Abstract:
An apparatus for working on both sides of a flat workpiece includes a first support surface having a first axial line thereacross, a second support surface juxtaposed to and associated with the first support surface and having a second axial line thereacross which lies in a common plane with the first axial line, and a first carrying assembly operably connected to the first support surface in a manner to movably reciprocate along the first axial line and having a first extending arm which is laterally displaced from one side of the common plane and a second extending arm which is laterally displaced from another side of the common plane. A second carrying assembly is operably connected to the second support surface in a manner to movably reciprocate along the second axial line, wherein the second carrying assembly includes a workpiece removably mounted in a manner to position the workpiece in the common plane between the arms such that the arms extend about the workpiece. A tool is removably operably connected to either of the extending arms in a manner to permit the tool to perform work on a face of the workpiece.

Description:
BACKGROUND 
     1. Field of Invention 
     This invention provides a method and apparatus for working on both sides of a flat workpiece such as a double sided printed circuit board (called hereinafter PCB) while maintaining high relative precision between sides. 
     2. Description of Prior Art 
     Various machines for drilling, engraving, and otherwise working flat stock such as a copper clad PCB substrate have been described, built and marketed in the past. Such machines are widely used for the production of PCBs in limited prototype quantifies. They have the advantage of fast production turn-around and require no processing chemicals. 
     Such machines are typically equipped with a working table and are frequently referred to as flatbed machines. Flatbed machines generally have a machining tool mounted to a spindle motor disposed above and perpendicular to the plane of the table. The tool is caused to traverse is about the bed by the use of X-axis and Y-axis driving apparatuses while the rotating tool is engaged or disengaged to the workpiece as defined by the controlling software. Such a machine is proposed in patent 5,462,512 to Hirioshima (1995). However, this machine has no provision for working a double-sided workpiece. 
     An alternative to the flatbed machine has been a design employing a rigid U-shaped frame which straddles the flat workpiece. On the end of one arm of the U-frame would be disposed a backup supporting surface. On the other arm would be disposed a spindle motor holding a machining tool perpendicular to the plane of the workpiece and facing the backup supporting surface. The spindle motor would be moveable towards or away from the workpiece surface by use of a z-axis solenoid or motor for the purpose of engaging the working end of the tool into the workpiece according to a depth defined by computer software or by manual settings. When engaged, the tool would apply pressure to the workpiece which, in turn, would be thrust against the backup surface. The workpiece would then be caused to traverse in its plane as the Z-axis machining tool is engaged or disengaged according to the controlling software specifications. Such a machine is proposed in patent 4,786,216 to Kitagawa et al (1988) for a drilling machine. This machine has no provision for working a double-sided workpiece. 
     Other drilling machines having U-shaped arms have been proposed which have opposing spindle heads but no provision to cause the heads to traverse over the plane of workpiece by a programmable device or calibrated manual positioners. Machines of this description include U.S. Pat. No. 4,215,958 to Jagers (1980) and U.S. Pat. No. 5,152,641 to Overmyer and Peitz, JR. (1962). 
     Patent 4,967,947 to Sarh (1990) claims, “in effect”, to have a C-frame configuration but the C-frame consists of three members having the two arms being independently slidable on the base, which would negate a prime advantage of my patent. This reference also provides for opposing detachable companionate tools, however it is limited to having a fixedly supported workpiece, has no provisions for engraving, and is a much more complex design than my patent. 
     In order to work both surfaces of a flat a workpiece it has been necessary to first place the workpiece over alignment pins on the machine surface such that the obverse workpiece surface faces the working end of a rotating machine tool. After the obverse surface was completely drilled and engraved, the workpiece would be flipped and again fitted over the alignment pins such that the inverse surface would face the machining tool and the engraving process would then continue to completion. 
     However, the flipping process created a number of problems. A major problem was an inability to obtain close overall alignment of related machined items on opposing surfaces of the workpiece. An obvious manifestation of this problem would be annular rings engraved on the inverse-side being misaligned with through-holes drilled from the obverse-side surface of a PCB. 
     Following is a list of some reasons that misalignment can not be eliminated when a workpiece is flipped in order to machine both sides: 
     The x-axis rails can not be adjusted perfectly perpendicular to the y-axis rails. 
     The X or Y-axis rails could not be set perfectly parallel or perpendicular to the workpiece pinning groove machined into the bed. 
     There may be irregularities in the pitch of the lead-screws. 
     The alignment pins may not be tight or may vibrate loose. 
     Pinning holes can not be drilled through the workpiece perfectly perpendicular to its plane. 
     Pins can not be set into the bed/table top perfectly perpendicular to the surface. 
     The rails can not be manufactured or installed perfectly straight. 
     Lead-screw wobble affects tool head position. 
     There could be imperfections in the axis driving motors. 
     Thermal expansion in lead-screws affects pitch. 
     None of the above listed reasons would necessarily be objectionable when machining only one surface of a workpiece. However, after being flipped, all the machined imperfections that were created on the workpiece from one table side of the alignment pins would now be associated with the table side opposite the alignment pins. The two machine-sides have totally different and sometimes additive imperfections. Compromise in machine alignment has been necessary in order to achieve overall acceptable double sided accuracy. 
     Other problems are as follows: 
     Obtaining acceptable overall alignment during machine manufacture has required many man-hours of highly skilled technical labor. 
     The larger the workpiece capacity the more difficult it has been to achieve overall machine calibration. 
     Machines are easily knocked out of alignment during shipping, handling, and normal wear and tear. 
     The table surface constitutes a considerable cost and weight percentage of a completed machine. 
     Debris from machining processes tends to settle on the working surface of flatbed machines. 
     Depth-of-cut is affected when the pressure foot “rides up” on the debris. 
     Horizontally positioned flat-bed machines require a considerable amount of table-top space. 
