Surface roughness is the roughness of a machined surface with small spaces between the peaks and valleys and small peaks and valleys themselves. The distance (wave distance) between two wave peaks or two wave valleys is very small (less than 1 mm), which is a micro-geometric error. The surface is smoother when there are fewer rough spots on it.
What are the effects of surface roughness on parts?
1) Affects wear resistance
The coarser the surface, the smaller the effective contact area between the mating surfaces, the greater the pressure, the greater the friction resistance and the faster the wear.
2) Affects the stability of cooperation
So that the clearance stays the same, the rougher the surface, the easier it is to wear down, and the clearance will gradually grow as the work is done. For interference fit, the micro summit flattens out during assembly, which weakens the connection and reduces how well it interferes.
3) Affects fatigue strength
The rough parts have deep grooves on their surfaces. They are very sensitive to stress concentration, just like sharp corners, grooves, and cracks. This means that they will change the strength of parts when they wear out.
4) Affects corrosion resistance
The surface of rough parts is very easy for corrosive gases and liquids to penetrate into the inner metal layer through the small valley of the surface, resulting in corrosion on the surface.
5) Sound seal
Rough surfaces cannot fit closely together, and gas and liquid will leak through the gap between the contact surfaces.
6) Influences contact stiffness
Contact stiffness is how well the joint surface of a part holds its shape when an outside force is applied. Most of the machine’s stiffness comes from how well the parts fit together.
7) Influences measurement precision
The surface roughness of the measured surface of parts and the measuring surface of measuring tools directly affects the accuracy of measurement, especially in the process of precision measurement.
What are the measurement methods for surface roughness?
Measurement methods can be broken down into two main groups: measurement with contact and measurement without contact. When it comes to measuring contact, there are mostly comparison methods, impression methods, stylus methods, and so on. Non-contact measurement methods include the light-cutting method, the real-time holographic method, the speckle method, the astigmatism method, the optical heterodyne method, the atomic force microscope (AFM), the optical sensor method, and many others.
The measuring tool is a common, easy-to-use, and reliable tool for measuring surface roughness. Based on how the sensors work, the stylus roughness instrument can be inductive, piezoelectric, photoelectric, laser, grating, or without a guide head. The guide type roughness meter can only be used to measure the surface’s roughness. The non-guide type roughness meter, on the other hand, can also be used to measure the surface’s waviness and shape.
What are the measurement methods for the roughness meter?
A roughness instrument is also called a surface roughness instrument, a surface finish instrument, a surface roughness detector, a roughness measuring instrument, a roughness meter, a roughness tester, and other names. So, how does the roughness meter measure roughness?
Interferometry uses the principle of light wave interference to measure surface roughness.
- Needle tracing
The needle tracing method is to use the stylus to gently scratch the measured surface directly, so as to measure the Ra value of surface roughness.
- Comparative method
A common method in workshops is to compare two things. When comparing the measured surface to the roughness sample, you can judge with your naked eye or use a magnifying glass and a comparison microscope to compare. You can also tell how rough a machined surface is by how it feels when you touch it and scratch it with your fingernail. Most of the time, this method is used to get a rough estimate of large roughness parameters.
- Light cutting method
The light cutting method uses the “light cutting principle” to measure surface roughness.
Working principle of digital roughness meter
Based on the needle-tracing method, the surface roughness measuring tool is made up of a sensor, driver, zero indicator, recorder, and inductance sensor. One of the most important parts of a profilometer is a sensor measuring rod with a diamond tip at one end. At the top of the stylus, the radius of the curve, called r, is very small. During measurement, the driver is used to move the sensor at a certain speed while the stylus is placed on the workpiece and touches the measured surface vertically.
Because the measured surface’s shape goes up and down, the touch will move up and down as the measured surface moves. This movement makes the magnetic core move up and down at the same time through the fulcrum. This changes the inductance of the two differential inductance coils that surround the magnetic core. The sensor’s coil and measuring circuit are connected right to the balance bridge.
The change in coil inductance throws off the bridge’s balance, so it sends out a signal that is directly related to how far the stylus has moved. After the electronic device boosts the change in this small amount of electricity and uses phase-sensitive detection, it can get a signal that shows how far the stylus moved and in what direction. After that, the signal is split into three channels.
One is added to the zero meter to show where the contact pin is, and the other is sent to the DC power amplifier. After the signal is amplified, the recorder is turned on to start recording. The third channel is filtered and amplified by the average meter amplifier, and then it goes to the integral calculator to be calculated. The indicator can then show the surface roughness Ra value.
What do you need to know before choosing a surface roughness meter?
