Method of and apparatus for measuring distances in numerically controlled machine tools

A method of machining the surface of a workpiece mounted on a numerically controlled machine tool, said machine tool having a cutting tool attached thereto which can be displaced relative to the surface of said workpiece, the cutting tool being employed for both machining said workpiece and sensing the location of the surface thereof, comprising the steps of

• storing a first signal corresponding to the desired distance of the finished machined surface of said workpiece from a datum plane;
• generating a second signal corresponding to the actual distance of said cutting tool from said datum plane;
• moving at least one of said cutting tool and said workpiece with respect to each other to machine the surface of said workpiece to a first distance from said datum plane at which said first signal is equal to said second signal, said first distance being greater than said desired distance by an amount equal to the relative deflection of at least one of said cutting tool and said workpiece with respect to each other;
• moving at least one of said cutting tool and said workpiece for separation of one from the other;
• generating a third signal when said cutting tool breaks engagement with said workpiece, said third signal corresponding to said first distance;
• determining the difference between said third and first signals, said difference being a deflection signal corresponding to the relative deflection between said cutting tool and said workpiece;
• generating a fourth signal equal to the difference between said first signal and said deflection signal;
• and
• moving at least one of said cutting tool and said workpiece with respect to each other to machine said workpiece to a distance from said datum plane at which said second signal equals said fourth signal, said cutting tool thereby machining the surface of said workpiece to the desired distance from said datum plane.

A numerically controlled machine tool comprising

• a first member for supporting a workpiece;
• a second member for supporting a cutting tool, said cutting tool being provided for machining the surface of said workpiece to a desired distance from a datum plane;
• motor means for displacing said first and second members with respect to each other;
• storage means for storing a first signal corresponding to said desired distance from said datum plane;
• position detection means for generating a second signal corresponding to the displacements of said first and second members relative to each other;
• comparator means having inputs coupled to the outputs of said storage and position detection means and an output coupled to said motor means, said comparator means driving said motor means to displace said first and second members with respect to each other until said first and second signals are equal;
• sensing means coupled to said first and second members for determining when said cutting tool and workpiece became disengaged from each other, said sensing means generating a third signal corresponding to the distance of the surface of said workpiece from said datum plane upon said disengagement;
• computer means coupled to said storage and sensing means for generating a deflection signal corresponding to the difference between said third and first signals, said difference being a deflection signal corresponding to the relative deflection between said cutting tool and said workpiece, said computer means further generating a fourth signal equal to the difference between said first signal and said deflection signal and inputting said fourth signal to said storage means, whereby said comparator drives said motor means to displace said first and second members with respect to each other until said fourth and second signals are equal to each other, said cutting tool thereby machining the surface of said workpiece to the desired distance from said datum plane.