     Workpiece warpage can cause the machine tool to drag on the work surface during “tool-up” moves. This can damage tools and create defects in the work-in-progress. 
     In addition to pinning the workpiece to the table, taping of the workpiece to the platen is usually required to assure the edges are held down close to the surface and no workpiece movement occurs during engraving. 
     The workpiece flipping process is time consuming. 
     On flatbed machines a sheet of backup material equivalent in size to the blank workpiece is required to prevent drilling through the platen surface and to prevent substrate breakout as the drill bit penetrates the bottom surface. 
     Even though very few holes may have been drilled into the backup material, it can not be reused because of the possibility of a drill bit in the new work stiking a hole from previous work, causing bit breakage or otherwise causing poor quality drilling. Thus, a substantial amount of once-used and substantially unpenetrated backup material is frequently discarded. 
     Previous machines, having permanently mounted tool heads, require the tool head to be positioned off the edge of the platen surface in order to replace or flip the workpiece or change milling tools and drill bits. This off-board positioning process consumes production time and requires longer rails and lead screws than would be necessary to traverse the bed surface only. 
     Software and operating procedures are complicated when board flipping is required. A software mirror-image must be created for the flip side which, in turn, must be centered precisely relative to the obverse side. Making multiple small double sided PCB&#39;s on one substrate sheet is further complicated because of the need to offset individual works-in-progress on alternate machine sides. Electrical cable routing to the spindle head of a flatbed design can be complicated by the fact that the head must traverse in both the X and Y axis. Fixed head machines are limited to the head type installed during manufacture of the machine. Additional head options such as a fluid dispensing head must be adapted to the existing head, rendering a more complex overall design. 
     On flatbed machines, the back side of the work-in-progress cannot be visually monitored, mechanically sensed, or electrically sensed because of the obstacle presented by the bed itself. For example, without first removing the workpiece, it would be difficult to determine whether or not a drill bit is completely penetrating the workpiece. 
     Boring individual holes from both sides of a workpiece (rather than completely through from one side) in order to prevent material breakout resulted in tool breakage because of flipping misalignment. 
     Presently, physical design of electronic assemblies requires closer tolerances than ever before and future designs will even be more critical as component dimensions continue to decrease in order to increase component densities and decrease electrical losses. The ability of current apparatus to machine close tolerances into double sided flat work pieces is limited, in large part, because of the need to flip the workpiece in order to access both sides. 
     SUMMARY OF THE INVENTION 
     Accordingly, several advantages of the present invention are: 
     to provide an apparatus having the ability to work both sides of a flat workpiece without having to reposition or flip the workpiece within its mounting; 
     to provide greatly improved relative work precision between opposite workpiece sides; 
     to enable improved machine alignment stability; 
     to enable a simplified machine alignment process; 
     to enable simplified software applications; 
     to enable simplified operating procedures; 
     to enable easy access of tool chuck for tool replacement; 
     to enable user mounting of heads having different functions; 
     to enable user replacement of defective heads; 
     to enable use of a small floating backup support surface rather than a precision machined flatbed table, thus: 
     eliminating associated precision alignment pinning groove in bed; 
     eliminating need to drill pinning alignment holes through workpiece; 
     significantly reducing overall weight; 
     reducing manufacturing material cost, reducing manufacturing labor time; 
     enabling warped areas of workpiece to make intimate contact with backup support, 
     enhancing control of depth-of-cut, and reducing disengaged tool drag and; 
     enabling use of small piece of backup material rather than full sheet as required for flatbed machines. Note: backup material is placed between workpiece and backup support surface or flatbed table to receive tip of drill bit as it completes workpiece penetration. 
     to enable the workpiece to be positioned vertically or at angle relative to support surface thus: 
     saving tabletop work space; 
     enabling workpiece angle to be regulated and hence controlling gravity pressure of substrate against backup surface; 
     enabling gravity to remove working debris from work surface and; 
     enabling visual, optical or electromechanical monitoring of work-in-progress from either workpiece side. 
     It is therefore an object of the present invention to provide a machine capable of precisely, speedily, and completely working both sides of a flat workpiece without having to reposition or flip the workpiece within its mounting. 
     According to the present invention, the forgoing and other objects can be achieved by providing an apparatus for working on both sides of a flat workpiece, which comprises: 
     a first support surface having a first axial line thereacross; 
     a second support surface juxtaposed to and associated with the first support surface and having a second axial line thereacross which lies in a common plane with the first axial line; 
     a first carrying assembly operably connected to the first support surface in a manner to movably reciprocate along the first axial line and having a first extending arm which is laterally displaced from one side of the common plane and a second extending arm which is laterally displaced from an opposing side of the common plane; 
     a second carrying assembly operably connected to the second support surface in a manner to movably reciprocate along the second axial line, wherein the second carrying assembly includes means for removably mounting the workpiece in a manner to position the workpiece in the common plane between the arms which extend about the workpiece; and 
     a tool removably operably connected to one of the extending arms in a manner to permit the tool to perform work on a face of the workpiece. 
     The method includes movably disposing the flat workpiece in a predetermined plane such that the flat workpiece is movably positionably maintained within the plane; and actuating a pair is of arms about the plane such that the arms are positionably maintained at a predetermined distance from the workpiece, wherein each arm has tool means for working a respective facing side of the workpiece and the tool means are actuatable to act on common coordinate locations of the respective facing sides. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1A is an overall isometric view of the apparatus of the present invention. 
     FIG. 1B is the same as FIG. 1 except with workpiece mounted thereupon. 
     FIG. 1C is the same as FIG. 1A without item numbers for reference clarity. 
     FIG. 1D is the same as FIG. 1B without item numbers for reference clarity. 
     FIG. 2A illustrates a plan top view of a prior art flat bed. 
