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Vol. 14
November Issue
Year 2013
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Good Vibrations


in Vol. 14 - November Issue - Year 2013
Innovative Media Expand The Range Of Mass Finishing Applications



Selection of different media shapes and sizes*


Polished knee joint*


Special pre-polish media for orthopedic implants & other parts*


Polished automotive wheel*


Milled acetate spectacle frames processed in a rotary vibrator*


Small media for finishing wrist watch components & other small parts* (*: Source of Pictures R

In the field of mass finishing - like with any other industrial manufacturing technology - the focus of attention is generally on the equipment like, for example, how do the parts move relative to the media, machine size, installed power, the degree of automation, etc. Of course, equipment is a lot sexier than the grinding & polishing media. But in recent years it has been the newly developed, innovative media that have opened the door to new mass finishing applications.

No doubt new equipment developments have greatly contributed towards making mass finishing the dominant surface finishing technology of today. For example, compared to vibrators, the high energy barrel and centrifugal disk systems increased the metal removal rate 10-fold. The advent of drag finishers allowed the mechanical surface finishing of delicate components with complex shapes. And more recently, the so-called "streamfinishing" has been touted as the latest and greatest, even though it is a decade's old technology, just with a new name. But of equal importance - though in a fashion a lot less spectacular - are the polishing & grinding media utilized in the equipment. And many of today's sophisticated finishing applications would not have become possible without the dramatically improved processing media.

A brief look back into the history of mass finishing

The birth of modern mass finishing coincides with the invention of the mass finishing vibrator in the 1950's. At that time, randomly shaped river pebbles, lava rock and slag from steel production where the only available grinding media. This limited the application of the newly-born technology to deburring and breaking sharp edges on simple mass-produced parts.

A drastic improvement occurred with the development of geometrically shaped grinding media in the 1960's. Suddenly, the newly available specific media shapes and sizes like cylinders, triangles, pyramids, etc., allowed the processing of parts with complex geometries without the risk of media lodging in the parts or some surface areas not getting treated. With this milestone,  mass finishing had evolved from a purely deburring into a true surface finishing technology.

A wide variety of media to choose from

Mass finishing media can be generally classified into ceramic, plastic, steel media, organic media and others like glass beads, polyamide micro beads and thermoplastic. Ceramic and plastic media account for approximately 90 - 95% of all media used in mass finishing. All other media are specialties used only for certain applications.

Ceramic and plastic media consist of a carrier containing certain types of abrasive. While for ceramic the carrier is clay, for plastic it is either polyester or urea resin. The most commonly used abrasives are silica (sand), aluminum oxide, silicon carbide and zirconium usually in sizes of 0.4 down to 0.04 mm. Densities (by specific weight) range from 1.0 - 2.3 kg/l for plastic to 2.3 - 3.9 kg/l for ceramic.

This means that the media developer can play with the following variables: (1) the carrier - usually ceramic or plastic, (2) type of abrasive, (3) abrasive size, (4) fill rate - the portion of abrasive material contained in the carrier, (5) media shape, and (6) media size.

Where the media made the difference

Below are a few examples where innovative media was the key to entirely new mass finishing applications:

Super finishing of orthopedic implants

In recent years, thanks to newly developed grinding and polishing media, the drag finishing technology has displaced numerous robotic systems for surface grinding and polishing of orthopedic implants like hip and knee joints. The new plastic grinding media, in conjunction with the powerful compact drag finishing machines, produces a pre-polish surface finish with surface readings of ra < 0.02 µm. The media is characterized by its high specific weight, an extremely fine abrasive with a grain size of < 20 µ and a fill rate of > 50%. Mass producing this media posed a significant technical challenge and required the development of numerous new proprietary production technologies.

In a two- or three-step process, this grinding media, in combination with maizorb (corn cob) polishing media containing a special bonding agent and special, extremely fine diamond powder, produces extremely fine, high gloss surfaces that would have been considered impossible just a few years ago.

High gloss polishing of mass produced automotive wheels

Considering that every year, millions of automotive wheels are produced, the surface finishing of cast or forged aluminum wheels represents a huge market. For many years, numerous manufacturers of mass finishing equipment attempted to develop a process for polishing aluminum wheels. But the breakthrough came only with the fine grinding plastic media developed by the global industry leader in surface finishing. The 3-stage process ¡V usually taking place in so-called plunge finishers -- consists of an intensive pre-grinding, a fine grinding and, finally, a polishing step.

Technical characteristics of this fine grinding, pre-polish media are again  extremely fine abrasive materials with grain sizes < 20 µm and a fill rate of > 50%.

Surface finishing of fashionable acetate spectacle frames

Nowadays the parts for fashionable spectacle frames are frequently milled from acetate sheets. After milling, the parts undergo an intensive edge radiusing followed by a fine grinding and polishing step. Until recently, this required an extremely time-consuming finishing process taking up to 40 hours. Thanks to the development of a special polyester media, the acetate spectacle frames can now be finished with a combined multi stage wet/dry process reducing the overall processing time down to about 25 hours! The media contains a special abrasive with grain sizes of about 50 µm.

Small is beautiful!

The surface finishing of small, delicate parts requires small finishing media! For this reason, a leading media manufacturer developed extremely small ceramic media for processing components used in luxury wrist watches. The cylindrically shaped media has a diameter of 0.4 mm and a length of 3 mm. Of course, the manufacturing engineers had to cope with numerous technical challenges until they were able to successfully produce such small media.

Outlook

Mass finishing has evolved from a simple mechanical deburring method into a technology that offers numerous special surface finishing solutions for a wide range of parts which usually require a very smooth and high-gloss polished finish. As we can safely expect this trend to continue, the development of new and innovative grinding & polishing media will be even more crucial in the future to open the door for new mass finishing applications. One R & D focus will certainly be on dry media that no longer requires the addition of water and chemical compounds, and makes waste water treatment superfluous.

by Eugen Holzknecht
Contributing Editor MFN and
Rösler Oberflächentechnik GmbH

E-mail: holzknecht.usa@gmail.com




Author: Eugen Holzknecht