Imagine a fabricator lands a contract involving critical stainless steel fabrication. Sheet metal and tubular sections flow through cutting, bending, and welding, then land at the finishing station. The part consists of a plate welded vertically to a tube. The weld looks OK, but it’s not the stack-of-dimes perfection the customer is looking for. So the grinder spends time removing a bit more weld metal than usual. Then, alas, some significant bluing emerges on the surface—a telltale sign of excessive heat input. In this case this means the part won’t meet customer requirements.
Usually performed manually, grinding and finishing require dexterity and finesse. Mistakes in finishing can be extraordinarily expensive, considering all the value that’s already been put into the workpiece. Add an expensive, heat-sensitive material like stainless steel, and costs for rework and scrap mount even more. Add complications like contamination and passivation failure, and a once profitable stainless steel job can become a money-losing, even reputation-losing misadventure.
How can fabricators prevent all this? They can start by developing a knowledge of grinding and finishing, what role each plays, and how each affects a stainless steel workpiece.
Grinding Versus Finishing
They aren’t synonymous. In fact, each has a fundamentally different goal. Grinding removes material like burrs and excess weld metal, while finishing puts a finish on the metal surface. The confusion is understandable, considering those grinding with a large-grit wheel remove a lot of metal quickly and in so doing leave a “finish” of very deep scratches. But in grinding, scratches are just the aftereffect; quick material removal is the goal, especially when working with heat-sensitive metal like stainless steel.
Finishing occurs in steps as the operator starts with a larger grit and steps down to finer-grit sanding discs, nonwoven abrasive, and perhaps a felt cloth and polishing paste to achieve a mirror finish. The goal is to achieve a certain final finish (scratch pattern). And each step (finer grit) removes the deeper scratches from the previous step and replaces them with smaller scratches.