What to Know About Surface Finish Measurement

Surface texture, or finish, can be looked at from two points of view. For the machinist, texture is a result of the manufacturing process. By altering the process, the texture can be changed. From the designer’s point of view, surface finish is a condition that affects the functionality of the part. By changing the surface finish specification, the part’s functionality can be altered.

Between the two is the viewpoint of the manufacturing engineer, who determines how the machinist is to produce the surface finish specified by the design engineer. The method chosen to measure surface finish depends on who needs to be satisfied.

Texture consists of the peaks and valleys that make up a surface and their direction on the surface. Machining processes, such as turning, milling and grinding, impose these irregularities on a part’s surface. Additional factors, such as cutting tool selection, machine tool condition, speeds, feeds, vibration and other environmental influences, further influence these irregularities. What components make up texture? Texture can be broken down into these three components:

• Roughness is essentially synonymous with tool marks.
• Waviness is the result of small fluctuations in the distance between the cutting tool and the workpiece during machining.
• Form (or rather bad form) is due to a lack of straightness or flatness in the machine tool’s ways.

All three components exist simultaneously, superimposed on one another. In many cases, it is desirable to examine each condition independently. Surface finish tends to be a stable condition and should not change from part to part unless process conditions change. Gages separate surface finish components using discrete units of length, called cutoffs. The length of the cutoff selected and used by various electrical filtering techniques permit the measurement of roughness by itself, waviness by itself or “total profile,” which combines roughness, waviness and form. By examining roughness, waviness or total profile separately, manufacturing engineers can narrow the search for sources of error, and take effective action to reduce or eliminate them.

Parameters are the quantitative methods used to describe and compare surface characteristics. These are defined by algorithms, which turn raw measurement data into numerical values. Although more than 100 parameters exist, machinists have traditionally relied upon just one or two.

Currently, roughness average (Ra) is the parameter most widely specified and measured. The algorithm for Ra calculates the average height of the roughness peaks on the surface, within the sampling length, from a mean line. It serves as an effective means of monitoring process stability, which explains why it is the predominant parameter in use today. Mean roughness depth (Rz) is widely preferred over Ra in Germany and elsewhere in Europe. Rz is based on the evaluation of five sampling lengths and averages the five largest peak-to-valley increments.

Through parameters, surfaces can be defined and described in great detail, and engineers have made correlations between parameters and part performance under various conditions. When a design engineer specifies a surface finish parameter and value, it is done with an understanding of how they will affect the part’s performance.

After the part has been designed and manufactured, it must, of course, be inspected. For surfaces specified only by a roughness parameter, this is simple. Pocket-sized, battery-powered gages that offer a small number of roughness parameters are available at low cost. They are easy to use and can be very flexible. More complex parameters require full-featured instruments that are run by computers and may cost thousands of dollars. 

Standards are written around the use of instruments that measure part texture by moving a stylus in a straight line across the surface and by monitoring the vertical movement of the stylus. Generally, the less-expensive stylus-type gages that measure only roughness use the surface of the part itself as a reference. These are called skidded gages. In contrast, full-featured instruments incorporate a precision internal reference surface, which enables them to measure waviness and total profile in addition to roughness. These are called skidless gages.

Inexpensive, compact “roughness” (skidded) gages, however, often retain their traditional utility, even where more complex parameters are specified. A shop may maintain one “roughness/profiling” (skidless) system for manufacturing engineering and quality assurance purposes, while making basic roughness gages readily available to machinists. Once the process is established and confirmed on the roughness gage, the design engineer uses the profiling gages to measure parts for their specific parameter call out.