WO1997012197A1

WO1997012197A1 - Universal measurement fixture

Info

Publication number: WO1997012197A1
Application number: PCT/US1996/015428
Authority: WO
Inventors: Darrel E. Brann; Roth K. Christopherson
Original assignee: The Molen Company Inc.
Priority date: 1995-09-28
Filing date: 1996-09-26
Publication date: 1997-04-03
Also published as: AU7373496A

Abstract

A universal measurement fixture for measuring a part, includes a base plate (50) having a flat upper surface in which a regular pattern of holes (52) opens. Brackets (54) are selectively attached to the base plate around the part. Each bracket has a sole plate (56) with a regular pattern of vertical holes (62) by which the bracket can be attached at desired, accurately known positions of the base plate, and has an upright wall (58) connected along one edge to the sole plate and extending upward at 90°. The upright wall has a regular pattern of horizontal holes (76) for receiving locating pins (82) and measuring devices. Elevation pins (108) are fit into the holes in the base plate for supporting the part at a reference elevation above the base plate. A reference surface (115) on each elevation pin adjacent the lower end at an accurately known distance from the top end engages a reference surface on the base plate for accurately locating the elevation pin on the base plate vertically and provides a reference elevation thereabove at the top ends of the elevation pins for support of the part at the reference elevation. Locator pins fit into the brackets for locating the part at a known distance from the brackets. A measurement device selectively is inserted in selected holes in the upright wall of a selected bracket to accurately locate the measurement device at a known position on the base plate for measuring the distance to the part positioned between the brackets.

Description

Universal Measurement Fixture

This invention pertains to accurate installation of bushings in holes provided in structures for that purpose, and more particularly to a method and apparatus for extremely precise installation of bushings in a base plate of a measurement fixture having assorted accessory elements that can be mounted on the base plate for accurately locating a part in a fixed and known position so that measurements can be taken from reference positions on the fixture to the part, which measurements can be converted to virtually any dimensions of the part that are of interest.

BACKGROUND OF THE INVENTION

Modern manufacturing has increasing come to require the use of part dimension measurements to accomplish various types of quality control systems. Statistical process control is one well know technique for tracking the dimensions of a part to anticipate quality degradation before it becomes serious, so that the causes of the problem can be identified and corrected before it causes the part to be made out of tolerance. The emerging techniques of statistical tolerancing which enable parts to be designed with tolerances more relaxed than the traditional worst case tolerance stack-up approach also require accurate part measurement to provide statistical information about the part dimensions. The old "go-no go" gauges that merely indicated whether the part dimension was within tolerance is useless for these new quality control techniques since it does not provide measurement data, so mechanisms and techniques for making actual measurements of the part dimensions are becoming more important to the operations of modern manufacturing operations.

Conventional part measurement techniques widely in current use include the coordinate measuring machine which has a probe on the end of a robot arm that is programmed to carry the probe and contact the part at designated points. The internal sensors in the arm provide a position read out when the probe makes contact with the part, and this information is processed to give the desired part dimensions.

The measurements provided by a coordinate measuring machine are usually accurate to the degree of precision needed for most applications when they are obtained from a machine that is properly calibrated and operated. However, coordinate measuring machines are large and expensive, so most manufacturing operations can afford to have only one or very few of them. As a consequence, the parts must be sent to the coordinate measuring machine and measured there by its expert operators. This usually creates a delay before the measurement data is made available, which could affect the currency of the quality control efforts for that part.

Ideally, each manufacturing unit in which parts are made would have its own measurement device so that the critical measurements could be made on the parts made in that, unit and that information could be made available to the machine operators in the unit, so corrections could be made immediately if the part dimensions began to drift toward an out of tolerance condition. Also, current information could be maintained and posted on the statistical nature of the part dimensions so the machine operators making the parts, and their supervisors, could be informed about the variations in the parts made by different operators, so corrections to the process or individual procedures of the different operators could be made on a timely basis.

This invention uses a base plate having bushings mounted accurately in vertical holes, in which hold-down devices are engaged to accurately locate and secure brackets. The part to be measured is positioned at a known location on elevation pins fastened into the bushing in the base plate, and against stop-pins mounted in other bushings in the brackets, and measurements are taken with digital indicators in the brackets. The accuracy of the fixture is dependent on the accuracy of the positioning and location and angle of the bushing axis. Thus, an urgent need exists for a method and apparatus for accurately locating and orienting the axis of a bushing in a structure such as the base plate and brackets of a universal measurement fixture for making part measurements that is inexpensive to procure and install, is quick and easy to use, and provides accurate part dimensional data for virtually any dimension of interest on the part.

