Patent Publication Number: US-2012041585-A1

Title: Adjustable locator for a workpiece fixture

Description:
This invention relates to an adjustable locator for a workpiece fixture and is particularly, although not exclusively, concerned with a locator for a workpiece fixture on which the workpiece, such as a casting, can be supported in an optimum orientation for a processing step, such as a machining operation. 
     Fixtures are commonly used in manufacturing operations, particularly automated manufacturing operations, and serve to locate a workpiece correctly in a given orientation with respect to a cutting tool or other components to be used in the manufacturing operation. 
     A fixture typically comprises a main body provided with at least one locator which contacts the workpiece to establish and maintain the position of the workpiece on the fixture. The fixture also typically comprises means for securing the main body to the bed of a machining centre, and clamping means for retaining the workpiece on the fixture. 
     Fixture locators may be adjustable. For example, it is known to employ fine-threaded screws as used in micrometer heads. Rotation of a thimble causes advancement or retraction of a locator element of the locator by way of the fine-threaded screw. Such devices are awkward to operate when installed on a fixture, since the thimble may not be easily accessible. 
     According to a first aspect of the present invention there is provided an adjustable locator for a workpiece fixture, the locator comprising a support element, a locator element which defines a datum point for contact with a workpiece, and an adjustment mechanism for adjusting the position of the locator element with respect to the support element, the adjustment mechanism comprising a ramp element, which is displaceable with respect to the support element and is lockable against displacement in at least one direction, and a plunger which engages the ramp element at a ramp surface whereby displacement of the ramp element is transmitted to the locator element by the plunger. 
     The ramp element and the plunger may cooperate with each other in such a manner that displacement of the ramp element causes displacement of the plunger in a direction transverse to the direction of displacement of the ramp element. The mechanical advantage between the ramp element and the plunger may be greater than one, for example it may be greater than five. 
     The ramp element may be lockable with respect to the support body by a ratchet mechanism which may comprise ratchet teeth on the ramp element and a pawl on the support body. 
     The ramp surface may be provided on the ramp element. The plunger may be resiliently biased into contact with the ramp surface. 
     The locator element may be provided on the plunger. In an alternative embodiment, the locator element may be provided on a locator element carrier which is coupled to the plunger for displacement in a direction transverse to the displacement direction of the plunger. 
     The support element may comprise a laminar component. The support element may have guide apertures within which the ramp element and the plunger are displaceable, and the ramp element and the plunger may also comprise laminar components. The support element may be disposed between side plates which retain the plunger and the ramp element within the guide apertures. At least one of the side plates may comprise a wall component of a workpiece fixture. 
     The present invention also provides a workpiece fixture including an adjustable locator as defined above. 
     According to a second aspect of the present invention, there is provided a method of orienting a workpiece on a fixture, the method comprising: 
     i) generating a computer model of a nominal workpiece shape and orientation; 
     ii) supporting the workpiece on the fixture by means of a plurality of locators, at least one of which is adjustable; 
     iii) scanning the supported workpiece and generating a computer model of the supported workpiece shape and orientation; 
     iv) comparing the computer model of the supported workpiece shape and orientation with the computer model of the nominal workpiece shape and orientation; 
     v) determining a required displacement of the supported workpiece to achieve a best fit between the supported workpiece shape and orientation and the nominal workpiece shape and orientation; and 
     vi) adjusting the adjustable locator, or at least one of the adjustable locators, to achieve the required displacement of the supported workpiece. 
     The workpiece may comprise a metallic casting. 
     The present invention also provides a machining operation, which may be a laser machining operation, which comprises orienting a workpiece in a method in accordance with the second aspect of the invention, and subsequently performing a machining step on the supported workpiece. 
     The or each adjustable locator may be an adjustable locator in accordance with the first aspect of the present invention. 
    
    
     
       For a better understanding of the present invention, and to show how it may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which: 
         FIG. 1  shows a fixture on which a workpiece is supported; 
         FIG. 2  is an external side view of an adjustable locator of the fixture of  FIG. 1 ; 
         FIG. 3  is an internal view of the adjustable locator of  FIG. 2 ; 
         FIG. 4  corresponds to  FIG. 3  but shows an alternative adjustable locator, and 
         FIG. 5  is an edge view of the adjustable locator of  FIG. 4 . 
     
