Method and apparatus for drill length compensation

A method of rapidly drilling holes in workpiece on a drilling machine, said drilling machine having a first and second spindle movable at least in a vertical direction, a first and second drill tool mounted in said first and second spindle, respectively, said first and said second spindle being adapted to cause rotation of said first and second drill tools to effect drilling, each of said drill tools having a tip depending a length below its respective spindle, a worktable positioned beneath said spindles on which a workpiece can be mounted and a first and second pressure foot mounted on said first and second spindle, respectively, and movable in a vertical direction relative to said respective spindles, each said pressure foot having a bottom surface adapted to engage a workpiece mounted on said worktable having a top surface, said method comprising the steps of:

• selecting a workpiece having a thickness;
• defining a nominal vertical home position for said first and second spindles such that in said home position there is a nominal clearance distance between said bottom surface of said pressure foot of said first and second spindle and said top surface of said workpiece;
• measuring a drill tool offset distance for said first and second spindle, said drill tool offset distance for a given spindle being the distance between said bottom surface of said pressure foot of said spindle and said tip of said drill tool mounted in said spindle;
• compensating for said drill tool offset distances by designating a vertical upper limit for said first spindle and a vertical upper limit for said second spindle such that when said first and second spindles are at their respective upper limits, the tips of their respective drill tools are positioned an equal distance from said top surface of said worktable;
• advancing said drilling spindles downward through a vertical drilling stroke to a lower limit;
• and,
• returning said drilling spindles upward to their respective upper limits.

A drilling machine in which the length of the drill stroke can be adjusted to improve the throughput of said drilling machine, said drilling machine comprising;

• a first spindle movable at least in a vertical direction, a first drill tool removably mounted in said first spindle, a tip of said first tool being positioned a nominal distance from the bottom of said first spindle, said first spindle being adapted to cause rotation of said first drill tool to effect drilling;
• a second spindle movable at least in a vertical direction, a second drill tool removably mounted in said second spindle, a tip of said second tool being positioned a nominal distance from the bottom of said second spindle, said second spindle being adapted to cause rotation of said second drill tool to effect drilling;
• a vertical position sensor which senses the absolute vertical position of said spindles and drill tools;
• a worktable positioned beneath said first and second spindle, said worktable adapted to have a workpieces mounted thereon, said worktable being movable in at least one direction in a horizontal plane so as position said workpieces at desired locations beneath said spindles;
• a first pressure foot mounted on said first spindle and movable in a vertical direction relative to said first spindle, a bottom surface of said first pressure foot adapted to engage a workpiece which is mounted on said worktable, said first drill tool passing through an aperture in said first pressure foot during drilling operations;
• a second pressure foot mounted on said second spindle and movable in a vertical direction relative to said second spindle, a bottom surface of said second pressure foot adapted to engage a workpiece which is mounted on said worktable, said second drill tool passing through an aperture in said second pressure foot during drilling operations;
• a first relative movement sensor which senses relative vertical movement between said first spindle and said first pressure foot, said first relative movement sensor sensing movement of said first pressure foot which occurs upon initial contact between said first pressure foot bottom surface and a workpiece;
• a second relative movement sensor which senses relative vertical movement between said second spindle and said second pressure foot, said second relative movement sensor sensing movement of said second pressure foot which occurs upon initial contact between said second pressure foot bottom surface and a workpiece;
• a computer controller which controls operation of said drilling machine, and which receives input from said first and second vertical position sensors and said first and second relative movement sensors, said controller comprising;
• a computer memory for storing data sensed by said relative movement sensor regarding the drill tool offset position for each spindle;
• means for calculating the difference between the drill tool offset positions for each spindle, said drill tool offset positions equal to being the distance between said bottom surface of said pressure foot of said spindle and said tip of said drill tool mounted in said spindle;
• and,
• means for controlling the vertical displacement of said spindles during a drill stroke, said control means adapted to set an upper limit for said first spindle and an upper limit for said second spindle having a distance between said upper limits equal to said difference between said drill tool offset positions.