by Jack Clark, President, Surface Analytics; [jclark@surfaceanalytics.com] and Colorado State University, Mechanical Engineering Dept. and SME’s Deburring and Surface Conditioning Technical Group

Edge and surface finish can be important factors in driving part performance of metal parts.

Top Figure 1 – Before finishing or honing. |  Bottom Figure – Figure 2 Plateaued surface after surface finishing  Diagrams Courtesy of Jack Clark, Surface Analytics.,  Optical Interferometry Graphics

Typical Gaussian machined surface vs. Plateaued or planarized surface

Problem:  Prior to the early 1970’s, there was little to no awareness of relating part function to the process that created a piece part.  Many areas of automotive power train manufacturing was starting to realize that component failure was causing increases in scrap and warranty costs even though the specified metallurgy, fit and form was specified and controlled properly.  There was no unified understanding of the root cause of premature wear, leakage, noise, unusually high operating temperatures and catastrophic failure, or, even if they were related.  A conventionally produced surface (turned, milled, ground, EDM) is typically Gaussian in nature, that is, the peak and valley distribution is pretty much equal in height. This type of surface can be very unstable and unpredictable when wear and load bearing factors are considered.

Definitions:

Gear and shaft with isotropic finish developed from centrifugal barrel finish operation
Gear and shaft with isotropic finish developed from centrifugal barrel finish operation

A conventionally produced surface (turned, milled, ground, EDM) is typically Gaussian in nature, that is, the peak and valley distribution is pretty much equal in height.  This type of surface can be very unstable and unpredictable when wear and load bearing is considered.  The images in Figure 1. demonstrate this type of surface.  There are many ways to produce plateaued surfaces.  They are varied in approach but all have the ability to control the surfaces peak characteristics separately for the valley characteristics.  This is summarized in Figure 2. showing that the peak and valley distributions can be controlled to allow substantial bearing load capabilities (broad, flat areas) with well defined lubrication, debris collecting valleys.

The constant that must be maintained to ensure part function of the sealing system is the specification and consistency the surfaces involved.  There are very specialized ISO Standard parameters that were designed to describe plateaued finished surfaces.  These are the Bearing Ratio parameters (Rpk, Rk and Rvk) and, more recently, the Probability Parameters (Rpq, Rmq, Rvq).  They monitor the peak and valley regimes of the surface independently allowing the production team to know that the process and therefore, the piece part, will function.

Gear Contact Surfaces: 

Drive train truck gears centrifugally finished for isotropic surface effect
Drive train truck gears centrifugally finished for isotropic surface effect

Gears of all types must be designed to maintain their integrity under very high local surface loads.  The industry has always battled what material, what form, what surface will survive the varying designs and environments.  Again the surface must distribute the lube well and, at the same time, have the capability to withstand very high unit area load.  The plateauing process generally will produce a surface free of local anomalies that can increase the local unit load and initiate a failure site.  The mechanisms for surface failure are many and, sometimes difficult to identify.  It has been found that when attention is paid to the integrity of the contacting gear surfaces by reporting the correct parameters during production, gear life is extended, noise and operating temperatures are reduced, and wear is minimized.

Conclusion:

The mechanism that allows for improved “functionality” for all surfaces is basic to surface performance – to accept the loads imposed and resist wear.  Traditional processes that generate form and control fit do not necessarily dictate whether that part or assembly will function.  Over the last three (3) decades, it has been realized that there is another contributor to part performance – Surface Finish.  If a manufacturer does not account for surface finish characteristics like lube retention, micro burr removal, identification of torn and folded material, directionality, and load bearing capability, he then cannot predict the performance of components in the system.

Through progressive process development, evolving measurands (new ISO Standard Parameters), and a mature understanding of the “Function” of surfaces, manufacturers can design parts that become assemblies that are in systems performing over predictable, extended lives.  This is the key to reduced warranty costs, reduced scrap, lower production costs, and satisfied customers.  □

See also the video on centrifugal barrel finishing at:

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

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