WO2004009440A2

WO2004009440A2 - Method of machining elongated workpieces utilizing ultrasonic workpiece modeling

Info

Publication number: WO2004009440A2
Application number: PCT/US2003/022620
Authority: WO
Inventors: Daniel R. Scherrer; John Barrington Mccullough; Jr. Joseph A. Sieber; Paul J. Sturdevant; Stephen G. Verrigni; Kevin Lee Komraus
Original assignee: Unova Ip Corp
Priority date: 2002-07-19
Filing date: 2003-07-18
Publication date: 2004-01-29
Also published as: WO2004009440A3

Abstract

The present invention is for a method of machining elongated workpieces (10), such as, for example, helicopter rotor blades and other aircraft spars, on a machine (20) such as a machining center, utilizing an ultrasonic gauge to prepare a “virtual” model of the workpiece (10) and to adjust positioning of the workpiece (10) relative to the machine automatedly so as to result in a finished workpiece (10) having negligible variations in the wall thickness thereof, i.e., an approximately constant wall thickness.

Description

METHOD OF MACHINING ELONGATED WORKPIECES UTILIZING ULTRASONIC WORKPIECE MODELING

BACKGROUND OF THE INVENTION The present invention relates to methods of machining elongated workpieces, such as, for example, helicopter blades and other aircraft spars. More particularly, the present invention relates to methods of machining elongated workpieces wherein an ultrasonic gauge is used in-process to create a model of the workpiece geometry for the purpose of accurately machining same. Elongated workpieces used as aircraft structural members, such as wing spars, helicopter rotor blades and the like, typically are constructed from extruded aluminum or aluminum alloy and may be hollow. A cross-sectional view of a typical extruded workpiece 10 intended for use as a helicopter rotor blade spar is shown in Figure la and includes an exterior rough contour 12 exhibiting surface and dimensional variations along the length of the workpiece 10. For example, it is typical for the extruded workpiece 10 to exhibit some degree of axial twist or lateral bending from one end to the other (or from root-to-tip, as the case may be) due to imperfections in the extrusion manufacturing process.

With additional reference now to Figure lb, it is customary to remove material from the rough contour 12 utilizing a contour milling machine 20, for example, to impart a finished contour 14 into the workpiece 10, where the finished contour 14 represents the desired exterior airfoil shape of the workpiece 10. In typical contour milling operations, the workpiece is affixed to the milling machine 20 by one or more fixtures 22, which operate to position the workpiece 10 in a substantially straight orientation (along the length of the workpiece 10) for lengthwise machining by a contour milling tool 24.

It has been observed, however, that the interior contour 18 of a hollow extrusion molded workpiece and the rough contour 12 thereof are not always congruous, again, due to imperfections in the extrusion molding process. For example, referring to Figures 2a-2d, at arbitrary locations along the length of the workpiece 10, the interior contour 18 may be positioned and/or oriented relative to the rough contour 12 such that the interior contour 18 is ballooned (Figure 2a), is not parallel (Figure 2b), is longitudinally offset (Figure 2c) or is laterally offset (Figure 2d) relative to the rough contour 12. Because the fixtures 22 used typically to position the workpiece 10 in a substantially straight orientation on the milling machine are clamped to (or otherwise located with reference to) the exterior of the workpiece 10, the interior contour 18 is not always in a substantially straight orientation when the workpiece 10 is fixtured to the milling machine, even though the exterior of the workpiece 10 may be. As such, variations in the position and/or orientation of the interior contour 18 result in variations in the wall thickness of the workpiece 10 once the desired finish contour 14 has been machined into the workpiece 10.

Those of ordinary skill in the art will appreciate that variations in the wall thickness of finished workpieces 10 may lead to significant detrimental effects in aerospace applications, to name only one of many applications where such detrimental effects may be observed. For example, excess material in wing spars increases the overall weight of the aircraft, thereby decreasing the efficiency thereof. Moreover, localized masses in rotating parts such as helicopter blades, for example, resulting from regions of greater wall thicknesses introduce unbalanced motion, thereby introducing unnecessary, inefficient and potentially catastrophic vibrations in the motion thereof. Accordingly, it is desirable to provide a method for orienting an elongated workpiece to be machined such that the final shape thereof includes negligible variations in its wall thickness. More particularly, it is desirable to provide a method for orienting an elongated workpiece to be machined relative to a desired finished contour thereof.

