by Dave Davidson | firstname.lastname@example.org | 509.230.6821 |https://dryfinish.wordpress.com
Developing “Isotropic” surfaces can be an important surface attribute to develop when seeking to improve performance, functionality, and longevity of components used in critical applications. Many components can have service life extended, wear resistance improved and premature fatigue or fracture prevented if surface characteristics are modified with high-energy and high-intensity mechanical surface finish methods. These kinds of surface finishing methods improve overall part quality by affecting the part surface in a number of different ways simultaneously.
(1) Isotropic Surface Development. In contrast to machined or ground surfaces, Isotropic surfaces are non-directional or random in character. They do not exhibit a surface pattern of parallel lines, grooves or notches common to all machining methods (S This is a desirable surface characteristic functionally as the overall amount of surface available for bearing loads can be increased dramatically and machining notches which provide potential failure related crack propagation points are attenuated substantially. Close in importance to isotropicity is the development of (2) negatively skewed surfaces that are plateaued or planarized
The photos above clearly show the difference between isotropic surfaces and surfaces that are only finely finished. In each case, only the part shown in the bottom left corner can be said to have isotropic surface attributes. It is only this part, finished with centrifugal isotropic finishing, that can be said to have a random surface characterization with the parallel lines or grooves evident in the other three examples removed (or blended in) (Photos by: Jack Clark, Surface Analytics, LLC and the SME Deburring and Surface ConditioingTechnicall Group)
(2) Negatively or neutrally skewed surfaces that are plateaued or planarized (see top set of diagrams in the photo). All conventional machining or fabricating methods (including: machining, turning, grinding, EDM, casting, forging etc.) develop positively skewed surfaces in which the predominant surface characteristic are the peaks and asperities of he surface profile. This can be disadvantageous as it can cause part life and performance deficits in many applications where parts are subject to wear and/or repeated stress or strain. Related to these is a further surface conditioning attribute of high-intensity mass finishing:
Isotropic Surfaces under high magnification: These scanning electron microphotographs show the different between a cast turbine blade surface prior to centrifugal isotropic finishing modification (upper) and a surface that has been finished (lower). Note that this is now a negatively skewed surface with much more functional from an aerodynamic sense, much more receptive to coatings, and much less prone to potential crack propagation. (Diagram by: Jack Clark, Surface Analytics, LLC and the SME Deburring and Surface ConditioingTechnicall Group)
(3) Compressive stress generation, like shot peening these kinds of finishing processes induce compressive stress and cold-hardening effects to the part while producing the sophisticated surface finish effects mentioned in (1) and (2). As all features and areas of the part are processed identically and simultaneously, stress equilibrium within the part can be developed that would be difficult to replicate with other surface conditioning methods.
Above: Isotropic Micro-Finishing Part Photography by Mark Riley, BV Products. Developing Isotropic Micro-Finishes can have a significant effect on performance values of parts such as these high-performance racing engine components among these are: Reduce Friction, Vibration and Noise, Improve shifting, Extend life, Reduce Lubricant Temperature
Improve performance and horsepower.
HOW DO YOU PRODUCE isotropic micro-finished surfaces that are negatively skewed? (See operational Videos below)
Centrifugal barrel finishing
Centrifugal barrel finishing (CBF) is a high-energy finishing method, which has come into widespread acceptance in the last 25-30 years. Although not nearly as universal in application as vibratory finishing, a long list of important CBF applications have been developed in the last few decades.
Similar in some respects to barrel finishing, in that a drum-type container is partially filled with media and set in motion to create a sliding action of the contents, CBF is different from other finishing methods in some significant ways. Among these are the high pressures developed in terms of media contact with parts, the unique sliding action induced by rotational and centrifugal forces, and accelerated abrading or finishing action. As is true with other high energy processes, because time cycles are much abbreviated, surface finishes can be developed in minutes, which might tie up conventional equipment for many hours.
