It’s a Handy Reference Tool for Metal Surface Finish Characteristics
A surface finish chart is a reference material we at Metal Cutting sometimes use internally as part of our quality assurance process. (You can read more about our quality measures on the Quality Commitment page of our website.) Typically, such charts provide guidelines regarding the measurement of surface finish, such as the parameters used, the typical roughness according to different processing methods, or the conversion from a unit such as microinches (µin) to microns (or micrometers, µm).
Surface finish is generally described as a measure of the texture of a surface, characterized by the lay (or direction) of the surface pattern, its roughness, and its waviness. What exactly will you find if you search the web for a “surface finish chart” and how does it relate to metal surface finish?
Units of Surface Finish Measurement
One type of chart shows the surface finish parameters — that is, the different units of measurement and arithmetic calculations that are used to describe surface finish. Generally, this chart would include the commonly used Roughness Average, Ra, which is a calculation of the average length between all the peaks and valleys (or the average height) from the mean line of the surface. Because it neutralizes any significantly outlying points, Ra is not sensitive to occasional spikes and gouges.
The Root Mean Square Roughness, or RMS, is similar to Ra but generally considered to be an approximation and therefore, less accurate than Ra. RMS is calculated using an algorithm that finds the square root of the average of the squares of the values; basically, RMS turns the surface profile in to a sine wave and measures the average deviation of the curve from the mean line.
Maximum Roughness Depth, Rmax, measures the vertical distance from the highest peak to the lowest valley within five sampling lengths and selects the largest of the five values. This is a very sensitive method of estimating surface finish and is vulnerable to burrs or scratches that lead to a higher reading, indicating a rougher surface. The Mean Roughness, Rz, is calculated by averaging the height of the five highest peaks and the depths of the five lowest valleys. Because it looks only at the extremes only, Rz produces values that tend to be high and may not be an accurate reflection of the average surface finish.
At Metal Cutting, we typically test for Ra, and it is what most of our customers ask for. Although Ra and RMS are sometimes used interchangeably because the conversion factor is RMS = Ra x 1.1, we encourage customers to use Ra, which is considered more accurate and is widely accepted in the industry.
Feasible Roughness for Different Processes
Another type of surface finish chart shows the average range of surface finish values that can be achieved using different types of manufacturing processes. This is important to know because surface finishes can vary greatly depending on the process used to produce them; this is also why deciding between different cutting methods is an important step if having a particular surface on your finished metal parts is critical to the success of your application. In addition, achieving a particular surface finish may require that more than one process be used.
Each metal cutting process has its own characteristics (what machinists call “witness marks”), which can be varied to a certain degree and with certain limitations, also depending on the process. Where there is a floor on how smooth a machined surface can be, polishing can be deployed to achieve a highly smooth surface or grinding can be used to create a sharp edge or shiny finish. Therefore, you can look for a surface finish chart that lists the relative surface finish roughness for various metal cutting methods, such as abrasive cutting, EDM, grinding, milling, turning, lapping, polishing, and so on.
A Tool for Visually Comparing Surface Finishes
Someone who is searching the web for a surface finish chart may actually be looking for a picture that shows what a particular surface — say, one with a roughness of 63 Ra — should look like. This is where another handy reference tool comes into play: surface finish comparators, also called surface roughness comparator plates.
Surface finish comparators are inspection tools that offer an alternative to mechanically testing each part and getting an actual measurement of its Ra. Mechanical testing generally involves using either a contact surface roughness profilometer, which rides a probe along the surface of the part to read it, or a non-contact, non-destructive tool that utilizes interferometry or other optical 3D metrology to measure without touching the part and therefore avoid the risk of scratching or otherwise altering the part’s surface.
Instead, you can look at a comparator plate that shows what 63 Ra, 32 Ra, 16 Ra, 8 Ra, and so on, looks like compared with the part you are inspecting. Using surface finish comparators is a quick, easy way of discerning just whether a part is in spec by looking at it rather than physically measuring the surface profile.
Naturally, just as there are different average surface finish ranges for different metal cutting processes, there are also different comparator plates for different processes. That’s because, for example, an EDM cut part will have the appearance of tiny “pits” on the surface of the end cut, while an abrasive method will produce what looks like lines across the end cut surface. Even the exact same Ra value may look very different depending on the cutting process used. Therefore, it is important to be sure you are looking at the comparator plate for the correct process.
In addition, when you get down to much lower (smoother) surface finish requirements, such as 2 Ra or 1 Ra, comparator plates are not helpful. That’s because unless they are under high magnification, the differences between 2 Ra and 1 Ra (or lower) will not be readily apparent. Or if your particular application has a critical range — for example, each part must be between 16 Ra and 32 Ra — then comparator plates may not be the best option. In these cases, it may be necessary to mechanically test the parts to ensure that their surface finish is within the specified range.
Our Surface Finish Standards
Here at Metal Cutting, the surface finishes we most often deal with are the end cut of small, precision metal parts and the body surface along the diameter of tubes and rods. Our abrasive cutting method conservatively produces an end cut of 63 Ra or better. We say conservatively and start with a high Ra value because different metal types play a crucial role in what can be achieved. (Note that with surface finish measurement, the lower the number is, the smoother — or less rough — the surface is; therefore, “or better” means “or lower.”) With a lapped part, the surface of the end cut is typically 32 Ra or better.
For part diameter surfaces, we typically produce ground finishes in the 8 Ra to 16 Ra range; 2 Ra to 4 Ra is possible by grinding under certain circumstances, and conversely, grinding can be used to intentionally develop a very rough surface, exceeding 128 Ra. And our best results are when we mechanically polish parts to end cuts as smooth as 1 Ra or better.
The other surface finish we deal with is the body of parts, where we typically achieve 8-16 Ra through grinding. When we tumble to deburr, an added benefit is it can creates a polishing action that produces an even smoother surface of 8 Ra or better. From there, we can use our mechanical polishing to bring the body surface finish down to better than 1 Ra.
It is important to remember that if there is a blemish, ding, or scratch somewhere on a part, it will not be included in the total surface finish measurement of the part. However, at Metal Cutting we take great care to not create these types of surface finish defects and inspect for any that might occur. By offering a full range of metal cutting processes and surface finish options, we are able to meet customers’ needs based on each customer’s critical dimensions and end use/application.
You can read more about our expertise and how we meet the Ra surface finish requirements of your small metal parts. Or to learn more about quality assurance and other things to look for in a potential partner, download our free guide, 7 Secrets to Choosing a New Contract Partner: Technical Guide to Outsourcing Your Precision Metal Fabrication.
This week’s blogger, Barbara Osborne, is the Quality Assurance Manager at Metal Cutting Corporation.