The Basics of Shaft Measurement

When precision matters, what is the best tool for the job?

Published: Monday, August 8, 2016 - 11:40

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A simple fact in manufacturing is that everyone has to measure. However, when precision is the ultimate goal, measurement is not simply about inspection, it’s about process control. Manufacturers need the right tools to increase quality, maximize productivity, and, ultimately, to make measurement a value-added process.

An introduction to shaft measurement systems

If a part is created to turn on an axis, a shaft is typically involved. Shafts contain a number of functional elements, such as gears, grooves, splines, tapers, and threads—which all have associated critical dimensions. Diameters, lengths, angles, distances between points and lines, groove widths, fillet radii, or chamfers on these parts are well-suited for shaft measurement systems.

Designed to facilitate precision control of complex turned parts, shaft measurement systems are ideal for automotive, aerospace, and medical industry applications, to name a few. Given the fact that the shafts used in these types of applications are often safety- and performance-critical, ensuring precision, quality, and high reliability is of the utmost importance.

Trends affecting shaft measurement

Ever-increasing accuracy requirements and declining cycle times are creating the need for rapid, precise measurement directly in the production environment. As shaft measurement systems gain popularity, the market demands increased capabilities.

Trends shaping the demand for advanced shaft measurement technologies include:
• Decreasing tolerances
• The need to move measurement closer to the manufacturing process
• The need for increased speed and economy in production lines
• User requirements for integrated systems to provide feedback directly to machine tools
• The need for more automation of the measurement process
• Increasing demand for more precise measurements
• Greater ease of use

These trends have led to the rise of advanced, fully automated optical shaft measurement systems that offer efficient and precise measurement of rotationally symmetrical workpieces in production. These systems also enable the highly accurate measurement of a wide variety of characteristics in just seconds—without operator contact. Fully automatic measuring sequences can be performed both in the lab and in the harsh environment of the shop floor, and eliminate operator influence from measurement results.

Innovative systems also feature easy-to-use graphical user interfaces to simplify programming and deliver easy-to-understand results. These benefits of high measurement speed, precision, and optimum ease of use in a noncontact process ensure the utmost in quality control during production.

Figure 1: Manual machines allow quick and easy measurement of a variety of features in an economical package.

Optical shaft measurement technology

There are two commonly used technologies in optical shaft measurement systems: high-resolution matrix array (also known as CCD cameras) and line scan technology. Line scan technology creates images of the dimensions of the part via cameras that contain a single row of pixels. As the object moves past the camera, the image is reconstructed line by line. Line sensors are often tipped slightly relative to the axis of the part to better measure dimensions such as edges and shoulders. Part and feature diameters are indicated as a series of connected points or dots, and measurement computations are made with a “calculated” image of the part. Because of the lower resolution, small features are harder to measure.

High-resolution matrix array is a more modern technology that was made possible by advances in computer processing speed. The higher resolution of matrix array has earned these systems a reputation for being the most accurate method, because they enable measurements to be more stable and precise, and allow much smaller features to be measured compared to the calculated image produced by a line scan. Zoom functions further allow even the most minuscule details to be measured, which with conventional measuring methods are difficult—if not impossible—to test.

Although matrix array measurements were previously seen as a slightly slower option to line sensors (due to the larger amount of data to be processed) a number of advancements including faster processing time and measurement optimization programs are quickly closing that gap.

Figure 2: Optical systems offer fast measurement of a wide variety of features and can measure many different parts with little or no changeover required.

Speed has been increased by several methods. Processing time has been sped up not only as a result of faster processors, but also the controllers that run them. Optimization routines are also utilized in newer, advanced systems to make the measurement process faster. Previously, a machine was moved to a certain location to take a picture. The area of interest (AOI) was defined within the picture where an edge, radius, or other feature was expected to be found. The process was repeated for each feature to be measured and the program would analyze the data within each AOI to determine the results.

With advanced optical shaft measurement systems, once the programming is finished, the system runs an optimization program that looks at the AOIs and determines, for example, that if several AOIs are close together they can all be captured in one image. The older system would have taken separate images whereas advanced systems can do the same with one image. These systems will also optimize the movement in between image captures to cover the minimum distances between locations.

Software matters: The benefits of an intelligent, integrated operating system

One of the most critical aspects that measurement tools can ultimately provide is valuable data that results in actionable intelligence. For maximum impact, every measurement machine in the production environment should deliver data in a standard format such as statistical process control (SPC) reports—and conform to a standard unified measuring language so that any operator can be easily trained on a single standard interface across machines.