The principle behind CBF is relatively straightforward. Opposing barrels or drums are positioned circumferentially on a turret. (Most systems have either two or four barrels mounted on the turret; some manufacturers favor a vertical and others a horizontal orientation for the turret.) As the turret rotates at high speed, the barrels are counterrotated, creating very high G-forces or pressures, as well as considerable media sliding action within the drums. Pressures as high as 50 Gs have been claimed for some equipment. The more standard equipment types range in size from 1 ft3 (30 L) to 10 ft3, although much larger equipment has been built for some applications.
Media used in these types of processes tend to be a great deal smaller than the common sizes chosen for barrel and vibratory processes. The smaller media, in such a high-pressure environment, are capable of performing much more work than would be the case in lower energy equipment. They also enhance access to all areas of the part and contribute to the ability of the equipment to develop very fine finishes. In addition to the ability to produce meaningful surface finish effects rapidly, and to produce fine finishes, CBF has the ability to impart compressive stress into critical parts that require extended metal fatigue resistance. Small and more delicate parts can also be processed with confidence, as the unique sliding action of the process seems to hold parts in position relative to each other, and there is generally little difficulty experienced with part impingement. Dry process media can be used in certain types of equipment and is useful for light deburring, polishing, and producing very refined isotropic super-finishes.
Further reading: Internet resources
(1) “Isotropic Mass Finishing for Surface Integrity and Part Performance”, Article From: Products Finishing, Jack Clark, from Surface Analytics, LLC and David Davidson, from SME Deburr/Finish Technical Group, Posted on: 1/1/2015, [Barrel, vibratory, centrifugal and spindle finish can improve part performance and service life.] http://www.pfonline.com/articles/isotropic-mass-finishing-for-surface-integrity-and-part-performance
(2) “Turbo-Charged Abrasive Machining Offers Uniformity, Consistency” Article From: Products Finishing, by: Dr. Michael Massarsky, President from Turbo-Finish Corporation, and David A. Davidson, from SME Deburr/Finish Technical Group. Posted on: 6/1/2012. [Method can deburr, produce edge contour effects rapidly] http://www.pfonline.com/articles/turbo-charged-abrasive-machining-offers-uniformity-consistency
(3) “Turbo-Abrasive Machining and Finishing”. MANUFACTURING ENGINEERING – Aerospace Supplement, by: Dr. Michael Massarsky, President from Turbo-Finish Corporation, and David A. Davidson, from SME Deburr/Finish Technical Group. [Method first developed for the aerospace industry can improve surface integrity and part performance] http://www.slideshare.net/dryfinish/turboabrasive-machining-me-aerospace-supplement-reprint
(4) “The Role of Surface Finish in Improving Part Performnce”, MANUFACTURING ENGINEERING, by Jack Clark, Surface Analytics.com and David A. Davidson, from SME Deburr/Finish Technical Group.
(5) “Free Abrasives Flow for Automated Finishing”, MANUFACTURING ENGINEERING, , by: Dr. Michael Massarsky, President from Turbo-Finish Corporation, and David A. Davidson, from SME Deburr/Finish Technical Group. [Exciting new methods of surface finishing that go beyond deburring to specific isotropic surface finishes that can increase service life] http://www.slideshare.net/dryfinish/october-2013-f2-deburring-1
(6) Turbo-Abrasive Machining Demonstration Video: https://www.youtube.com/watch?v=jYxqCxMIHNo
(7) SME Spokane, WA Factory Floor video, Centrifugal Finishing in the Precision Machine Shop: Demonstration) https://www.youtube.com/watch?v=dUdKjaysTYM
AUTHOR BIOGRAPHY – David A. Davidson, [email@example.com]
Mr. Davidson is a deburring/surface finishing specialist and consultant. He has contributed technical articles to Metal Finishing and other technical and trade publications and is the author of the Mass Finishing section in the current Metal Finishing Guidebook and Directory. He has also written and lectured extensively for the Society of Manufacturing Engineers, Society of Plastics Engineers, American Electroplaters and Surface Finishers Association and the Mass Finishing Job Shops Association. Mr. Davidson’s specialty is finishing process and finishing product development.