How to Measure Surface Finish | Surface Roughness Conversion Chart | CNCLATHING


With the changes in manufacturing specifications and tightening of dimensional tolerances, the impact of surface finish is growing. How do designers and CNC machinists determine what surface finishes the product requires? Before we get started on how to measure surface finish, let’s understand the definition of surface texture and what is it composed of. Aside from that, you can check out the surface roughness conversion chart here as well.

What is Surface Finish?

Surface finish, also known as surface texture or surface topography, which is produced in every manufacturing process, refers to the nature of a surface of a part, defined by characteristics of lay, surface roughness and waviness. The surface roughness or texture can be optimized or changed through some techniques like the industrial etching to get the desired surface finish. 

How to Measure Surface Finish?

A surface consists of three basic components: lay or form (the result of errors when the lathe producing a CNC turned part), waviness (the result of vibrations from inside or outside sources) and roughness (the result of tool geometry, condition, hardness, variation, etc.). The characteristic of surface geometry is produced due to different causes, they can be separated during the analysis. There are different measuring systems that can be used to measure roughness only or all three. 


The measurement of surface finish can be divided into two types: contact and non-contact, the following are the surface finish measurement instruments, equipment and tools.

Contact: use profilometers to drag a measurement stylus across the surface

Non-contact: interferometry, confocal microscopy, focus variation, structured light, electrical capacitance, electron microscopy, atomic force microscopy and photogrammetry.


The method (technology) for measuring surface finish:

– Profiling techniques: use a high-resolution probe to measure the surface

– Area techniques: measure the finite area and calculate a statistical average of the peaks and valleys

– Microscopy techniques: utilize the measurement of contrast to obtaining information about peaks and valleys


How to determine surface finish?

In the stage of designing a CNC part, the engineers or designers usually need to decide the surface finish. The required surface roughness is depending on the friction of the applications or whether the parts need to reduce wear. The function and uses are critical for the surface texture of a machined component. For example, the optical components used with X-Rays may be required to achieve the finest surface finish. The purpose of a design is to specify surface finishes that are as rough as possible but also function well. 

Surface Roughness Conversion Chart

Here is a table shows comparisons of different surface roughness scales.

Ra = Roughness Average in micrometers or microinches

RMS = Root Mean Square in microinches

CLA = Center Line Average in microinches

Rt = Roughness Total in microns

N = New ISO (grade) scale numbers

Cut-off Length = Length required for sample