Photo above: Isotropic Micro-Finishing of High-Performance Automotive Parts, Photo Credit: Mark Riley, BV Products
The role of mass finishing processes as a method for removal of burrs, developing edge contour and smoothing and polishing parts has been well established and documented for many years. These processes have been used in a wide variety of part applications to promote safer part handling (by attenuation of sharp part edges) improve the fit and function of parts when assembled, and produce smooth, even micro-finished surfaces to meet either functional or aesthetic criteria or specifications. Processes for developing specific edge and/or surface profile conditions on parts in bulk are used in industries as diverse as the jewelry, dental and medical implant industries on up through the automotive and aerospace industries. 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. Industry has always been looking to improve surface condition to enhance part performance, and this technology has become much better understood in recent years. Processes are routinely utilized to specifically improve the service life of parts and tools subject to failure from fatigue and to improve their performance. These improvements are mainly achieved by enhancing part surface texture in a number of different, and sometimes complimentary ways.
– (Before) This photograph was taken with an electron microscope at 500x magnification. It shows the surface of a raw unfinished “as cast” turbine blade. The rough initial surface finish as measured by profilometer was in the 75 – 90 Ra (µin) range. As is typical of most cast, ground, turned, milled, EDM and forged surfaces this surface shows a positive 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, generally considered to be an undesirable surface finish characteristic from a functional viewpoint. SEM Photo by Jack Clark, Surface Analytics
(After) This SEM photomicrograph (500X magnification) was taken after processing the same turbine blade in a multi-step procedure utilizing orbital pressure methods with both grinding and polishing free abrasive materials in sequence. The surface profile has been reduced from the original 75 – 90 Ra (µin.) to a 5-9 Ra (µin.) range. Additionally, there has been a plateauing of the surface and the resultant smoother surface manifests a negative skew (Rsk) instead of a positive skew. This type of surface is considered to be very “functional” in both the fluid and aerodynamic sense. The smooth, less turbulent flow created by this type of surface is preferred in many aerodynamic applications. Another important consideration the photomicrographs indicate is that surface and sub-surface fractures seem to have been removed. Observations with backscatter emission with a scanning electron microscope (SEM) gave no indication of residual fractures. SEM Photo by Jack Clark, Surface Analytics
Isotropic Micro-Finishing can be accomplished with high-energy finishing equipmentsuch as the type shown above.
To understand how micro-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:
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.
These profilometer graphs graphically illustrate in 2-D the difference between an as cast positively skewed surface, and the same typical surface processed with a sequence of high energy loose media operations to produce an improved surface topology that that is potentially very useful for mission critical components that require improved wear, fracture or fatigue resistance.
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.
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. Certain high-energy mass finishing processes can be implemented to modify this surface stress condition, and replace it with uniform residual compressive stresses.
3-D Micro-Surface Topographical maps are coming into increasing use to better quantify surfaces as they relate to part service life and performance. This surface is one that has been processed to blend in parallel rows of surface peaks left behind from fine grinding operations. The resultant surface is one that is more isotopic or random in nature. This type of surface can be an important surface attribute to parts that are subjected to repeated stress or strain and parts that undergo high force loading of opposing surfaces.
Isotropic Tool Honing is a group of processes that improves edge condition and surface physical properties on tooling. The processes involve complete envelopment and processing of all edge and surface areas of the tool and are non-selective in nature. These processes develop surface compression and stress relief on all surfaces, as well as uniformly micro-honing and polishing all edge areas, regardless of tool or edge geometry.
The need for some edge preparation on cutting tools in many applications has been well documented. Most of the processes currently utilized (including manual ones) concentrate on modifying the edge to produce an edge land to strengthed the edge. Although these types of proceses improve cutting edge geometries they do not typically address surface profile topography issues at the cutting edge such as profile skewness, isotropiciy, load bearing ratio and residual stress correction. Specialized high-energy methods can and do.
Although the processes promote changes that are significant and crucial to improved tool performance, the changes in the physical cutting edge are so subtle that dimensional integrity of the tool is maintained. These alterations of the cutting edge surface area and supporting structures contributes to an improvement in mechanical strength and can avert premature wear or fracture of the tool.
The processes also have a tendency to improve overall symmetry of the cutting features, this is especially contrasted with manual honing methods, in which abrasive filament wheels, abrasive sticks or even coated abrasive paper are utilized to pre-condition edges by hand methods. Surface profile improvement is isotropic in nature, minimizing the fatigue and fracture problems associated with uni-directional surface patterns in surfaces created by production grinding methods. Smoother more isotropic surfaces provide greater lubricity for improved chip flow, often facilitating higher feed and speed rates with processed tools.
This edge conditioning procedure is an outgrowth of research to improve the tribological functionality of wear surfaces, with a view to extending useful service life and preventing premature failure of critical components that are highly stressed or strained. It has been found that surface treatment processes that can develop some measure of compressive stress and an isotropic micro-finish to edges and surfaces can improve load bearing ratios and other wear resistant factors far more than improvement in one of these areas alone.
When utilized to develop tool surface and edge improvements these processes can dramatically improve tool performance and maximize productivity in a number of areas:
(1) Greater tool life and durability
(2) Increased machine feed rates
(3) Increased tool speed rates
(4) Improved surface finish on parts
(5) Reduced tool change-over and downtime cycles
(6) Improved tool performance meeting high tolerance requirements
Summary. Many parts or tools that are subject to fatigue, fracture or wear can gain substantial improvements in life and performance from alterations to their overall surface texture. Improvements in overall smoothness, load bearing ratio, surface profile skewness and isotropicity can in many instances, improve life and performance and cut operational costs dramatically. Manufacturer s that have not subjected their parts to an analysis to determine the potential benefits of this kind of processing may be making parts “that are not all that they can be.”
(1) Davidson, D. A., “Mass Finishing Processes”, Metal Finishing 2002 Guidebook and Directory, pp. 104-117, New York, NY: Elsevier Science, 2002
(2) Massarsky, Dr. M. L., Davidson, D.A., “Turbo-Abrasive Machining and Polishing”, ABRASIVES Magazine, Oct/Nov 1999
(3) Davidson, D. A., “High Energy Dry Process Polishing”, SME Technical Paper MR90-389, Dearborn, MI: Society of Manufacturing Engineers, 1990
See below: SME-Spokane video of Centrifugal Isotropic Finishing in a precision machine shop [Case Study]