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The basics of metal surface finishes

Do you know how to get to achieve the finish your customer wants?

Finishing is both an art and a science and often poses challenges for today’s welding fabricators and mechanical contractors.

Whether you are trying to achieve a linear, nondirectional, or mirror finish, you need to follow a specific process to arrive at the desired finish. Following the correct steps and using the correct power tool and corresponding consumable result in a process that is efficient and effective, reducing frustration and quite possibly production costs in the long run.

The desired finish of a job depends on the product application, material, and type of finish your customer requires. When formal training in the finishing process is lacking, it can lead to undesirable results. This includes the frustration of not achieving the required finish and the use of an excessive amount of consumables, which can have a large impact on the cost of the project. Following the required steps and processes to achieve different finishes can alleviate this frustration and help to prevent problems that fabricators may face when tasked with finishing.

An Explanation of Finishing Values

Before a discussion of a step-by-step process to metal finishing success takes place, you need to understand Ra (which is the average roughness determined by an algorithm) and the different values required to achieve a specific finish. Customers typically reference a desired Ra value when describing their finishing requirements.

This “roughness average,” which is expressed in units of microinches, is by far the most common parameter in the U.S. for quantifying the surface roughness of a material. Using a device called a profilometer with a highly sensitive diamond stylus, you can measure the so-called “peaks and valleys” of the finished surface. It is important to keep in mind that the resulting number is in fact an average of the data points that the profilometer collects, meaning that it is possible to have an inconsistent surface finish that still yields a desirable Ra. This is why it’s important to thoroughly understand and follow the steps of the finishing process.

So, after your shop is done cutting, grinding, and blending, you are then charged with performing the final step of finishing. The job instructions call for a mirror finish—an Ra of 4 to 8 microinches. How does that differ from a sanitary finish or even a fine finish? One process does not fit all, and having a clear understanding of the proper tool speed, the right consumable, and how to use those elements are critical to achieving the desired outcome.

First, let’s focus on the different types of finishes.

Sanitary Finish. If you are dealing with food-grade products, achieving a sanitary finish (Ra of 30 to 35 microinches) is vital. Preventing bacteria growth is the primary goal in these types of applications. Because of this, no fine particulates or scratches are allowed because they can trap bacteria. A sanitary finish devoid of marring and particulate helps to facilitate proper washing and cleaning.

To achieve a sanitary finish, you need to apply the right abrasive at the right speed, so that the scratch pattern is created evenly and consistently (see Figure 1). A sanitary finish should not leave behind any deep scratches.

A common problem in finishing stainless steel is discoloration. Stainless steel does not conduct heat well. If a grinder is running too fast, it generates excessive heat, which burns out the nickel and chromium in stainless steel. This also can have a negative impact on the life and performance of the consumable product. The end result is that you are left with just steel and a dull, yellow finish on your product. Here’s a fabrication tip: It is essential that you run the abrasive product at the recommended speed to control heat and mitigate this issue.

Fine Finish. Products used in medical and pharmaceutical applications, as well as in the aerospace industry, require a fine finish (Ra of 12 to 16 microinches) to prevent corrosion and improve structural integrity and durability (see Figure 2). Additionally, a fine finish allows for sanitary cleaning with smaller particulates such as powders, which are common in these environments.

Mirror Finish. Probably the most difficult and least understood finish is the mirror finish (see Figure 3). You’ll find these finishes are required in architectural and decorative applications. They may not be all that common, but when your shop gets one of these jobs, you want to ensure you use the proper technique and achieve the end result in as few steps as possible.

The Step-by-Step Process to Achieve the Finish

For best results, use a product with a flexible backing pad that can be used with multiple abrasive discs. This saves valuable tool-change and labor time.

These are the steps to get to the desired finish (see chart):

Figure 4 shows the different stages to achieve a mirror finish. Note: It’s important to rotate the scratch pattern 90 degrees at each step to even out scratches from the previous step.

Finishing is an important part of the fabrication process and can be easily achieved if you have a thorough understanding of the requirements for the desired finish. Material knowledge and the proper pairing of consumable to power tool speed are the keys to achieving the desired finish in the most cost- and time-effective manner.