Multi-Sensory Capability in Gear Inspection

February 14, 2014

CNC gear metrology machines provide a stable, robust platform, able to position and control movement in both linear and rotational axes.

In today’s competitive manufacturing environment, especially for products as specialized as gears, many of the world’s leading gear OEM’s and their top suppliers are demanding greater system capability and utilization from their investments in quality control. Just measuring gear geometry is no longer sufficient. For gear metrology, where the demand for accuracy is at sub-micron levels, performance in workshop environments and support of other materials testing is becoming more common every day.

The answer is obvious—expand the capabilities of the machine. CNC gear metrology machines provide what high accuracy measurement systems require most: a stable, robust platform, able to position and control movement in both linear and rotational axes. Adding the functionalities of surface finish measurement, form measurement, grind burn detection, and general prismatic (CMM) measurements increases value for the user of this equipment.  Figure 1

The ability to accurately control motion to a micron level of performance, combined with a robust system for measuring the complex geometries of gear tooth forms, offers a system platform that can be expanded in capability. It also eliminates the need for additional highly expensive, dedicated equipment that can only perform a single task, is costly to maintain, takes up valuable space, and adds redundant costs for fixturing, setup, programming, calibration, and repairs.

When a gear is set up for measurement on a Gleason GMS inspection machine, for example, it is convenient to test for other quality characteristics called out on most part prints today (surface roughness, for instance). Although such machines had this feature in the late 1990s, it was limited. Digital probing systems were still in their infancy and were not as versatile and reliable as they are today—the integration was not as seamless as most had hoped. Graph 1

Fast-forward to today’s motion control technology—systems integration techniques and capabilities, coupled with market demand, makes the picture of meeting manufacturer’s needs much clearer.  Today’s systems utilize devices kinematically coupled to motion control systems, which allows the stylus to automatically:

• Move normal to the surface of the workpiece
• Deflect the probe at a constant force
• Gather data
• Determine roughness to whatever parameter required

—all while not requiring manual intervention with the measurement, or manipulation of the part.  No special fixturing or material handling, outside of loading the part, is needed for comprehensive measurement.

Since we understand it is not just surface finish but also waviness that affects gear behaviors, the Gleason GMS systems add the ability to analyze this waviness. All analysis is shown after measurement on screen and is printed in additional pages in the inspection report. Figure 2

In adding these capabilities to the GMS series of machines, careful consideration must be given to a number of factors.  Actual capacity of the gear metrology machine must be taken into account, as well as the design and integration of accurate sensors for surface roughness measurement and grind burn detection (Barkhausen Noise Analysis), in order to properly accommodate these added applications.  In addition, a thorough review of parts requiring this testing must be conducted and factored into the design phase.

In 2011, Gleason Metrology Systems partnered with American Stress Technologies, a global supplier for Barkhausen Noise Analysis (BNA) and a recognized world leader, especially in gear inspection. Gleason devised a way to mount a StressTech RollScan® unit on GMS machines, sizes 350 and up and apply motion control to follow the lead profile at various radial positions in order to create a full flank analysis of residual and compressive stresses induced by hard finishing processes.  This type of testing is non-destructive so that when the part tests as “accepted,” it can be used as a production workpiece, whereas the more traditional Nital Etching process often renders it unusable.

This capability has proven extremely valuable for large gears where the lot size is typically small (1-5 pieces), materials and process times are expensive, and cost savings are quickly realized. BNA is also a great predictor of sub-surface defects in the microstructure of the material and is a valuable tool for assessing hardness, especially when compared to older, more destructive testing.

GMS has taken the output from the StressTech unit and devised a charting technique that is easy for the average operator to interpret. If all the trace outputs are uniform, as in the first graph, it’s easy to see that tooth form is consistent and the material is uniform in respect to residual and compressive stresses. In contrast, when levels of residual and compressive stresses are vastly dissimilar, measurement of a tooth form surface becomes very erratic makes the determination of where in the signature, the surface anomalies actually exist.  Being able to decipher the output is quite difficult even for skilled operators. Figure 3

Enter GMS software innovation; extrapolating those output signals to create an easily understandable graph that shows, at a glance, where the material defects occur.   The output graph is rectangular represents the tooth surface area. The graph’s horizontal, or X axis shows the measureable tooth surface in the lead direction of the part. Not all of the tooth surface can be measured, as the sensor must stay fully on the tooth surface.  The sensor, in this case, covers a surface area equivalent to one square milimeter. The vertical or Y axis depicts the radial depth or number of scans that were taken along the lead direction. Again, this measurement does not go into the root or to the very edge of the tooth tip. The rectangular graph is then color coded based on the analog signal generated by the RollScan unit, ultimately displaying a colorized topographical map of the surface, red in color where grind burn is present and blue in color where the surface is in “normal” process condition.

As with all metrology systems, certain limitations may exist and consulting with our applications department will best determine which models and compliment of software will best fit your particular applications. All rotationally symmetrical work pieces can apply these technologies. Graph 2

A third addition to GMS inspection machine functionalities is a CAD-based inspection package.  Geometric analysis is no longer confined to the tooth form, though admittedly this is still quite complex, given the number of profile and lead modifications used today to make gear sets perform to design intentions. More and more gear sets are designed in CAD and as part of assemblies that require measurement at various stages of assembly.

Recognizing this, Gleason Metrology Systems now offers CAD-based inspection to complement other inspection packages. The company has integrated the capability to measure non-gear features into a measurement platform.  Programming with popular CAD models is supported.  

This opens the door for GMS to support manufacturers of non-gear parts who require high-accuracy inspection on any rotationally symmetrical workpiece, where the rotation of the part is integral to the inspection and not just handled as an indexing axis to present the part to the measurement probe.  

The modular structure of this CAD inspection system allows users to purchase simple, basic, and advanced versions, with upgrade costs well below industry standards. This allows users to enhance system capabilities as they learn or their needs change. Advanced CMM software capabilities, including direct cad model programming:

• Supports
• Unlimited stylus calibration library
• Reverse engineering
• Digitizing
• Graphical reporting
• On-line and off-line programming with tool path debug and crash detection  

About The Author

Dennis Traynor

is sales manager for Gleason Metrology Systems. Founded in 1865, Gleason Corporation is a global leader in the technology of gearing. Products and services include machinery for the production, finishing and testing of gears as well as a worldwide support system which provides cutting tools, workholding, replacement parts, field service, application development services, gear design and inspection software, training programs, engineering support and machine rebuild and upgrade services. For more information, visit