Processing apparatus for guiding workpiece using bellows with floating means

A processing apparatus including an expansible and contractable bellows unit mounted on a workpiece holding unit for covering an area of movement of the workpiece holding unit in a feeding direction and a guide unit for guiding the expansion and contraction of the bellows unit in the feeding direction. The guide unit has an opening for allowing the movement of the workpiece holding unit in the feeding direction and a gutter extending in the feeding direction adjacent to the opening. The bellows unit has a bellows member having one end connected to the workpiece holding unit and the other end connected to the guide unit. The processing apparatus further includes a floating unit for floating the bellows member along the gutter of the guide unit in a noncontact manner.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to a processing apparatus such as a cutting apparatus for cutting a workpiece such as a semiconductor wafer and a laser processing apparatus for performing laser processing to such a workpiece.

Description of the Related Art

In a semiconductor device fabrication process, a plurality of crossing division lines are formed on the front side of a substantially disk-shaped semiconductor wafer to thereby define a plurality of separate regions where a plurality of semiconductor devices such as ICs and LSIs are respectively formed. The semiconductor wafer thus having the plural semiconductor devices is cut along the division lines to thereby divide the regions where the semiconductor devices are formed from each other, thus manufacturing the individual semiconductor devices as chips. Further, a gallium nitride compound semiconductor or the like is formed on the front side of a sapphire substrate to obtain an optical device wafer. The optical device wafer is also cut along the division lines to thereby divide the regions where a plurality of optical devices such as light emitting diodes and laser diodes are formed from each other, thus manufacturing the individual optical devices as chips.

Cutting of such a wafer along the division lines is usually performed by using a cutting apparatus or a laser processing apparatus. The cutting apparatus includes workpiece holding means for holding a workpiece, cutting means for cutting the workpiece held by the workpiece holding means as supplying a processing water to the workpiece, feeding means for moving the workpiece holding means in a feeding direction, bellows means provided on the opposite sides of the workpiece holding means in the feeding direction so as to cover the feeding means and adapted to be moved together with the workpiece holding means in the feeding direction, the bellows means being expansible and contractable in the feeding direction, and guide means for guiding the movement of the bellows means in the feeding direction (see Japanese Patent Laid-open No. 2002-66866, for example).

Further, the laser processing apparatus includes workpiece holding means for holding a workpiece, laser beam applying means for applying a laser beam to the workpiece held by the workpiece holding means, feeding means for moving the workpiece holding means in a feeding direction, bellows means provided on the opposite sides of the workpiece holding means in the feeding direction so as to cover the feeding means and adapted to be moved together with the workpiece holding means in the feeding direction, the bellows means being expansible and contractable in the feeding direction, and guide means for guiding the movement of the bellows means in the feeding direction (see Japanese Patent Laid-open No. 2007-201178, for example).

In each case, the bellows means is provided with a plurality of support members arranged at several positions in the feeding direction for preventing the slack of the bellows means in such a manner as to slide in contact with the guide means. Each support member is formed of a low-friction resin such as Teflon (registered trademark) and nylon or provided by a bearing or the like, so as to reduce the friction between each support member and the guide means.

SUMMARY OF THE INVENTION

However, when the workpiece holding means holding the workpiece is moved in the feeding direction at a high speed in order to improve the productivity, there arises a problem such that each support member provided on the bellows means cannot follow the high feed speed of the workpiece holding means, causing damage to each support member.

It is therefore an object of the present invention to provide a processing apparatus which can prevent the slack of the expansible and contractable bellows means mounted on the workpiece holding means so as to cover the feeding means and adapted to be moved together with the workpiece holding means in the feeding direction, wherein even when the workpiece holding means is moved in the feeding direction at a high speed, the support members for supporting the bellows means can follow the movement of the workpiece holding means, thereby preventing damage to the support members.

In accordance with an aspect of the present invention, there is provided a processing apparatus including workpiece holding means for holding a workpiece; processing means for processing the workpiece held by the workpiece holding means; feeding means for moving the workpiece holding means in a feeding direction; bellows means mounted on the workpiece holding means for covering an area of movement of the workpiece holding means in the feeding direction, the bellows means being expansible and contractable in the feeding direction; and guide means for guiding the expansion and contraction of the bellows means in the feeding direction; the guide means having an opening for allowing the movement of the workpiece holding means in the feeding direction, a gutter extending in the feeding direction adjacent to the opening, and an end wall provided at an end of the opening in the feeding direction; the bellows means having a first connecting member connected to the workpiece holding means, a second connecting member connected to the end wall of the guide means, and a bellows member extending between the first connecting member and the second connecting member; the processing apparatus further including floating means for floating the bellows member along the gutter of the guide means in a noncontact manner.

