Above: Isotropic Micro-Finishing of Gears, Photo by Mark Riley, BV Products

by Dave Davidson, SME Tech Community Advisor, dryfinish@gmail.com

Mass Finishing is a term coined to describe a group of mechanical finishing processes in which energy is imparted to loose abrasive (and in some cases non-abrasive) material contained in a work chamber to produce surface conditioning effects on parts in bulk. These processes include barrel finishing, vibratory finishing, centrifugal disc and barrel finishing and to some degree spindle finishing (even though the latter process does not neatly fit within the above definition). Although many innovative procedures have been employed to counter difficult surface finishing problems in recent years, the actual concept of finishing work pieces in bulk is not a new one. Some authorities cite evidence which indicates that crude barrel finishing techniques may have been in use in Roman and early Medieval and Chinese cultures for edge finishing various metal parts.

30 x 36 in hardwood lined barrel for dry process finishing of plastics and jewelry

The advent of the “Industrial Revolution” and the acceptance of the “interchangeable part”, “assembly line” and “mass production” concepts necessitated the commercialization of finishing techniques in which repeatable and uniform edge and surface finish effects could be generated on large numbers of parts. An increasing variety of parts can be mass finished with the technology variants available today including: die-castings, forgings, extrusions, castings and machined parts and others, as well as parts made from ceramics, plastic, wood and composite materials. The trend for the last several decades has been toward automating processes and eliminating or minimizing what has traditionally been the manual component in the manufacturing sequence. New technologies just now emerging, will facilitate the automation of deburr and edge finish procedures on parts with very complex geometries and rotational parts which had defied previous efforts to mass finish.

plastic finishing 2016
Dry process finishing was used for fine finishing and polishing of plastics and jewelry.  Typically natural materials were used including treated media made from wood, corn cob and walnut shell

Much of the work and the technology developed earlier this century centered around developing and improving existing barrel finish technologies. Some of the earliest work on which we have documentation is the barrel finish formulations and processes patented by Joseph Lupo in the 1920’s and 1930’s. Mr. Lupo’s Lupoline Corporation was responsible for proliferating dry process barrel finishing for developing fine finishes and/or decorative finishes on a wide variety of smaller metal and plastic parts as well as promoting burnishing techniques utilizing metal media and submerged and vented tumbling equipment. One of the weaknesses inherent to the industry as it developed from this time period into the war years was the heavy dependence on what we will call “black art”. In many cases, successful finish processing was highly dependent on the knowledge retained by a few specific individuals, which was passed along as oral tradition, in some cases, literally from father to son. Most of the knowledge obtained was often based on extensive trial and error experience and little effort appears to have been made to develop a comprehensive knowledge base from which future applications process development would be benefited. This problem continued to dog the industry until well after the war years, and was compounded by the fact that part finishing remained the step child of manufacturing processes, lacking the attention engineering and manufacturing academia lavished on understanding and improving metal forming or metal working processes. This deficiency has only begun to be corrected in relatively recent years. It is paradoxical when you stop to consider how odd it is that many manufacturing companies remain relatively indifferent to the importance of their mechanical finishing processes when these processes are performed on parts after they have almost their entire manufacturing cost expended. To be sure, the problem was at first exacerbated by the very nature and conditions surrounding barrel finishing processes. In many instances, early production barrel finishing areas were wet, noisy and dirty places which were best approached with rubber gloves, rubber aprons, hip boots and not a little trepidation. In the early days, assignment to the finishing area of a manufacturing plant was not generally considered to be a promotion, nor was it something that engineering candidates would often aspire to when considering career options.

Dry process finishing of eyeglass frame components
Hardwood Pegs and other hardwood preformed shapes were used in closed tumbling systems to produce polished surfaces and large numbers of plastic items such as eyeglass frame fronts and temples, button, acrylic and a wide variety of plastic and soft metal consumer items

Having commented on the unpleasant aspects of early barrel finishing operations perhaps we should give an overview of what was involved. Prior to the advent of preformed media shapes most deburring and edge finishing that was done by tumbling utilized graded crushed rock or natural abrasive materials of some sort along with a soap/water compound mixture. Water levels were commonly approximately that of the media mass, and the optimum media/part level load for barrels was considered to be 45-55%. Barrel finishing was a positive displacement method; rotation of the barrel moved the mass of material to a turnover point, gravity would overcome inertia and whatever cohesive properties the mass might have and a portion of it would slide to the lower area of the barrel chamber. Almost all the abrading work performed was within this slide zone area which might not be more than 10-20% of the total mass at a given time. This partial utilization characteristic led to sometimes lengthy process times, and the necessity of stopping the operation to drain, flush and rinse the media mass to prevent the deposition of objectionable materials on the part surfaces. The very “tumbling” nature of the process also meant that effects would be heavily concentrated on exterior corners and edges, with less attention to other areas of the part. Some barrel finishing OEM’s did studies on slide zone flow and motion by utilizing clear plate plastic panels on the octagonal end of the barrels to observe motion within the barrel to determine optimum barrel rotational speed and slide zone characteristics for various applications. It soon became obvious that there would be definite economic and processing advantages to a finishing method that could somehow impart energy and movement to the entire media mass simultaneously, and make more efficient use of work chamber space.   (To be continued….)

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