For example, it is known that if the workpiece 10 is affixed to the machine 20 such that the orientation thereof is substantially straight relative to the interior contour 18 (rather than relative to the exterior thereof), then after the finished contour 14 has been machined into the workpiece 10, the variation in wall thicknesses at arbitrary locations along the length of the workpiece 10 will be negligible. However, the only method known to applicants of orienting the workpiece 10 relative to the interior contour 18 requires a machine operator to manually gauge the wall thickness of the workpiece 10 at numerous locations thereon and to manually shim the workpiece 10 accordingly. Methods of determining the wall thickness of unmachined workpieces, and more particularly, of long, slender workpieces, are difficult -- if not impossible ~ in most machining environments. It is desirable therefore to provide a method for automatedly detecting the wall thickness of a workpiece at a number of preselected locations thereon. It is desirable moreover to provide a method of machining a workpiece wherein the workpiece is mounted to a machine adapted to automatedly detect the wall thickness of the workpiece at a number of preselected locations thereof prior to performing one or more machining operations thereon. It also is desirable to provide a method of mounting a workpiece to a machine for performing one or more machining operations on the workpiece, wherein the machine is adapted to automatedly detect the wall thickness of the workpiece at a number of preselected locations thereon and wherein the workpiece is mounted to the machine using a single fixture or single set of fixtures. It is desirable even furthermore to provide a method of mounting a workpiece to a machine for performing one or more machining operations on the workpiece, using one or more adjustable fixtures.

SUMMARY OF THE INVENTION The present invention is for a method of machining elongated workpieces, such as, for example, helicopter rotor blades and other aircraft spars, on a machine utilizing an ultrasonic gauge to prepare a "virtual" model of the workpiece and to adjust positioning of the workpiece relative to the machine automatedly so as to result in a finished workpiece having negligible variations in the wall thickness thereof. According to one form of the present invention a method for mounting a workpiece to a machine for performing one or more machining operations is provided, wherein the position and/or orientation of the workpiece is selected automatedly so as to provide a finish machined workpiece having substantially a desired wall thickness. In an exemplary, preferred embodiment of the present invention, the workpiece is an elongated, slender, hollow extruded aluminum aircraft spar, such as, for example, a helicopter rotor blade, and the machine is, for example, a U5® 5-axis universal mactøning center sold by Cincinnati Machine, A Division of UNOVA Industrial

Automation Systems, Inc., of Cincinnati, Ohio. The machine may be a moving gantry-style machine, such as the machine shown in Figure lb, or may be a stationary bridge-style machine. The workpiece is mounted to a worktable of the machine by one or more adjustable fixtures provided at one or more axial fixture locations along the length of the workpiece. Each adjustable fixture includes one or more adjustable locators which contact and support the workpiece. One or more axial gauge locations are selected along the length of the workpiece, preferably intermediate two of the one or more adjustable fixtures.

The machine is adapted to measure one or more external dimensions of the workpiece at each gauge location and to store such external dimension measurements in an electronic look-up table located within the machine control memory or within another data storage location in communication with the machine control or the machine control memory. According to one form of the present invention, the external dimension measurements are obtained by a conventional touch probe held by a spindle of the machine and positioned relative to the workpiece thereby under operative motion control of the machine control.