    
    
     The fixture shown in  FIG. 1  comprises a main body  2  provided with adjustable locators  4 ,  6 ,  8 ,  10  provided with respective locator elements  12 ,  14 ,  16 ,  18 . Further locators are provided on the body  2 , for example as indicated by their respective locator elements  20 ,  22 . 
     A workpiece  24  (represented only partially in  FIG. 1  for clarity) is supported on the fixture by the locator elements  12 ,  14 ,  16 ,  18 ,  20 ,  22 . 
       FIGS. 2 and 3  show a locator similar to the locator  6  in  FIG. 1 . It is, in fact, a mirror image of the locator  6 , and consequently can be regarded as the locator having the locator element  22  in  FIG. 1 . 
       FIG. 2  shows an outer side plate  26  of the locator, and so is visible from the outside of the fixture.  FIG. 3  shows a support element  28  of the locator which is sandwiched between the outer side plate  26  and an inner side plate. (not shown) which may be a separate component having the same outside profile as the outer side plate  26 , or may be constituted by a wall  30  of the main body  4 . The support element  28 , like the outer side plate  26  and the inner side plate  30  (if a separate component from the wall of the fixture), is a laminar component and may be formed, for example, by laser cutting from a metal sheet. 
     The support element  26  has two guide apertures  32 ,  34  which accommodate respectively a ramp element  36  and a plunger  38 , both of which are laser cut sheet metal components. The ramp element  36  and the plunger  38  are constrained for translational movement between parallel edges of the apertures  32 ,  34 , and are retained within the apertures  32 ,  34  by the outer side plate  26  and the inner side plate  30 . In the embodiment shown the locator element  22  is provided in the form of a ball defining a datum point  23  at its top (as viewed in  FIG. 3 ), to engage a horizontal surface of the workpiece  24  in the cases of the locator elements  12   14  and  22 . It will be appreciated, however, that the datum point may engage surfaces at other orientations. For example, the locator element  20  contacts a vertical surface of the workpiece  24 . 
     The ramp element  36  has a ramp surface  40  which cooperates with a correspondingly oblique end surface  42  of the plunger  38 . A spring  44  acts between the support element  28  and the plunger  38  to bias the plunger  38  towards the ramp surface  40 , ie downwardly as viewed in  FIG. 3 . Consequently, displacement of the ramp element  36  along the aperture  32  causes displacement of the plunger  38  in a direction transverse to that of the ramp element  36 , so adjusting the position of the locator element  22  with respect to the support element  28 . As shown in  FIG. 3 , the ramp surface  40  has a relatively shallow inclination with respect to the longitudinal direction of the aperture  38  (ie the direction of displacement of the ramp element  36 ). By way of example, the angle of inclination of the ramp surface  40  may be such as to give a mechanical advantage between the ramp element  36  and the plunger  38  of approximately 10. The mechanical advantage is defined as the ratio of the displacement of the ramp element  36  to that of the plunger  38 . The mechanical advantage must be greater than one if the movement of the plunger  38  is to be less than that of the ramp element  36  and is preferably greater than five if the locator element  22  is to be displaceable in small steps. 
     The ramp element  36  has a slot  46 , one edge of which is formed with ratchet teeth  48 . A sprung pawl  50  is pivotably supported by the outer and inner side plates  26 ,  30 . The pawl  50  has a pawl tooth engaging the ratchet teeth  48  in a manner which provides a ratchet mechanism enabling displacement of the ramp element  36  to the left, as viewed in  FIG. 3 , so as to raise the locator element  22 , while preventing reverse movement while the pawl  50  engages the ratchet teeth  48 . The ratchet mechanism  48 ,  50  thus serves to lock the ramp element  36  against displacement in one direction (to the right in  FIG. 3 ) while permitting such displacement in the opposite direction. 
     Referring to  FIG. 2 , a pawl release lever  52  is pivotably supported on the outer side plate  26 , and has a projection  54  which projects through an opening in the outer side wall  26  to engage the pawl  50 . The release lever  52  can thus be operated by hand from outside the adjustable locator to release the pawl  50  from the ratchet teeth  48 , permitting displacement of the ramp element  36  to the right as shown in  FIG. 3 , so as to lower the locator element  22 . The outer side plate  26  is provided with a slot  56  through which projects a sliding adjustment knob  58  carried by the ramp element  36 . The knob  58  can be used to displace the ramp element  36  in either direction when the pawl  50  is released by means of the pawl release lever  52 , or towards the left, as seen in  FIG. 3 , when the pawl is engaged. A scale  60  is provided on the outer side panel  26  along the slot  56 , and a calibration point  62  is marked on the ramp element  36 . Positioning of the calibration point  62  against the zero position on the scale  60  (mid-way between the ends of the scale  60 ) corresponds to a nominal initial position of the locator element  22 . 
     A locking thumb wheel  64  cooperates with the outer side plate  26  and can be turned to engage the ramp element  36  to lock the ramp element  36  and consequently the locator element  22  in a set position. 
