Analysing Surface Finish Samples with the MTI-1000 Fotonic Sensor from MTI Instruments

Topics Covered

Introduction
Surface Preparation
Advantages over Stylus Measurements
Results
Conclusion
About MTI Instruments

Introduction

MTI Instruments, a company specializing in non-contact measurement systems, used a standard MTI-1000 Fotonic Sensor to perform a range of experimental tests to analyze a variety of surface finish samples. To perform these tests, the MTI-1000 sensor was adjusted for the usual operation against a finely lapped and mirror-like surface, 2L. Then, without modifying this adjustment, other surface finish samples were replaced.

The extra samples used were 8L, 16G, 63P, 63G, and 125M, which are deemed as standard surface finish reference numbers and specify a significant difference in finishes. They are utilized across the machine tool industry.

Surface Preparation

When the MTI-1000 Fotonic Sensor is used to determine the surface finish or smoothness, the probe is normally isolated from the sample at the distance, and this places the curve of the MTI-1000 sensor at its output peak. The output is not sensitive to small deviation from perpendicularity of the probe or displacement ensuing from non-flat surfaces. The sensor becomes sensitive only to color changes or smoothness.

It is important to ensure that surfaces being viewed are clean and free from dirt, rust or oil. Also, the calibration standards and the item to be viewed should have the same common color. Given the fact that the frequency response of the pickup extends to 2MHz and no carrier frequencies are utilized in the electronics, larger surfaces can be scanned more rapidly.

Another viable method is to perform in-line assessment of the whole surface of the parts. The MTI-1000 is capable of detecting even surface scratches that are less than a microinch wide. Without motion, the system can integrate smoothness across the entire area under observation.

Advantages over Stylus Measurements

With different fiber optic configurations, the Fotonic Sensors have relatively different capabilities. Smoothness can be determined sans shifting the part or probe, but in stylus measurement motion is important. Most surfaces exhibit a different roughness in different directions or planes.

As a result, it is difficult to define the actual overall finish. For instance, a groove can have an extremely smooth bottom, but may also have an irregular peak and side. When measured with a stylus, the surface roughness may differ significantly, depending upon the measurement direction.

Since the MTI-1000 sensor integrates the surface being viewed, the results obtained are far more convincing than the results achieved by passing a stylus down a groove. In fact, more reliable indication of fluid seal leakage can be realized with the MTI-1000 sensor than with the stylus versions of surface finish measurement.

Although fluid may leak through the apparent scratch, the leakage appears to be associated with ridge or peak roughness which improves the seal from the surface at the static or dynamic level.

Results

Each output curve was normalized and plotted separately in contrast to the reference 2L surface. Irrespective of the wide difference in surface condition and finish, these results reveal a small error in the usual calibration procedure. This precision is present up to a surface finish of No. 63 with around 0.5 mil equivalent displacement error up to a surface finish of No. 500.

The slope of calibration curve, that is the dynamic error, is almost indiscernible and could be below the experimental error of the tests themselves. Results of the experiments are shown in Figure 1.

Figure 1. Results show a relatively small error in the standard calibration procedure regardless of the wide variation in surface finish and condition.

Conclusion

The smoothness of the surface can be measured at the time of machining during the last or before the final machining operation. Conditions can then be changed prior to scrapping a one dimension part, but this may display a poor surface finish. Hence, on line and adaptive measurements of finish provide a practical solution and could form a part of present and future machines.

About MTI Instruments

MTI Instruments is a worldwide leader in the design, manufacture and engineering of non-contact measurement systems and sensors.

MTII’s main products consist of computerized general gauging instruments for position, displacement, thickness and vibration applications based on laser triangulation, fiber-optic and capacitance measurement technologies.

The Semiconductor Products sensor group manufactures manual, semi-automated and fully automated wafer characterization tools designed to measure wafer thickness, total thickness variation (TTV), bow, warp and flatness of semi-insulating and semiconducting materials.

MTII’s Aviation Balancing Instruments group provides state-of-the-art portable balancing and vibration analysis systems for turboprop and jet aircraft engines.

This information has been sourced, reviewed and adapted from materials provided by MTI Instruments.

For more information on this source, please visit MTI Instruments.

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