Portable Measurement Arm Cuts Digitizing Time In Half

A portable measurement arm is helping a West Virginia firm manufacture pump replacement parts that, in many cases, are more accurate and thus, perform better than the original OEM parts they replace.

For more than 25 years, Sturm, Inc., Barboursville, West Virginia, has served the utility companies, paper mills, chemical processing firms and other users of large, industrial, centrifugal pumps. The firm serves primarily as a source for pump parts that are no longer available from the original manufacturer. By supplying the needed part, Sturm helps the user keep its existing pump operating—and avoid the expense and disruption to operations of installing a new one.

When a company comes to Sturm for a replacement part, the firm searches its library of 70,000 pump-part drawings for a match. If it finds the drawing, the manufacturing process can begin. If the drawing cannot be found, Sturm can replicate the needed part from a like-new sample provided by the customer. When the customer cannot provide a usable sample of the part, Sturm may be able to borrow one from another customer with the same kind of pump and replicate it.

The impeller, the main rotating member of a centrifugal pump, is the component Sturm is most frequently called on to replace. As often as not, the firm must replicate it by reverse engineering: the process begins by making enough measurements of the sample to enable Sturm to prepare an accurate, fully-dimensioned drawing of the part.

Until recently, Sturm personnel typically measured pump parts using scales, micrometers, calipers and similar manual gages. "Hand gages are fine for measuring features like bore diameters and shaft ODs (outside diameters), but measuring an impeller with hand tools is a tedious, time-consuming task, and the measurements frequently include errors," explained Jim Pike, engineer for Sturm, Inc. Even if the measurements are completely accurate, there is still potential for error from inspectors writing their measurements in the wrong place on the sheet, transposing numbers, and so on.

Over the years, Sturm computerized its metallurgical and metal-casting operations and added computer numerically controlled (CNC) machine tools to its parts machining department. However, the firm's measurement methods, based on manual inspection tools, remained a generation or two behind the times and threatened to compromise the accuracy and efficiency of its modern production operations. The firm needed an inspection method that would allow it to get measurements right the first time, every time.

Mr. Pike had some experience with coordinate measuring machines (CMMs) at a previous employer, realized their potential and began to sell management on the advantages of such a machine for Sturm. At the same time, he began taking parts to a nearby industrial institute to measure them on the gantry-style CMM there.

While the part-measurement experiments validated Mr. Pike's confidence in CMM technology, they exposed some shortcomings of a gantry-style machine for Sturm's applications. In the first place, the machine's working envelope (about 2 by 2 by 3 feet) was too small to handle parts at the large end of Sturm's part size range. Certainly, Sturm could buy a larger machine, but it would carry a correspondingly higher price tag, and cost was a factor.

The complex impeller geometry also created problems. Mr. Pike discovered that it was difficult—sometimes impossible—to probe areas of one vane surface hidden from view by another with the institute's gantry-style CMM.

Also, Sturm needed to be able to measure parts with consistent accuracy, but wanted a level of accuracy suited to its dimensional requirements. It saw no advantage in buying a machine with a level of measuring precision that it would probably never need.

Mr. Pike found what he was looking for in the FaroArm portable CMM made by Faro Technologies, Inc., Lake Mary, Florida. The FaroArm's most prominent feature is an articulated measuring arm made of anodized aircraft aluminum, with precision bearings and rotary transducers at each of its six joints. The base of the arm is a mounting plate that permits direct attachment to a stable base, such as a transit stand or a rigid fixed surface. The arm's freedom of movement provides a spherical measurement envelope ranging from six to 12 feet in diameter depending on the model selected. Accuracies range from plus or minus 0.012-inch to plus or minus 0.003-inch. Accuracy reported as 2 sigma single point accuracy, according to ANSI B89 standards.

Two point probes and two ball probes come with the unit; each screws into the handle on the end of the arm. The handle has two buttons that the operator pushes to collect and record measurement data, either one point at a time (point-to-point measurements), or in a continuous stream as the probe is moved along a surface.

The measurement data is fed to a controller box that converts each recorded probe position to a precise location in 3D space. The controller is supplied with Faro's maintenance software, which is capable of many 3D measurement tasks such as setting new coordinate systems and leapfrogging to measure objects larger than the arm's reach.

Sturm purchased the S06-01 Silver Series model with a six-foot diameter, spherical, measurement envelope and a single-point accuracy of plus or minus 0.003-inch. It also purchased FaroArm's optional AnthroCAM software, and from a third party, a stainless steel measurement table with a ground surface drilled and tapped at regular intervals to permit securing the part for measurement. The table also serves as a mounting point for the FaroArm.

