Tool touch probe system and method of precision machining

Claim 2

Numerically controlled apparatus for precision machining at least one surface of a work piece to selectively determined final dimensions; said apparatus comprising: programmable numerical control means (46); machining apparatus including a stationary bed (10), work piece holding means (36, 38, 82, 84), and tool holding means (26, 30) adapted to mount one or more tools (31, 98) and to place one of said tools into operating position; means for providing rapid relative motion between the cutting edge of said tool and said work piece, said tool being adapted when stationed in its operating position to effect the removal of material from said work piece during said rapid relative motion while said cutting edge is forced against said work piece; means controlled by said numerical control means for positioning said respective holding means relative to each other in at least first and second axes of motion of said machining apparatus; position feedback means adapted to signal to said numerical control means information representative of the actual position of said holding means at least in said first and second axis respectively; and position reference means associated with said machining apparatus and including at least a pair of position reference surfaces each corresponding to one of said axes, each of said reference surfaces providing a calibrated position reference with respect to the origin position of a machining program adapted to direct the operation of said numerical control means; the apparatus being characterised by: transducer means (37) positioned to sense vibrations indicative of contact between said work piece or one of said reference surfaces respectively and said cutting edge during rapid relative motion therebetween and being adapted to convert said sensed vibrations to electrical signals; circuit means coupling said transducer means to said numerical control means adapted to discriminate against spurious signals on the basis of predetermined time domain characteristics, said circuit means being effective to select signals generated by said transducer means in response to vibrations transmitted as a consequence of an actual probing contact between said cutting edge and a surface; whereby the position of said cutting edge may be calibrated in at least said first and second axes through probing contacts with said reference surfaces during rapid relative motion therebetween and the precision of machining said work piece surface in accordance with said machining program is enhanced by means of modifying compensating offsets determined at least in part by said calibration.

Claim 3

Apparatus as claimed in claim 1, characterised in that said machining apparatus comprises a lathe including a rotatable spindle (16), a saddle (18). and a cross slide (24) mounted on said saddle; said tool holding means (26, 30) being mounted on said cross slide; said means for providing rapid relative motion including means for rotating said work piece holding means with said spindle at least for in a forward direction; said reference surfaces being adapted to rotate jointly with said work piece holding means; at least one of said reference surfaces comprising a cylindrical surface (42 or 170) coaxial with said spindle (16) and positioned at a calibrated radial distance from said origin; at least one other reference surface comprising a planar surface (44, 174) normal to the spindle axis (14) and positioned at a calibrated axial distance with respect to said origin; said means for positioning said respective holding means relative to each other comprising means for moving said saddle and said cross slide in said first and second axes respectively relative to said work piece holding means; and said transducer means being mounted to sense vibrations transmitted through said tool when said cutting edge makes probing contact with a surface.

Claim 4

Apparatus as claimed in claim 2, characterised in that said transducer means (37) is mounted on said tool holding means.

Claim 5

Apparatus as claimed in claim 3, characterised in that said position reference surfaces (42, 44) constitute surfaces of said work piece holding means.

Claim 6

Apparatus as claimed in claim 3, characterised in that said position reference means comprises a separate structure (168) mounted on said work piece holding means and adapted to move in unison therewith.

Claim 7

Apparatus as claimed in any preceding claim, characterised in that said machining apparatus comprises a milling machine; said means for positioning said respective holding means relative to each other comprising means for moving said work piece holding means at least in said first and second axes and means for moving said tool holding means at least in a third axis at an angle to the plane defined by said first and second axes; said position reference means being mounted on said work piece holding means and including at least first, second and third reference surfaces each normal to one of said axes; said position feedback means including means for indicating the position of said work piece holding means at least in said first and second axis respectively, and means for indicating the position of said tool holding means at least in said third axis; said means for providing rapid relative motion including means for rotating said tool around an axis of rotation parallel to said plane adapted to operate at least during said removal of material from said work piece; said transducer means being mounted to sense vibrations transmitted through said work piece when said cutting edge makes contact with the latter.

Claim 8

Apparatus as claimed in any of claims 1 to 5, characterised in that said means for providing rapid relative motion between said cutting edge and said work piece comprises: means for providing substantially continuous motion of one of said work piece or said cutting edge respectively relative to the other during contact therebetween adapted to remove material from said work piece; means for inducing reciprocating motion in one of said work piece or said cutting edge respectively adapted to produce vibrations through intermittent contact therebetween substantially without removing material from said work piece; and means for disabling said continuous motion during the occurrence of said reciprocating motion.

Claim 9

Apparatus as claimed in claim 2, characterised in that said means for providing rapid relative motion further includes means for jointly inducing reciprocating motion in said work piece holding means and said reference surfaces while the rotation of said work piece holding means is halted.

Claim 10

Apparatus as claimed in claim 6, characterised in that said means for providing rapid relative motion further includes means for inducing reciprocating motion in said tool holding means while positioning of said work piece holding means in said first and second axes is halted.

