Method of determining position of openings adapted to receive fret saw blade of numerically controlled fret saw machines

In a numerically controlled fret saw machine comprising:

• blank drive means for clamping a flat plate shaped blank to move the same to any position on a X-Y plane of rectangular coordinates;
• fret saw blade rotation means for controlling rotation of said fret saw blade such that a tip of said blade is normally oriented in directions of grooves cut through said blank by said fret saw blade;
• a tool disposed on said X-Y plane with a predetermined spacing between the tool and the fret saw blade for forming openings through said blank;
• drive means for driving said tool;
• and
• a digital control device for controlling the direction of movements of said blank in X and Y axes directions of rectangular coordinates and for controlling rotation of said fret saw blade, thereby forming with said tool openings adapted to receive said fret saw blade at points respectively corresponding to cutting start points of said grooves,
• a method of determining a position of each opening comprising the steps of:
• producing a movement instruction data regarding a cutting start point and a cutting end point of a portion of a groove to be cut, said points defining a path A.fwdarw.B;
• determining a point P having coordinates (XP.multidot.YP) and representing a center of said opening for receiving said fret saw blade, said point P being located on a straight line passing through a point A having coordinates (XA.multidot.YA) and a point B having coordinates (XB.multidot.YB) and said point P being spaced from said point A by one half of the width SW of said fret saw blade in a direction opposite to the direction of cutting of the groove starting from said point A when said section is straight, whereas when said section is a circular arc, said point P is selected to be a line which is tangent to said ciruclar arc at said point A and spaced from said point A by one half of the width SW of said fret saw blade.