Sturm currently uses its FaroArm primarily to reverse engineer replacement pump parts from customer samples, measuring the samples to generate the dimensional data needed to completely define the parts.

The part the firm is most frequently called on to duplicate is a one-piece, cast-steel impeller for a centrifugal pump. Centrifugal pumps are made for specific applications, and the design of their impellers varies. An open impeller consists of several evenly spaced vanes extending from the impeller hub and looks similar to an airplane propeller.

The semi-open impeller includes a disc or plate (called a shroud) that supports the vanes. Measurement of a semi-open impeller starts with alignment of the sample to established coordinates. Machined details, such as the diameter and thickness of the shroud, hub bore and keyway, and overall impeller height, are quickly and easily measured. However, it's the contoured surfaces of the impeller vanes that present more of a challenge.

"The curvature of an impeller vane can be described by a single radius or can include as many as 15 different radii," Mr. Pike explained. "Trying to measure that curvature with scales or height gages simply doesn't work—the accuracy isn't there. With our FaroArm, we simply scan the surface and get all to the dimensional data we need in a single pass."

"FaroArm gives us the luxury of measuring in a scanning mode, which generates a stream of points as quickly as we can move the probe along the surface being measured, or in a point-to-point mode," Mr. Pike continued. "We use one or the other depending on the type and configuration of the vane. Scanning measures every surface detail faithfully, including the occasional surface flaw that can skew a contour line. At such times, we sometimes use the point-to-point measurement mode. We can, for example, measure three points on the surface and the computer will generate a smoother arc through them.

"We bought the FaroArm primarily to reverse engineer our vane wraps (the angle between the starting point of the vane and the end of the vane), our shrouds, our contours, to measure the many features of pump components that are impossible to accurately measure any other way," Mr. Pike emphasized.

For Mr. Pike, the increased accuracy that the FaroArm brings to part measurements is its biggest advantage: "Before, it was a real challenge to accurately measure to a given point from a bore to centerline or reference point using scales and hand measuring tools," he conceded. "Now, we can measure any point within the FaroArm's reach to an accuracy of plus or minus 0.003-inch."

Mr. Pike also likes the fact that the FaroArm measurements are less likely to be compromised by human error. "With our previous method of measuring with hand tools, the opportunities for incorrectly measuring the part were numerous," he says. "Committing the measurements to paper provided an opportunity for still more errors; the inspector could write down the wrong number, write the measurement in the wrong place, transpose some numbers...it was easy to make a mistake."

FaroArm is designed to prevent such errors, he says. "Measurements are convened to digital data automatically and captured instantaneously, preventing human error."

FaroArm's measuring speed is another important advantage for Sturm. "It would take us two hours to measure an impeller vane by hand," he recalls. "We now do it in half the time using the FaroArm, and we are getting faster the more familiar we become with the machine."

"We keep discovering shortcuts that help reduce inspection times," Mr. Pike says. "For example, instead of measuring numerous points along the contour of a vane to determine vane wrap, we found that we could do it by measuring just two points, at the start and the finish."

Ability to measure faster also means being able to make more measurements in the same time. Sturm can take advantage of this expanded measuring capability to perform more comprehensive inspections of the sample. For example, instead of reverse engineering an impeller based on measurements of only one vane, Sturm can now measure two or more vanes and average the readings. More extensive analysis of the sample also reveals imperfections in the sample that are probably also present in other parts currently in use—many customers operate multiple pumps—and that may be adversely affecting effective performance of the customer's pumps.

Still another advantage of FaroArm is the compatibility of its AnthroCAM software with Sturm's existing computer-aided design (CAD) software. Measurement data collected and processed by AnthroCAM is transferred to Sturm's AutoCAD CAD system, where it becomes a part design file that can also be printed out as a part drawing. Time from measurement of a sample to printout of a part drawing is short, the process prevents random human error.

Sturm sends the accurately drawn and dimensioned part drawing to a pattern shop, which uses it to make a wood pattern of the part. Sturm uses the pattern to make a casting mold for the part, sand casts the part in one of several corrosion-resistant steel alloys, and then machines it as needed, primarily on CNC-turning machines.

The FaroArm brings to Sturm's part measurement requirements the same level of sophistication that characterizes the firm's computerized casting and machining operations. The unit's measuring ease, speed, accuracy and reliability are the most important features for Sturm; to date, the firm has preferred to use it in a fixed workstation, bringing work to it, rather than it to the work. Recently, Sturm has been considering taking its FaroArm to customers' facilities to measure the pump components on location—an extra, time-saving service that only a portable CMM can provide.