What is Ra Value in Surface Finishing | Ra Value Chart & Difference Between Ra and Rz

Both Ra and Rz values are representations of surface roughness, what do they mean and what are the differences between them? Let’s learn about what is Ra value in surface finishing, the Ra value chart for different machining processes, and Ra vs Rz.

What is Ra Value in Surface Finishing?

Ra is a measurement unit of surface roughness, Ra is defined as the arithmetic average of the absolute value of the Z-direction deviation relative to the mean line in a sampling length. Ra values are average measurements of surface peaks and valleys. Ra is a commonly used parameter in industry, it ensures the stability of the parameters and is not affected by accidental stray spikes or scratches. Surface finish often refers to surface roughness.

Ra vs Rz - Difference Between Ra and Rz

Ra refers to the average surface roughness or individual variances of the peaks and valleys average, it is calculated by measuring the average length between the peaks and valleys and the deviation from the mean line on the surface within the sampling length.

Rz refers to the average maximum height of the profile, or the largest difference from peak to valley, it is calculated by averaging the measurements of the vertical distance from the highest peak to the lowest valley within five sampling lengths.

Ra can’t reflect the surface features of the tested part, and the surfaces obtained by different processing methods can have exactly the same Ra value. So sometimes Rz is required to eliminate errors.

When to use the Rz value?

– For other extremely rough or smooth surfaces, considering working and testing conditions, Rz is probably needed.

– When the part material is soft, Rz should be used. Because the value of Ra is generally measured by needle tracing, it will scratch the surface of soft material parts, and the measurement result is inaccurate.

– For a very small measurement area, Rz should be used when the sampling length has less than 5 internal contour peaks or valleys.

– Parts with fatigue strength requirements.

How to Measure Surface Roughness?

Surface roughness is the measure of the deviation of the surface of an object from its ideal shape. There are several methods for measuring surface roughness.

1) Contact Methods: a probe is brought into contact with the surface, and the surface roughness is measured based on the amount of deflection of the probe. The most common contact method is the stylus method, which involves a stylus that is moved across the surface, and the deflection is recorded.

2) Non-contact Methods: In this method, the surface is scanned with a non-contact method such as a laser or an optical system, and the surface roughness is measured based on the reflected or scattered light. The most common non-contact method is the optical method, which involves projecting light onto the surface and measuring the reflected light.

3) Profilometry: involves scanning the surface with a probe or stylus and measuring the height of the surface at various points. The surface roughness is then calculated based on the profile data obtained.

4) Image Analysis: the surface is imaged using a camera or a microscope, and the surface roughness is measured by analyzing the image. This method is suitable for surfaces with fine features that cannot be measured by other methods.

Ra Value Chart - Ra Value for Different Machining Processes

Surface roughness measures the irregularities on the part surface, it will be affected by the manufacturing and finishing process.

Ra Value for Different Materials in Machining

The surface finish requirements for different materials in CNC machining are affected by the process, cutting speed, tool geometry, depth of cut, and other factors. There is a typical range of common metals.

– Steel: Ra values for steel can range from 0.2 µm to 3 µm depending on the machining process used. For example, turning typically produces Ra values between 0.4 µm and 1 µm, while grinding can achieve Ra values as low as 0.2 µm.

– Aluminum: Ra values for aluminum are typically lower than those for steel, ranging from 0.1 µm to 1 µm. Machining processes such as turning and milling can produce Ra values between 0.2 µm and 0.5 µm, while grinding can achieve Ra values as low as 0.1 µm.

– Copper: Ra values for copper can range from 0.1 µm to 1 µm. Machining processes such as turning and milling can produce Ra values between 0.2 µm and 0.5 µm, while grinding can achieve Ra values as low as 0.1 µm.

– Titanium: Ra values for titanium are typically higher than those for steel, ranging from 0.5 µm to 4 µm. Machining processes such as turning and milling can produce Ra values between 0.8 µm and 1.5 µm, while grinding can achieve Ra values as low as 0.5 µm.

What is Ra Value in Surface Finishing | Ra Value Chart & Difference Between Ra and Rz