In recent years, many detectors for measuring surface roughness have become available, and are you wondering which one is the best?
Be careful, because if you make a mistake in selecting a detector, you may end up with a completely unreliable value to evaluate. For example, if you want to measure a soft object but use a stylus-type detector, it may deform the surface of the workpiece. Also, some optical detectors, which are easy to use, may not be able to measure correctly depending on the shape of the workpiece.
In this issue, we will introduce four types of detectors that can measure surface roughness. Let’s make use of this information in selecting a detector.
1. Tentacle detectors are highly accurate and reliable
Surface roughness is called surface texture, also defined as “characteristics of a stylus-type surface roughness measuring instrument.
A stylus-type detector is a detector with a contactor called a stylus with a pointed tip. Since this stylus touches the surface of the workpiece directly, it is sometimes called a contact detector.
When measuring, the stylus traces the surface of the workpiece and moves up and down in accordance with the unevenness of the workpiece surface. This up-and-down movement is detected electrically, which is the principle of measurement.
Advantages of Tactile Detectors
Advantages of the stylus detector include the following:
- High accuracy
- Highly reliable
- Low cost of both detector and consumables
- Long distance (wide range) measurement
- Can measure inside holes
The most important advantage of the stylus detector is its high reliability. As long as the shape of the stylus is the same, almost the same surface roughness value can be obtained no matter which manufacturer’s measuring instrument is used.
This may seem obvious for a measuring instrument, but unfortunately the non-contact detector introduced next cannot guarantee the values. Therefore, when presenting surface roughness values externally, we basically present the values measured with a stylus type detector. Another advantage is that we can measure inside holes, which is difficult with optical detectors.
Disadvantages of Tactile Needle Detectors
The disadvantages of a stylus-type detector include the following:
- The stylus wears out
- Workpiece surface may be scratched
- Soft or sticky surfaces cannot be measured
A major demerit of the stylus-type detector is the possibility of scratching the workpiece. This is not a concern for steel parts, but for parts softer than aluminum, there is a risk of scratching.
However, this does not mean that soft surfaces cannot be measured, as some detectors can be set to lower measuring force.
2. Optical detectors (2D measurement) are non-contact measurement
Optical detectors are detectors that measure the surface roughness of a workpiece by irradiating light onto the workpiece instead of the stylus of a stylus-type detector.
There are many types of optical detectors, but they can be broadly classified into the following two types
- Those that measure the distance between the detector and the workpiece surface from the light reflected from the workpiece without vertical movement of the detector.
- Those that measure the height from the movement of the detector by moving the detector up and down with autofocus and maintaining a constant distance between the detector and the workpiece surface.
Here we introduce the point auto-focus method using a laser, which is the most commonly used of the latter.
In the point auto-focus method, a laser is irradiated onto the workpiece surface and auto-focused using the triangulation method. By autofocusing, the distance between the workpiece surface and the objective lens is kept constant.
In other words, by detecting the height of the objective lens, the height of the workpiece surface can be determined.
Advantages of Point Auto Focus Type Detectors
Advantages of point auto-focus type detectors include the following:
- Non-contact measurement
- Excellent angular characteristics
The most important merit of the point auto-focus type detector is that it can measure without contact. Since it is non-contact, the surface of the workpiece is not deformed.
This means that measurements can be taken even on soft surfaces such as sponges and rubber, which could not be measured with a stylus type detector. Depending on conditions, even liquid surfaces can be measured.
In addition, optical detectors are not good at measuring slopes with angles, but point auto-focus has the advantage of being relatively strong on slopes.
Aisadvantages of Point Auto Focus Type Detectors
The disadvantages of the stylus type detector include the following:
- There are incompatible surface geometries, and it is not possible to measure them correctly
- Light cannot penetrate groove widths of 0.5 μm or less
Although not limited to point auto-focus detectors, optical detectors are more selective about the workpiece compared to stylus detectors. Sometimes they produce values similar to those of a stylus-type detector, and sometimes they produce values that are completely different.
Therefore, while they can be used for in-house evaluations, they are less reliable as values to be submitted to external parties. It is tempting to trust the same value after repeated measurements, but it is important to understand that repeatability and accuracy are not the same thing.
Also, although many people misunderstand, an optical detector cannot get into finer grooves than a stylus detector. This is because light cannot be focused to a range narrower than the wavelength of light. Keep in mind that the wavelength of light (laser) is about 0.3 μm to 0.6 μm, so it cannot be narrowed down to less than about 0.5 μm.
3. Optical detector (three-dimensional measurement) is fast
Unlike the above detectors, optical detectors (3D measurement) are detectors that can measure three-dimensional surface properties in one shot.
There are many types of optical detectors (3D measurement), but we will introduce the following two commonly used ones:
- Laser microscope (confocal microscope, confocal microscope)
- White light interferometer
Principle of Laser Microscope
The principle of laser microscopy is to measure the height of a surface by acquiring microscopic images while changing the height of the objective lens and looking for the height at which it is in focus. The focus is determined by whether the laser passes through a hole called a pinhole.
By performing this process on every pixel of the camera, a three-dimensional three-dimensional shape can be acquired.
Principle of White Interferometry
White interferometers look the same as microscopes. The difference, however, is that it uses interference fringes to determine whether the object is in focus. White interferometry uses a special objective lens to generate interference fringes.
Advantages of Optical Detectors (3D Measurement)
The advantages of laser microscopy and white interferometry include the following:
- Non-contact measurement
- 3D shapes can be acquired in seconds
As with point auto-focus type detectors, non-contact measurement is possible. Another advantage is that measurement of 3D shapes, which would take an hour with a point auto-focus detector, can be completed in a few seconds.
Disadvantages of Optical Detectors (3D Measurement)
The disadvantages of laser microscopy and white light interferometry include the following:
- Narrow measurement range
- Inclined surfaces are difficult to measure
- Incompatible surface topography, not sure if it can be measured correctly
- Light cannot penetrate groove widths of 0.5 μm or less
Laser microscopes and white light interferometers can measure within the field of view of the microscope in a short time. However, their disadvantage is that they cannot measure an area wider than the field of view at once. Data from multiple fields of view can be stitched together using a function called stitching, but this is time consuming.
Other disadvantages unique to optical detectors are the same as those of point auto-focus detectors.
Other detectors include those that do not measure shape. For example, there are those that shine a light on the workpiece surface and measure the average value of the overall surface roughness from the reflected light. This is very convenient, but the absolute value is not reliable.
However, since relative comparisons can be made, it works well enough for simple inspections. It is recommended to use it for mass-produced products while checking the correlation with a stylus type detector.
In this article, we briefly introduced four types of detectors that can measure surface roughness.
The stylus type detector can be said to be an indispensable detector when providing parts externally due to its high reliability. For other detectors, it is necessary to check whether they are measuring properly based on the results of the stylus type detector.
However, in many cases, absolute values are not important for surface roughness, but relative comparisons are used to determine roughness. For example, it may be important whether a workpiece is rougher or rougher than a certain standard workpiece.
For this reason, there is a method of measuring good and bad workpieces and daring to select a detector that produces a large difference. For example, a detector whose value is larger than it actually is when there are scratches is best suited for determining the presence or absence of scratches.
If many workpieces are manufactured, it is necessary to determine which detector to use for each workpiece. If in doubt, gain experience with a stylus detector.
Once you get used to it, you will be able to tell which detector is best just by looking at the shape.