Roundness measurement systems can help

you run circles around the competition.

by Kennedy Smith

If you're spinning your wheels, running in circles or going round and round, you're

probably wasting your time. But if you manufacture round parts, going round and round--using a roundness testing system--may be the only way to accurately measure your parts.

In the grand scheme of things, roundness testing systems are fairly new (only about 20 years old). The advent of computer technology was the catalyst for creating a mechanized roundness measuring system, but many quality control engineers can remember "the old days" before modern roundness testing.

"During the infancy of roundness testing, people used a V-block," explains Michael Kambeitz, national sales manager of surface-finish and form measurement at Carl Zeiss IMT Corp. "Operators would place a part--a hydraulic spool for example--in a V-block, manually rotate it and record out-of-roundness using a dial indicator." This was a very crude method because measurements were heavily based on operator influence. Plus, there are only so many points along a round part that a human being is capable of measuring with a dial indicator.

In the past two decades, however, roundness testers have evolved into dedicated form-measurement machines and emerged as essential tools on the shop floor. Any company that manufactures round parts, especially those supplying the automotive industry, is probably already familiar with roundness testing systems. Originally designed to measure a part's roundness, today's systems are capable of several other form measurements, too.

Structure of a roundness tester

A roundness testing system consists of a base, on which a vertical column stands. A radial arm--which houses the machine's probe--extends from the vertical column. Within the base, there is either a mechanical or air-bearing spindle, which accurately rotates the part. The machines are capable of measuring thousands of data points per part, and data are stored through an accompanying measuring station and software (which will be discussed in more detail later in the article).

Specifications vary from system to system, and almost all roundness testing machine manufacturers also custom-build systems. Some systems are capable of measuring parts weighing hundreds of pounds, so finding a machine to fit your load-bearing needs shouldn't be difficult.

Less common are roundness measurement systems that feature an overhead spindle design. These systems are specially designed for cases in which the part itself cannot be moved. For example, the part is attached to another piece of equipment or is simply too heavy to place on a traditional roundness tester.

Although a roundness tester's probe is essential for collecting thousands of data points, the heart of the machine lies within its base, where you'll find the spindle. Roundness measuring machines are equipped with either an air-bearing or mechanical- bearing spindle. The fundamental difference between the two is how they rotate the part--either mechanically or utilizing a constant stream of air. Most experts prefer air-bearing spindles, but mechanical-bearing spindles are also quite popular.

"Lower-end machines make use of mechanical bearings that will eventually wear," notes Kambeitz. "There's less degradation with air bearings because friction is eliminated. This allows the user to maintain accuracy and repeatability over a long period of time."

Systems with mechanical-bearing spindles are typically less expensive and are perfectly suitable for applications in which tolerances are more relaxed. Air-bearing spindle-equipped machines are ideal for cases in which tolerances are tight and submicron-type accuracy is critical.

Although its name implies that a roundness measurement system is only good for one thing, the latest models are more dedicated and lend themselves to many more types of form measurement.

Other than roundness…

Roundness testing systems are often referred to as form measurement systems because of the several part features they're capable of measuring. Along with roundness, today's roundness testing systems are equipped to measure runout, coaxiality, flatness, squareness, concentricity, cylindricity, parallelism and perpendicularity, among other things (See the chart below).

Additionally, surface roughness measurement will soon be introduced as an advanced feature of roundness testing systems, predicts Robert Wasilesky of Mitutoyo America Corp.

Why a roundness tester?

Some quality professionals might ask, "If I can get similar results with a coordinate measuring machine or a contour measurement system, why buy the roundness tester?" One obvious consideration is the type of part(s) you need to measure. Simply put, a roundness testing system is essential for any organization that measures round, spherical and cylindrical parts.

There are a few other reasons you might be better off choosing a roundness tester over other dimensional measurement systems. Sebastian LaBella, sales manager at Detroit Precision Hommel, urges roundness tester shoppers to consider:

Data collection. A major difference between a roundness tester and a CMM is the ways in which they gather data. A roundness measuring machine is capable of continuous form measurement (thousands of data points around the circumference of part), giving you the ability to thoroughly map the part's form. In contrast, a nonscanning CMM can only grab a few data points around the circumference of the part, which isn't enough information to map it as accurately. In addition, a contour measurement system--although ideal for part profiling--can't measure the circumference of a part.

Cost. Low-end roundness measurement systems start as low as $15,000. Companies that have a tight budget and are interested in basic form measurement may want to consider investing in a roundness tester. "With the lower price tag, companies can afford to buy two or three systems and place them at various locations on the shop floor, as opposed to spending thousands of dollars on a bigger piece of equipment that needs to be housed in a central location," explains Mike Colicci, product manager at Mahr Federal.

What to look for

Once the decision to purchase is made, there are a few basic functions that a potential buyer should seek. The importance of each function depends on the intended use, so consider which are most crucial to your application and buy accordingly.

"Companies need a machine that is well-built--one that's going to give longevity to the accuracy and repeatability of the measurements," notes Kambeitz. "It should also be solid and sturdy, with tooling flexibility and analytical software."

Other features to look for include:

The machine's design. Is it solidly designed? How freely does each part move? If portability is an issue, how easily can it be moved around the shop floor?

Ease of use. How easy is it to use? How much training will new users need? How much time will it take an operator to make a measurement? Does the machine come with a user console, computer, touch screen and keypad?