SUMMARY OF THE INVENTION Accordingly, it is an object of this invention to provide an improved measurement fixture that is adaptable to enable accurate measurements to be made quickly and inexpensively of virtually any size and shape of part. Another object of this invention is to provide an improved part dimension measuring process that is fast, easy, repeatable and reproducible

These and other objects are attained in a universal measurement fixture and a process of making selected dimensional measurements on a part wherein a first datum of the part is located by contact with three fixed points set on the fixture, a second datum of the part representing a line orthogonal to the first datum is located against two fixed points set on the fixture, and a third datum of the part offset from the first and second datu s is located against a third fixed point on the fixture. This positioning locates the part in a unique position and orientation in space relative to the fixture. Hold-down devices may be used where necessary to hold the part against the fixed points on the fixture. Accessory elements such as right angle brackets fixable at selected known positions on a base plate, and bridge plats across the tops of angle brackets spanning the part, have receptacles for measurement gauges, such as a conventional digital indicator, by which measurement may be made from the known position of the accessory element to the part.

DESCRIPTION OF THE DRAWINGS

The invention and is many attendant objects and advantages will become better understood upon reading the description of the preferred embodiment in conjunction with the following drawings, wherein: Fig. 1 is a perspective view from above of a universal measurement fixture in accordance with this invention;

Fig. 2 is an enlarged perspective view of the universal measurement fixture shown in Fig. 1 with the part and bridge removed for clarity of illustration;

Fig. 3 is a sectional elevation on a vertical plane through the axis of one of the receptacles in the base plate, showing a bracket fastened to the base plate with a hold-down bolt; Fig. 4 is an sectional elevation like Fig. 3, but showing a cam-action hold down device instead of a bolt;

Fig. 5 is a perspective view of the universal measurement fixture shown in Fig. 2, showing a hypothetical part located against stand-off pins and digital indicators in the brackets;

Fig. 6 is a sectional elevation of a stop pin in a hole in the upstanding flange of a bracket;

Fig. 7 is an enlarged perspective view on a stop pin shown in Fig. 6; Fig. 8 is a perspective view of the universal measurement fixture shown in Fig. 5, showing a bridge element spanning the tops of two brackets, and a digital indicator taking a vertical measurement to the part; Fig. 9 is a perspective view of the universal measurement fixture shown in Fig.l, but with bridges supported on support blocks fastened to the brackets; Fig. 10 is a perspective view of an elevation pin for supporting the part at the desired height on the base plate;

Fig. 11 is a perspective view of a sweep fixture and two locator devices for positioning a part on the universal measurement fixture shown in Fig. 1;

Fig. 12 is an enlarged perspective view of the sweep fixture shown in Fig. 11;

Fig. 13 is a schematic elevation of a base plate blank positioned over a base plate master in preparation for installation of the inserts in the blank;

Fig. 14 is an enlarged sectional elevation of one hole in the base plate blank positioned over a hole in the base plate master; and Fig. 15 is a sectional elevation of the insert

(bushing) being installed in the base plate blank using the hole in the base plate master to locate a mandrel on which the insert is mounted while the adhesive around the insert sets.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Turning now to the drawings, wherein like reference characters designate identical or corresponding parts, and more particularly to Fig. 1 thereof, a universal measurement fixture in accordance with this invention is shown having a thick steel base plate 50, on the order of about 1-3" thick, having a matrix of receptacles 52 opening in the top surface of the base plate 50. The base plate 50 could also be made of other materials to suit the requirements of the user. For example, if aluminum parts are the primary articles to be measured on the fixture, the base plate could be made of aluminum to match the coefficient of thermal expansion of the aluminum parts, thereby simplifying the calculations to account for differing CTE of the parts and the measurement fixture. Granite and graphite/epoxy are two other materials contemplated for the base plate 50. The receptacles 52 are positioned very accurately at regular intervals, for example, every 2" in an orthogonal pattern, over the surface of the base plate 50. The receptacles 52 are numbered along one axis and are lettered along an orthogonal axis to provide a unique identifier for each receptacle on the base plate 50. Each receptacle 52 is configured to receive and hold various hardware items such as stand-off pins and hold- down devices that are inserted into the receptacles, as will be explained in more detail below.