Preferably, the floating means includes a magnet rail extending along the gutter in the feeding direction and a floating magnet provided on the bellows member so as to be opposed to the magnet rail; the magnet rail being composed of a plurality of magnets juxtaposed in the feeding direction, the plurality of magnets being oriented so that their N poles or S poles are opposed to the bellows member; the floating magnet being opposed to the magnets of the magnet rail so that the poles having the same polarity face each other.

Alternatively, the floating means includes an air discharge rail extending along the gutter in the feeding direction and a slider provided on the bellows member so as to be opposed to the air discharge rail; the air discharge rail having a plurality of fine holes for discharging jets of air toward the slider.

As described above, the processing apparatus according to the present invention includes the expansible and contractable bellows means mounted on the workpiece holding means for covering the area of movement of the workpiece holding means in the feeding direction and the guide means for guiding the expansion and contraction of the bellows means in the feeding direction. The guide means includes the opening for allowing the movement of the workpiece holding means in the feeding direction, the gutter extending in the feeding direction adjacent to the opening, and the end wall provided at an end of the opening in the feeding direction. The bellows means includes the first connecting member connected to the workpiece holding means, the second connecting member connected to the end wall of the guide means, and the bellows member extending between the first connecting member and the second connecting member. The processing apparatus further includes the floating means for floating the bellows member along the gutter of the guide means in a noncontact manner. Accordingly, no friction is generated between the bellows means and the guide means, so that even when the feed speed of the workpiece holding means is high, the bellows means can follow the movement of the workpiece holding means, thereby improving the productivity.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A preferred embodiment of the processing apparatus according to the present invention will now be described in detail with reference to the attached drawings.FIG. 1is a perspective view of a cutting apparatus according to this preferred embodiment. The cutting apparatus shown inFIG. 1includes a substantially boxlike base housing1. As shown inFIG. 2, there are provided in the base housing1a stationary base2, a chuck table mechanism3for holding a workpiece, the chuck table mechanism3being provided on the stationary base2so as to be movable in a feeding direction (X direction) shown by an arrow X, a spindle supporting mechanism4provided on the stationary base2so as to be movable in an indexing direction (Y direction) shown by an arrow Y perpendicular to the X direction, and a spindle unit5as cutting means provided on the spindle supporting mechanism4so as to be movable in a cutting direction (Z direction) shown by an arrow Z perpendicular to both the X direction and the Y direction.

The chuck table mechanism3includes two guide rails31and32provided on the stationary base2so as to extend parallel to each other in the X direction, a chuck table33as workpiece holding means supported to the guide rails31and32so as to be movable in the X direction, and a chuck table supporting mechanism34for supporting the chuck table33. The chuck table supporting mechanism34includes a chuck table supporting base341provided on the guide rails31and32so as to be movable in the X direction, a cylindrical member342mounted on the chuck table supporting base341, and a waterproof cover343mounted on the upper end of the cylindrical member342and positioned below the chuck table33by a predetermined level. The chuck table33is mounted on the upper end of a rotating shaft (not shown) provided in the cylindrical member342. The chuck table33has an upper surface for holding a workpiece such as a disk-shaped semiconductor wafer under suction by operating suction means (not shown).

The chuck table mechanism3further includes feeding means35for moving the chuck table33in the X direction along the guide rails31and32. The feeding means35includes an externally threaded rod351extending parallel to the guide rails31and32so as to be interposed therebetween and a servo motor352as a drive source for rotationally driving the externally threaded rod351. The externally threaded rod351is rotatably supported at one end thereof to a bearing block353fixed to the stationary base2and is connected at the other end through a reduction gear (not shown) to the output shaft of the servo motor352so as to receive the torque thereof. The externally threaded rod351is engaged with a tapped through hole formed in an internally threaded block (not shown) projecting from the lower surface of a central portion of the chuck table supporting base341supporting the chuck table33. Accordingly, the chuck table33is moved in the X direction along the guide rails31and32by operating the servo motor352to normally or reversely rotate the externally threaded rod351.

