The word “metallic”: a term synonymous with adjectives such as bright, gleaming, and reflective. However, newly-produced metal straight from the mill isn’t very shiny; in fact, it looks and feels rather rugged. Its appearance is referred to as “mill finish”, and it has a rough and irregular surface texture. The finish is dull, and its color often patchy due to metal oxidation or contamination from mill oil. While it looks unrefined, depending on its use mill finish can be acceptable. Why spend extra time and money to make unfinished metal look better when appearance is unimportant?
However, the majority of consumers prefer their metal products to be smooth and shiny. Beyond the look of mill finish, other processes such as shearing can leave metal with rough or sharp edges. To improve the overall quality of the metal, polishing is often done to create a smooth and shiny surface. But when a finer degree of finishing is needed for certain applications, additional processing such as superfinishing is done.
Superfinishing
Simply put, superfinishing is a level beyond the typical processing done to metal products. The metal has already gone through some type of finishing such as polishing or another surface modification process. To the naked eye, polishing leaves a gleaming surface and metallic shine, but the process can actually result in microscopic structural irregularities on its surface. These irregularities are referred to as the residual amorphous layer. Unlike the rest of the metal which retains its crystalline structure, the amorphous layer has scattered atoms creating a disordered surface. Superfinishing is necessary for situations where removal of that amorphous layer is critical to the metal component’s use.
The Superfinishing Process
During a primary finishing process like polishing, a great deal of friction is needed to smooth away any pits or bumps marring the metal’s surface. On the other hand, the secondary process of superfinishing is a much more precise operation using lower friction. The best way to create lower friction is by the use of a very fine abrasive agent. The height of a superfinishing abrasive agent may only measure from 5 to 8 micrometers: too small for successful polishing, but effective in wearing away the amorphous layer.
After the selection of stone or sandpaper with an extremely fine grit, the abrasive is applied on the metal’s surface while the item rotates in the opposite direction. This is done one small section at a time, both for precision and to keep friction to a minimum. Additional thermal protection is provided by using a lubricant, which helps reduce any heat produced from friction.
Advantages of Superfinishing
Superfinishing is useful for both functional and aesthetic purposes:
• Appearance: the metal is left with tiny crosshatch marks, creating an attractive brushed-metal finish.
• Low Tolerances: the amorphous layer removal performed during superfinishing means the metal’s measurements are much more precise. Polishing may leave metal looking shiny, but on a microscopic level, the surface is still uneven. Superfinishing ensures that the metal’s surface is uniform, because the crosshatching is the same height on every part of the component. The metal’s dimensional accuracy is improved, creating better seals and more precise assembly when the part is used in applications.
• Prolonged Use: precise assembly allows for less friction created during use, because pieces are less likely to press and slide against each other. This reduces wear on the metal, which then requires less maintenance and enables a longer period of use. Automotive parts like rods and shafts are typically superfinished to help extend the lifetime of the part.
Disadvantages of Superfinishing
• Finish: depending on the consumer’s taste, a polished finish may be considered more appealing than a superfinished one.
• Cost: additional equipment is needed to perform superfinishing, adding to the metal’s overall cost.
• Time: while superfinishing is not a particularly lengthy process, the operation does add to the overall processing period for that part. This prolongs the time before a finished product can be shipped or sold.