# Determining the Optimum Surface Finish for the Tooth Flanks of Gears

## Minimizing Refining Steps for Gears

Researchers at Lund University have created a simulation model to estimate the suitable machine parameters for a form milling cutter, to help prevent expensive post-processing. This step was undertaken to make sure that the tool would generate tooth flanks with improved surface quality. The researchers applied Alicona systems at Sandvik Coromant to authenticate the mathematical models and substantiate their suitability for practical application.

Thanks to the high working distance, we were able to measure the roughness of tooth flanks that were previously inaccessible to us.

Mattias Svahn, Lund University

Since there is competition across the world, cost pressure is continuously on an upward trend. This makes it essential to optimize the efficiency of methods used in gear manufacture. One of the chief factors for cost increase is post-processing, which comprises of refining steps, including honing and grinding to guarantee the accurate roughness of tooth flanks.

This method could be reduced if it were possible to manufacture almost perfect gears with better surface quality that require little to no post-processing. To make this possible and to guarantee that gears are manufactured with the necessary roughness, it is important to estimate the exact machine parameters for the tool used, for instance for a form milling cutter.

The fatigue, uniform transmission of motion, and service life of the gears are impacted by the roughness. The roughness is mainly established by potential errors linked the tool, the feed rate, and the machining process. As a result, it is of major economical interest to estimate which roughness values arise from varied machine parameters, and the way in which potential error sources in milling impact the cut surface.

Based on this reason, Lund University began a research project to analyze this very question using a simulation. The team built a mathematical model to study how machine parameters and potential isolated or combined error sources impact the cut surface roughness.

This feat was achieved in partnership with Sandvik Coromant, a well-known Swedish tool manufacturer that recently introduced a novel range of form milling cutters. Here, Alicona systems were utilized to confirm whether the roughness values measured in the model could in fact be manufactured in reality, and detect error sources in the milling method. Measurement of areal roughness allowed Lund University to verify the model at the required level of quality.

“We carried out the areal roughness measurement on-site at Sandvik Coromant and got to know Alicona in the process. The high precision and speed of the measurements immediately convinced us to purchase our own InfiniteFocus system,” explains professor Carin Andersson.

Figure 1. Calculated roughness versus measured roughness: The team formulated a mathematical model so as to test how possible error sources and machine parameters find their effect on the cut surface roughness.

Figure 2. Variation between measured surface and calculated surface. Alicona systems were used to validate whether the roughness values calculated in the model could be created in reality.

Figures 1 and 2 show calculated roughness vs. measured roughness, and variation between measured surface and calculated surface, respectively.

## Roughness and Positional Tolerance

A tooth flank’s quality is established by both its profile precision and its roughness. The roughness of the tooth flank plays a significant role in many ways. For instance, it directly impacts noise generation - the rougher the surface, the noisier the gear. However, uniform transmission of motion largely relies on the positional tolerances and form of the tooth flank.

It is crucial to determine both form and roughness to ensure good quality assurance of gears. During roughness measurement, it is vital to consider the gears’ dominant surface structure and select a suitable measurement technology for this reason. An Alicona system was used by Mattias Svahn, lead researcher and measurement expert, because he is well aware of the fact that simple profile-based roughness measurement would not bring about useful results.

Profile-based measurement allows me to map the surface only partially. A lot of important information is lost by only a few line measurements along the tooth height and the tooth width. The resulting measurement values are simply not useful for validating the calculation model.

Mattias Svahn, Lund University

On the contrary, the measurement systems supplied by Alicona help in mapping the roughness of the whole surface, even of the tooth flanks - repeatedly, quickly, and at high resolutions. The surface texture parameters Sa, Sq , and Sz enable accurate evaluation of the surface quality.

By using difference measurement, discrepancies in the form can be made visible. This is realized by comparing measurement results to a CAD dataset and/or positional tolerances and form. Along with roughness and form measurement, Lund University also makes use of the visualization of 3D data sets. The large vertical and lateral scanning areas make it helps in mapping the topography of the whole gear cutting.

”Thanks to Alicona, we have been able to minimize the time and cost-intensive refining steps of gears. We were blown away by the capabilities of the InfiniteFocus system we got to know at Sandvik Coromant. There is no measurement system we know that is capable of measuring critical form and positional tolerances and roughness of tooth flanks in this way with just one system.“ Mattias Svahn.

This information has been sourced, reviewed and adapted from materials provided by Alicona Imaging GmbH.

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