     FIG. 2B illustrates an undesirable result of a misaligned flatbed of FIG.  2 A. 
     FIG. 3A illustrates a side view of the preferred embodiment which has been similarly misaligned. 
     FIG. 3B illustrates a side view of the embodiment shown in FIG. 3A compensating for the misaligned. 
     FIG. 4 is a variation of the invention having positionally fixed workpiece. 
     FIG. 5 is a variation of the invention having positionally fixed tools. 
     FIG. 6 is a top perspective variation having two pairs of heads. 
     FIG. 7 illustrates a computer controller variation of the invention and a vertical counterbalance. 
     FIG. 8 illustrates sensing of tool position. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Reference Numerals in Drawings 
       10  base plate 
       11  first support surface 
       12  X-axis balance rail support 
       15  common positioner member 
       20 A Y-axis upright support plate 
       20 B Y-axis upright support plate 
       20 C Second support surface 
       21  Y-axis guide rail 
       22  Y-axis carriage assembly consisting of: 
       23 A,B Y-axis slide units 
       23 C and  23 D threaded open surfaces 
       24  workpiece mount 
       25 A,B thumb screws 
       28 ,  28 N,  28 P workpiece, ZN face, ZP face 
       30  X-axis guide rail 
       32  U-shaped carriage assembly consisting of: 
       34 A,B upright arms 
       36  X-axis slide unit 
       38 A,B head mounts 
       40  tool head assembly consisting of: 
       42  tool head bracket 
       43  trapezoidal grooved surface 
       44 A,B thumb screws 
       45 A, 45 B threaded open surfaces 
       46  solenoid 
       48  solenoid plunger 
       50  z-axis guide block 
       52  z-axis shafts 
       54  spindle motor 
       55 A,B solenoid brackets 
       56  adjustable depth foot 
       58 ,  58 D,  58 K  58 R tool, drill bit, milling tool, or routing tool 
       60  spring-loaded pressure foot 
       62  spindle housing 
       70  back-up head assembly consisting of: 
       72  back-up bracket 
       73  trapezoidal grooved surface 
       74 A,B thumb screws 
       75 A,  75 B threaded open surfaces 
       76 ,  76 D,  76 M back-up tool, drilling backup tool, milling backup tool 
       80  balance rail 
       82  balance rail slide unit 
       90  A, B, C, D, E, F, G, H through-holes, (A-D are prior art) 
       91  A, B, C, D, E, F, &amp;, H Annular rings—board side one 
       92  A, B, C, D, E, F, G, H annular rings—board side two 
     Items  110  through  185  relate to a conventional flatbed machine design: 
       110  table/bed 
       120  Y-axs carriage assembly 
       121  Y-axis rail 
       130 A,B X-axis rail 
       154  tool head 
       158  tool 
       185 A,B board mounting/alignment pins 
     Items  400 - 499  are unique to FIG. 4 (fixed workpiece variation) but have similar functions to similarly numbered items  00  to  99 . 
       410  Base Plate 
       420  Y axis support—X axis slideable 
       421 A,  421 B Y axis guide rails 
       424  workpiece mount 
       430 A, B X axis guide rails 
       432  U-shaped carriage assembly consisting of: 
       434 A,  434 B Y-axis carriage arms 
       436  Y-axis slide unit 
     Items  500 - 599  are unique to FIG. 5 (fixed tool variation) but have similar functions to similarly numbered items  00  to  99 . 
       520  Y-axis support—X-axis slideable 
       521 A,  521 B Y axis guide rails 
       523  Y-axis slide unit 
       524  Workpiece mount 
       530 A, B X-axis rails 
       534 A,  534 B fixed upright arms of U-shaped assembly 
     Items  600 - 699  are unique to FIG. 6 (two head pair variation) but have similar functions to similarly numbered items  00  to  99 . 
       632  U-shaped Carriage Assembly 
       634 C,  634 D,  634 E,  634 F upright arms 
       636  X-axis slide unit 
       640 A,  640 B Tool head assembly 
       670 A,  670 B Backup head assembly 
     Items  700 - 799  are depicted in FIG. 7 
       714  Computer 
       716  Controller 
       720 B Y-axis upright support plate 
       722  Y-axis carriage assembly 
       726  cable 
       727  idle pulley 
       729  counterbalance weight 
       734 A, B Upright arms 
       740  Tool Head Assembly 
       758  automated tool 
       770  Backup Head assembly 
       786 A,  786 B,  786 C—Electrical conductors 
       787  electrical signal conductors cabling 
       788 A,  788 B electrical receptacles 
       789 A,  789 B Axis drive motors 
     Items  800 - 899  are depicted in FIG. 8 
       810  Cylindrical wall of hole 
       811  Plunger 
       812  first switch contact 
       813  second switch contact 
       814  switch assembly 
     Preferred embodiments for the present invention will be described hereunder with reference to FIGS. 1A to  8 . While this invention is susceptible of embodiment in many different forms, there is shown in the drawings and will herein be described in detail, specific embodiments, with the understanding that the present disclosure is to be considered as an exemplification of the principles of the invention and is not intended to limit the invention to the embodiments illustrated. 
     For example, while a flat workpiece could be worked through the use of more exotic methods such as laser cutters, electrochemical machining, and the like, my discussion is limited to conventional drilling and milling methods as applied to a PCB working machine. 
     Note: certain parts of the apparatus can be interchanged with each other which would change their spatial relationship to other parts. All references to parts in this “description” section assume they are positioned as depicted in the figure being actively described. 
     A first embodiment of the flat workpiece working machine of the present invention is illustrated in FIG. 1A and 1B (isometric views). In general there are forces and reciprocating motions acting in this embodiment in three different axes which are arbitrarily labeled the X-axis (front to back), the Y-axis (top to bottom), and the Z-axis (left to right). In general the embodiment complies with the rules of a three-dimensional rectangular Cartesian coordinate system. I have further labeled the positive and negative components of the three axes (indicated by the three directional arrows on FIG. 1) as follows: 
     XN=X-Negative direction or X-negative side of machine parts. 
