Smooth Operators: Attaining Isotropic finish through chemically accelerated vibration

 It seems recently there has been more talk about the advantages of an isotropic, non-directional finish on machined surfaces obtained through a chemically accelerated vibratory process.

It isn’t necessary to utilize a chemically accelerated chemistry to achieve an isotropic finish, because the vibratory finishing process is what is producing the isotropic non-directional finish which reduces or eliminates the surface stress on machined components.

But the addition of a chemical accelerant to the vibratory process has actually increased the use of vibratory finishing as a means to produce very smooth finishes because it drastically reduces the time required to achieve this finish.

What is isotropic finishing?

Webster’s definition of isotropic is “exhibiting properties with the same values when measured along axes in all directions”.  Since its evolution, the vibratory finishing process has produced an isotropic non-directional finish which improves the surface finish and edges of machined components. The manufacturing process inherently creates stress risers, and vibratory finishing reduces or removes them. This improved isotropic surface enhances any subsequent final finish in addition to increasing the overall strength and condition of the component due to the relief of the surface stress.

The beauty of vibratory finishing is it provides a repeatable, non-directional, uniform finish from part to part at a reasonable cost. It is widely recognized as the first step to finishing a component requiring deburring, radiusing or micro surface improvement. There are tremendous advantages to applying an isotropic finish to the surface of a part, however most people see deburring as the biggest or only advantage to vibratory finishing.

So where does this chemically accelerated vibratory process fit into isotropic surface improvement, and what is its true advantage over the conventional vibratory process?

Since it is a vibratory process, it inherently produces an isotropic non-directional surface finish, and therefore provides the benefits of removing stress from the surface of the part.

However, these accelerants have dramatically reduced the time to achieve very low surface finishes without heavy metal removal and radiusing of the part.

How ASF works

Accelerated surface finishing uses a surface active compound that forms a soft film on the surface of the metal part. Nonabrasive media removes the soft film from the high or peaks of the part surface leaving the valleys untouched. The soft film is removed, exposing a new metal surface which will reform the soft film. This continuous forming and removal of the soft coating accelerates the finishing of the metal surface. The peaks are removed (high media contact) while the valleys are untouched (no media contact).

The advantages of accelerated surface finishing are:

• Achieve low RMS finishes without heavy metal removal or radiusing
• Achieve negative skew values by removing surface peaks while retaining micro valleys for lubricant retention
• Reduce friction, lower torque requirements and minimize surface wear, thereby reducing operation temperatures
• Refine surfaces to mirror-like brightness and very low surface roughness
• Precision components are imparted with a “Run-In” surface condition
• Increase fatigue life

 

The incorporation of accelerated chemistry has increased the versatility of vibratory finishing and has enabled manufactures reap the benefits of the isotropic finish generated by the process economically. The racing industry has recognized these benefits for some time and you can see that other industries are now beginning to see potential that wasn’t available to them previously.

Successes

Some examples we have seen first-hand of cases using accelerated surface finishing:

A local screw machine house manufactures a nut for a motorcycle component which requires a very smooth and bright finish prior to getting plated. The company was tumbling this part for 20 plus hours in order to achieve a desired RMS finish and brightness. Using the ASF process enabled them reduce this time down to 6 hours and achieve a smoother, brighter finish with much less radius on the corners of the nut.

A local machine shop manufactures a S. Steel and Steel ultrasonic calibration test block for the medical industry. The requirement is the test block must be very smooth and polished with crisp edges. The only way the company knew to achieve this result was by hand polishing, which took a very long time. It switched to an ASF process on both alloys, which has resulted in a huge time savings over hand polishing, and is actually providing a more uniform and consistent result at a fraction of the cost. Using the conventional vibratory process to achieve this finish would have resulted in to much radius of the blocks edges, but the ASF process was able to provide a very smooth, bright surface without radiusing the edge. The ASF process was better than hand polishing and much less expensive.

The ASF process has assisted gun manufacturers in reducing their hand preparation required to remove grind lines of the slides prior to blasting. In addition, gun triggers and hammers are run in the ASF process to achieve a very smooth finish which reduces the risk of fatigue and cracking. Again, conventional tumbling would have taken much longer and created too much radius on the parts.

The racing industry has been a proponent of this accelerated finishing process for many years and has been a driving force for other industries to recognize its’ advantages. Custom Cams Inc. (Martinsville, VA) wanted to improve on the RMS finish they were currently getting from their current finishing process. Incorporating the ASF process has enabled them to achieve lower RMS finishes which out much metal removal. The smooth finish on the crankshafts sheds the oil better and thus reduces amount of oil required.

Ring and pinion and gear sets are also run using the ASF process to improve the surface finish on the teeth. This results in increased fatigue life, and reduced friction thereby reducing operating temperatures.

A leading manufacture of rods in California wanted to reduce its hand preparation required on the rods. The ASF process has eliminated its hand preparation and provides a more uniform, consistent and stronger part.

The powdered metal industry is beginning to recognize the advantages of the ASF process for parts  it manufactures for the automotive and lawn and garden industry, to name a few. With the ASF process, they are able to get a very smooth surface without much radiusing of the part, and thereby increasing the fatigue life of the parts. This could not have been achieved in conventional tumbling due to the time required to achieve this improved surface.

The vibratory finishing process does a good job of producing isotropic non-directional finish, but combined with a chemical accelerant will produce smoother finishes and drastically reduced time requirements. n

Jeffrey Bell is president of Precision Finishing Inc., in Sellersville, PA. He can be reached at 800-523-6232, or at precisionfinishinginc.com.