Hey guys, welcome to MechGeni! In this post we are going to explain about different types of surface finish measurements and its detailed explanation.
Before going to start about verify whether you read our past article about
- surface finishing and its Terminologies
so that you may get more accuracy in understanding the types.
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Surface Finish Measurements
There are two methods which are used for measuring the finish of the machined part.
- Surface inspection of the comparison method
- Direct instrument measurements
Surface Inspection of Comparision Method
In comparison methods, the surface finish is assessed by observation of the surfaces. But these methods not reliable as they misleading if the comparison is not made with a surface made by the same techniques.
There are various methods under methods are:
- Touch inspection
- Visual inspection
- Scratch inspection
- Microscopic inspection
- Surface photographs
- Micro-interferometer
- Wallace surface dynamometer
- Reflected light intensity
Touch inspection
Visual inspection
In this type of comparison method, the surface is inspected by the naked human eye. Sometimes maginifiers(magnifying lenses) are also used by human to observe the surfaces.
In human eye visual inspection is often likely to have a error or misleading in measurements because the results may vary from one person to another since every person has a unique perspection of his own.
So, this type of inspection is not recommended for inspecting rougher surfaces and uneven surface measurements. surface finish particularly when surfaces having a high degree of finish are inspected.
Scratch Inspection
In this method, some of the softer materials like lead and plastics are rubbed over the surface which is to be measured. By doing the rubbing processes, the impression of this scratches is produced on the surface which can easily visualized for measuring the surface finishing.
Microscopic Inspection
It is one of the finest method for evaluation of surface finish. In this method, the surface which is to be inspected is compared to the master surface on the microscope for inspection. Sometimes a light beam is also used to check the finish by projecting it at 60° to the surface under inspection.
The main disadvantage of this method is, that this inspection can be done only in a small portion at a particular time.
For inspecting a larger surfaces this method consumes more time since many readings can be calculated for getting an average value.
Magnified Surface photographs
In this method of surface inspection, the magnified photographs of the surfaces are taken under different types of illumination.
In vertical illumination the defects like irregularities appear to be as dark spots and flat surfaces appears as bright area.
The photographs taken different types of illumination are compared with one another and then the results of the surfaces are assessed.
Micro-interferometer
In this method, an optical flat is placed on the surface to be inspected and illuminated by a monochromatic source of light. Interference bands are studied through a microscope.
Defects, i.e. scratches in the surface appear as interference lines extending from the dark bands into the bright bands. The depth of the defect is measured in terms of the fraction of the interference band.
Wallace surface dynamometer
This is a sort of friction meter and consists of a pendulum in which the testing shoes are clamped to a bearing surface and a predetermined spring pressure can be applied.
In this method the pendulum is lifted to its initial starting position and allowed to swing over the surface to be tested. If the surface is smooth, then there will be less friction and pendulum swings for a longer period.
Thus time of swing is a direct measure of surface finish.
Reflected light intensity
In this method a beam of light of known quantity is projected upon the surface. This light is reflected in several directions as beams of lesser intensity and the change in light intensity in different directions is measured by a photocell.
The measured intensity changes are already calibrated by means of reading taken from surface of known roughness by some other suitable method.
Direct instrument measurements
In this type of inspection the surfaces are directly inspected by the machines.
Some of the direct measurement instruments are
- Stylus probe instruments.
- Tomlinson surface meter.
- Profilometer
- Taylor-Hobson Talysurf
That's all for now, Hope that you understand about the types surface finishing measurements and its explanation.
If you have any queries or doubts put on the comment box. See you soon in the next topic. Please don't forget to share this with your friends and colleagues so that they can also learn the topics that you have learnt from here.
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