     XP=X-Positive direction or X-Positive side of machine parts. 
     YN=Y-Negative direction or Y-Negative side of machine parts. 
     YP=Y-Positive direction or Y-Positive side of machine parts. 
     ZN=Z-Negative direction or Z-Negative side of machine parts. 
     ZP=Z-Positive direction or Z-Positive side of machine parts. 
     The machine framework of FIG. 1A includes a base plate  10 , a Y-axis upright support plate  20 A, a Y-axis upright support plate  20 B and a X-axis balance rail support  12 , all rigidly affixed to each other orthogonally forming a common positioner member  15  as follows: 
     1) the YP-side plane of base plate  10  lies substantially parallel to the X-axis and Z-axis, 
     2) the ZP-side plane of upright support plate  20 A lies substantially parallel to the X-axis and Y-axis, 
     3) the XN-side plane of balance rail support  12  lies substantially parallel to the Z-axis and Y-axis, and 
     4) the XP-side plane of upright support plate  20 B lies substantially parallel to the Z-axis and Y-axis. 
     A Y-axis carriage assembly  22  includes a plurality of slide units  23 A and  23 B which are rigidly connected by a workpiece mount  24 . A Y-axis guide rail  21  may be rigidly affixed or integrally formed to upright support plate  20 A. Y-axis carriage assembly  22  is slidably mounted to Y-axis guide rail  21  via its slide units  23 A and  23 B. Rail  21  is aligned on plate  20 A such that carriage  22  will slide substantially along the Y-axis. 
     Precise reciprocating Y-axis positioning of Y-axis carriage assembly  22  is provided by a power lead screw mechanism (not shown) or other means well known in the art. Threadably disposed in a pair of threaded open surfaces  23 C and  23 D of workpiece mount  24  are a plurality of thumb screws  25 A, and  25 B, respectively. Thumb screws  25 A and  25 B enable a flat workpiece  28  to be securely mounted at one side in an X and Y-axis plane. Workpiece  28  could be of any flat, rigid material but, for the purpose of this discussion, is considered to be a printed circuit board (PCB). An inverse, or second, workpiece ZP Face  28 P can be worked from the ZP machine side. 
     An X-axis guide rail  30  may be rigidly affixed or integrally formed to base  10 . A U-shaped carriage assembly  32  is slidably connected to X-axis guide rail  30  via an X-axis slide unit  36 , thus permitting reciprocation substantially along the X-axis. U-shaped carriage assembly  32  includes a plurality of upright arms  34 A and  34 B which are rigidly connected or integrally formed to X-axis slide unit  36 . Disposed on the YP end of arm  34 A is a head mount  38 A and on the YP end of arm  34 B is a head mount  38 B. Together arms  34 A and  34 B, mounts  38 A and is  38 B, and X-axis slide unit  36  form the rigid U-shaped X-axis carriage assembly  32 . Upright arms  34 A and  34 B and their respective mounts  38 A and  38 B are substantially identical except that they are positioned on opposite Z sides of X-axis slide unit  36  and upright arm  34 B is connected to a balance guide rail  80 . 
     Balance guide rail  80  is disposed between X-axis balance rail support  12  and Y axis upright support plate  20 B such that a balance rail slide unit  82 , being affixed to U-shaped carriage assembly  32 , assists in supporting upright arms  34 A and  34 B in their upright positions substantially parallel to the Y axis. The purpose of balance rail  80  and balance rail slide unit  82  is to steady U-shaped carriage assembly  32  on X-axis guide rail  30 . 
     The plane formed by the YP-side of base plate  10  represents a first support surface  11  and a line lying on surface  11  and parallel to the slidable motion of carriage  32  represents a first axial line. The plane formed by the XP-side edge of plate  20 A represents a second support surface  20 C and a line lying on surface  20 C, parallel to the slidable motion of carriage  22 , and intersecting the first axial line represents a second axial line. The first axial line and second axial line lie in a common plane which is also common to a plane which lies substantially parallel to ZP face  28 P and a ZN face  28 N of workpiece  28  when mounted to mount  24 . 
     X and Y-axis guide rails  30  and  21 , respectively, are mutually and permanently aligned such that when workpiece  28  is secured in workpiece mount  24 , the space representing the X-Y plane halfway between upright arms  34 A and  34 B substantially superimposes the center X-Y plane of workpiece  28 . 
     Precise reciprocating X-axis positioning of carriage  32  is provided by a power lead screw mechanism (not shown) or other means well known in the art. A tool head assembly  40 , shown mounted to arm  34 A, and a backup head assembly  70 , shown mounted to arm  34 B, complete FIG.  1  and are further detailed in the following paragraphs. 
     In general, though, the locations of head assembly  40  and  70  are interchangeable in that either head assembly  40  or  70  head can be mounted on either upright arm  34 A or  34 B by use of respective head mounts  38 A and  38 B. 
     Tool head assembly  40  and back-up head assembly  70  could take a variety of forms depending on the application for which the flat workpiece working apparatus is intended. The application chosen for the preferred embodiment is conventional drilling, milling, and routing. 
     As viewed from the YP side of the apparatus, head mounts  38 A and  38 B are trapezoid shaped. The functional components of tool head assembly  40  are disposed on a tool head bracket  42  which has a complimentary trapezoidal grooved surface  43  machined therein. 
     Likewise, the functional components of back-up head assembly  70  includes a tool head bracket  72  which has a complimentary trapezoidal grooved surface  73  machined therein. 
