Computer controlled automatic shift drill

A method for controlling the speed of a tool driven by a machine having a first speed that is faster than a second speed, said method comprising:

• sensing a force exerted on the tool;
• comparing the sensed force to a first threshold value;
• shifting the speed of the machine to the second speed in response to the sensed force exceeding the first threshold value, and to the first speed in response to the sensed force being below the first threshold value;
• determining the average force exerted on the tool during a given period of time after a shift in speed;
• and
• employing the determined average force to redefine the first threshold value.

A drill for rotationally driving a bit into a workpiece to form a hole comprising:

• a prime mover;
• a transmission, coupled to said prime mover, for selectively driving the bit at either a first or a second speed;
• means for sensing a force applied to the bit;
• means for comparing the sensed force to a plurality of threshold values;
• means, responsive to the means for comparing, for shifting the speed of the transmission (1) to the first speed for a first interval upon the sensed force exceeding a first threshold value, (2) thereafter shifting the speed of the transmission to the first speed in response to the sensed force being above a second threshold value and to the second speed in response to the sensed force being below the second threshold value, and (3) shifting the speed of the transmission to the first speed whenever the sensed force is below a third threshold value;
• means for determining the value of the average force exerted on the bit during a second interval of time after each shift in speed in response to the force crossing the second threshold value;
• and
• means, responsive to the average force values, for redefining the second threshold value.

A drill having a bit for forming a hole in a workpiece comprising:

• a means for selectively rotationally driving the bit at either a first or a second speed;
• means for sensing a force applied to the bit;
• means for comparing the sensed force to a threshold value;
• means, responsive to the means for comparing, for shifting the speed of said means for driving (1) to the first speed in response to the sensed force being above the threshold value, and (2) to the second speed in response to the force being below the threshold value for a first predetermined period of time.