The roughness Ra is, in mechanics, an average characterizing the surface condition of a part or material. In industry, roughness control is part of the dimension validation process and ensures part conformity during machining. There are other indicators that characterize the roughness of a surface, but in this article we will focus on the general surface condition of the control area.

What is the roughness Ra?

Definition

The Arithmetic Average Roughness, noted as Ra, is the absolute average relative to the base length. For simplicity, the Ra value indicates the average surface roughness for the length of the measurement performed, i.e., the average difference between peaks and valleys.

To schematize what roughness is, we can imagine a two-dimensional cut of the surface. The higher the roughness of this surface, the greater the difference between the peaks and valleys of the surface.

The roughness Ra is an excellent way to quickly check that the surface corresponds well to the customer’s request, its value is representative of the general condition of the surface.

Why should you care about the surface roughness?

In some sectors, particularly high technology, it is necessary that the roughness of the components produced be in line with expectations. Roughness plays an important role in mechanics, it allows to control phenomena such as flow or adhesion. This is an important dimension to consider in precision mechanics because it has a significant impact on friction force, wear and mechanical anchoring (among others).

If the average roughness of a surface is determined using the Ra indicator, it is possible to quickly determine the micro- or macro-geometric irregularities of the surface. This is how the part can be designated as compliant or non-compliant during the quality control process.

Other roughness indicators

In addition to Ra, when trying to assess the surface condition, it is possible that we also look at indicators like Rz or Rt to gain more information:

  • The Rt defines the total roughness recorded, i.e. the sum of the maximum peak with the maximum valley. This is the largest distance recorded for the measurement.
  • The Rz defines the regularity in the recorded measurements. It is an average of the five largest total deviations (Rt) recorded for length.

These indicators make it possible to know more precisely the macro-geometry of the surface. They are sometimes used as selection criteria in quality control and modern measuring instruments mention them in their results.

Measure the roughness of a surface

Several tools are available to measure the roughness of a surface. The choice of the tool to be used depends essentially on the needs of the control and its specificities.

Roughometers

The roughometer is a contact measuring machine that allows you to quickly determine the roughness of a surface. The main advantage of these devices is that they are very simple to use and often easy to move, so all you have to do is place them on the surface to be inspected and start them up to perform a measurement.

There are two types of roughometers: skid and non-skid roughometers.

In the first case, the machine has a small skid at the end of its head that slides on the control surface and serves as a straight guide. The skid can affect the measurement if the surface is not flat, so you should only use it on the flattest possible surfaces. Despite this defect, roughometers with skid are a very handy and efficient way for measuring flat surfaces.

In the second case, machines without skids, the devices use an internal guide that calculates the orientation of the probe in the 3D space, ensuring measurements on non-plane surfaces. The ability of devices without skids to recalculate trajectories allows for more accurate control, where devices with skids could complicate or even distort control when the surface differs slightly.

Optical Measurement

Roughness Ra can be calculated very easily with roughness Ra, but they are mechanical devices, which require physical contact between the probe and the surface. Therefore, if more accuracy is required, it is preferable to use an optical measuring system such as an interferometer.

Interferometers are measuring machines capable of giving the surface topography of any material. They are much more accurate and efficient than physical contact measuring systems because they use the interference effects produced when light is superimposed when it is projected and reflected by the measurement area. This is currently the most efficient method to obtain very accurate values for surface roughness.

Indirect control of the roughness Ra

The surface condition may sometimes be impossible to measure because it is impossible to access the measuring devices. There are many reasons for this, mainly one of these three:

  • It would be necessary to dismantle a seemingly assembled part to bring it to the measurement laboratory.
  • The part is too bulky and it is difficult to position it for the machine.
  • The area to be checked cannot be reached by your control instrument.

It is in this kind of situation that it would be extremely practical to be able to control the part indirectly, and that is exactly why Plastiform has developed impression products capable of performing such operations.

Indirect, delocalized Ra roughness control.

Plastiform has developed impression products. These products allow the properties of the control area, including its surface finish, to be copied with great precision. If for any reason it is impossible for you to check the part, Plastiform will undoubtedly help you solve this problem.

P80 Ra is a pasty impression product (which adheres to the surface, even if it is overhanging). By using it on your part, you can copy its properties, the product then stiffens to 80 shore A, which will allow you to perform your measurements easily on the impression, directly and with your usual measuring devices.

Check the Roughness Ra on a Plastiform impression

Depending on the measuring instrument you have, the accuracy of the measurement will be more or less important. We recommend the use of non-contact measurement systems, such as a white light interferometer, which are much more accurate and suitable for measuring the surface condition of impressions.

If you can’t get one, choose a skidless roughometer, which is much more suitable for measuring the roughness of an impression. Whether with the roughometer or the interferometer, the Ra values will be accurate, the only difference will be with respect to the other indicators that will be given with much more precision with an interferometer.

Roughometer with skid also works very well on our footprints. However, it will give you rough estimates of Ra, and the other values (Rt and Rz) will be distorted because of the pressure exerted by the skate on the print.

At Plastiform, we recommend performing the impression checks with the devices for which we have validated the compatibility, i. e. Accretech and Sylvac.

Conclusion

Roughness is an indicator of the macro-geometry of the surface of a part or material that depends on the treatments that the production line has subjected it to. To measure this indicator, we use various measurement instruments that measure the difference between “valleys” and “peaks” recorded over a given length and allow us to calculate several characteristics:

  • The Total roughness, noted Rt, which indicates the maximum deviation recorded over the length. Rt is calculated by adding the maximum peak added to the maximum trough recorded over the length.
  • The regularity, noted as Rz, which indicates the average of the main 5 Rt over the length.
  • The arithmetic mean roughness, noted as roughness Ra, which indicates the average roughness of the controlled surface.

An important parameter is the roughness Ra, because it directly characterizes the condition of the surface in general. It makes it possible to evaluate the latter in its entirety.

To check the surface condition, it is possible to use various devices, with or without contact. Generally, Roughometers or interferometers are used. Sometimes it is impossible for devices to access the desired control area. It is in these cases that Plastiform can save your life. Use Plastiform to copy the control area and perform your measurements directly on the impression with your measuring devices! Opt for our personalized case, you will have everything you need to try the impression controls!