     Brackets  42  and  72  are machined trapezoidally such that they are slidable in the y-axis over respective trapezoidally machined mounts  38 A and  38 B. Brackets  42  and  72  are tapped with a plurality of threaded open surfaces  45 A,  45 B and  75 A and  75 B, respectively, to receive a plurality of threaded thumb screws  44 A,  44 B, and  74 A, and  74 B, respectively, for the purpose of firmly securing them to their respective head mounts  38 A and  38 B. 
     Other than the fact that tool head assembly  40  is demountable, it is similar to those presently used in the trade. A brief description follows to further familiarize the reader with FIG.  1 . 
     Rigidly affixed to tool head bracket  42  is a solenoid  46  and a Z-axis guide block  50 . All remaining tool head assembly  40  parts are rigidly linked together. They reciprocate in the Z-axis as commanded by solenoid  46  and are introduced in the following paragraph. 
     A solenoid plunger  48  is operably connected with solenoid  46  and a plurality of Z-axis shafts  52  are associated with block  50 . Plunger  48  and shafts  52  are joined by a solenoid bracket  55 B. Also, disposed on solenoid bracket  55 B is a spindle housing  62  which, in turn, supports a spindle motor  54 . 
     Another solenoid bracket  55 A further rigidly connects the ZN side of motor  54  to shafts  52 . A tool chuck (not shown) disposed on the rotor of motor  54  holds a machine tool  58  such that the tool axis is positioned substantially within the Z-axis. An adjustable depth foot  56  is provided to limit or adjust the depth that tool  58  can penetrate into or through workpiece  28 . 
     All interconnecting parts of U-shaped carriage assembly  32  and tool head assembly  40  are mutually calibrated such that the axes of Z-axis shafts  52 , and tool  58  are aligned to reciprocate substantially within the Z-axis as commanded by solenoid  46 . A spring-loaded pressure foot  60  is disposed on bracket  42  and formed such that constant pressure is applied against the ZN side of workpiece  28 . The function of foot  60  is to ensure separation of workpiece  28  and tool  58  at all times except when engaged by solenoid  46 . 
     A primary purpose of the back-up head assembly  70  is to contain workpiece  28  substantially within the common X-Y plane lying between upright arms  34 A and  34 B. Disposed on back-up bracket  72  is a back-up tool  76 . In general, the form, design, or material of back-up tool  76  would depend upon what type of tool is installed in tool head assembly  40 . For example, a laser cutting tool installed in tool head assembly  40  may require a laser sensor or receiver in back-up head assembly  70 . For conventional milling, routing, and drilling though, back-up tool  76  appears in two different forms, both in the shape of pads having a re-positionable adhesive applied to the ZP side to temporarily hold them onto the ZN side of bracket  72 . Construction material of a back-up tool  76 D, as used during a drilling process, would be of a special back-up material commonly utilized in the trade. Its purpose is to receive a drill bit  58 D as it penetrates the back side of workpiece  28  in order to minimize backside breakout of workpiece  28  material. 
     The same material would be used when a routing tool  58 R is being utilized to cut the outline of a small circuit board as well as making oversized or irregularly shaped holes in workpiece  28 . 
     On the other hand, during a milling process, tool  58  and foot  56  are engaged against workpiece  28  as head assemblies  40  and  70  traverse in various X-Y vectors relative to workpiece  28 . This relatively strong force is transferred through workpiece  28  creating a relatively higher tension of workpiece ZP face  28 P against a milling back-up tool  76 M during X-Y motion. In this case, the primary concern for material selection of back-up tool  76 M is that it presents minimum friction against workpiece ZP Face  28 P in order to not affect X-Y positioning and not damage workpiece ZP Face  28 P. One option would be selection of material for back-up tool  76 M in the form of a felt pad, again applied to bracket  32  with re-positional adhesive or other mechanical means. 
     In FIG. 1, the tool head assembly  40  is shown mounted onto the upright arm  34 A with working end of the tool  58  facing the ZP direction. It can also be mounted to the arm  34 B with the tool  58  facing the ZN direction and the ZP side of the workpiece  28 . In fact, the arms  34 A and  34 B and the mounts  38 A and  38 B are factory aligned such that when the tool head assembly  40  is thus reversed, the axis of tool  58  will be substantially coaxial with a line extended from the axis of the tool  58  when it was mounted on the opposing the arm  34 A or  34 B. While such alignment is not absolutely necessary, it will be seen that maximum benefit of the unique features of the apparatus would be gained by doing so. 
     It will be readily apparent that the operation of the present invention lends significant advantages of the art. 
     As seen in FIG. 7, digital instructions that control operation of the apparatus are prepared in a computer  714  and, when ready, sent through a driving controller  716 , and through a plurality of electrical signal conductors  787  to an X-axis drive motor  789 A, a Y-axis drive motor  789 B, and a pair of electrical receptacles  788 A and  788 B. Like receptacles  788 A and  788 B are disposed on respective uprights  34 A and  34 B for convenience of plugging in an automated tool head  740  which contains electrical components including spindle motor  54  and solenoid  46  and could be mounted to either of a pair of automated uprights  734 A or  734 B. 
     A blank copy of workpiece  28 , to be drilled and engraved, is fitted into workpiece mount  24  and secured by thumb screws  25 A and  25 B such that it lies in the X-Y plane between arms  34 A and  34 B. Workpiece  28  will remain thus mounted throughout the entire process of drilling, engraving, and routing of both ZN and ZP faces  28 M and  28 P. 
     As seen in FIGS. 3A and 3B for this example, a set of four drilled holes  90 E through  90 H, a set of four respective engraved annular rings  91 E through  91 H and another set of four milled annular rings  92 E through  92 H will be described. 
