A Guide to Surface Finish

Surface Finish – Definition

In manufacturing processes, a design engineer ensures the quality of the manufactured part by using three important parameters.

  • Dimensions
  • Tolerances
  • Surface Finish

These parameters are critical to the function of the manufactured part. In this article, we discuss the important attributes of the surface finish process.

Surface Finish, sometimes referred to as surface texture or surface topology by machinists, is the quantification and characterization of the texture of the surface. This texture is paramount to the certain physical attributes of the final product. The wear and tear of the machined part, the friction characteristics, and the lubrication requirements are all dependent upon the texture of the part which in turn is based upon the surface finish process. The surface finish texture is quantified by the following parameters:(Complete Surface Finish Chart, Symbols & Roughness Conversion Tables, n.d.)

  • Lay of the surface
  • Roughness of the surface
  • Waviness of the surface

Texture Lay

Different machining processes result in different surface patterns which may be vertical, horizontal, circular, or any other shape. These surface patterns are unique to the machining process used to manufacture that part and this unique and dominant pattern is called texture lay.

Surface Roughness

Surface Roughness, or simply roughness, is the measure of the irregularities in a surface. It is the most measured and quantified aspect of the surface finish, and this is the reason most technicians refer to surface roughness as surface finish.

Surface Waviness

While surface roughness quantifies the irregularities in the surface at a finer level, surface waviness deals with coarse irregularities. These coarse irregularities are at a scale larger than the surface roughness but still, the scale is small enough to be regarded as a defect.

Surface Finish – Units

The surface Finish of a machined part is described using some parameters which are called surface finish units. These units help design engineers as well as technicians to understand the desired surface finish. These units are:(Machining Surface Finish Chart, Conversion, Comparator, Method, Degree, Ra, Rz, RMS, n.d.)

Average Roughness, Ra:

The most common unit to quantify roughness is the Average Roughness (Ra). Average Roughness (Ra) is an arithmetic average between the peak of the roughness profile and a mean line. Since it is an arithmetic average, average roughness eliminates the effect of any outliers in the form of occasional spikes.

However, different roughness profiles can have the same average roughness therefore to completely describe surface finish, we need some additional parameters.

Maximum Roughness Depth, Rmax:

Maximum roughness Depth is the highest vertical distance between the peak and lowest trough in a selected sample length. Since it is the highest value, maximum roughness depth is not an accurate description of surface roughness since any scratch can result in an increased value of surface roughness than the actual value.

Mean Roughness, Rz:

This surface finish unit is preferred across Europe over average roughness. Instead of measuring the distance between the peak and mean line, mean roughness measures the distance between the few highest peak heights and depths of roughness profile in the sample length and then averages the value. This parameter, however, is not an accurate one since it relies on the highest peaks and depths.

Root Mean Square, RMS:

Root mean square, as the name indicates, is the surface finish unit that is similar to the average roughness parameter but instead of averaging the distance, it takes the average of the square of the distance between the peak height and mean length and then takes the square root of the value. This value represents the surface roughness in the form of a sinusoidal wave, each representing the distance from the mean line.
Root mean square is, however, an approximation of the surface roughness and is not an actual representation of the final surface finish. It relates to average roughness Ra, by a factor of 1.1.

Conversion:

Following is an important chart to allow readers to convert between various surface finish units according to the requirements. (Why Use a Surface Finish Chart? | Metal Surface Finish, n.d.)

Surface Finish – Machining Processes:

Surface Finish is highly dependent upon the machining process. Therefore, it is paramount to choose the correct machining process for the surface finish required, or depending upon the need, additional surface finish techniques must be employed to get desired surface finish characteristics. The following table will help our readers to understand the impact of the machining process on the surface finish.


Surface Finish – Symbols and Callouts:

Different industrial standards are followed across the world. However, the majority follows International Organization for Standardization (ISO) and American Society of Mechanical Engineers (ASME) standards. Both standards are listed below:

ISO Surface Finish Standard 1302:

This standard is followed across the world except for the United States of America. (Complete Surface Finish Chart, Symbols & Roughness Conversion Tables, n.d.)

ASME Surface Finish Standard:

This standard is followed mostly in the USA.