There’s more than meets the eye, or the touch of your fingertips, when it comes to surface finish on small diameter stainless steel tubing. Surface finish is a measure of the roughness of a given profile, and it’s usually quantified as roughness average (Ra) or Root Mean Square (RMS) in microinches. Evaluating surface finish gets rougher (no pun intended) when it comes to actually measuring product.
Historically, reliance on visual charts/standards and comparison to representative product samples were among the earliest ways to gauge finish. Although still used today in some shops, results can be inconsistent due to subjective interpretations by different operators. In modern times, profilometers measure surface finish on tubing by pulling a stylus across a sample, detecting peaks and valleys in the surface and feeding that information back to a computer unit that plugs the data into the selected algorithm and issues a result. Profilometers can be finicky though: the size of the product, stylus, fixturing, ambient test environment and operator can all dramatically affect results. There is almost always some variation if one of those variables changes! Optical profilometers are an improved method (performing the same calculation but via microscope) but the equipment cost is dramatically higher than mechanical profilometers and overkill for many applications.
All of this talk about ways to measure finish and the potential pitfalls is important: it hopefully ensures the customer and supplier measure finish using a common standard and stay aware of the potential for variations during QC inspection. From the supplier standpoint, there are ways to improve finish on the tubing OD via centerless grinding, polishing, chemical, electropolishing and other methods or some combination thereof. ID finish improvement options for tubing are more limited and preferably rely upon multiple mandrel or plug passes to achieve better finish. Mechanical options to improve tube ID finish are limited to larger diameters. Typically, as a stainless or alloy tube is drawn down, its finish gets worse once the plug or mandrel is removed (a process limitation).
From the customer standpoint, it’s important for buyers and engineers to be aware of measurement limitations and know that smoother surfaces require more processing and in turn typically have higher costs.