A tool assembly (10, 110) comprising: a tool having a working end configured to perform an operation on a workpiece along a line of cut; a support fixture (11, 111) for supporting said tool on the workpiece, said support fixture (11, 111) including: a clamping assembly (12, 112) configured for clamping onto the workpiece; a miter plate (40, 172) mounted on said clamping assembly (11, 111) to permit pivoting of said miter plate (40, 172) to variable angular orientations relative to said clamping assembly (11, 111); a guide rail assembly (15, 150) mounted on said miter plate (40, 172) and including an elongated member (15, 151, 152), the tool configured to move along a path defined by the elongated member (15, 151, 152); a carriage (25, 160) slidably supported on said elongated member (15, 151, 152); and a tool mount (29, 170) carried by said carriage (25, 160) and configured to support said tool; characterized in that said miter plate (40, 172) is configured to pivot about a virtual pivot point defined along said line of cut, and is mounted to said clamping assembly (12, 112) offset from said virtual pivot point, said clamping assembly (12, 112) includes a line of cut indicator at said virtual pivot point, and said miter plate (40, 172) includes: two parallel arcuate slots (173, 178) radially offset from each other relative to said virtual pivot point; a mounting pin passing through one of said arcuate slots (173, 178) radially closer to said virtual pivot point and connected to said clamping assembly (12, 112); and a locking member passing through the other of said arcuate slots (173, 178) and connected to said clamping assembly (12, 112), said locking member configured to selectively lock said miter plate (40, 172) to said clamping assembly. a tool having a working end configured to perform an operation on a workpiece along a line of cut; a support fixture (11, 111) for supporting said tool on the workpiece, said support fixture (11, 111) including: a clamping assembly (12, 112) configured for clamping onto the workpiece; a miter plate (40, 172) mounted on said clamping assembly (11, 111) to permit pivoting of said miter plate (40, 172) to variable angular orientations relative to said clamping assembly (11, 111); a guide rail assembly (15, 150) mounted on said miter plate (40, 172) and including an elongated member (15, 151, 152), the tool configured to move along a path defined by the elongated member (15, 151, 152); a carriage (25, 160) slidably supported on said elongated member (15, 151, 152); and a tool mount (29, 170) carried by said carriage (25, 160) and configured to support said tool; a clamping assembly (12, 112) configured for clamping onto the workpiece; a miter plate (40, 172) mounted on said clamping assembly (11, 111) to permit pivoting of said miter plate (40, 172) to variable angular orientations relative to said clamping assembly (11, 111); a guide rail assembly (15, 150) mounted on said miter plate (40, 172) and including an elongated member (15, 151, 152), the tool configured to move along a path defined by the elongated member (15, 151, 152); a carriage (25, 160) slidably supported on said elongated member (15, 151, 152); and a tool mount (29, 170) carried by said carriage (25, 160) and configured to support said tool; characterized in that said miter plate (40, 172) is configured to pivot about a virtual pivot point defined along said line of cut, and is mounted to said clamping assembly (12, 112) offset from said virtual pivot point, said clamping assembly (12, 112) includes a line of cut indicator at said virtual pivot point, and said miter plate (40, 172) includes: two parallel arcuate slots (173, 178) radially offset from each other relative to said virtual pivot point; a mounting pin passing through one of said arcuate slots (173, 178) radially closer to said virtual pivot point and connected to said clamping assembly (12, 112); and a locking member passing through the other of said arcuate slots (173, 178) and connected to said clamping assembly (12, 112), said locking member configured to selectively lock said miter plate (40, 172) to said clamping assembly. two parallel arcuate slots (173, 178) radially offset from each other relative to said virtual pivot point; a mounting pin passing through one of said arcuate slots (173, 178) radially closer to said virtual pivot point and connected to said clamping assembly (12, 112); and a locking member passing through the other of said arcuate slots (173, 178) and connected to said clamping assembly (12, 112), said locking member configured to selectively lock said miter plate (40, 172) to said clamping assembly.