Visual Finishing Options In Mechanical Finishing

Visual Finishing Options

Q. We have a customer asking us to be creative with the finishes we are producing on their stainless parts. What are some visual finishing options and how are they produced?

A. There are only three visual finishes (with variances of each) with mechanical finishing when visual effects are important: bright-reflective, satin-matte and directional or non-directional lines.

Bright-reflective finishes. Bright finishes through abrasive (buffing) and massing finishing processes are often used for cosmetic value, as well as for light reflection and a smooth, refined surface. Because bright finishes accentuate surface imperfections, most brightening applications require preceding finishing processes to create a smooth finish without lines, scratches, pits or dings prior to the final brightening process. A finish brightened over a dinged or scratched surface is not desirable, so bright finishes are often accompanied by multiple prefatory finishing operations to reduce surface imperfections.

The brightening abrasive process may consist of four or five operations from the start to the final finish. For example, a typical abrasive process may consist of: 1) 120-grit polish; 2) 180-grit polish; 3) 320-grit polish; 4) cut buff and 5) final color finish buff.

The initial finishing operation begins with the finest abrasive. Once this fine abrasive polishes and scratches the surface, then further abrasive polishing refines the initial scratch. A lighter initial scratch pattern requires fewer additional processes for a highly reflective, polished and buffed finish.

The mass finishing process for brightening, which is much more cost effective than abrasive finishing, begins with one or two cutting processes that refine the surface. These cuts enable the final burnishing process to get the maximum brightness over a smooth finish. Mass finishing seldom achieves an as-buffed finish on large flat surfaces, though it does well on smaller contoured parts.

Satin and matte finishes. A satin finish is a tight-patterned, diffused, semi-reflective surface, while a matte finish is a larger, patterned version of a satin finish. This finish can be obtained with abrasive, blasting and mass finishing processes and is often used for non-reflective finishing applications. The satin-matte finish hides minor surface imperfections and is less expensive to achieve than a smooth-bright finish. Because some surface imperfections are too large to hide with finer satin finishes, a surface refinement process is used prior to the final finish.

Blast finishing processes with most medias (glass beads, ceramic beads, aluminum oxide, silicon carbide, garnet, steel and stainless media) produce a pleasant, tight satin pattern. Each media type and size will affect the brightness and satin pattern.

Mass finishing produces a larger patterned matte finish and is one of the least expensive ways to produce a satin-matte finish. Smaller media produce a tighter pattern matte than larger media and ceramic media produce a brighter matte than the softer matte produced by plastic media.

Directional or non-directional lined finishes. Only abrasive finishing will produce a lined, swirled or random scratch finish. Abrasive polishing with coated abrasive belts or wheels produces scratched finishes of various sizes and directions, depending on the abrasive grit type, size and machinery used. Some finishes referred to in this category are brushed, grained, orbital, No. 3 and No. 4 on stainless and straight-lined finishes.

Media Size Recommendation for Vibratory Finishing

Q. We are currently using a fast-cut, 5/8-inch, cylindrical wedge-shaped ceramic media on our 7-inch by 10-inch steel parts for deburring, and we would like to decrease the process time. What media would you recommend?

A. To shorten your deburring time on steel, I recommend increasing your media size. Stay with the same shape but try going to a 7/8-inch or 1-inch size.

Size is an important factor in selecting mass finishing media. Larger media generate higher energy because of the mass of each piece (like a larger hammer delivers more energy to a nail). The larger media also creates more voids within the mass, which increases energy to the part. Larger media cuts and finishes faster and also wears faster, making it a good choice to support larger parts.

Smaller media holds more water/compound, which cushions the part, creating less part on part damage. Smaller media have a gentler impact on the part, resulting in longer process time cycles, better finishes and less media wear.

Media sizing is also a factor with automated part unloading equipment using mechanical screening for media/part separation. Generally, media has to be a different size (larger or smaller) than the part in order to use mechanical screening separation.

The majority of mass finishing uses media sizes ranging between 1/4 inch to 2 inches, while general finishing and deburring media are in the ½-inch to 1 1/4-inch range and burnishing media (brightening and cleaning) use smaller media ranging from 1/16 inch to 3/8 inch.

Bright-lined satin.