Surface Roughness

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Surface roughness.

Surface roughness indicates the state of a machined surface.

For example, when representing the surface of a component, surface roughness can be examined by eye or rubbed with a fingertip. Expressions used to describe surface roughness include "shiny and pretty," "lusterless and rough," "like oxidized silver," or "like a mirror." Those differences are caused by the differences in the irregularities of the component surfaces.

When a surface's level of shininess or asperity is clearly quantified, it is called surface roughness, which plays an important role in defining the character of a surface. The surface irregularities of a component or material may be intentionally created by machining, but they can also be created by a wide range of factors such as tool wobbling caused by motor vibration during machining, the quality of the tool edge, and the nature of the machined material. The form and size of irregularities vary, and are superimposed in multiple layers, so differences in those irregularities impact the quality and functions of the surface. The results of these irregularities can control the performance of the end product in aspects such as friction, durability, operating noise, energy consumption, and airtightness. If the products in question are printing paper or exterior panels, aspects of quality such as glossiness and adhesion of paint and ink can also be affected by surface roughness.

Why do we measure surface roughness?

The shape and size of irregularities on a machined surface have a major impact on the quality and performance of that surface, and on the performance of the end product. The quantification and management of fine irregularities on the surface, which is to say, measurement of surface roughness, is necessary to maintain high product performance.

Quantifying surface irregularities means assessing them by categorizing them by height, depth, and interval. They are then analyzed by a predetermined method and calculated per industrial quantities standards. The form and size of surface irregularities and the way the finished product will be used determine if the surface roughness acts in a favorable or an unfavorable way. Painted surfaces should be easy for paint to stick to, while drive surfaces should rotate easily and resist wear. It is important to manage surface roughness so that it is suitable for the component in terms of quality and performance.

Many parameters have been established regarding the measurement and assessment of surface roughness. As machining technologies progress and higher-quality products are demanded, the performance of digital instruments continues to improve. The surface roughness of more diverse surfaces can now be assessed.

What are the methods for measuring surface roughness?

Surface roughness measurement methods include linear roughness measurement, which measures a single line on the sample surface, and areal roughness measurement, which measures an area of the surface. Linear roughness measurement has been the norm until now, but expectations for areal roughness measurement, that can see a surface as it really is, has been rising in recent years.

Profile method type (referred to as "linear roughness")

linear roughness
• The degree of roughness in the surface is measured along an arbitrary straight line

• Compliant with ISO and other national standards
• Long, continuous dimensions can be measured

Areal method type (referred to as "areal roughness")

areal roughness
• The degree of roughness in the surface is measured over an arbitrary rectangular range

• The surface is seen as it really is, giving a more accurate grasp of the state of the surface

There are two ways to measure surface roughness.

The instruments for measuring surface roughness can be broadly divided between contact and non-contact types.

Contact Type

Contact Type
Contact-type roughness instrument
With this type, the tip of the stylus directly touches the surface of the sample. As the stylus traces across the sample, it rises and falls together with the roughness on the sample surface. This movement in the stylus is picked up and used to measure surface roughness. The stylus moves closely with the sample surface, so data is highly reliable.

Non-Contact Type

Non-Contact Type
Laser microscope (focus detection system)
The leading method of this type is light. Light emitted from the instrument is reflected and read, to measure without touching the sample. Various non-contact systems include the focus detection type, the confocal microscope type, and the interferometer type. As they are non-contact, these systems never harm the sample and can even measure soft or viscous materials.

Challenging the limits of surface roughness measurement. Laser Technology

The appeal of an extremely small spot diameter

The tip radius of a general contact-type stylus is 2~10μm which causes the roughness data to be "filtered" by the size of the stylus. In contrast, the radius of a laser spot from a laser microscope is only0.2μm, so it can measure surface roughness that a contact-type stylus cannot enter.
The appeal of an extremely small spot diameter

Measurement without contact or damage

With contact-type measurement, the stylus directly touches the sample surface, so it may scratch the surface of a soft sample or pull on a viscous sample, which makes it impossible to obtain accurate values. A laser microscope is non-contact, so it has no impact on the state of the surface or the accuracy of the roughness measurement.

The stylus can scratch soft samples in contact-type measurement

Observation image from a laser microscope
Sample: Viscous tape 256×256μm

Measurement of fine, narrow surfaces

With a contact-type instrument, it is extremely difficult to measure narrow areas such as fine wires. With a laser microscope, however, positioning can be determined accurately and it is simple to perform areal roughness measurement for a small target area.

Measurement of fine, narrow surfaces1
Measurement of fine, narrow surfaces2
It is extraordinarily difficult to lower a stylus onto a wire surface only tens of microns across

Observation image from a laser microscope
Sample: Extremely fine wire (φ50um), objective lens magnification 100x

> Click here for detail about OLS5000, laser confocal microscopes
> Click here for detail about OLS4500, nano search microscopes

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