1. A method for machining a workpiece (18) with a programmable, numerically
controlled machining system (8), the system (8) including numerical control means
adapted to operate under the direction of a predetermined machining program
including a predetermined toolpath (34) extending between an initial machining point
(36) and an end machining point (38) and at least one rotating cutting tool (12)
powered by a computer numerically controlled machine (10), the method comprising
the following steps: (1) probing the workpiece (18) with the rotating tool (12)
during relative motion therebetween,
(2) monitoring a vibration signal from a vibration sensor (28) mounted in the machining system (8),
(3) recording a contact position (29) of the workpiece (18) when the monitored vibration signal indicates contact between the rotating tool (12) and the workpiece (18),
(4) comparing the recorded contact position (29) to a predetermined initial machining point (36) of the predetermined toolpath (34) and calculating or retrieving a compensated toolpath (35) based on the comparison wherein the predetermined toolpath (34) extends between the initial machining point (36) and the end machining point (38), and
(5) machining the workpiece (18) along the compensated toolpath (35).

2. The method as claimed in claim 1, further comprising machining the workpiece (18) with multiple passes (40) wherein steps (1) through (5) are repeated for one or more of the passes (40).
3. The method as claimed in claim 2, further comprising calibrating the machine

(10) by probing a calibration part (60) having known dimensions with the cutting tool (12) while monitoring a vibration signal from the vibration sensor (28) during the probing with the cutting tool (12).
4. The method as claimed in claim 3, further comprising determining tool
parameters of the cutting tool (12) during the calibrating that are influenced by at
least one of a tool diameter, a tool geometry, and distance between the tool and a
workpiece guide (30) mounted to the machine (10).
5. The method as claimed in claim 1, further comprising the compensated toolpath (35) including an angle offset (48) to a machining path coordinate system (46) of the predetermined toolpath (34).
6. The method as claimed in claim 1 further comprising:
mounting the workpiece (18) in a multi axis manipulator (16) of the numerically controlled machining system (8) before step (1),
during step (1) the probing includes moving the multi axis manipulator (16) holding the workpiece (18) to provide the relative motion between the rotating tool (12) and the workpiece (18), and
during step (5) the machining the workpiece (18) along the compensated toolpath (35) includes moving the multi axis manipulator (16) holding the workpiece (18).
7. The method as claimed in claim 6 further comprising the machine (10) being a grinder and the rotating tool (12) a grinding wheel (14).
8. A method for repairing an airfoil (21) tip (52) using a programmable, numerically controlled machining system (8), the system (8) including numerical control means adapted to operate under the direction of a predetermined machining

program including a predetermined toolpath (34) extending between an initial machining point (36) and an end machining point (38), a grinding wheel (14) powered by a computer numerically controlled machine (10), the method comprising:
adding repair material (54) at the tip (52) of the airfoil (21),
using the grinding wheel (14) powered by the machine (10) for making machining passes (40) to machine away excess repair material and blend the airfoil surface 22 of the airfoil (21) at different chordwise locations (42) along the airfoil (21) between leading and trailing edges (LE, TE) of the airfoil (21),
at least one of the machining passes (40) including the following steps:
(1) probing the airfoil (21) with the rotating grinding wheel (14) during relative motion therebetween,
(2) monitoring a vibration signal from a vibration sensor (28) mounted in the machining system (8),
(3) recording a contact position (29) of the airfoil (21) when the monitored vibration signal indicates contact between the rotating grinding wheel (14) and the airfoil (21),
(4) comparing the recorded contact position (29) to a predetermined initial machining point (36) of the predetermined toolpath (34) and calculating or retrieving a compensated toolpath (35) based on the comparison wherein the predetermined toolpath (34) extends between the initial machining point (36) and the end machining point (38), and
(5) machining the airfoil (21) along the compensated toolpath (35).
9. The method as claimed in claim 8, further comprising mounting a gas turbine

engine blade (19) containing the airfoil (21) in a multi axis manipulator (16) of the numerically controlled machining system (8) before step (1) and the probing the airfoil (21) with the rotating grinding wheel (14) during relative motion therebetween includes moving the multi axis manipulator (16) holding the blade (19) relative to the machine (10).
10. The method as claimed in claim 9, further comprising the compensated toolpath (35) including an angle offset (48) to a machining path coordinate system (46) of the predetermined toolpath (34).
11. The method as claimed in claim 10, further comprising using a workpiece guide (30) mounted on the machine (10) for guiding the blade (19) and the airfoil (21) through the machining passes (40) and using a compliance means (32) for urging the grinding wheel (14) against the workpiece (18).