by Dave Davidson, SME Machining/Material Removal Tech Community,
dryfinish@gmail.com  | 509.230.6821 and Jack Clark, Surface Analytics LLC,  jclark@surfaceanalytics.com
cbf-isotropic
The SEM photos shown above show typical cast, machined or ground surfaces under high magnification with the photos to the left.  The photos shown to the right show surfaces that have been modified to be isotropic and planarized.  These modified surfaces can help parts perform better and last longer in demanding applications

For Tech Support or information on on free sample part processing

Contact Dave Davidson | dryfinish@gmail.com | 509.230.6821 

 Mass finishing processes have been widely adopted throughout industry as the optimum methodology for producing advanced edge and surface finish effects on many types of machined and fabricated components. America industry has long been in the forefront in aggressively deploying these methods to improve edge and surface finishing operations.
All too often, situations still exist where archaic, even primitive hand or manual finishing methods are used to produce edge and surface finishing effects.  This is not to say that some industrial part applications are not going to require a manual deburring approach – some do.  In many cases, however, hand or manual methods are still being utilized because more automated or mechanized methods have not been considered or adequately investigated.

An often observed dichotomy in precision manufacturing operations, is a situation that is still all too common.  This is that many manufacturers, after spending vast sums on CNC machining equipment to produce parts to very precise tolerances and specifications consistently, in the end, hand off these expensive parts to a deburring and finishing department that utilizes hand methods, with all the inconsistency, non-uniformity, rework and worker injury potential that implies.  Even when manual methods cannot be completely eliminated, mass media finish techniques can and should be used to produce an edge and surface finish uniformity that simply cannot be duplicated with manual or single-point-of-contact methods.  Developing an overall edge and surface finish continuity and equilibrium can have a significant effect on performance and service life of critical components.

Boeing-Titanium-Coupon -- Test1

 The titanium test coupons shown above give a good example of the difference between isotropic and non-isotropic surfaces.  On the coupon on top, machining marks and step-overs are clearly visible, in the part shown on the bottom, these marks have been completely removed with centrifugal isotropic finishing.  This type of surface is much more useful and functional in a wide variety of applications, especially where moving parts  or parts subjected to stress and faigue failure are concerned.
High speed centrifugal finishing in the lean context at MacKay Manufacturing, Spokane, WA
The part to the right shows surface machining and milling patterns and step-overs from machining operations.  The part to the left has been finished with a hands-free high-energy centrifugal finishing operation that has completely cleared and blended machining patterns leaving behind a uniform, homogenous surface in a hands-free automated finishing operation

In recent years, mass media finishing processes have gained widespread acceptance in many industries primarily as a technology for reducing the costs of producing edge and surface finishes. The economics are especially striking when manual deburring and finishing procedures are minimized or eliminated.