A plurality of brackets 54 is provided for attachment at selected positions onto the base plate 50. Each bracket 54 includes a base flange or sole plate 56 and an upstanding flange or upright wall 58 attached to an inner end of the base flange 56 at 90° thereto. The upstanding flange is connected to or integral with a buttress 60 at the inner end of the base flange 56 for stiffening against deflection in use.

A plurality of holes 62 is provided in the base flange 56 of the brackets 54. The holes may be in any suitable pattern, but in this preferred embodiment are in a regular pattern of equally spaced holes in a 7X7 matrix on 1/2" centers shown in Fig. 1. The rows of holes are numbered from one edge, and the columns are lettered from an orthogonal edge. As shown in Fig. 3, each of the holes 62 has a smooth-bored insert 63 fixed in the hole on a vertical axis at precisely spaced dimensions from the other inserts, using the process for installing the inserts described below. The inserts 63 receive hold-down devices for holding the brackets 54 down at selected locations on the base plate 50. The hold-down devices include bolts 64, shown in Figs. 2 and 3, which have accurately machined smooth shank sections 65 dimensioned to fit with small clearance into the bore of the insert 63, and having a lower threaded portion 67 that is received in a threaded lower portion 66 of the receptacles 52, shown in Fig. 3. The hold-down devices could also be conventional cam-action grippers 68, shown in Fig. 4, which have split rings 71 around a shank that is pulled by a cam 69 on the end of a lever to radially expand the rings 71 and grip the smooth sides of a counter-bore 70 in the receptacles 52 above the threaded portion 66. The counter-bore 70 is preferably in a hardened insert 72 set into a hole 74 drilled accurately through the base plate 50. The technique for accurately locating the inserts 72 in the holes 74 is described in detail below.

As shown in Figs. 1 and 6, a pattern of holes 76 on parallel horizontal axes is drilled in the upstanding flange 58 of each bracket 54, and a smooth bored insert 78 is set into each hole. One axis, for example the vertical columns of holes, are numbered from the top, and the other axis, the horizontal rows of holes, are lettered from the left. In the example shown, the pattern of holes is in a 6X7 matrix lettered A-F across the top and 1-7 down one edge. Any particular hole can then be designated by a single letter/number designator to make set-up and measurement instructions easy to write and follow, in the same manner as the receptacle designators on the base plate 50.

The bore 80 of each insert 78 is very accurately made with a precisely known internal diameter, and the insert is located in the hole 76 on a known horizontal axis to a very small tolerance. The bore diameter is sized to receive, with a small clearance, a stand-off pin 82 or the shaft 84 of a digital indicator probe 86, as shown in Figs. 1 and 5. Suitable digital indicators are widely available from several sources, such as Mitutoyo. Conveniently, a digital indicator probe 86 with a signal generating and transmitting unit is preferred so that the reading from the digital indicator can be read and recorded on a digital recorder automatically by a push of a button. The locator pin 82, shown in Fig. 5, is shown installed in the upstanding flange 58 in Fig. 6 and in perspective detail in Fig. 7. The pin 82 includes a cylindrical shank section 90 having a diameter just slightly smaller than the inside diameter of the insert 78 so that the pin shank 90 slides with a small clearance into the bore 80 of the insert 78. A threaded section 92 of the pin 82 receives a nut 94 which holds a head 96 of the pin 82 against the back side of the upstanding flange 58 of the bracket 54. The pin has a shaft portion 98 extending beyond the threaded section 92 and terminates in a rounded hardened distal end 100 which contacts the part to be measured. The locator pins are conveniently provided in several standard lengths from the reference surface, that is, the underside of the head 96, to the end 100, for ease of set-up. The locator pins may also be adjustable by standard micrometer type adjustment so the stand-off distance from a bracket 54 can be set at any desired length within the range of the adjustable length locator pin.

A bridge beam 102, shown in Figs. 1 and 9, is provided for attachment to reference surfaces, such as the tops of the upstanding flanges 58 of the brackets 54 as shown in Fig. 1, or the tops of blocks 106 as shown in Fig. 9. The bridge beam 102 has a pattern of holes 104 like the holes in the brackets, which preferably are all provided with hardened inserts like those set into the brackets 54. Vertical measurements may be taken from the bridge beam 102 to the part using digital indicators 86 as shown in Figs. 1 and 9. If the vertical position from the bridge beam to the part is too long, the bridge beam 102 may be mounted on a support block of known dimensions lying on the base plate 50, or on another support block 106 fastened at a known position to the upstanding flange of the bracket 54 by bolts, as shown in Fig. 9.

