For additional information contact Contributing Editor: Dave Davidson | Deburring/Finishing Technologist
ddavidson@deburring-tech-group.com | +1.509.563.9859 Mobile | https://dryfinish.wixsite.com/iso-finish
ABOVE: Packaged and ready for shipment, this crankshaft has been super-finished utilizing a BV Products tub-style vibratory finishing machine. The part has been "fixtured" in the vibratory work-chamber during the finishing process. This is a two-step process that involves both abrasive processing (1) for initial smoothing of surfaces, followed by a (2) non-abrasive chemically assisted process for final low micro-inch reduction of the surface texture to single digit micro-inch Ra surfaces that are highly polished. These types of surfaces can have highly important functional attributes that improves both the performance and potential service life of cooperating components such as this. Typical parts that are candidates for this type of processing include: Crankshafts, camshafts and gear train components for high-performance motorsports engines. Other industries (aerospace and wind turbine gearboxes for example) are also ale to take advantage of the operational improvements this type of surface finish can offer to cooperating parts.
ABOVE: Additional examples of precision machined components super iso-finished with the CASF method with the fixtured vibratory finishing method in tub style vibratory finishing machines. (Note: smaller parts can be processed in vibratory bowl machines without resorting to fixturing the components.
ABOVE: A Model VT-5 Vibratory machine manufactured by BV Products shown here with a crankshaft that has been fixtured in preparation for Chemically Assisted Super-Finishing (CASF)
ABOVE: See the videos: A tub Style Model VT-SF machine is shown in operation finishing a large gear that has been fixtured.
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.
The question that comes up is why do commonly used mass media finishing techniques produce this effect? There are several reasons. These methods produce isotropic surfaces with negative or neutral surface profile skews. Additionally, they consistently develop beneficial compressive stress equilibriums. These alterations in surface characteristics often improve part performance, service life and functionality in ways not clearly understood when the processes were adopted. In many applications, the uniformity and equilibrium of the edge and surface effects obtained have produced quality and performance advantages for critical parts that can far outweigh the substantial cost-reduction benefits that were the driving force behind the initial process implementation.
Wherever metals come into contact with each other contact stresses and friction occur. Both these conditions regulate and reduce the performance and compromise the design of the component. Micro-finishing is a means of regaining those losses by producing a superfine finish where it is most needed – at the point of contact.
The benefits are greatest with high contact stress applications and high fatigue life requirements. It is particularly beneficial in mating gear applications where it is proven to reduce contact stresses but also to reduce individual tooth bending which is key to maintaining good fatigue life with a resultant reduced operating temperature.
While it is known that reduced operating temperatures are an indication of increased performance the secondary benefit is that there is less heat to dissipate and as such the specific cooling cross section or cooling flow rate can be reduced all of which complement efficiency and performance.
Equipment: vibratory machines The process is carried out in specially designed vibratory finishing bowls and or tub/trough machines. These durable machines have been around for more than 60 years. Vibratory machines are available in sizes from 15 L to 1000 L working capacity. This means gears and other components can be finished ranging in size from less than six mm in diameter to more than two meters in diameter and quantities from one to hundreds at a time.
Consumables: high density, non-abrasive ceramic media The process utilizes high density, non-abrasive ceramic media in the vibratory finishing machine. It is considered non-abrasive since it does not contain discrete abrasive particles and alone is unable to abrade material from the hardened surface of the components being processed. The media is selected from a range of shapes and sizes best suited for maintaining the geometry of the parts. The selection of the correct media is a critical part of the process.
Process chemistry
The unique and significant feature of the process is the surface leveling/smoothing mechanism utilized to achieve the surface finish. A reactive chemistry is used in the vibratory machine in conjunction with the media. When introduced to the vibratory machine this chemistry produces a stable, soft conversion coating across the asperities (peaks and valleys) of the components. The rubbing motion across the components developed by the machine and media effectively wipes soft conversion coating off the ‘peaks’ of the parts surfaces, thereby removing a micro-layer of metal. After this continual process is complete, the conversion coating is wiped off one final time using a neutral soap to produce a mirror-like surface. This process does not affect the integrity of the parts either structurally or dimensionally and any very sensitive part areas can be effectively masked if required prior to processing with CASF (Chemically Accelerated Surface Finishing.
Performance Benefits • Reduced friction • Increased part durability • Improved corrosion resistance • Reduced wear • Reduced lubrication requirements and cost • Improved oil retention • Reduced contact and bending fatigue • Improved pitting resistance • Reduced vibration and noise attenuation • Reduced applied torque requirements • Improved surface finish uniformity (part-to- part, feature-to-feature and lot-to-lot) • High-quality, micro-finished surfaces
Reduced Friction Benefits • Increased fuel economy • Reduced contact fatigue • Increased power density • Lower operating temperature • Extended mean time between maintenance overhauls • Reduced maintenance costs • Eliminated break-in • Extended component life • Reduced metal debris • Reduced part failures • Minimized overheating
Many cooperating parts including gears and gear sets in a variety of industries remain subject to fatigue, fracture and wear. Such parts 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. Manufacturers that have not subjected their parts to an analysis to determine the potential benefits of this kind of isotropic processing may be making parts that are not all that they can be.
