pressure foot for precision drilling of holes to designated depths and, more particularly, to a drilling machine with a pressure foot having a contact pad. Drilling machines are employed in numerous industrial situations for fabricating a large variety of products. One situation of particular interest is the fabrication of electronic printed circuit boards and similar products which require the drilling of very fine holes to a high degree of precision. Circuit boards are frequently constructed as a laminate of copper and dielectric layers, with the dielectric being formed typically of a mixture of fibrous glass and epoxy. Certain fabrication processes require the drilling of a hole part way into the laminated circuit board, the hole passing through an outer copper layer and part way into the dielectric layer. As an example of such a precision hole, sometimes referred to as a "spotface hole", a hole having a diameter of 0.125 inch (3,175 mm) and a depth of 0.004 ± 0.005 inch (0,102 ± 0,013 mm) serves as a window for x-ray examination of internal plane alignment of a laminated panel such as that of the foregoing circuit board. A problem arises in that commercially available drilling equipment is not able to drill the foregoing spotface hole with sufficient precision to repeatably attain the foregoing depth requirement. In the usual construction of such equipment, the drilling machine includes a spindle which rotates a drill bit formed as a boring bar, and further includes a pressure foot which encircles the drill bit. The pressure foot has a registration contact pad which contacts the surface to be drilled. The pressure foot serves as a depth gauge for stopping a forward advance of the drill bit to attain the desired depth of hole. The pressure foot also is provided with apertures through which air is drawn under vacuum to clear debris from the area of the workpiece being drilled. In the foregoing equipment, an interface between a contacting surface of the contact pad and the surface of the workpiece serves as the reference plane from which the hole depth is measured. Any undulation or unevenness in the surface of the workpiece, or the presence on the workpiece surface of debris such as is produced by a drilling operation, detracts from the positional accuracy of the reference plane with a consequent loss of control in maintaining the desired hole depth. The foregoing equipment appears to be unable to accommodate adequately to the undulations and unevenness in the surface, and the apertures, or vacuum ports, in the pressure foot appear to be unable to provide sufficient direction of the vacuum air draft for reliable removal of all interfering debris from the interface between the contact pad and the workpiece surface. The foregoing problem is overcome and other advantages are provided by a drilling machine incorporating the invention, and having a drill bit and a pressure foot encircling the drill bit, the pressure foot having a contact pad with central bore which serves as a passage for receiving the drill bit. It is an object of the invention to reduce the susceptibility of the reference plane position to undulations and unevenness in the surface of the workpiece by reducing the area of interface between the contact pad and the workpiece, and by minimizing spacing between the drill bit and the encircling contact pad. It is a further object of the invention to improve direction of vacuum air flow for better removal of debris from the area of interface between the contact pad and the workpiece. These objects are met in accordance with the invention by making the diameter of the bore within the pressure foot fractionally larger than the diameter of the drill bit so as to provide only enough space between the drill bit and the bore surface for clearance of the drill bit during passage through the bore. Also, the contact pad is made with a circular shape and has an outer diameter which is less than approximately twice the diameter of the bore. This feature insures that any distant undulations in the workpiece surface will not affect the location of the reference plane, and minimizes the interfacing area and undulations therein between the contact pad and the workpiece surface. A vacuum post can be located alongside the bore at the contact surface and in communication with the bore, for delivery of vacuum directly to the area of interface for more reliable removal of drilling debris. This feature permits the contact pad to set directly on the surface of the workpiece essentially without the interposition of debris between the pad and the workpiece. The foregoing aspects and other features of the invention are explained in the following description taken in connection with the accompanying drawing wherein: Figure 1 is a simplified and stylized view of a drilling machine including a pressure foot of the prior art positioned between a drill chuck and a workpiece, the machine including a vacuum operated debris removal system; Figure 2 is a plan view, looking up from a workpiece, of a pressure foot constructed in accordance with the invention with a drill passage having a side extension service as a duct for direction of vacuum; and Figure 3 is a side