Abrasive wheel

A grinding machine for grinding a tip of a hard metal pin of a button-studded drilling bit, to an obtusely profiled shape in an annular tapered band having an axially outer, smaller diameter and an axially inner, larger diameter, comprising:

• a rotatable holder in which the hard metal pin is disposed at a grinding position and rotated about a holder rotational axis for rotating the hard metal pin while the tip of the hard metal pin is being ground by an abrasive wheel;
• an abrasive wheel;
• and
• a driven spindle on which, in use, the abrasive wheel is disposed at a grinding position and rotated about a spindle rotational axis for rotating the abrasive wheel while the tip of the hard metal pin is being ground by the abrasive wheel;
• at least one of the holder and spindle being linearly reversibly movable along said spindle rotational axis, in a plane containing both said rotational axes;
• the spindle rotational axis being inclined from perpendicularity to the holder rotational axis, by an acute angle .alpha.;
• said abrasive wheel comprising:
• a body which is rotationally symmetrical about an abrasive wheel rotational axis which, in use, coincides with said spindle rotational axis, said body including two circumferentially extending, radially outwardly projecting flange portions which sandwich between them a circumferentially extending, radially outwardly opening groove portion;
• said groove portion having a surface profile which, in longitudinal cross-section of said abrasive wheel, is symmetrical about a line of symmetry which intersects said abrasive wheel rotational axis at an angle which is equal in magnitude to said angle .alpha.;
• said surface profile including a radially innermost base segment two axially opposite radially outer segments having respective radially outer ends, and two axially opposite intermediate segments extending between axially opposite ends of said base segment and respective ones of said radially outer segments;
• said surface profile being radially outwardly generally concave and flaring in width from said axially opposite ends of said base segment towards said radially outer ends of said radially outer segments;
• said base segment being shorter transversally of said line of symmetry than the magnitude of said axially outer, smaller diameter of said obtusely profiled shape of said hard metal pin, and said two radially outer ends of said radially outer segments of said surface profile being located further apart transversally of said line of symmetry, than the magnitude of said axially inner, larger diameter of said obtusely profiled shape of said hard metal pin.