The machine also is adapted to measure the wall thickness of the workpiece at one or more perimetrical locations of the workpiece at each gauge location and to store such thickness measurements in the electronic look-up table of the machine control. According to one form of the present invention, such wall thickness measurements are obtained by a conventional ultrasonic gauge transducer held by the spindle of the machine and positioned relative to the workpiece thereby under operative motion control of the machine control. An adjustment table is then constructed within the machine control memory comprising adjustment values for each adjustable locator of each adjustable fixtures. Adjustment values correspond to position adjustments that should be made to each adjustable locator so that the interior contour of the workpiece is placed in a substantially straight orientation. Once the adjustable locators are adjusted according to the calculated adjustment values, an elongated retaining boss, such as a dovetail, is machined into a base wall of the workpiece and clamped by a stiff clamping fixture provided on the machine, thereby rigidly securing the workpiece in an orientation such that the elongated interior contour or the workpiece is substantially straight. All surfaces but the base wall of the workpiece is machined under conventional machine control, as instructed from the machine control part program, after which the workpiece is returned to the adjustable locators/fixtures and gripped thereby. The retaining boss is then machined from the workpiece and the workpiece is removed from the machine.

It is an object of the present invention to provide a method for orienting an elongated workpiece to be machined such that the final shape thereof includes negligible variations in its wall thickness.

It is another object of the present invention to provide a method for orienting an elongated workpiece to be machined relative to a finished contour thereof. It is yet another object of the present invention to provide a method for automatedly detecting the wall thickness of a workpiece at a number of preselected locations thereon.

It is still another object of the present invention to provide a method of machining a workpiece wherein the workpiece is mounted to a machine adapted to automatedly detect the wall thickness of the workpiece at a number of preselected locations thereof prior to performing one or more machining operations thereon.

It is yet another object of the present invention to provide a method of mounting a workpiece to a machine for performing one or more machining operations on the workpiece, wherein the machine is adapted to automatedly detect the wall thickness of the workpiece at a number of preselected locations thereon and wherein the workpiece is mounted to the machine using a single fixture or single set of fixtures.

It is another object of the present invention to provide a method of mounting a workpiece to a machine for performing one or more machining operations on the workpiece, using one or more adjustable fixtures.

These and other objects, features and advantages of the present invention become apparent to those of ordinary skill in the art from the description which follows, and may be realized by means of the instrumentalities and combinations particularly pointed out therein, as well as by those instrumentalities, combinations and improvements thereof which are not described expressly therein, but which would be obvious to those of ordinary and reasonable skill in the art. According to one embodiment of the present invention, a method of machining an elongated hollow workpiece having a length and a wall thickness is provided, comprising the steps of: positioning the workpiece relative to a machine for performing machining operations on the workpiece; securing the workpiece relative to the machine by one or more fixtures at one or more preselected fixture locations along the workpiece length; obtaining one or more external dimension measurements of the workpiece at one or more preselected gauge locations along the workpiece length; obtaining one or more thickness measurements of the workpiece at each location of the one or more external dimension measurements obtained at each of the one or more preselected gauge locations; comparing the thickness measurements to the external dimension measurements to determine an internal dimension of the workpiece at each of the one or more preselected gauge locations; adjusting the orientation of the workpiece at at least one of the one or more fixture locations until the internal dimension of the workpiece at each of the one or more preselected gauge locations is substantially aligned; and, machining the workpiece such that the wall thickness of the workpiece is substantially constant along the workpiece length.

BRIEF DESCRIPTION OF THE DRAWINGS

A better understanding of the invention will be had upon reference to the following description in conjunction with the accompanying drawings in which like reference numerals represent like parts, and wherein:

Figure la is a cross-sectional view of a typical workpiece suitable for machining according to a preferred embodiment of the present invention;

Figure lb is a perspective schematic representation of a milling machine equipped to machine the workpiece of Figure la

Figure 2a is a cross-sectional schematic view of the workpiece of Figure la, before machining, showing ballooning; Figure 2b is a cross-sectional schematic view of the workpiece of Figure la, before machining, showing non-parallelism;

Figure 2c is a cross-sectional schematic view of the workpiece of Figure la, before machining, showing longitudinal offsetting;

Figure 2d is a cross-sectional schematic view of the workpiece of Figure la, before machining, showing lateral offsetting;

Figure 3 is a flow chart depicting generally a method of machining elongated workpieces according to a preferred embodiment of the present invention;

Figure 4 is a perspective schematic representation of a workpiece being machined according to a preferred embodiment of the present invention; Figure 5 is a cross-sectional view of the workpiece of Figure 4, shown along section line 5-5 of Figure 4, and showing three adjustable locators;