elevation view of the pressure foot of Figure 2 showing a registration contact pad of reduced cross section. Figure 1 shows a stylized view of a drilling machine 20 which is typical of machines currently employed in drilling holes in electronic circuit boards exemplified by the workpiece 22 set on a table 24. The drilling machine 20 includes a cabinet 26 having a movable arm 28 extending therefrom horizontally and holding a motor 30 at the outer end of the arm 28. A spindle 32 and chuck 34 extend from the motor 30 and are driven in rotation by the motor 30. A drill bit 36 is held within the chuck 34 and passes through a pressure foot 38 to contact the workpiece 22. The pressure foot 38 includes a chamber 40 connected by a flexible hose 42 to a vacuum pump 44, and opens into a vacuum port 46 in a contact pad 48 on the underside of the pressure foot 38. The contact pad 48 is integrally formed with the pressure foot 38 and is provided with an outer rim surface which contacts the upper surface of the workpiece 22. The vacuum port 46 is located within the rim of the pad 48, and the drill bit 36 passes along a central axis of the port 46 and the pad 48. In operation, the arm 28 is moved vertically by a mechanism (not shown) within the cabinet 26 for advancing the drill bit 36 into the workpiece 22 during a drilling of a hole therein, and for retracting the drill bit 36 from the workpiece 22 upon completion of the drilling. During the drilling, the motor 30 rotates the drill bit 36 to provide a cutting action whereby the hole is bored. Debris resulting from the drilling operation is drawn away from the site of the drilling by the vacuum within the chamber 40. The pressure foot 38 may be used as a stop for engaging with a housing of the motor 30 to limit the depth of penetration of the drill bit 36 into the workpiece 22. When the pressure foot 38 is used to limit the vertical travel of the drill bit 36, the pressure foot 38 is, in essence, being used as a depth gauge for determining the depth of a hole bored by the drill bit. A reference plane for such depth gauge is established at the interface of the rim of the contact pad 48 with the surface of the workpiece 22. Any irregularities in the surface of the workpiece 22 will displace the reference plane resulting in a lack of precision and repeatability and the drilling of a succession of holes in the workpiece 22. The vacuum in the chamber 40 also aids in clearing debris from the interface between the pad 48 and the workpiece 22, so as to reduce errors in depth associated with such debris. The mechanism whereby the pressure foot 38 comes into abutment with the housing of the motor 30 has been deleted in the figure to simplify the presentation, such mechanisms being well known. Figures 2 and 3 show a pressure foot 50 incorporating the invention. The pressure foot 50 comprises a base 52 with a contact pad 54 extending away from the base 52. The pad 54 comprises a housing 56 having a frusto-conical shape extending from the base 52 to a rim 58 of the pressure foot 50. Within the housing 56 is located a passage 60 formed as a central bore 62 along an axis of the housing 56 and being provided with a chamber 64 of elongated cross-section extending radially outward from the bore 62 into the side wall of the bore 62. The chamber 64 has an elongated shape extending parallel to the axis of the housing 56, this shape of the chamber 64 enabling the chamber 64 to serve as a duct for the application of vacuum to the area of a workpiece such as the workpiece 22 of Figure 1, in which a drilling operation is proceeding. The chamber 64 terminates at the rim 58 and also extends sideways into the frusto-conical surface at 66 to provide a vacuum port 68. Thus, a portion of the port 68 opens directly on the rim 58, while a further portion of the port 68 opens on the surface 66. The portion of the bore 62 at and near the base 52 is enlarged in diameter to provide a region 70 into which the chuck 34 (Figure 1) can descend during a drilling operation. Also, the base 52 is provided with countersunk holes 72 which receive screws (not shown) for mounting the pressure foot 50 to a drilling machine such as the machine 20 of Figure 1. In particular, it is noted that such machines have a well-known frame, indicated in phantom at 74 in Figure 1, whereby a pressure foot, such as the foot 50 is secured in registration with a spindle, such as the spindle 32 of Figure 1. The frame 74 may include a spring 76 (Figure 1) which exerts a predetermined pressure between the pressure foot 38 or 50 against the workpiece 22 as the spindle 32 advances toward the workpiece 22. In the construction of the pressure foot 50 the size of the bore 62 is selected in accordance with the size of the drill bit 36. Thus, the diameter of the bore 62 is only fractionally larger, for example, 10% larger than the diameter of the drill bit 36 so as to provide adequate clearance for the drill bit 36 to enable the drill bit 36 to travel along the passage 60 during a drilling operation. By minimizing the diameter of the bore 62, the size of the rim 58 is also minimized to provide a two-fold benefit to the precision of drilling a hole in the workpiece 22. The outer diameter of