     The tool head assembly  40  is then mounted to upright support arm  34 A by sliding bracket  42  over mount  38 A and securing wilt thumb screws  44 A and  44 B. Through the computer keyboard (not shown) the operator commands the Y-axis to traverse to the maximum YN position, which is considered a tool-load-point. In this position the axis of spindle motor  54  is further YP than the YP edge of workpiece  28 , thus enabling tool  58  to be inserted into the chuck of [the] motor  54 . Appropriate adjustments are made to tool head assembly  40  to achieve correct throw and depth of cut as is common in the practice. 
     Backup tool  76 D is installed on back-up bracket  72  and back-up head assembly  70  is mounted to upright support arm  34 B by sliding bracket  72  over mount  38 B and securing with thumb screws  74 A and  7413 . Typically drill bit  58 D will penetrate through workpiece  28  into backup tool pad  76 D. Therefore, because the same hole first drilled into backup pad  76 D will be used repeatedly for all drill bits  58 D, the smallest diameter drill bit  58 D is first called for by the computer program. After a set of small diameter holes  90 E through  90 H are drilled through workpiece  28 , the next larger diameter is called for etc. until all required diameter holes  90 E through  90 H are drilled through workpiece  28  as instructed by the computer program. 
     The machine operator again commands heads  40  and  70  to traverse to the tool-load-point whereupon a milling tool  58 M is installed into the chuck (not shown) of spindle motor  54  for the purpose of engraving annular circuit pads  91 E through  91 H around drilled holes  90 E through  90 H as well as interconnecting land circuit traces (not shown). Back-up tool  76 D is replaced by milling back-up tool  76 M and workpiece ZN Face  28 N is then completely engraved. In this way the operation accomplishes objectives as did previous flat-bed machine designs commonly used in the trade. 
     Now workpiece  28  has been completely drilled and the ZN Face  28 N engraved. In order to engrave workpiece ZP Face  28 P, the positions of tool head assembly  40  and the backup head assembly  70  are reversed. This is accomplished by loosening thumb screws  44 A,  44 B,  74 A, and  74 B, sliding tool head assembly  40  and  70  off their the respective mounts  38 A and  38 B, and re-installing on opposite mounts  38 B and  38 A as previously described. Engraving of annular rings  92 E through  92 H around the drilled holes  90 E through  90 H and interconnecting land traces (not shown) on ZP Face  28 P then continues to program completion. 
     FIGS. 2A and 2B illustrate the problem inherent in drilling and engraving double-sided PCB&#39;s on machines of current technology which requires the workpiece to be flipped over to access opposing sides. FIGS. 2A and 2B represent such a flatbed machine well known to those in the art. A pair of flatbed X-axis rails  130 A and  130 B are disposed on a table/bed  110  such that a flatbed Y-axis carriage  120  is confined to sliding in the XP/XN directions when so commanded by controlling software (not shown). 
     A flatbed Y-axis rail  121  is disposed on Y-axis carriage assembly  120  such that when commanded, a flatbed tool head  154  should slide in the YP/YN directions. However, in this case, Y-axis carriage assembly  120  is misaligned such that Y-axis rail  121  is grossly non-perpendicular to rails  130 A and  130 B. On these simplified illustrations, a flatbed tool axis  158  of tool head  154  is represented by an “X”. 
     A plurality of board mounting/alignment pins  185 A and  185 B are permanently pressed into table/bed  110  such that they lie substantially within a Y-axis line halfway between rails  130 A and  130 B. A blank copy of workpiece  28 , which is pre-drilled with a pair of holes to match the spacing of pins  185 A &amp; B, is fitted over the pins in preparation to drill and engrave the first-side circuitry. 
     FIG. 2A illustrates the status of process after the obverse side of workpiece  28  has been drilled and milled. First four through-holes  90 A,  90 B,  90 C, and  90 D are drilled in what should have been a rectangular pattern. However, because Y-axis guide rail  121  is skewed, the pattern appears as a parallelogram. Then a set of four annular rings  91 A,  91 B,  91 C, and  91 D are milled around four holes  90 A through  90 D. As intended, four annular rings  91 A through D are substantially coaxial with respective four holes  90 A through D because, thus far, drilling and milling have been done on the same workpiece face  28 P. 
     On the flatbed machine as depicted in FIGS. 2A and B, in order to mill annular rings on the inverse side of workpiece  28 P, workpiece  28  is lifted off pins  185 A and  185 B, flipped over, and re-inserted such that the same guide holes are fitted over same guide pins  18 SA and B. This step is now completed as illustrated on FIG.  2 B. 
     FIG. 2B illustrates the status of process after workpiece  28  has been flipped and a set of four inverse-side annular rings  92 A,  92 B,  92 C, and  92 D have been milled around respective through-holes  90 A,  90 B,  90 C, and  90 D utilizing mirror-imaged software. The intention was for these four annular rings  92 A through  92 D, respective holes  90 A through  90 D, and respective obverse side annular rings  91 A through  91 D to be coaxial as like work appears in FIG.  3 B. Prior to milling inverse-side annular rings, the machine is manually offset such that when drilled, a first annular ring  92 A will be coaxial with hole  90 A. Under program control, ring  92 B will also appear substantially coaxial with respective hole  90 B and ring  91 B. However, as milling continues under program control, rings  92 C and  92 D are milled substantially offset from their ideal coaxial positions around respective holes  90 C and  90 D and rings  91 C and  91 D. This undesirable offset is a result of Y-axis rail  121  being skewed. 
     FIGS. 2A and 2B illustrate but one of a myriad of alignment afflictions that similarly limit the ability of machines to accurately converge machined work on one side of a flat workpiece to work performed on the opposite side when flipping of the workpiece is involved. These problems directly affect the resolution or fineness of the printed circuit artwork being performed on double sided PCBs. 