Figure 6 is a cross-sectional view of the workpiece of Figure 4, shown along section line 5-5 of Figure 4, and showing exemplary locations of width measurements; Figure 7 is a representation of a look-up table showing data cells used for storing measurement data;

Figure 8 is a cross-sectional view of the workpiece of Figure 4, shown along section line 5-5 of Figure 4, and showing exemplary locations of wall thickness measurements; Figure 9 is a representation of a look-up table showing data cells used for storing position adjustment data;

Figure 10 is a cross-sectional view of the workpiece of Figure 4, shown along section line 5-5 of Figure 4, and showing a retaining boss being gripped by a clamping mechanism; and,

Figure 11 is a cross-section view of the workpiece of Figure 4, shown along section line 5-5 of Figure 4, and showing the retaining boss at one end thereof.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention is for a method of machining elongated workpieces, such as, for example, helicopter rotor blades and other aircraft spars, on a machine utilizing an ultrasonic gauge to prepare a "virtual" model of the workpiece and to adjust positioning of the workpiece relative to the machine automatedly so as to result in a finished workpiece having negligible variations in the wall thickness thereof. According to a preferred embodiment of the present invention, the workpiece is an elongated, slender, hollow extruded aluminum helicopter rotor blade, and the machine is, for example, a U5® 5-axis universal machining center sold by Cincinnati Machine, A Division of UNO A Industrial Automation Systems, Inc., of Cincinnati, Ohio, adapted to operate as described herein. It will be appreciated by those of ordinary skill in the art, upon reading the within detailed description of the exemplary and preferred embodiments of the present invention, that other workpieces, such as short or non-slender workpieces, may be machined according to the present invention without departing from either the spirit or the scope of the present invention.

With reference to Figure 3, the method of machining workpieces according to a preferred embodiment of the present invention is described with reference to a flow chart showing schematically the general steps thereof, each of which will be described in greater detail below. For the moment, however, it can be said that the present invention includes the steps of: 100 - securing the workpiece to a worktable of the machine using one or more adjustable locator fixtures; 110 - obtaining one or more external dimension measurements of the workpiece and storing these measurements in a look-up table located within the memory (or other storage medium) of the machine control; 120 - obtaining one or more wall thickness measurements of the workpiece and storing these measurements in the look-up table of the machine control; 130 - using the external dimension measurements, the wall thickness measurements and the look-up table to create a "virtual" model of the workpiece and calculating adjustment values for each of the locator fixtures; 140 - adjusting the location and/or position of the locator fixtures to load and to position the workpiece in a substantially "straight" orientation; 150 - machining a retaining boss in the workpiece and clamping the workpiece by the retaining boss so as to maintain the substantially "straight" orientation thereof after the adjustable locator fixtures have been removed from the workpiece; 160 - machining a desired contour into the workpiece and retiirning the machined workpiece to the locating fixtures; 170 - machining the retaining boss from the workpiece; and, 180 - removing me finished workpiece from the machine.

Each of these steps will now be described in greater detail with reference to exemplary features of the machine and its various components provided for the purpose of functioning as described herein.

With reference to Figure 4, the workpiece 10 is secured to a worktable 26 of the machine 20 by one or more adjustable locator fixtures 200 securely fastened to the worktable 26 by conventional mechanisms. Preferably, fixtures 200 are spaced uniformly along a longitudinal axis "X" of the workpiece 10, one fixture 200 being located at each fixture location Xi, X₂, X₃... along the longitudinal axis "X".

Each fixture 200 includes a base 202 and one or more axial locators 204 protruding from the base 202. At least one of the locators 204 is movable relative to the base 202 along its axis. For example, referring now also to Figure 5, three locators 204_A, 204_B, 204c are provided in the base 202 of an exemplary fixture 200 positioned near the midspan of the workpiece 10. Each locator 204 _A, 204_B, 204c is movable relative to the base 202 along a locator axis ZA, Z_B, YC_» respectively. When positioned, the workpiece 10 rests on the locators 204_A, 204_B, 204c (and is located with reference to the machine, for example, by locating pins (not shown) or by another locating device) and is constrained from movement thereby.