Referring toFIGS. 1 and 2, the cutting apparatus according to this preferred embodiment further includes bellows means6for covering an area of movement of the chuck table33as workpiece holding means in the X direction and guide means7for guiding the expansion and contraction of the bellows means6in the X direction. The bellows means6includes first bellows means61and second bellows means62provided on the opposite sides of the chuck table33in the X direction, thereby covering the feeding means35and its associated parts.

The first bellows means61is composed of a bellows member611, a first connecting member612mounted on one end of the bellows member611, and a second connecting member613mounted on the other end of the bellows member611. The bellows member611is formed from a foldable sheet member like a cloth such that a plurality of ridges and grooves are alternately formed so as to be expansible and contractable. Each of the first and second connecting members612and613may be formed from a metal plate. The first connecting member612mounted on one end of the bellows member611of the first bellows means61is connected to the waterproof cover343adapted to move with the chuck table33. The second connecting member613mounted on the other end of the bellows member611is connected to an end wall of the guide means7, which will be hereinafter described.

As similar to the first bellows means61, the second bellows means62is composed of a bellows member621, a first connecting member622mounted on one end of the bellows member621, and a second connecting member623mounted on the other end of the bellows member621. The bellows member621is formed from a foldable sheet member like a cloth such that a plurality of ridges and grooves are alternately formed so as to be expansible and contractable. Each of the first and second connecting members622and623may be formed from a metal plate. The first connecting member622mounted on one end of the bellows member621is connected to the waterproof cover343adapted to move with the chuck table33. The second connecting member623mounted on the other end of the bellows member621is connected to the other end wall of the guide means7, which will be hereinafter described.

The guide means7for guiding the expansion and contraction of the first bellows means61and the second bellows means62in the X direction includes an opening70for allowing the movement of the chuck table33in the X direction, a first gutter71extending in the X direction adjacent to one side of the opening70, a second gutter72extending in the X direction adjacent to the other side of the opening70, a first end wall73provided at one end of the opening70in the X direction, and a second end wall74provided at the other end of the opening70in the X direction. A first communication gutter75is provided outside the first end wall73, and a second communication gutter76is provided outside the second end wall74. The first and second gutters71and72are in communication with each other through the first and second communication gutters75and76. A drain hose77is connected to the second communication gutter76. The second connecting member613of the first bellows means61is connected to the first end wall73of the guide means7by means of fastening bolts (not shown). Similarly, the second connecting member623of the second bellows means62is connected to the second end wall74of the guide means7by means of fastening bolts (not shown).

The cutting apparatus according to this preferred embodiment further includes floating means for floating the bellows member611of the first bellows means61and the bellows member621of the second bellows means62along the first gutter71and the second gutter72of the guide means7in a noncontact manner. A first preferred embodiment of this floating means will now be described with reference toFIGS. 3 and 4. InFIGS. 3 and 4, reference numeral8denotes floating means as the first preferred embodiment. The floating means8includes a first magnet rail81extending along the first gutter71in the X direction, a second magnet rail82extending along the second gutter72in the X direction, a first floating magnet831provided on the lower surface of each of the bellows member611of the first bellows means61and the bellows member621of the second bellows means62so as to be opposed to the first magnet rail81, and a second floating magnet832provided on the lower surface of each of the bellows member611of the first bellows means61and the bellows member621of the second bellows means62so as to be opposed to the second magnet rail82. The first magnet rail81is composed of a plurality of magnets811juxtaposed in the X direction, and these plural magnets811are oriented so that their N poles or S poles are opposed to the bellows member611of the first bellows means61and the bellows member621of the second bellows means62. Similarly, the second magnet rail82is composed of a plurality of magnets821juxtaposed in the X direction, and these plural magnets821are oriented so that their N poles or S poles are opposed to the bellows member611of the first bellows means61and the bellows member621of the second bellows means62.