       FIGS. 4 and 5  show a variant of the locator corresponding to the locator  10  of  FIG. 1 . As with the locator shown in  FIGS. 2 and 3 , the locator  10  comprises three laminar components  26 ,  28  and  30 , of which the components  26  and  30  comprise outer and inner side plates respectively, and the component  28  comprises a support element, shown in  FIG. 4 . As with the locator of  FIGS. 2 and 3 , the locator of  FIGS. 4 and 5  comprises a ramp element  36  and a plunger  38  guided in respective apertures  32 ,  34  in the support element  28 . The position of the ramp element  36  can be adjusted and locked in the same manner as that of the locator of  FIGS. 2 and 3 , and so will not be described in detail again. 
     In the locator of  FIGS. 4 and 5 , the locator element  18  is in the form of a hooked tip providing a datum point  76  positioned, in the embodiment of fixture shown in  FIG. 1 , to engage a vertical, or at least upwardly extending, surface of the workpiece  24 . The locator element  18  is carried by a locator element carrier  66  which is slidable in a further aperture  68  in the support element  28 . Like the support element  28 , the outer and inner support plates  26 ,  30 , the ramp element  36  and the plunger  38 , the carrier  66 , including the locator element  18 , is laser cut from sheet metal material, the locator element  18  then being bent out of the plane of the rest of the carrier  66  to stand upright in the fixture as shown in  FIG. 1 . 
     A spring  70  acts between the support element  28  and the carrier  66 , the spring being thin in the direction perpendicular to the plane of  FIG. 4 , so that it can be accommodated in the aperture  68  between the outer and inner side plates  26 ,  30 . 
     In operation of the locator shown in  FIGS. 4 and 5 , displacement of the ramp element  36  causes corresponding transverse displacement of the plunger  38  with a mechanical advantage of 10. The plunger  38  engages the carrier  66  at cooperating ramp surfaces  72 ,  74 , which are inclined at 45° to the direction of displacement of the plunger  38 . Consequently, displacement of the plunger  38  by the ramp element  36  is accompanied by displacement of the carrier  66  in a direction perpendicular to that of the plunger  38 , ie parallel to, and in the same direction, as that of the ramp element  36 . 
     The fixture shown in  FIG. 1  may be used to support an alloy workpiece in an as-cast state (or with only minor modifications after casting) for a subsequent machining operation. As an initial step, a computer model is generated of the nominal shape of the casting in a nominal orientation on the fixture. In practice, shrinkage and distortion which occur during the casting process mean that the finished casting does not conform exactly to the nominal shape. In order to compensate for the inaccuracies in the finished casting, the positioning of the casting on the fixture is adjusted. 
     The workpiece is initially placed on the fixture with the locator elements  12  to  22  in nominal initial positions. These nominal initial positions may correspond to the “zero” position on the scale  60 , but they may be varied from this position in the light of experience with the particular casting concerned. The ramp element can be displaced to the required position by releasing the pawl  50  by means of the release lever  52  and sliding the ramp element  36  in the required direction using the knob  58 . 
     The three locator elements  12 ,  14 ,  22  will support the weight of the workpiece  24 , while the locator element  20  will position the workpiece in the longitudinal direction of the fixture and the locator elements  16  and  18  will determine its angular position about the lengthwise direction. 
     The casting is then scanned by any suitable process, and the scan is used to generate a computer model of the actual shape and orientation of the casting as supported on the fixture. The computer model of the supported casting is then compared with the computer model of the nominal casting and calculations are made to determine adjustments to the locating elements  12  to  22  which will result in the supported casting being a “best fit” with the computer model representing the nominal casting. 
     The locating elements  12  to  22  are adjusted as required by reference to the scale  60 . The appropriate ramp elements  36  are displaced to move the respective locator elements to the desired positions. It will be appreciated that, with a mechanical advantage of 10, a displacement of 1 mm of the ramp element  36  will produce a displacement of 0.1 mm of the locator element  12  to  22 . Once the “best fit” position of the workpiece has been achieved, the machining operation on the casting can proceed. 
     The pitch of the ratchet teeth  48  and the slope of the ramp surface  40  may, for example, be selected in conjunction with the maximum travel of the ramp element  36  so as to enable adjustment of the datum point defined by each locator element  12  to  22  over the range of plus or minus 3 mm in steps of 0.1 mm. It will, of course, be appreciated that the characteristics of each locator can be altered by varying the pitch of the ratchet teeth  48  and/or the angle of inclination of the ramp surface  40 . 
     It will be appreciated that the wall  30  of the main body  4  of the fixture may serve as either of the outer and inner side plates  26 ,  30 , or the support element  28  of each of the locators. The three components  26 ,  28 ,  30  are secured together by bolts or screws passing through the stacked components and securing the assembly to the fixture wall. Alternatively, each locator may be a self-contained unit which is separately secured to the main body  2  of the fixture. However, integrating the locators with the main body  2  of the fixture has the advantage that the load path from the locator elements to the main body  2  is simplified, with the result that each locator is able to bear substantial loading from the workpiece while it is supported during the machining operation. Locators as described above are thus robust and compact, and can form an integral part of a fixture, rather than being a separate unit to be attached to the fixture.