Thrust-responsive two-speed drill and method of operation

A pneumatically powered positive feed drill for drilling a hole in a workpiece consisting of a layer of material of a first hardness overlaying a material of a second hardness greater than the first hardness, the hole in the material of the first hardness to be drilled at a first speed and the hole in the material of the second hardness to be drilled at a second and slower speed, said positive feed drill comprising:

• a housing;
• a rotatable drill spindle mounted in said housing for feed and retract motion and including means for mounting a drill bit;
• a first drive train operatively connected to rotate said rotatable drill spindle;
• a second drive train operatively connected to feed and retract said rotatable drill spindle to and from a drilling position with respect to said workpiece;
• a first air motor to drive said first drive train and said second drive train at the first speed;
• a second air motor to drive said first drive train and said second drive train at the second and slower speed;
• control means connected to said first air motor and said second air motor to select between said first air motor and said second air motor;
• means to activate said first air motor when said positive feed drill is started so the positive feed drill will feed to said drilling position and drill the layer of material of a first hardness at said first speed;
• and
• thrust sensing means to sense the thrust on said rotatable spindle and to cause said control means to select said second air motor to drive said first drive train and said second drive train when the thrust on said rotatable spindle increases as the hole is drilled in said material of a second hardness and to cause said control means to select said first air motor to drive said first drive train and said second drive train when the thrust decreases after the hole has been drilled in said material of a second hardness.