Additional locators (not shown) may be provided, for example, on a positionable clamping fixture (not shown) to hold the workpiece 10 against the locators 204A, 204_B, 204_c along directions Z_A', Z_B', Yc, for example, and to further constrain movement of the workpiece 10 relative to the machine 20. The positionable clamping fixture may be a swing-type fixture affixed to an upright portion 203 of the base 202 and movable relative thereto by conventional means. Alternatively, the positionable clamping fixture may be a slide-type fixture adapted to be moved into position by a conventional plunging apparatus (not shown) of the machine 20.

Locators 204_A, 204_B, 204c are adapted to move along their respective axes Z_A, Z_B, Y_C, respectively, under programmed control from the machine 20. For example, each locator 204A, 204_B, 204_C may include a threaded portion (not shown) which is matingly received by a threaded bore (not shown) in the base 202. Rotation of the locator 204_A, 204_B, 204_C, then, results in linear movement thereof in and out of the base 202. Preferably, each locator 204_A, 204_B, 204c is operatively coupled to an independent drive (not shown), such as, for example, a servo motor, which operates under programmed control from the machine 20 to position the locator 204_A, 204_B, 204c precisely an adjustable, lockable distance relative to the base 20.

According to a preferred embodiment of the present invention, the method hereof begins by mounting the workpiece 10 to the worktable 26 by positioning the workpiece 10 on one or more fixtures 200 and setting the workpiece 10 on the protruding locators 204 thereof, each of which having been first controUably adjusted until the workpiece 10 is in a substantially "straight" or "square" orientation relative to the machine 10 when resting freely on the locators 204. The positionable clamping fixture (if provided) is then moved into position to securely grip the workpiece and to constrain movement thereof.

Once the workpiece 10 is secured to the worktable 26 by the fixtures 200, a conventional touch probe, such as those manufactured by Renishaw Inc. of Hoffman

Estates, Illinois, is gripped by the spindle of the machine 10, according to and utilizing conventional techniques, and the touch probe is used to obtain external dimension measurements of the workpiece 10 at one or more locations thereon. For example, referring now back to Figure 4, one or more gauge locations X_A, X_B are selected, preferably spaced between adjacent fixture locations Xi, X₂, X₃.

Referring now to Figure 6, for each gauge location X_A, X_B> the touch probe (not shown) is moved under machine control to contact an outer surface of the workpiece 10 at two or more locations, for example, at ten preselected locations Wπ, W₁₂, W₂₁, W₂₂, W₃₁, W₃₂, W i, W ₂, W₅₁, W₅₂ spaced around the perimeter of the workpiece 10. Preselected locations are arranged in pairs representing endpoints of width measurements of the workpiece 10. Using conventional algorithms programmed into the machine control, location data for each endpoint of each pair of width measurements are compared with one another to calculate a "width" measurement of the workpiece 10 related end points of a pair of location readings. For example, a first width measurement Wi is calculated by comparing the position data of location readings Wπ and W₁₂, a second width measurement W₂ is calculated by comparing the position data of location readings W₂₁ and W₂₂, and so on. In the present example, five such width measurements Wi, W₂, W₃, W and W₅ are calculated using the ten locations Wπ, W₁₂, W₂₁, W₂₂, W₃₁, W₃ , W ₁, W₄₂, W5₁ and

W₅₂, wherein locations Wπ, W₂ι, W₃ι, W ₁ and W₅₁ are considered first endpoints of a distance measurement and wherein locations W₁₂, W₂2, W₃₂, W ₂ and W₅₂ are considered second endpoints of the distance measurement to respective first endpoints. Referring now to Figure 7, for the gauge location X_A, X_B being gauged, width measurement values Wi, W₂, W₃, W₄ and W₅ are stored in a unique memory cells located within a first region 232 of a look-up table 230 residing within the machine control memory or wilhin another data storage location in communication with the machine control or the machine control memory. The machine spindle then returns the touch probe to a staging area (not shown) and selects a conventional ultrasonic thickness gauge transducer (not shown), such as those manufactured by Panametrics Inc. of Waltham, Massachusetts, which has been adapted (such as by a housing) to be gripped by a clamping mechanism (not shown) of the machine spindle. Referring now to Figures 6 and 8, the machine spindle positions the ultrasonic gauge to measure the thickness T_11? T₁₂, T₂₁, T₂₂, T₃₁,