Elevation pins 108, shown in Figs. 1,2, 5 and 10, have a threaded end 110 and a cylindrical shank 112 depending from an enlarged diameter body portion 114. A shoulder 115 formed at the junction of the shank 112 and the body 114 is a reference surface for establishing the height of the top 116 of the elevation pin above the surface of the base plate 50. In use, the elevation pin is inserted into a selected receptacle 52 and the shank 112, whose diameter closely matches the inner diameter of the insert 72 which very accurately locates the position and orientation of the elevation pin. The threaded end 110 is screwed into the threaded portions 66 of the receptacle 52 and turned with a certain torque by engaging a torque wrench with one or more holes 118 in the body 114. A hole 120 accurately drilled in the top of the body 114 can be used to locate holes in the part on top of the elevation pin, and can also receive an extension pin to add a standard increment to the height of the pin 108.

Turning now to Figs. 11 and 12, a sweep 122 is shown in use to measure the horizontal distance from the vertical projection of a reference position on the base plate 50 to the closest surface of the part P. The sweep has a vertical pin 124 (shown in Fig. 12A) that fits closely into a selected receptacle. The pin 124 is attached to a base 125 coaxially with a vertical cylinder 126 having a hole in which the shank 84 of a digital indicator 86 is inserted.

The sweep 122 may also be used to measure the distance to the center of a hole in the part P. As shown in Fig. 12B, a self centering pin 128 is fitted into the hole in the part, and a closely fitting bushing 130 is placed onto the self-centering pin 128. The outside diameter of the bushing 130 is known exactly, as is the radial thickness of the cylinder 126 and the distance from the cylinder 126 to the bushing 130 is measured by the indicator 86. The addition of the measured distance plus the radius of the bushing 130 and less the radial thickness of the vertical cylinder 126 on the sweep 122 gives the distance of the centeriine of the hole in the part from the selected reference on the base plate. In Fig. 11 the part P is held against a reference surface 134 of a locating fixture 136 by a spring loaded pin 138 on a hold-down device 140. The locating fixture 136 has circular disc 142 attached coaxially to a cylindrical base 144, which is mounted at desired locations on the base plate 50 by threading a threaded extension of a pin, like the bolt 64, into a selected receptacle 52 in the base plate 50. The radius of the circular disc is known to a high degree of accuracy, thereby enabling the accurate location of the part P on the base plate 50. The hold-down device 140 has a cylindrical locating pin projecting from the underside of a base 146. The diameter of the locating pin closely matches the diameter of the inserts 72 in the base plate 50, so the hold-down fixture 140 stays in place when fitted into the selected insert despite the spring force exerted by the spring loaded pin 138 against the part P. The positioning of the inserts in the base plate and the brackets is important to the accuracy of the universal measurement fixture. Extreme accuracy is normally very expensive, but extreme accuracy is possible using the procedure described below at a reasonable cost. The major expense is in the manufacture of a master from which the other elements of the universal measurement fixture are made. The precision of the elements is a function of the precision of the masters, so they must be made to the highest precision. However, once they are made, they can be used indefinitely to make additional copies, so the cost can be amortized over a great many elements.

Once the master base plate 50M is made, the holes in the element blank are drilled a few thousandths oversized, for example .003" over sized. The blank plate 50B is laid on the master, as indicated in Fig. 13, and an insert 72 is placed loosely in the oversized hole. A locating mandrel 150 is slid through the insert and into the accurately drilled and ground hole in the master plate 50M. Adhesive is poured into the space between the insert 72 and the walls of the hole in which it is place, and the adhesive is permitted to cure. The locating mandrel holds the insert securely in place while the adhesive cures and afterwards may be extracted, leaving the accurately located insert 72 fixed in place.

The mandrel can be a helically cut expanding collet 152 on a tapered locating pin 154. One useful example of such a device is known as a lap, used for very accurate grinding of holes. The lap may be slightly tapered and the locating pin on which it is mounted may have the same taper, so that the axial position of the tapered pin in the tapered lap helix determines its diameter. The helical configuration ensures that the bore of the insert and the hole of the master are engaged uniformly by the lap so its axis remains coincident to an extremely high degree of accuracy with the axis of the hole. The adhesive can be may types, but the preferred adhesive is a cyanoacrylate available from 3-M Corporation, Pacer Technology, or Hernon. Epoxy adhesive is also a preferred material for setting the inserts 72 in the holes in the blank plate 50B

Set-up is preferably, but not necessarily, done with the aid of computer modeling which specifies the location for the brackets and the locations and heights of the elevation pins. The computer model specifies the location for the digital indicator and calculates the part dimensions from the digital indicator measurements. If these calculations are done manually, they are slower but not complicated.