In this preferred embodiment, the upper surface of each magnet811of the first magnet rail81(the surface opposed to the bellows member611of the first bellows means61and the bellows member621of the second bellows means62) is set as an N pole. Similarly, the upper surface of each magnet821of the second magnet rail82(the surface opposed to the bellows member611of the first bellows means61and the bellows member621of the second bellows means62) is set as an N pole. On the other hand, the lower surface of the first floating magnet831(the surface opposed to the plural magnets811of the first magnet rail81) is set as the same pole (N pole) as that of the upper surface of each magnet811. Similarly, the lower surface of the second floating magnet832(the surface opposed to the plural magnets821of the second magnet rail82) is set as the same pole (N pole) as that of the upper surface of each magnet821.

Thus, the plural magnets811of the first magnet rail81are opposed to the first floating magnet831so that the poles having the same polarity face each other. Similarly, the plural magnets821of the second magnet rail82are opposed to the second floating magnet832so that the poles having the same polarity face each other. Accordingly, the first floating magnet831floats over the plural magnets811of the first magnet rail81, and the second floating magnet832floats over the plural magnets821of the second magnet rail82. As a result, the bellows member611of the first bellows means61and the bellows member621of the second bellows means62are floated over the first and second magnet rails81and82in a noncontact manner.

The first floating magnet831may be provided by a single magnet or a plurality of magnets arranged in the X direction on the lower surface of each of the bellows members611and621. Similarly, the second floating magnet832may be provided by a single magnet or a plurality of magnets arranged in the X direction on the lower surface of each of the bellows members611and621. The first and second floating magnets831and832provided on the lower surface of each of the bellows members611and621function as support members for preventing the slack of the bellows members611and621.

There will now be described with reference toFIGS. 5 and 6a second preferred embodiment of the floating means for floating the bellows member611of the first bellows means61and the bellows member621of the second bellows means62along the first gutter71and the second gutter72of the guide means7in a noncontact manner. InFIGS. 5 and 6, reference numeral80denotes floating means as the second preferred embodiment. The floating means80includes a first air discharge rail85extending along the first gutter71in the X direction, a second air discharge rail86extending along the second gutter72in the X direction, a first slider871provided on the lower surface of each of the bellows member611of the first bellows means61and the bellows member621of the second bellows means62so as to be opposed to the first air discharge rail85, and a second slider872provided on the lower surface of each of the bellows member611of the first bellows means61and the bellows member621of the second bellows means62so as to be opposed to the second air discharge rail86.

The first air discharge rail85is a hollow member having a rectangular cross section, and the upper surface thereof (the surface opposed to the bellows member611of the first bellows means61and the bellows member621of the second bellows means62) is formed with a plurality of fine holes851for discharging jets of air. Similarly, the second air discharge rail86is a hollow member having a rectangular cross section, and the upper surface thereof (the surface opposed to the bellows member611of the first bellows means61and the bellows member621of the second bellows means62) is formed with a plurality of fine holes861for discharging jets of air. The first air discharge rail85and the second air discharge rail86are connected to air supplying means (not shown). On the other hand, the first slider871and the second slider872are formed of a low-friction resin in this preferred embodiment. In operation, the air supplying means (not shown) is operated to discharge jets of air from the plural fine holes851of the first air discharge rail85and the plural fine holes861of the second air discharge rail86. Accordingly, the first slider871floats over the first air discharge rail85, and the second slider872floats over the second air discharge rail86. As a result, the bellows member611of the first bellows means61and the bellows member621of the second bellows means62are floated over the first and second air discharge rails85and86in a noncontact manner.

The first slider871may be provided by a single slider or a plurality of sliders arranged in the X direction on the lower surface of each of the bellows members611and621. Similarly, the second slider872may be provided by a single slider or a plurality of sliders arranged in the X direction on the lower surface of each of the bellows members611and621. The first and second sliders871and872provided on the lower surface of each of the bellows members611and621function as support members for preventing the slack of the bellows members611and621.

Referring back toFIG. 2, the spindle supporting mechanism4includes two guide rails41and42provided on the stationary base2so as to extend parallel to each other in the Y direction and a movable support base43provided on the guide rails41and42so as to be movable in the Y direction. The movable support base43is composed of a horizontal portion431slidably supported to the guide rails41and42and a vertical portion432extending vertically upward from the upper surface of the horizontal portion431. Further, two guide rails432aare provided on one side surface of the vertical portion432so as to extend parallel to each other in the Z direction. The spindle supporting mechanism4further includes indexing means44for moving the movable support base43in the Y direction along the guide rails41and42.