Software is a huge differentiator in terms of delivering valuable, actionable data and ensuring maximum ease of use. Many matrix-based optical shaft measurement systems already provide feedback and integrate with the machine tools and manufacturing systems in their environment. Some also now incorporate their gauges into innovative metrology platforms with intelligent, integrated operating systems that promise even simpler operation, better performance, and faster customization.

If all metrology systems in a manufacturer’s facility are modularized on an operating system level, they use the same basic command and interface structure and therefore will essentially all operate in the same way. As a result, users can more easily utilize different types of measurement tools because the setup looks similar and the same basic steps are taken to control it. Data across all systems can also be exported in a single, standard format such as SPC reports, further maximizing its usability and value.

As a result of operators being trained on a single standard interface across machines, cross-pollination is possible; surface and contour measurements can now be taken on a form machine, and form and contour measurements can be taken on shaft measurement systems. This can dramatically improve the entire dimensional measurement process and increase productivity.

Customization for specialized applications is also simplified. With all of the tools and software libraries from different types of gauges available, the ability to create application or customer specific solutions is greatly enhanced.

In short, intelligent integrated operating systems enable all systems in a manufacturer’s line to operate the same way, minimizing training and maximizing ease of use. Time savings in writing and updating system software is also an added benefit.

Larger systems are frequently used directly on the shop floor to provide fast feedback to the machining centers.

Selecting the right shaft measurement tool

Shaft measurement can be performed with a variety of metrology tools, from handheld devices such as micrometers and snap gauges to high-end coordinate measuring machines (CMMs). However, specific shaft measurement systems can accomplish the required tasks significantly faster, with more precision, and do so directly in the production environment.

Precision-turned parts manufacturers
As the trend continues toward increasingly smaller features and tighter tolerances, noncontact optical shaft measurement devices that are easy to program, perform efficient and highly accurate measurements, and can accommodate a wide range of parts are becoming the norm in this environment.

Figure 3: Larger systems are frequently used directly on the shop floor to provide fast feedback to the machining centers.

Optical shaft measurement systems such as the high-resolution, matrix array systems detailed above can complete many different measurements from a single image. The higher resolutions make measurements more stable and precise, and allow for measurement of much smaller features.

These flexible systems also enable operators to move instantly from one piece to the next to rapidly service several projects/machine parts, which make them an ideal solution for precision-turned parts manufacturers. Optical shaft measurement systems also help ensure consistent and accurate production while delivering the high precision, detail, and efficiency needed to deliver the excellence demanded by precision-turned parts manufacturers.

Dedicated systems for a single part
For companies that produce high volumes of the same shaft parts every day, over long runs, dedicated shaft measurement systems offer great efficiency in that they can perform a number of dimensional checks with a single setup and scan. The construction is custom-built for the specific shaft being measured but typically consists of a collection of gauges mounted onto a dedicated fixture, mechanized to engage simultaneously to provide any number of dimensional checks.

This type of shaft measurement system offers economy and speed of measurement for the specific application.

Figure 4: Dedicated systems are ideal for speed in high-volume production lines.

Smaller volume, more part diversity
For those that need more flexibility in the measuring process but cannot justify the investment in an advanced optical system, a more modularized and generalized version of the fully dedicated gauge described above is a good option. This system is manually operated, but typically features a computer interface for electronic data capture, and consists of a series of measuring modules mounted on a precision bed next to a lathe-like part mounting assembly.

This universal, modular shaft measuring instrument is designed for fast and flexible measurement, so precision workpieces can be cost-effectively produced to a high level of quality across all manufacturing stages. Depending on the configuration, this type of shaft measurement system can perform a number of basic measurement tasks such as length, diameter, radial, and axial run-out, as well as more complex measurements such as distance, width of recesses, depth, diameter of recesses, roundness, taper, angle, radius, coaxiality, concentricity, and more.

About The Author

Patrick Nugent

Patrick Nugent, the vice president of metrology systems for Mahr Federal Inc., is responsible for the metrology systems product line of the Mahr group throughout North America. Nugent works out of the U.S. headquarters in Providence, Rhode Island. He has been with the Mahr group since 1998 and held several different positions prior to the vice presidency at the corporate headquarters in Goettingen, Germany. In addition to his work at Mahr, Nugent is a member of a number of ASME B89 committees working on standards for form measurement and measurement of internal and external standards.