T₃₂, T41, T₄₂, T51, T₅₂of the workpiece at each of the ten locations Wπ, W₁₂, W2₁, W₂₂, W31, W₃₂, W41, W42, W5i, W₅₂, respectively. Those of ordinary skill in the art will appreciate that, whereas width measurements Wi, W₂, W₃, W and W₅ represent the distance between locations Wπ and W₁₂, for example, thickness measurements Tπ, T12, T₂₁, T22, T₃₁, T₃₂, T ₁, T₄₂, T₅₁, T₅₂ measure the thickness of the workpiece at each of the ten individual locations Wπ, W₁₂, W₂₁, W₂₂, W₃₁, W₃₂, W₄₁, W₄2, W₅₁, W52. As such, ten thickness measurements are obtained rather than five width measurements.

Referring back to Figure 7, each of the ten thickness measurements are stored in its own unique memory cell located within a second region 234 of the look-up table

230. Having thus acquired both external workpiece widths and thicknesses, the machine control can construct a 2-dimensional "virtual" model of the geometry of the cross-section of the workpiece at that gauge location. Of course, the accuracy of the 2-dimensional model can be increased by increasing the number of measurement locations; however, this increases the time necessary to construct the model. The position of the workpiece -- relative to the machine spindle — is adjusted such that the above steps of acquiring workpiece width and thickness measurements can be repeated for each of the remaining gauge locations and stored in respective memory cells of the look-up table 232.

Once all of the width and thickness measurements have been stored for each of the gauge locations along the axis "X" of the workpiece 10, a 3-dimensional "virtual" model of the entire workpiece 10 is constructed using conventional interpolation techniques. Of course, the accuracy of the 3-dimensional model can be increased by increasing the number of gauge locations (and spacing them closer to one another); however, this increases the time necessary to construct the model.

With reference to Figure 9, a second look-up table 330 is constructed using workpiece width and wall thickness values stored in the measurement data look-up table 230 and using conventional interpolation techniques. That is, workpiece geometry is known at each gauge location on the workpiece and can be used tp predict workpiece geometry at each fixture location, for example, by interpolating measurement data acquired at adjacent gauge locations and stored in look-up table 230. Adjustment values are thereafter calculated corresponding to each adjustable locator to load the workpiece 10 such that the interior contour 18 thereof will be substantially straight.

Referring to Figure 10, once the workpiece 10 has been loaded so that the interior contour 18 thereof is in a substantially straight orientation, a retaining boss 17 _ is machined into a base wall 19 portion of the workpiece 10. The retaining boss 17 may span the entire length of the workpiece 10, although it is preferable for the retaining boss 17 to comprise one or more boss segments located, for example, between fixtures 200. In one form of the present invention, the retaining boss 17 is a segmented dovetail.

One or more clamps 15 are provided by the machine and operatively engage the retaining boss 17 securely, at which point, the workpiece 10 is removed from the fixtures 200. Clamps 15 are sufficiently stiff to preserve the loaded orientation of the workpiece 10, whereat the interior contour 18 thereof is in a substantially straight orientation. Now free of the fixtures 200, the entire exterior surface (except the base wall 19) of the workpiece 10 can be oriented by the clamping mechanism 15, rough/finish machined by the machine under conventional machine control and returned to the fixtures 200 (Figure 11) for removing of the retaining boss 17 and machining of the base wall 19 of the workpiece 10.

According to one aspect of the present invention, the present invention provides a method for orienting an elongated workpiece to be machined such that the final shape thereof includes negligible variations in its wall thickness. According to another aspect of the present invention, the present invention provides a method for orienting an elongated workpiece to be machined relative to a desired finished contour thereof.