Reputation of the vendor. "Everybody has someone whom they love to buy from, and there are reasons for that," says Colicci. "Go with a vendor that provides the best technical assistance or service if needed."

Customer needs. If you're a supplier, keep the customer's specifications in mind. For example, if your customer demands tight tolerances for a part's coaxiality and cylindricity, make sure you buy a roundness tester than can accomplish both. This eliminates having to use more than one machine to measure one part.

Measuring to standards. "Measuring to current North American standards is a must," notes Wasilesky. "This includes individual companies' standards and those of the American Society of Mechanical Engineers." ASME's codes and standards, as well as membership and educational information, can be found at www.asme.org.

Software application and integration. Look for multifunctional analysis, graphic capabilities and the ability to upgrade the system to different ports or sensors, says Wasilesky.

Roundness tester software

To give you an idea of different software packages that accompany roundness testing systems, here's a look at four major providers' offerings:

Mahr Federal's metrology software works with its MMQ 6100 roundness testing system. Using a touch screen, the operator has the ability to set up, analyze and store information for future recall, as well as monitor system status. A Roundness Results screen displays chart and measurement information, and users can generate hard-copy printouts. A "measure" key allows the operator to take repeat measurements using the same conditions as previous applications. The Roundness Analysis screen lets the operator zoom in on segments or profiles of parts. It allows evaluation and elimination of unwanted details like scratches and burrs. A harmonic analysis feature relates the part's lobing conditions to the manufacturing process to predict the part's performance.

Mitutoyo's Roundpak v. 4.0 dedicated data-processing software works with its Roundtest roundness measuring machine. The Windows-based package analyzes roundness and cylindricity and other geometric features. The software provides multiple displays of analysis results, one-key measurement analysis and a variety of 3-D displays. The multiple analysis/recalculation function allows for simultaneous analysis of multiple items, which permits the user to change filter cutoff values, delete unnecessary data, reapply data for the analysis of different items, and perform other recalculation functions based on already recorded data.

Zeiss's TIMS form measurement software works with its Rondcom roundness measurement system. The software offers clearly defined access to functions such as controlling motorized axes, performing computer-aided calibration, inputting workpiece data, defining measuring conditions, measuring polar and linear parameters, and setting automatic functions. Analysis functions include profile editing and processing with different filter settings and analytical methods. It allows for 2-D and 3-D display options, plus linear, bearing area, amplitude density and Fourier analysis. Profiles can be exported to a contour-evaluation mode to analyze angles and distances, and all information can be printed in hard-copy format.

Taylor Hobson Precision's roundness testing software works with its Talyrond line of machines. The software provides programming for automatic measurement, harmonics analysis, cylindricity analysis with a choice of displays, asperity removal and edge detection to remove unnecessary data, and vertical and horizontal straightening.

Other companies provide similar software (refer to the Web directory on this page to learn more). There is constant development of more user-friendly, dedicated and intuitive software, capable of performing not only the aforementioned functions, but also advanced part analysis and gage R&R. Most experts agree that roundness testing software is only going to get better over time.

Future improvements

Besides the development of more user-friendly and intuitive software, experts speculate on other improvements we can expect from roundness testers in the years to come.

"The ideal case is to have a machine that allows an operator on the shop floor to make the most reliable measurements with the least amount of training and the fewest number of errors," says Colicci. "The goal is to make the machine more user-friendly, more intelligent, less expensive and faster."

"A major fault with roundness machines today is subjectivity," adds LaBella. "As time goes by, operator influence should be eliminated with easier-to-program software and computer-numeric-controlled functions."

Common human errors

Although they're becoming more automated, roundness testers still call for a fair amount of human interaction. And any time a person is introduced in the measurement process, subjectivity (and the potential for less accuracy) is also introduced.

"The most common human-related difficulty encountered when using roundness testing equipment is a lack of experience," comments Colicci. "These gages used to be delicate and expensive, used only in laboratories by trained specialists. Today, it's very common to see roundness testers out on the shop floor used by machinists. Operators must understand the meanings of various parameters and how they're called out on part prints. They must also know how to set up a part on a gage and the purposes of their various functions."

It's critical that operators learn how to best utilize the gage and be consistent in how they take their measurements to ensure accurate and repeatable results, Colicci continues.

"The most common alignment errors result from incorrectly centering the workpiece's angular alignment," adds Wasilesky. This is referred to as the Limacon error." (To learn more about the Limacon error, visit www-gap.dcs.st-and.ac.uk/~history/Curves/Limacon.html.) Wasilesky says the second most common human error is misaligning the workpiece's tilt prior to measurement.

There are steps that any roundness testing machine operator can take to prevent inaccurate results and ensure better repeatability. Wasilesky and Colicci offer the following tips:

Measure with a constant measuring force and speed.

Thoroughly clean the work piece prior to measuring.

Minimize vibration, temperature change and air flow.

Invest in CNC alignment, which automates the part alignment process and takes away some operator subjectivity. (This feature is now available for less than $50,000.)

Circle the best answer

Measuring round parts without using one of these machines is like trying to fit a round peg into a square hole. If you've ever experienced inaccurate measurements and unreliable repeatibility, or if you've simply been running in circles trying to find the answer to your round part-measuring woes, check out what roundness testing systems have to offer.

About the author

Kennedy Smith is Quality Digest's assistant editor. Letters to the editor regarding this article can be sent to letters@qualitydigest.com.