How to Use and Choose Surface Roughness Comparators
A surface finish or surface roughness is the composition and appearance of the final surface of the part. A part’s surface is defined by main surface features: roughess, waviness and form, that of which is the composition of the surface finish and surface finish applications manage those features in different ways.
Surface finishes are accomplished through many types of machining processes, and different projects will have different surface finish requirements. If you are removing material to finish a harder material, you’ll probably look at precision grinding, honing, polishing, buffing and super finishing. On the other hand, if your goal is to finish a soft metal without removing any material, roller burnishing, bearizing and ballizing could be your go-to options. Regardless of the material and process, making sure that the surface of the final product is finished correctly is key to make sure that the part is suitable for the given purpose.
Why surface finish matters
If a part is to be adhered to something, a rough surface would be preferred. Rough surfaces are however prone to corrosion and don’t wear as quickly as smoother surfaces. But if the parts will be in an environment in which they are sliding against each other, that smoother surface will be necessary. Therefore the required surface finish will be determined by the environment in which the part you’re machining will live.
Checking the surface finish
Once you’re finished with the surface finish process, you’re going to have to check it to ensure the requirements are met. This can be done with a handy-dandy roughness comparator. A surface comparator allows you to ensure that the surface finish meets the project specifications. The different surfaces on the comparator are literally visually compared to the surface finish of your part.
Here is an image of a surface roughness comparator below.
The roughness comparator should be selected based on the manufacturing process that was used to complete the surface finish. You can use a roughness comparator when the following processes have been used to achieve the given surface finish: casting, conventional machining, cylindrical ground, surface turned, electrical discharge machining (EDM), shot blast and grit blast.
How to use a surface roughness comparator
Step 1: Put the comparator beside the workpiece and ensure that the scale is adjacent to the surface you’re measuring.
Step 2: Use your fingernail to draw a right angle to the tool marks.
Step 3: When the feeling of your fingernail against the comparator is the same as against the workpiece, then the surface finish is the same or similar. If a match isn’t found then you’ll need the next roughness comparator scale.
YouTuber and Machinist mrpete22 walks through the process very well in the video above.
Check out these surface comparators
This surface comparator is AS9000 compliant. This pocket size scale has 22 precision surfaces for specifying and controlling surface finishes. This comparator is a corrosion resistant electroformed nickel duplicate of actual machined surfaces.
Ideal for use in the drafting room, engineering department or in the shop. This set has 30 specimens and each surface finish, such as flat lapping, reaming, grinding, horizontal milling, vertical milling, and turning, is truly and consistently reproduced to give the operator or designer a realistic idea of the feel, appearance, and texture of the machined components.
This set comes with 30 specimens, each 7/8″ x 3/8″ in size and made of electro-formed solid nickel. This set supports quick and easy comparisons and includes instructions and a vinyl wallet for the set.
In this set there are nine replicated cast surface finish specimens ranging from 20 to 900 microinches. The C-9 cast microfinish comparator is rectangular in shape (4 1/2″x 3 1/4″) and is a corrosion resistant electroformed nickel duplicate of actual cast surfaces. Supplied in a leatherette case with engineering data and instructions.
This set is compatible with surface finishes obtained from blanchard grinding, grinding, lapping, milling, profiling and shape turning. There are 22 specimens that are made from nickel.
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