Obviously, numerous modifications and variations of the preferred embodiment are possible and will occur to those skill in the art in like of this disclosure. Accordingly, it is to be expressly understood that these modifications and variations, and the equivalents thereof, are intended to be encompasses within the spirit and scope of this invention as defined in the following claims, wherein I claim:

Claims

1. A universal measurement fixture for measuring a part, comprising: a base plate having a flat upper surface in which a regular pattern of holes opens; a plurality of brackets, each having a sole plate for attachment to said base plate on at least two sides of said part, and an upright wall connected along one edge to said sole plate and extending at 90° thereto; said sole plate having a regular pattern of holes therethrough on axes perpendicular to said sole plate, and said upright wall having a regular pattern of holes therethrough on axes perpendicular to said upright wall; elevation pins, each having a distal end and opposite thereto a proximal end portion sized to be received in selected base plate holes; each elevation pin having a reference surface adjacent said proximal end portion at a accurately known distance from said distal end, said elevation pin reference surface engagable with a reference surface on said base plate for accurately locating said elevation pin on said base plate vertically and for providing a reference elevation thereabove at said distal ends of said elevation pins for support of said part at said reference elevation; locator pins having one end sized and configured to be received with a tight fit in said holes in said upright walls of said brackets; said locator pins having a reference surface adjacent to said one end engagable with a reference surface on said upright walls; said locator pins having a distal end remote from said reference surface at a accurately known distance therefrom; a measurement device selectively insertable in selected holes in said upright wall of a selected bracket to accurately locate said measurement device at a know position on said base plate for measuring the distance to said part positioned between said brackets; whereby, said distal ends of said elevation pins engage and vertically support said part on said base plate at a known height thereabove, and said part is positioned in contact with said locator pins in said brackets to position said part at a known position and orientation

2. A method of measuring a part, comprising: accurately locating a part on a universal measurement fixture by: a) placing a part on a base plate of a universal measurement fixture, with at least three points of a first datum of said part in contact with three reference pins at a known distance from a first reference plane on said universal measurement fixture; b) moving said part to bring two points on a second datum of said part, on a surface at an angel from said first datum, into contact with two locator pins in two brackets at a known distance from a second reference plane on said brackets; and c) moving said part to bring a point on a third datum on said part, separate from said first and second datums, into contact with a third locator pin in a third bracket at a known distance from a third reference plane on said third bracket; inserting at least one measuring device in selected holes in said upright wall of said brackets, engaging a reference surface of said measuring device with at least one reference surface of said brackets and measuring the distance between said bracket reference surface and said part; using said measured distance plus known distances from said reference surfaces to obtain selected dimensions of said part.

3. A method of making a universal measurement fixture for measurement of a part, comprising: selecting a flat base plate; drilling a regular pattern of holes in said base plate; inserting bushings in said holes, said bushings having through bores accurately drilled and reamed to the desired diameter; selecting a plurality of flat sole plates and flat upright walls for fabricating a plurality of brackets; drilling a regular pattern of holes in said flat sole plates and said upright walls; attaching said upright walls along one edge to said sole plates to make said brackets; connecting a plurality of said brackets to said base plate at selected positions according to the shape and size of said part using fasteners extending through selected holes on said sole plates and said base plate; installing a plurality of locator pins in said upright wall of at least two brackets orthogonally located on said base plate with said pins projecting beyond said upright walls to a position at which distal ends of said pins can contact said part; said holes in said upright wall being sized and configured to receive a measuring device inserted into selected holes on said upright wall for measurement of the distance from said upright wall to said part.

4. A method for fixing the location and orientation of a bushing in a hole to a desired degree of accuracy, comprising: drilling a hole in a structure into which said bushing is to be installed, said hole being larger in diameter than an outside diameter of said bushing; mounting said bushing on a lap, in turn mounted on a lap pin; inserting said lap pin on which another lap is mounted in a hole drilled in a master structure having holes drilled with extreme accuracy; locating said structure adjacent said master structure, with said lap and said bushing mounted thereon positioned in said hole to said desired degree of accuracy; potting said bushing in said hole with potting compound and allowing said potting compound to harden; and removing said lap from said bushing.