Claim 11

Apparatus as claimed in claim 1, characterised in that said circuit means comprises: means for successively providing a count window; an amplifier/filter circuit (104) coupled to the output of said transducer means and including amplifier means and band-pass filter means; at least a counts channel (151) and an events channel (157) coupled to said amplifier/filter circuit; said channels including first voltage comparator means (148) adapted to convert each positive-going analog signal excursion coupled thereto from said amplifier/filter circuit to a corresponding digital pulse if said signal excursion exceeds a predetermined first low-level threshold; said counts channel further including second voltage comparator means (158) adapted to pass all vibration-originated digital pulses in excess of a predetermined second low-level threshold; counter means (164) coupled to said comparator means and adapted to provide a total count of said last-recited pulses passed in said counts channel during one count window; means (118) for generating a counts channel success signal in the form of a logic level shift when said last-recited total count exceeds a first predetermined digital count threshold, the duration of said logic level shift extending from the time said count threshold is exceeded to the end of said count window; said events channel further including third voltage comparator means (60) for passing only vibration-originated digital pulses in excess of a predetermined high-level threshold; event clock means (116) coupled to said third comparator means, said event clock means being adapted to provide clock frequency pulses throughout the duration of each digital pulse received from said third comparator means if such received pulse falls within said count window; means (123) for providing a count of said event clock pulses generated during said count window; means (122) for generating a digital pulse representative of a significant event whenever said clock pulse count exceeds a second predetermined digital count threshold during said count window; means (170) for providing a count of said significant event pulses generated during said count window; means (125) for providing an events channel success signal in the form of a logic level shift whenever said significant event pulse count exceeds a third predetermined digital count threshold, the duration of said last-recited logic level shift extending from the time said third count threshold is exceeded to the end of said count window; and means (132) responsive to the concurrence of success signals from said respective channels to provide a tool touch digital signal indicative of said actual probing contact.

Claim 12

In a numerical control system of the type in which a machine tool including work piece holding means for holding a work piece, tool holding means for holding a tool, a reference surface at a predetermined location with respect to a home position of the tool holding means, positioning motors for effecting relative motion between the tool and work piece, apparatus for providing a continuous indication of actual tool displacement from the home position and apparatus coupled to the tool for providing a tool touch signal when the tool contacts the reference surface or work piece is operative in a closed loop mode and in response to a stored program, a method of numerical control operation comprising the steps of: a) providing, in response to the stored program, first move command signals to the positioning motors to cause the tool to move from the home position into contact with the reference surface whereby a tool touch signal is generated; b) sensing the indicated displacement of the tool from the home position when the tool touch signal is generated; c) comparing the indicated tool displacement to a programmed displacement obtained from the stored program; d) computing a deviation between the indicated tool displacement and the programmed displacement; e) establishing a first tool offset value based upon the computed deviation to compensate for such deviation; f) providing, in response to the stored program, a plurality of control signals to effect machining of the work piece based upon the first tool , offset value; g) terminating the control signals prior to finish machining of the work piece; h) providing second move command signals to effect movement of the tool into contact with the reference surface; i) sensing again the indicated tool displacement from the home position with the reference surface; j) recomputing the deviation between the again sensed tool displacement and the programmed tool displacement; k) modifying the first tool offset value in accordance with the recomputed deviation; 1) providing third move command signals to cause the tool to move into contact with the work piece; m) sensing tool contact with the work piece; n) comparing the sensed tool displacement upon contact with the work piece to the programmed tool displacement; o) computing the remaining machining necessary to finish the work piece based upon said last mentioned comparison and the actual desired dimensions of the work piece; and p) providing a plurality of control signals to effect finish machining of the work piece.

Claim 13

In a numerical control system of the type in which a machine tool including work piece holding means for holding a work piece, tool holding means for holding a tool, a reference surface at a predetermined location with respect to a home position of the tool holding means, positioning motors for effecting relative motion between the tool and work piece, apparatus for providing a continuous indication of actual tool displacement from the home position and apparatus coupled to the tool for providing a tool touch signal when the tool contacts the reference surface or work piece is operative in a closed loop mode and in response to a stored program, a method of numerical control operation comprising the steps of: a) providing, in response to the stored program, first move command signals to the positioning motors to cause the tool to move from the home position into contact with the reference surface whereby a tool touch signal is generated; b) sensing the indicated displacement of the tool from the home position when the tool touch signal is generated; c) comparing the indicated tool displacement to a programmed displacement obtained from the stored program; d) computing a deviation between the indicated tool displacement and the programmed displacement; e) establishing a first tool offset value based upon the computed deviation to compensate for such deviation; f) providing, in response to the stored program, a plurality of control signals to effect machining of the work piece based upon the first tool , offset value; g) terminating the control signals prior to finish machining of the work piece; h) providing second move command signals to effect movement of the tool into contact with the reference surface; i) sensing again the indicated tool displacement from the home position with the reference surface; j) recomputing the deviation between the again sensed tool displacement and the programmed tool displacement; k) modifying the first tool offset value in accordance with the recomputed deviation; 1) providing third move command signals to cause the tool to move into contact with the work piece; m) sensing tool contact with the work piece; n) comparing the sensed tool displacement upon contact with the work piece to the programmed tool displacement; o) computing the remaining machining necessary to finish the work piece based upon said last mentioned comparison and the actual desired dimensions of the work piece; and p) providing a plurality of control signals to effect finish machining of the work piece.

Claim 14

A method as claimed in claim 11, wherein the steps of providing first, second and third move command signals to effect movement of the tool to a surface each includes the substeps of: generating a feedhold signal in response to the tool touch signal to terminate said first move command signal; generating a negative error signal in response to said feedhold signal; applying said negative error signal to cause the positioning motors to rapidly halt tool advancement.

Claim 15

A method as claimed in claim 12, wherein said negative error signal is of sufficient magnitude to effect a rapid reversal of the positioning motors to cause the tool to move a short distance away from the contacted surface or work piece.