1) Clarify your actual measurement needs, such as the shape and size of the workpiece to be measured
2) Whether the material is metallic or non-metallic, and what is the hardness
3) Whether the workpiece has been precision machined or shot-blasted
4) Need roughness meter or roughness profiler
5) Whether the measuring surface on the back is flat or has inner holes, grooves, or curved surfaces
6) Is it necessary to print the measurement results
How many factors should be considered in purchasing a surface roughness instrument?
1) Measurement application industry. This depends on whether the contact or non-contact type is selected.
2) Measurement accuracy requirements. This item is very important and mainly depends on the resolution of the sensor, the accuracy index of the hardware part, the type of probe, the radius of the probe, the requirements of the service environment, etc.
3) The size of the sensor range is based on the peak valley drop of the surface profile of the piece being measured. The length and height of the piece to be measured depend on how far the surface roughness instrument can move in the X direction (measurement travel) and how high it can go in the Z direction.
4) Measure the contact force of the needle. This item affects the degree of scratch on the workpiece surface during measurement.
What is a portable roughness meter?
A portable roughness meter is a type of instrument for measuring surface roughness that is small and easy to carry. It is used a lot at places where things are made or analyzed further in measuring rooms. The roughness measuring tool, which is about the size of a hand, can be taken anywhere that surface roughness needs to be measured.
What is the roughness meter’s measuring stroke length?
The stroke length is the length of a trimmed contour that is used to measure the roughness of the surface. This means that the movement track of the stylus and the sensor are in sync during the measurement because of the action of the filter. The length of the stroke that was measured is made up of one or more sampling lengths. The evaluation length is the length needed to measure the roughness of the surface of the contour. It can be one or more sampling lengths. The length of the measuring stroke of the instrument tells you how long the profiler (gauge) is.
What is the function of the roughness meter filter?
The filter is used to break up the surface contour curve into waveforms with different lengths. The roughness instrument’s filter mostly gets rid of the surface waves and only keeps the surface roughness.
What is the difference between surface roughness RA, RZ, and RY?
RA is the average value of the offset value of the contour over the sampling length L.
RZ is the sum of the average heights of five big peaks and the average depths of five big valleys along the sampling length.
Within the sampling length L, RY is the distance between the contour peak line and the contour bottom line.
If RA/RZ/RY is not written in the symbol, it usually defaults to RA.
What is the difference between surface waviness and surface roughness?
Surface waviness is an unevenness in the shape of the surface that is bigger than surface roughness but smaller than surface geometric error. It is a geometric error between the small and the big. It is caused by the shaking of the machine tool and tool system when it is being used. The surface roughness has a direct effect on the mechanical properties of the part’s surface, such as its contact stiffness, fatigue strength, bonding strength, wear resistance, vibration resistance, and sealing.
Surface roughness is usually caused by the way the part is processed and other factors, such as the friction between the tool and the part surface, the plastic deformation of the surface metal when the chip is separated, and the high-frequency vibration in the process system. The depth, density, shape, and texture of the marks left on the machined surface are different depending on how the piece was made and how it was processed.
Surface roughness is closely related to how well parts fit together, how well they don’t wear out, how strong they are against fatigue, how stiff their contacts are, how much they vibrate, and how loud they are. It also has a big effect on how long mechanical products last and how reliable they are.
Wave distance λ＜ When 1mm, it shall be treated according to the surface roughness
Wave distance: 1mm ＜ λ＜ 10mm, treated according to surface waviness
10 mm wave distance, treated based on shape and position error
What sensors can the roughness meter be equipped with?
When measuring different workpieces, they will be measured with different sensors. So what sensors can the roughness meter match?
1) Standard sensor
The standard sensor is the one that is most often used. It can measure the roughness of planes, inclined planes, conical surfaces, inner holes, and grooves, among other things. It can be used to measure things by hand. In addition to the standard sensor, other sensors must use the measurement platform to measure.
2) Curved surface sensor
The curved surface sensor is mostly used to measure how smooth a cylinder with a radius of more than 3 mm is. It can also get a good approximation for other curved surfaces, like a smooth spherical surface with a larger radius. The surface will be smoother and the measurement effect will be better if the radius of curvature is bigger.
3) Hole sensor
Using a small hole sensor, the inner surface roughness with a hole diameter greater than 2 mm can be measured.
4) Deep groove sensor
Deep groove sensor can be used to measure grooves with a groove width of more than 3mm and a groove depth of less than 10mm. Surface roughness of steps with a height of less than 10mm can also be measured on the flat cylindrical surface, which can be used with the platform.
With a roughness meter, you can use any of the four sensors listed. Different sensors measure different types of workpieces.