     FIGS. 3A and 3B illustrate the solution provided by the preferred embodiment of this patent to the problems described in the above paragraphs of this section. Both FIGS. 3A and 3B are views from the YN-side of the preferred embodiment. General operation was previously described in the operation section of this patent As in FIGS. 2A and 2B, FIGS. 3A and 3B illustrate Y-axis plate  20 A being grossly misaligned such that Y-axis rail  21  is not perpendicular to X-axis rail  30 . 
     FIG. 3A illustrates that as described for FIG. 2A, the programmed drilling of an intended rectangular pattern of four through-holes  90 E,  90 F,  90 G, and  90 F actually results in a parallelogram pattern when the preferred embodiment is so misaligned. As intended, four annular rings  91 E,  91 F,  91 G, and  91 H appear substantially coaxial to respective holes  90 E through  90 H when milled from the same workpiece side under program control. Drilling and milling, in FIG. 3A, were performed with tool head assembly  40  mounted on the YN-side arm  34 A. Work, thus far, was performed on the obverse side of workpiece  28 . 
     FIG. 3B illustrates the programmed milling of four inverse-side annular rings  92 E,  92 F,  92 G, and  921 H around their respective holes  90 E through  90 H. In this case, however, workpiece  28  was not flipped in order to access the inverse side. Rather, tool head  54  was removed from the YN-side arm  34 A and reinstalled on the YP-side arm  34 B. Therefore, annular rings  92 E through  92 H all appear substantially coaxial with their respective holes  90 E through  90 H and obverse side annular rings  91 E through  91 H, the desirable effects of machine misalignment being similarly reproduced on opposing workpiece  28  sides. 
     Another illustrated benefit is that it is not necessary to manually offset the position of either axis prior to beginning inverse-side machining. 
     FIG. 4 illustrates another variation of the present invention. In this variation workpiece  28  remains fixed to a type-four base  410  (hereinafter, “type” followed by a number designation refers to the embodiment of the invention represented in the figure of that number designation, i.e., “type-four mount” indicates a mount of the embodiment illustrated in FIG. 4) by a type-four mount  424  rigidly connected thereto throughout machine operation rather than reciprocating in either the X or the Y-axis. Head assemblies  40  and  70  traverse in unison in both the X and the Y axes. This is accomplished as follows: 
     a type-four Y-axis support  420  is X-axis-slidably mounted over a pair of type-four workpiece rails  430 A and  430 B which are mounted on base  410 ; 
     a type-four U-shaped carriage assembly  432 , which includes a type-four Y axis slide unit  436 , is Y-axis-slidably mounted over a pair of type-four common rails  421 A and  421 B; and 
     a pair of type-four carriage arms  434 A and  434 B serve same function as previously described arms  34 A and  34 B except that they traverse in unison in both X and Y-axes, which may be manually positioned or automatically positioned according to pre-determined programmed instructions. 
     FIG. 5 illustrates another variation of the flat workpiece working machine. In this variation, heads  40  and  70  remain fixed to base  10  through a pair of fixed uprights  534 A and  534 B which virtually serve same function as previously described arms  34 A and  34 B respectively throughout machine operation. 
     In this version, heads  40  and  70  do not reciprocate in X or Y direction. Rather, workpiece  28  is able to traverse in both the X and the Y-axes. This is accomplished as follows: 
     a type-five Y axis support  520  is X-axis-slidably mounted over a pair of type-five X-axis rails  530 A and  530 B; 
     a type-five Y-axis-slide unit  523  is slidably mounted over a pair of type-five rails  521 A and  521 B to Y-axis support  520 ; 
     a type-five workpiece mount  524  is rigidly disposed on slide unit  523 ; and 
     workpiece  28  is removably mounted to mount  524  and thus is capable of traversing in either the X or Y-axes via manual positioning or automated positioning via predetermined programmed instructions. 
     FIG. 6 is a top (YP) perspective of a modified FIG. 1, which illustrates a dual-spindlemotor variation of the apparatus for working double sided workpiece. A type-six U-shaped carriage assembly  632 , consisting of a plurality of paired upright arms  634 C through  634 F disposed on a dual tool X-axis slide unit  636 , serves the same function as carriage  32  in FIG. 1, with the provision of an added function. Upright arm  634 C is disposed on the XP/ZN comer of slide unit  636  and disposed on arm  634 C is a fixed tool head  640 A. Upright arm  634 D is disposed on the XP/ZP corner of slide unit  636  and disposed on arm  634 D is a dual backup head  670 A. Arm pair  634 C and  634 D and head pair  640 A and  670 A serve the same function as arms  34 A and  34 B and heads  40  and  70  previously described except that the heads can remain fixed rather than removable such that tool head  640 A would be limited to working on or from the ZN side of workpiece  28 . 
     Additionally, upright arm  634 E is disposed on the XN/ZP comer of slide unit  636  and disposed on arm  634 E is a tool head  640 B. A paired upright arm  634 F is disposed on the XN/ZN comer of slide unit  636  and disposed on arm  634 F is a fixed backup head  670 B. Arm pair  634 E and  634 F and head pair  640 B and  670 B also serve the same function as arms  34 A and  34 B and heads  40  and  70  previously described except that the heads can remain fixed rather than removable such that tool head  640 B would be limited to working on or from the ZP side of workpiece  28 . 
     The object of the head arrangement depicted in FIG. 6 is that both sides of a double sided workpiece could be worked without having to reposition the tool/backup heads from one Z-side of the workpiece to the other. However, tool offsetting would be required in the X-axis when working opposing sides of workpiece  28  which could degrade from the fall benefits of the preferred embodiment as described for FIG.  1 . 
     FIG. 7 Illustrates, in simplified form, a preferred embodiment connected to programmable computer  714 . The reader is spared in-depth details of computer programming as it will be apparent to those skilled in the art. Digital instructions from computer  714  are downloaded to a controller  716  through an electrical conductor  786 A. The controller  716  converts the received digital instructions to a suitable protocol to be received by various electrical devices required on the preferred embodiment to properly position an automated tool head  740  and an automated backup head  770  over workpiece  28  (not shown in FIG.  7 ), cause an automated tool  758  to rotate, and engage tool  758  to workpiece  28 . 
