If the tool machining the seating surface of a valve seat develops a chip in its cutting surface it will not cut a smooth surface on the part. Instead, a ridge will be created by the void in the surface of the cutting tool. This ridge can prevent the valve from properly seating, causing the cylinder to lose compression. These ridges on the seating surface may be almost microscopic and could be very difficult to detect visually, yet could still have an effect on engine performance. An IOMS probe is able to detect a ridge if one is formed on a valve seat surface. This is shown in the figures below that show the probe signal from an acceptable surface (a) and from a surface with a defective surface finish (b).
The signals shown are single line scans across a valve seat. The defective surface has a dip at the peak because the ridge on the seating surface scatters incident light and prevents some of it from returning to the probe and being detected by the detector in the probe. Since a defective tool will produce a ridge around the complete circumference of the valve seat, it is only necessary to perform a line scan to identify the defect. The probe can quickly identify when a cutting tool tip has become chipped so that it can be quickly changed and not produce additional defective engine heads.