Surface Finish Measurement Errors

The aim of any measurement is to provide the best results possible. The same is true for surface finish measurement. The goal of surface finish metrology is to provide a measured value that is as accurate as possible.

Therefore, it’s good to understand what adverse influences affect the results. If the controllable influences are known and considered before the measurement is made, then the results can only become more reliable.

The sources of surface finish error can be broken into the following categories:

Non-Adherence To Measurement Conditions

When making a surface finish measurement, there are a number of setup conditions that need to be met. These are cutoff length, measuring length and type of tracing system.

One of the most frequent causes of incorrect measurement is the selection of the wrong cutoff wavelength. These are usually dependent on reference tables found in standards. Selecting the incorrect cutoff length could make the values appear larger or smaller than the desired values.

Likewise, the measurement length is also specified by a standard. It depends either on the roughness spacing (with periodic profiles) or the roughness depth (with random profiles). For some parameters, the measuring length has considerable influence on measurement results because the long wave profile components (form error) being assessed depend on the measurement length.

Finally, the tracing system can also influence results. This is true especially with skidded tracing systems. Measurement values are affected by surface structure and the type of tracing system used. Skidded systems are generally used only to measure smaller surface irregularities such as roughness because the skid mechanically suppresses the entire form and some of the waviness error.

Different Quality Grades Of Surface Finish Measuring Equipment

The accuracy of the measurement system is affected by a number of factors inherent in the surface measuring instrument. Some of these factors include:

• Quality grade of the instrument. Surface measuring instruments fall into different grades, which can have inherent errors of ±5 percent to ±25 percent.
• Stylus or radius tip. Stylus tips are available in 2µm, 5µm and 10µm, with tolerances associated with each size. Depending on the stylus radius and the surface structure, the tip itself may act as a filter and affect the results.
• Stylus force. The force the stylus exerts on the surface may lie between 0.4 mN and 16 mN depending on the system. With higher forces, the surface may be damaged by the stylus tip and the results falsified. On the other hand, if the force is too light, then the tip may “float” and not stay in contact with the surface continuously.
• Tracing speed. The speed of the measurement is not standardized and may differ according to the instrument used. However, if the trace speed is too high, it is likely that the fine profile structures will not be truly recorded.
• Sample rate. With digital surface measuring instruments, the resolution of the traced surface profile depends on the sample rate. The sample rate is one of the key factors in collecting the most data correctly, because having too few measuring points may cause important features to be missed.

Faulty Measurement Setup

The measurement setup normally consists of the measuring stand, the drive unit, the tracing system and the part to be measured. The uncertainty of the measurement is caused by a series of systematic disturbances in the measuring loop. The most common causes of trouble are:

• Vibrations in the environment.
• Part movement during measurement.
• Driving and guiding difficulties in the drive unit.
• The wrong tracing system causing distortions.
• Tracing system not aligned parallel to the test piece.
• Dirt on the surface.

If skidless tracing systems are used to assess fine surface irregularities, the measuring accuracy can be highly affected by vibration. In these cases, the measurement setup should be checked prior to making a measurement in order to assess the extent of environmental influence. Also, to help reduce these influences, keep the measurement loop as small as possible.

Finally, one of the biggest enemies of the precision measurement, as always, is dirt. An insufficiently cleaned surface will result in both inaccurate and unrepeatable measurements. Make sure that the surface area to be measured is free of coolant, oil, chips, dirt, dust and other contaminants.