the rim 58 is selected so as to provide a contact area to the pad 54 which can support the force of the pressure foot 50 against the workpiece 22 without deforming or otherwise injuring the surface of the workpiece 22. The minimization of the inner and outer radii of the rim 58 insure that any perterbations in the surface of the workpiece 22 at a considerable distance from the drilling site will have no effect on the establishment of the reference plane from which hole depth is measured. In addition, the minimization of the contact area of the rim 58 minimizes the possibility of interference in the establishment of the reference plane because of nearby perterbations in the surface of the workpiece 22. With respect to the joining of the pressure foot 50 to a drilling machine such as the machine 20 of Figure 1, the pressure foot 50 may be secured by its base 52 to a housing (not shown) similarly shaped to the base portion of the pressure foot 38 to permit the connection of the vacuum hose 42 to the pressure foot 50. Thereby, suction may be applied to the chamber 64 for initiating a flow of air into the port 68 for drawing debris into the chamber 64 and further into the hose 42 for removal from the drilling site. A feature of the invention is found in the enplacement of the port 68 directly at the rim 58. In view of the relatively small size of the rim 58, there is reduced space in which debris can collect at the interface between the contact pad 54 and the workpiece 22. Thus, the vacuum in the chamber 64 is better able to clear the region of interface between the pad 54 and the workpiece 22. The extension of the port 68 onto the surface 66 at the side of the rim 58 further allows the withdrawal of debris from the immediately surrounding surface of the workpiece 22. As an example in the dimensions of an embodiment of the pressure foot 50, the diameter of the bore is 0.1875 inch (4,763 mm), this also being the cross-sectional width of the port 68. The over-all cross-sectional length of the port 68, as viewed in the plan view of Figure 2, is 0.4375 inch (11,113 mm). The height of the port 68, as viewed in the elevation view of Figure 3, is 0.145 inch (3,683 mm). The spacing between the center of a hole 72 and the center of the base 52 is 0.880 inch (22,352 mm). The diameter of the housing 56 at the base 52 is 1.000 inch (25,400 mm). The height of the housing 56 and of the contact pad 54 above the surface of the base 52, as viewed in the elevation view of Figure 3, is 0.325 inch (8,255 mm). The thickness of the base 52 is 0.125 inch (3,175 mm). The diameter of the region 70 is 0.3125 inch (7,938 mm). The diameter of the base 52 is 2.125 inch (53,98 mm).

Claims: 1. Drilling machine having a drill bit and a pressure foot encircling the drill bit, wherein said pressure foot (50) comprises a contact pad (54) extending away from a base (52), there being a bore (62) extending through said pad for receiving said drill bit (36); characterized in that - the diameter of said bore (62) is fractionally larger than the diameter of said drill bit (36) to provide clearance of said drill bit during passage thereof through said bore; - said contact pad (54) has an outer diameter less than approximately twice the diameter of said bore for contacting only the area of a workpiece adjacent to said drill bit during a drilling operation for improved precision in drilling; and - said pressure foot further comprises a vacuum port (68) formed at a side of said bore and opening at said contact pad for removal of debris associated with a drilling operation. 2. Drilling machine according to claim 1, characterized in that said contact pad includes a frusto-conically shaped housing (56) for positioning a contacting surface (58) of said pad away from said base. 3. Drilling machine according to claim 2, characterized in that said bore passes through said housing (56) and said base (52). 4. Drilling machine according to claim 2 or 3, characterized in that said bore (62) is coaxial to a center axis of said housing (66). 5. Drilling machine according to claim 2, characterized in that said vacuum port (68) opens at said contacting surface (58) and along the frusto-conical surface (66) of said housing (56) adjacent to said contacting surface. 6. Drilling machine according to claim 1 or 5, characterized in that said vacuum port (68) communicates with said bore (62). 7. Drilling machine according to claim 1, characterized in that said vacuum port (68) is formed as an extension of a side wall of said bore (62). 8. Drilling machine according to claim 5, characterized in that said vacuum port (68) is formed as an extension of a side wall of said bore (62). 9. Drilling machine according to claim 7 or 8, characterized in that the cross-section of said vacuum port (68) is elongated in a direction away from said bore (62). 10. Drilling machine according to claim 7 or 8, characterized in that the cross-section of said vacuum port (68) is symmetrically located about a radius of said bore (62).

Inventors: SCALES, EUGENE MONAGON, WAGNER, FREDERICK
Applicants: INTERNATIONAL BUSINESS MACHINES CORPORATION
Application year: 1986
Main classification: B23B47
IPC codes: B23Q3, B23Q16, H05K3, B23B47, B23Q11