According to yet another aspect of the present invention, the present invention provides a method for automatedly detecting the wall thickness of a workpiece at a number of preselected locations thereon. According to still another aspect of the present invention, the present invention provides a method of machining a workpiece wherein the workpiece is mounted to a machine adapted to automatedly detect the wall thickness of the workpiece at a number of preselected locations thereof prior to performing one or more machining operations thereon. According to another aspect of the present invention, the present invention provides a method of mounting a workpiece to a machine for performing one or more machining operations on the workpiece, wherein the machine is adapted to automatedly detect the wall thickness of the workpiece at a number of preselected locations thereon and wherein the workpiece is mounted to the machine using a single fixture or single set of fixtures.

According to yet another aspect of the present invention, the present invention provides a method of mounting a workpiece to a machine for performing one or more machining operations on the workpiece, using one or more adjustable fixtures.

While the invention has been described and illustrated with reference to one or more preferred embodiments thereof, it is not the intention of the applicants that the invention be restricted to such detail. Rather, it is the intention of the applicants that the invention be defined by all equivalents, both suggested hereby and known to those of ordinary skill in the art, of the preferred embodiments falling within the scope hereof.

Claims

CLAIMS We claim:

1. A method of machining an elongated hollow workpiece having a length and a wall thickness, comprising the steps of: positioning said workpiece relative to a machine for performing machining operations on said workpiece; securing said workpiece relative to said machine by one or more fixtures at one or more preselected fixture locations along said workpiece length; obtaining one or more external dimension measurements of said workpiece at one or more preselected gauge locations along said workpiece length; obtaining one or more thickness measurements of said workpiece at at least one location of said one or more external dimension measurements obtained at each of said one or more preselected gauge locations; comparing said thickness measurements to said external dimension measurements to determine an internal dimension of said workpiece at each of said one or more preselected gauge locations; adjusting the orientation of said workpiece at at least one of said one or more fixture locations until said internal dimension of said workpiece at each of said one or more preselected gauge locations is substantially aligned; and, machining said orkpiece such that said wall thickness of said workpiece is substantially constant along said workpiece length.

2. The method according to claim 1 , wherein said step of securing said workpiece relative to said machine includes the step of providing one or more adjustable fixtures.

3. The method according to claim 2, wherein said step of adjusting the orientation of said workpiece includes the step of adjusting at least one of said adjustable fixtures.

4. The method according to claim 1, wherein said step of obtaining one or more external dimension measurements includes the step of obtaining one or more external width measurements.

5. The method according to claim 4, wherein said step of obtaining one or more external width measurements includes the steps of: selecting a first endpoint at an exterior location of said workpiece at one of said one or more gauge locations; selecting a second endpoint at an exterior location of said workpiece opposite said first endpoint; and, measuring the distance between said first endpoint and said second endpoint, said distance being said external width measurement.

6. The method according to claim 5, wherein said workpiece is symmetrical about an axis and wherein said step of selecting said first end point and said second endpoint includes the step of selecting said second endpoint such that said second endpoint is symmetrical to said first endpoint about said axis.

7. The method of according to claim 5, wherein said step of obtaining one or more thickness measurements includes the step of obtaining at least one of said one or more thickness measurements at said first endpoint.

8. The method according to claim 1 , wherein said step of obtaining one or more external width measurements includes the step of providing a touch probe.

9. The method according to claim 1 , wherein said step of obtaining one or more thickness measurements includes the step of providing an ultrasonic transducer.

10. The method according to claim 1, further comprising the step of recording at least one of said one or more external dimension measurements in a look-up table of a computer after said step of obtaining said one or more width measurements.

11. The method according to claim 1 , further comprising the step of recording at least one of said one or more thickness measurements in a look-up table of a computer after said step of obtaining said one or more thickness measurements.

12. The method according to claim 1 : further comprising the following steps prior to said step of machining said workpiece: machining a retaining boss in one surface of said workpiece, gripping said workpiece by said retaining boss, and releasing said workpiece from at least one of said one or more fixtures; and, further comprising the following steps after said step of machining said workpiece: re-securing said workpiece relative to said machine by said one or more fixtures, releasing said retaining boss, and machining said retaining boss from said workpiece.