The indexing means44includes an externally threaded rod441extending parallel to the guide rails41and42so as to be interposed therebetween and a pulse motor442as a drive source for rotationally driving the externally threaded rod441. The externally threaded rod441is rotatably supported at one end thereof to a bearing block (not shown) fixed to the stationary base2and is connected at the other end through a reduction gear (not shown) to the output shaft of the pulse motor442so as to receive the torque thereof. The externally threaded rod441is engaged with a tapped through hole formed in an internally threaded block (not shown) projecting from the lower surface of the horizontal portion431at a central portion thereof. Accordingly, the movable support base43is moved in the Y direction along the guide rails41and42by operating the pulse motor442to normally or reversely rotate the externally threaded rod441.

The spindle unit5includes a unit holder51, a spindle housing52mounted to the unit holder51, and a rotating spindle53rotatably supported to the spindle housing52. The unit holder51is formed with two guided grooves51afor slidably engaging the two guide rails432aprovided on the vertical portion432of the movable support base43. Accordingly, the unit holder51is supported to the movable support base43so as to be movable in the Z direction by the slidable engagement of the guided grooves51awith the guide rails432a. The rotating spindle53projects from the front end of the spindle housing52, and a cutting blade54is mounted on the front end portion of the rotating spindle53. The rotating spindle53thus having the cutting blade54is rotationally driven by a servo motor55as a drive source. A pair of cutting water nozzles57(one of which being shown inFIG. 2) for supplying a cutting water as a processing water to the workpiece in cutting are provided on both sides of the cutting blade54.

The spindle unit5further includes moving means58for moving the unit holder51along the guide rails432ain the Z direction. Like the feeding means35and the indexing means44mentioned above, this moving means58includes an externally threaded rod (not shown) extending parallel to the guide rails432aso as to be interposed therebetween and a pulse motor582as a drive source for rotationally driving this externally threaded rod. Accordingly, the whole of the unit holder51, the spindle housing52, and the rotating spindle53is moved in the Z direction along the guide rails432aby operating the pulse motor582to normally or reversely rotate this externally threaded rod.

The cutting apparatus according to this preferred embodiment further includes imaging means50provided at the front end portion of the spindle housing52of the spindle unit5. The imaging means50includes a microscope, CCD camera, etc. for imaging a subject area such as division lines formed on the workpiece such as a semiconductor wafer. An image signal output from the imaging means50is transmitted to control means (not shown).

Referring back toFIG. 1, the cutting apparatus according to this preferred embodiment further includes a cassette11for storing a semiconductor wafer W as a workpiece. The semiconductor wafer W is stored in the cassette11in the condition where the wafer W is attached to a protective tape T supported to an annular frame F. The cassette11is placed on a cassette table12vertically movable by elevating means (not shown). The cutting apparatus shown inFIG. 1further includes a temporary setting table13for temporarily setting the semiconductor wafer W thereon, handling means14for handling the semiconductor wafer W to take it from the cassette11to the temporary setting table13before cutting and also to take it from the temporary setting table13to the cassette11after cutting, first transfer means15for transferring the semiconductor wafer W from the temporary setting table13to the chuck table33, cleaning means16for cleaning the semiconductor wafer W after cutting, and second transfer means17for transferring the semiconductor wafer W from the chuck table33to the cleaning means16after cutting. The first transfer means15also functions to transfer the semiconductor wafer W from the cleaning means16to the temporary setting table13after cleaning. The cutting apparatus shown inFIG. 1further includes display means18for displaying an image or the like obtained by the imaging means50.

The operation of the cutting apparatus described above will now be described. The cassette table12is vertically moved by the elevating means (not shown) to thereby vertically move the cassette11and set the semiconductor wafer W stored at a predetermined position in the cassette11to a given exit position (the semiconductor wafer W supported through the protective tape T to the annular frame F will be hereinafter referred to simply as the semiconductor wafer W). Thereafter, the handling means14is operated to take the semiconductor wafer W set at the exit position from the cassette11to the temporary setting table13. Thereafter, the first transfer means15is pivotally operated to transfer the semiconductor wafer W from the temporary setting table13to the chuck table33set at a standby position shown inFIG. 1. In this condition, the suction means (not shown) is operated to hold the semiconductor wafer W on the chuck table33under suction. The chuck table33thus holding the semiconductor wafer W is next moved in the X direction from the standby position to an alignment position directly below the imaging means50. At this alignment position, the imaging means50is operated to detect a plurality of division lines formed on the semiconductor wafer W. The chuck table33is next moved in the X direction from the alignment position to a working position directly below the cutting blade54. Further, the spindle unit5is moved in the Y direction as the indexing direction to thereby precisely align the cutting blade54with a predetermined one of the division lines.