TAM Isotropic Edge-Contour
 Above: one emerging technology Turbo-Abrasive Machining is a, is a high-speed dry spindle finishing technology for rotating parts can deburr, as well produce edge-contour and isotropic surface effects in cycles of  2 – 5 minutes 
The first casualty of over-reliance on a manual deburring and finishing approaches is the investment the manufacturer has made, often in the millions, for precise and computer-controlled manufacturing equipment. The idea behind this investment was to have the ability to produce parts that are uniformly and carefully manufactured to exacting specifications and tolerances.  At this point, in too many cases, the parts are then handed off to manual deburring and finishing procedures that will guarantee that no two parts will ever be alike.
Moreover, the increased complexity and precision requirements of mechanical products have reinforced the need for accurately producing and controlling the surface finish of manufactured parts. Variations in the surface texture can influence a variety of performance characteristics. The surface finish can affect the ability of the part to resist wear and fatigue; to assist or destroy effective lubrication; to increase or decrease friction and/or abrasion with cooperating parts; and to resist corrosion. As these characteristics become critical under certain operating conditions, the surface finish can dictate the performance, integrity and service life of the component
 CBF - Gear before and after
The role of mass finishing processes (barrel, vibratory and centrifugal finishing) as a method for removal of burrs, developing edge contour and smoothing and polishing parts has been well established and documented for many years. Less well known and less clearly understood is the role specialized variants of these types of processes can play in extending the service life and performance of critical support components or tools in demanding manufacturing or operational applications.
To understand how edge and surface topography improvement can impact part performance, some understanding of how part surfaces developed from common machining, grinding and other methods can negatively influence part function over time.  A number of factors are involved:
(1) Positive vs. Negative Surface Skewness.  The skew of surface profile symmetry can be an important surface attribute.  Surfaces are typically characterized as being either negatively or positively skewed. This surface characteristic is referred to as Rsk  (Rsk – skewness – the measure of surface symmetry about the mean line of a profilometer graph).  Unfinished parts usually display a heavy concentration of surface peaks above this mean line, (a positive skew).  It is axiomatic that almost all surfaces produced by common machining and fabrication methods are positively skewed.  These positively skewed surfaces have an undesirable effect on the bearing ratio of surfaces, negatively impacting the performance of parts involved in applications where there is substantial surface-to-surface contact.  Specialized high energy finishing procedures can truncate these surface profile peaks and achieve negatively skewed surfaces that are plateaued, presenting a much higher surface bearing contact area.  Anecdotal evidence confirms that surface finishing procedures tailored to develop specific surface conditions with this in mind can have a dramatic impact on part life.  In one example the life of tooling used in aluminum can stamping operations was extended 1000% or more by improved surface textures produced by mechanical surface treatment.
Unfinished Bearing
This photo shows bearing surface as seen by 2D and 3D scanning measurement. The surface peaks and asperities are typical of machined or ground surfaces.
Finished bearing surfaces
After high energy centrifugal finishing, the surfaces are far more functional for the bearing application. More isotropic, more plateaued, more negatively skewed and have imparted a beneficial compressive stress. All of these attributes improving the load bearing and tribological properties of the part. Photo courtesy of Jack Clark, Surface Analytics.
(2) Directionalized vs. Random (Isotropic) Surface Texture Patterns.  Somewhat related to surface texture skewness in importance is the directional nature of surface textures developed by typical machining and grinding methods. These machined surfaces are characterized by tool marks or grinding patterns that are aligned and directional in nature.  It has been established that tool or part life and performance can be substantially enhanced if these types of surface textures can be altered into one that is more random in nature. Post-machining processes that utilize free or loose abrasive materials in a high energy context can alter the machined surface texture substantially, not only reducing surface peaks, but generating a surface in which the positioning of the peaks has been altered appreciably.  These “isotropic” surface effects have been demonstrated to improve part wear and fracture resistance, bearing ratio and improve fatigue resistance.
Bearing Picture 6
Magnified view of bearing surface after surface peaks have been removed (top series of diagrams). This high-intensity isotropic  CBF method is an economical way of blending in machining or grinding lines to develop isotropic surfaces. Optical Interferometry by Jack Clark at Surface Analytics

(3) Residual Tensile Stress vs. Residual Compressive Stress.  Many machining and grinding processes tend to develop residual tensile stresses in the surface area of parts.  These residual tensile stresses make parts susceptible to premature fracture and failure when repeatedly stressed.   High-energy mass finishing processes can be implemented to modify this surface stress condition, and replace it with uniform residual compressive stresses.  Many manufacturers have discovered that as mass finishing processes have been adopted, put into service, and the parts involved have developed a working track record, an unanticipated development has taken place.  Their parts are better—and not just in the sense that they no longer have burrs, sharp edges or that they have smoother surfaces.  Depending on the application: they last longer in service, are less prone to metal fatigue failure, exhibit better tribological properties (translation: less friction and better wear resistance) and from a quality assurance perspective are much more predictably consistent and uniform.

image014
Centrifugal barrel machines such as these can produce exceptional edge and surface finishes in very short cycle times. Accelerated process effects can be developed because of the high-speed interaction between abrasive media and part surfaces, and because media interaction with parts are characterized by high pressure by virtue of the high centrifugal forces developed in the processes. Smaller turbine blades can be processed in the 5 x 8 inch compartments in the 12-liter capacity machine shown to the right. Larger centrifugal machines such as the 220 liter or 330 liter capacity machine shown to the left can handle much larger parts as the barrel compartments are as much as 42 inches in length. Larger parts processed in this type of machinery can be processed one at a time within the barrel compartment suspended within the media mass or be fixtured. Barrel compartments can be divided into processing segments to accommodate more than one part.
SUMMARY:
Mass media finishing techniques (barrel, vibratory, centrifugal and spindle finish) can be used to improve part performance and service life, and these processes can be tailored or modified to amplify this effect.  Although the ability of these processes to drive down deburring and surface finishing costs when compared to manual procedures is well known and documented, their ability to dramatically affect part performance and service life are not.

 

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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.
http://www.slideshare.net/dryfinish/november-2012-f4-deburring-1-final

(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, [dryfinish@gmail.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.

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2 thoughts on “Understanding Surface Finishing’s Role in Part Performance and Service Life Improvement

  1. Ductile steel improves fatigue resistance when residual stress due to for example shot peening is introduced. The texture is nearly neutral if the grooves, for example caused by either mechanical finishing or by coating with brushes, are directed parallel to the main positive principle load stress.

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