What is Surface Roughness

This page introduces basic knowledge about surface roughness and Ra, which are important indicators in precision polishing and precision machining.

・A list of surface roughness and Ra achievements for various materials that have undergone mirror finishing at TDC
・Precision measurements conducted at TDC
Please also check these related sections.

What is surface roughness?

The surface of the workpiece has complex undulations with continuous peaks and valleys of different heights, depths, and intervals. Those with large undulations are called “flatness”, and those with small undulations are called “surface roughness”.
Surface roughness affect not only the precision of processed products, but also the gloss and texture of the surface, so they are one of the factors that influence the product image and product brand.
Especially in the field of ultra-precision machining, not only machining accuracy but high surface treatment ability is required, so the surface roughness is extremely important.

Measurement Methods

There are basically two types of Measurement Methods for surface roughness; one is contact type that uses a stylus and non-contact type that uses a laser or light.

Contact measurement method

A method of measuring the surface condition of a part by bringing the tip of a stylus into contact with the object to be measured. Detects vertical changes and displacements of the stylus and records the surface roughness.
In addition to being able to obtain a clear shape waveform because it is touched directly, it has the advantage of being able to measure long distances.
The standard for evaluating and analyzing surface roughness is “JIS B 0601 (ISO4287)”.

 Non-contact measurement method

The “non-contact type” is a method of measuring surface roughness using light or laser instead of a stylus. There are multiple measurement methods for non-contact type due to the difference in the principle of measurement method. In addition to surface roughness, there are many that measure the shape, and by converting the measurement results into three-dimensional data, various measurements such as “height”, “flatness”, and “plane profile” can be performed.

The non-contact type has the major advantage of being able to solve the problems of the contact type, such as wear of the stylus, indentation on the object, and limitation of the minimum measured value. Is also a feature.

The standard for evaluating and analyzing surface roughness is “ISO 25178”.

Surface Roughness Parameters

Surface roughness use different calculation formulas (parameters) depending on the measurement content.
Typical parameters for surface roughness are as follows.

Ra

It is a parameter in the height direction called “arithmetic mean roughness”, and the arithmetic mean roughness when the contour curve is a roughness curve is called “Ra”.
A part of the roughness curve is used as the reference length, and the unevenness of that section is expressed as an average value.
Since the average value is used, it is not easily affected by one scratch, and it is a parameter that can obtain stable results when evaluating surface roughness.
It is written as Sa in “ISO 25178”.

Rz

It is a parameter in the height direction called “maximum height”.
A part of the roughness curve is used as the reference length, and it is calculated by the sum value of the highest part and the deepest part in the section.
It is possible to check the presence or absence of protruding scratches, and together with Ra, it is one of the indicators of quality stability.
In “ISO 25178”, it is written as Sz.

Rq（Rms）

This is a parameter called “root mean square roughness”.
There are always peaks and valleys in the surface roughness of an object, and the average roughness on the peak side (height direction) is calculated.
In “ISO 25178”, it is written as Sq.

Standard	ISO 25178	JIS B 0601-2001 (ISO 13565-1)
Maximum profile peak height	Sp	Rp
Maximum profile valley depth	Sv	Rv
Maximum height of profile	Sz	Rz
Arithmetical mean height	Sa	Ra
Root mean square deviation	Sq	Rq
Skewness	Ssk	Rsk
Kurtosis	Sku	Rku
Spatial Parameters	Sal、Str、Std	–
Hybrid Parameters	Sdq、Sdr	RΔq

Necessity of smooth surface

Measurement of surface roughness is important not only for controlling the finished state and appearance quality of processing, but for controlling product life and mechanical efficiency.

By measuring surface roughness such as Ra / Rz / Rq, you can mainly know the following functionality.

・ Appearance
Appearance such as scattered light reflection and glare.

・ Air density
The roughness between the contact surfaces has an effect on leakage from gaps.

・ Texture

・ Friction force

・ Adhesiveness
Optimal coating for packaging and adhesives. Coating adhesion.

・ Print quality
Paper texture and ink ride.

・ Noise / vibration
Vibration of the surface that rolls at high speed.

・ Easy to peel off
Easy to peel off the mold and molded product.

・ Optical performance
Disturbance or scattering of refraction of luminous flux.

·Abrasion resistance
Ease of scraping due to load concentration during sliding.

・ Fatigue fracture / strength
Strength against fatigue fracture due to stress concentration.

・ Joint surface rigidity
Increased deflection due to load concentration on the joint surface.

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