VOL. 6 March ISSUE YEAR 2005
in Vol. 6 - March Issue - Year 2005
Vibratory Finishing adds Appeal to Parts
Author: Sandy Fryer, President Dougco
Dougco was established in 1964 as a Mass Finishing Job Shop offering mechanical surface finishing on metal stampings, castings, forgings, extrusions, and much more. The processes produce clean, dry parts with surface finishes suitable for direct use, or for additional surface treatments such as: plating, anodizing, painting and powder coating. The American company will match the surface finish on an existing part, or develop and customize a new finish to meet the requirements. Dougco also provides resin impregnation to seal porous metal castings. This is a vacuum/pressure impregnation process with a high temperature cure. Impregnation will not physically affect castings in any way except to seal the pores. Impregnated parts may then be plated, painted or powder coated.
Vibratory finishing and its older cousin tumble finishing are widely recognized in the metal trades as economical ways to clean and deburr metal components. Still, relatively few are aware of the range and quality of the finishes that can be achieved using this technology. Vibratory and tumble finishing processes can be used to create surface finishes that range from a distressed look on an aluminum table leg to a mirror-like finish on a bicycle component. By varying the media, compound, run time, and type of machine, a variety of different looks can be achieved. When parts are put through multiple steps, the improvement in appearance can be quite dramatic. However the cost is still quite economical since mass finishing processes are used.
Vibratory and tumble finishing can be divided into four basic categories based on the media used. The first category, ceramic media is used in the mill or barrel. The objective is to achieve a significant cut down on the part to radius edges and remove any polishing lines, burrs or other defects. The larger the burr or defect, and the courser the grit used in the polishing step the longer run time in the vibratory mill or tumble barrel.
The second category is finishing with plastic media. In this process the surface is cleaned and smoothed. If it is a first processing step, sharp edges are softened. For more delicate parts this may be the only operation performed.
The third category, burnishing, is accomplished in a vibratory mill or tumble barrel with hardened steel or hard nonabrasive ceramic media. The hard media, in the presence of special compounds, peens the metal component surface, producing mirror-like reflective surfaces.
The „shiny“ surfaces are hardened and are wear and corrosion resistant, making this finish ideal for many applications. It is a standard finish for many aluminum tools and components. The aluminum castings in Picture 2 have been burnished.
The fourth category, dry finishing, is used where a more nearly perfect reflective surface or softer surface is required. Dry finishing is most often accomplished in a high-energy machine such as a centrifugal barrel. This process utilizes a medium, such as ground corncob, with a compound similar to a buffing compound. If the parts are smoothed prior to dry finishing, reflective surfaces that rival buffed parts can be obtained. This process is particularly effective for smaller intricate parts that are difficult to buff.
Picture 1 is one example of how vibratory finishing process steps can be put together to create a finished part. As shown starting on the left, a raw aluminum forging is robotically polished to remove the parting line. The part is then run through the vibratory finishing process in ceramic media (not shown) to remove the grit lines, and then in plastic media to create a very soft smooth clean surface as shown. The part is then run through a dry finishing process and a near buffed look is obtained as shown on the right in the picture.
Replacing the dry finishing step with a burnishing step will give the part a more durable, but less perfect finish. Burnishing is generally more economical than dry finishing because it is less sensitive to prior surface preparation, run times are shorter, and part loading can be higher. In addition, the burnishing media will last for years if properly maintained while the dry media has a relatively short life cycle.
Picture 2 illustrates finishing can enhance product appearance. In the original design for a product, shown at the top of the picture, the sand casting was burnished in steel media and paired with stainless steel tubing. The casting and tubing appearance did not blend well and the grit lines and rough casting texture detracted form the appearance of the product. To improve product appearance, the parting line on the casting was removed using finer grit than is normally used in a foundry. The casting was given a heavy ceramic vibratory finish to remove the polishing grit lines and most of the surface texture before the burnish step. The tubing in the product was changed to anodized aluminum tubing which blended better with the finished casting. The savings in tubing costs offset the extra finishing costs, so the customer got a better look for about the same price.
Vibratory and tumble finishing are very versatile and economical manufacturing processes. Properly applied, vibratory finishing can add significant value to metal components. Part designers can minimize manufacturing costs if they keep vibratory finishing process capabilities and limitations in mind during the design phase.