Thereafter, the chuck table33holding the semiconductor wafer W is moved in the X direction as the feeding direction (the direction perpendicular to the axis of rotation of the cutting blade54), thereby cutting the semiconductor wafer W along the predetermined division line by the cutting blade54. More specifically, the cutting blade54is moved in the Z direction as the cutting direction by a predetermined amount in the condition where it is aligned with the predetermined division line in the Y direction. Further, the cutting blade54is rotated at a high speed (e.g., 30,000 rpm) by the servo motor55. In this condition, the chuck table33holding the semiconductor wafer W is moved in the X direction at a predetermined feed speed (e.g., 50 mm/sec). As a result, the semiconductor wafer W held on the chuck table33is cut along the predetermined division line by the cutting blade54(cutting step). At this time, a cutting water as a processing water is supplied from the cutting water nozzles57to a subject area of the semiconductor wafer W to be cut. The cutting water supplied to the subject area is allowed to flow from the waterproof cover343and the first and second bellows means61and62constituting the bellows means6down to the first and second gutters71and72constituting the guide means7. Thereafter, the cutting water is drained through the drain hose77connected to the second communication gutter76.

In the cutting step mentioned above, the waterproof cover343is moved together with the chuck table33, so that the first bellows means61and the second bellows means62provided on the opposite sides of the waterproof cover343are expanded and contracted in the X direction. In this expansion and contraction of the first and second bellows means61and62, the floating means8shown inFIGS. 3 and 4is operated in the following manner. The first and second floating magnets831and832functioning as the support members for preventing the slack of the bellows member611of the first bellows means61and the bellows member621of the second bellows means62are allowed to move along the first and second magnet rails81and82, respectively. As described above, the plural magnets811of the first magnet rail81and the plural magnets821of the second magnet rail82are opposed to the first floating magnet831and the second floating magnet832, respectively, in the condition where the same poles face each other. Accordingly, the first floating magnet831floats over the plural magnets811of the first magnet rail81in a noncontact manner, and the second floating magnet832also floats over the plural magnets821of the second magnet rail82in a noncontact manner. Accordingly, no friction is generated between the first floating magnet831and the first magnet rail81and between the second floating magnet832and the second magnet rail82. As a result, even when the feed speed of the chuck table33is high, the first and second floating magnets831and832as the support members for the bellows members611and621can follow the movement of the chuck table33, thereby improving the productivity.

Further, in the expansion and contraction of the first and second bellows means61and62in the cutting step mentioned above, the floating means80shown inFIGS. 5 and 6is operated in the following manner. The first and second sliders871and872functioning as the support members for preventing the slack of the bellows member611of the first bellows means61and the bellows member621of the second bellows means62are allowed to move along the first and second air discharge rails85and86, respectively. At this time, air is discharged from the plural fine holes851of the first air discharge rail85and the plural fine holes861of the second air discharge rail86. Accordingly, the first slider871floats over the first air discharge rail85in a noncontact manner, and the second slider872floats over the second air discharge rail86in a noncontact manner. Accordingly, no friction is generated between the first slider871and the first air discharge rail85and between the second slider872and the second air discharge rail86. As a result, even when the feed speed of the chuck table33is high, the first and second sliders871and872as the support members for the bellows members611and621can follow the movement of the chuck table33, thereby improving the productivity.

Having thus described specific preferred embodiments of the present invention, it should be noted that the present invention is not limited to the above preferred embodiments, but various modifications may be made within the scope of the present invention. For example, while the present invention is applied to a cutting apparatus in the above preferred embodiment, the present invention may be applied also to a laser processing apparatus such that the workpiece holding means for holding a workpiece is moved in the feeding direction or a grinding apparatus such that the bellows means is mounted on the workpiece holding means for holding a workpiece.