     Appropriate electrical currents flow from controller  716 , through conductors  787 , to X-axis positioning motor  789 A, and Y-axis positioning motor  789 B. Appropriate currents also flow through conductors  787  to electrical receptacles  788 A and  788 B, which are disposed respectively on arms  734 A and  734 B. As previously noted, heads  740  and  770  could be interchangeable. The receptacles  788 A and  788 B provide a convenient means to plug electrical components (not shown) from heads  740  and  770  into controller  716  through a multiplicity of electrical head conductors  786 B and  786 C, as well as conductors  787 . 
     The preferred embodiments of FIG.  1 B and FIG. 7 depict carriage assembly  22  or a vertical carriage  722  and other associated Y-axis components which reciprocate in a direction vertical to the earth&#39;s surface. Earth&#39;s gravity would therefore cause an imbalance in motion that could cause imperfections in the resulting artwork to be engraved on workpiece  28 . FIG. 7 illustrates a method and device to minimize the undesirable effects of the force of gravity. A cord or cable  726  has one end connected to carriage  722  and the other end connected to a counterweight  729 . Disposed on the YP end of a type-seven upright plate  720 B is a pulley assembly  727 . Cable  726  is of an appropriate length that when wrapped over Pulley  727 , counterweight  729  will remain suspended and thus keep cable  726  taut throughout the vertical travel of carriage  722 . The weight of counterweight  729  is selected to be substantially equal to the sum weight of all vertical moving components and thus will serve as a counterbalance to these parts. 
     FIG. 8 is a XP-side perspective of U-shaped carriage  32  of preferred embodiment having a means for sensing the Z-axis position of tool  58 D. A hole having a cylindrical w all  810  is bored through backup bracket  72  such that wall  810  is substantially coaxial with drilling tool  58 D. A plunger  811  having a Z-axis length slightly longer than the Z-axis width of bracket  72  is loosely fitted within wall  810  such that it&#39;s ZP end is resting against a first electrical contact  812  of switch assembly  814  which is disposed on the ZP side of bracket  72 . Contact  812  is normally electrically open from a second switch contact  813 . In FIG. 8, drill tool  58 D is shown having been actuated towards the ZP direction such that it has bored through workpiece  28  and backup tool  76  and has contacted and pushed plunger  811  against contact  813  such that contacts  812  and  813  are electrically closed. In this manner it will be clear to those in the art that the tool position could be sensed for the purpose of signaling computer  714  that work has progressed to the point that the tool could be backed off from shown engaged position in preparation of continuing to the next X/Y coordinate. 
     Accordingly, the reader will see that the opposing interchangeable heads of this invention facilitate a method to work both sides of a flat workpiece without disturbing the workpiece mounting and thus maintain high relative precision between both sides regardless of machine&#39;s X/Y alignment imperfections. Furthermore, a design incorporating opposing interchangeable heads has additional advantages for a machine producing double-sided work in that: 
     machine stability is greatly improved; 
     machine alignment during manufacture is greatly simplified; 
     software programs are simplified because it is not necessary to “mirror-image” opposing sides; 
     operating procedures are simplified because offsetting X-axis or Y-axis is not necessary when changing sides, this is especially important when producing multiple pieces of artwork on a single workpiece substrate; 
     easy access to tool chuck, for tool replacement purposes, is provided; 
     tool head can be removed for this purpose rather than positioning head off the side of machine table; 
     progress of work can be monitored or sensed on surface of workpiece opposite the tool head; 
     a variety of tool heads having special functions could be easily installed by the operator; 
     a defective head could be easily replaced by the user; 
     cost, material, and weight of precision flatbed tabletop eliminated; 
     hole in backup material, produced by first drilled hole in workpiece, can be reused for all remaining holes, again saving operating costs; and 
     workpiece can be positioned vertically saving work space, enabling gravity to remove work debris, and permitting visual observation of both sides of workpiece. 
     Although the description above contains many specificities, these should not be construed as limiting the scope of the invention but as merely providing illustrations of the preferred embodiment of this invention. Some examples of alternatives are that: 
     the workpiece could have photosensitive surfaces and the tool could produce a laser or light beam; 
     the workpiece could have writing surfaces and the tool could be a writing instrument such as an ink pen, which could be especially useful for verifying work prior to utilizing more expensive materials; 
     the tool could be a laser engraver/cutter, additional tools could be for through-hole fluid dispensing, riveting, or insertion of electronic components; 
     the workpiece could be of multi-layered plastic laminate such as is commonly used for engraving signs, nameplates and such; 
     the machine could be designed to produce items as small as integrated circuit chips, as large as sheets of wood for cabinet making etc., or even larger; 
     the machine could be positioned in any conceivable attitude relative to a support surface, 
     multiple heads could be provided on both sides of the workpiece to enable production of multiple duplicate PCBs in a single operation; 
     one-piece head/arm assemblies could be interchangeable rather than just the head/tool assemblies or even the complete u-shaped carriage assembly could be made reversible; 
     both heads, having spindle motors, could be permanently mounted with facilitation for converting either side to a backup head; 
     the backup head could be designed to rotatably step a disk made of backup material in order to provide a new piece of material for each hole drilled; 
     the backup head could be fitted with a multi-directional wheel assembly or air bearing to virtually eliminate any marring or scratching of the inverse side during the milling process; 
     although thumb-screws were selected for use in the illustrations, a final product would likely utilize more time-efficient fasteners such as cam-levers. 
     Thus the scope of the invention should be determined by the appended claims and their legal equivalents, rather than by the examples given.