The expectations on manufacturing industries have increased in recent years. Different mechanical processes such as turning, milling etc. lead to surfaces with inherent irregularities. Therefore, manufacturing industries are faced with the responsibility of finding improved finishing operations that will both nullify these effects as well as improve other properties in these materials.
Surface plastic deformation processes were created to resolve these impediments. This process does not involve material removal but rather deforms the surface plastically under compressive load. The surface of the component under this external load is therefore said to be subjected to cold working.
Burnishing is one such surface plastic deformation technique. It has been in existence for a long time and has continued to gain general acceptance in the manufacturing industry today.
1. What is Burnishing?
Burnishing is a surface modification process through which smooth surface finishes are achieved. This is possible by means of planetary rotation of a tool over a board of turned surface. In general, burnishing is a cold working process that smooths, removes discoloration and polishes a metal surface into a sparkling finish which is nearly as good as new.
This specifically targets the peaks and valleys as contained in all machine or other processed metal surfaces. The burnishing tool is applied with a calculated amount of force. This force drives the materials which hitherto were on the peak to flow into the valleys. This effect brought about will be a reduction in the height of the peak as well as in the depth of the valley.
This method was first developed in America in the 1930s. Burnishing was applied in a bid to impart residual comprehensive stress to layers of various metal parts, specifically the railroad, car axle, and machinery shafts. Over the years, it came to be widely accepted and has found applications in almost every industry the world over.
2. Types of Burnishing
This process can be classified into different types depending on the type of tool used, the geometry of the workpieces being worked on, and so on.
Considering geometry of tools used, burnishing is further classified into two groups: Ball burnishing and Roller burnishing. However, for the purpose of this article, we’re going to consider ball burnishing, its definition, advantages and application in various industries and businesses in the world today.
Ball burnishing: In this type of burnishing, one or more spherical balls are supported in shank by the hydraulic pressure of a fluid, spring or the relative force of the workpiece. The ball is kept constantly in contact with the workpiece by fluid which is circulated using a hydraulic pump. As the tool is fed along the workpiece, the ball becomes pressed against it, resulting in a burnishing operation.
Depending on the desired effect, the force of burnishing can then be controlled by varying the hydraulic pressure of the fluid.
3. Advantages of Ball Burnishing
- This process allows parts to be produced with high control over the dimension and allowing for very accurate sizes.
- It produces a very smooth surface finish
- Saves cost and is more economical compared to the other burnishing processes
- Creates improvements in physical properties and increases the fatigue life of components
4. Benefits of Ball Burnishing with Stainless Steel Media
Stainless steel media is particularly popular in surface finishing due to its superior characteristics. This has led it to become the most preferred and widely accepted in industries the world over.
Below are some inherent characteristics which make the stainless steel media the most widely used today:
- Higher Surface Pressure and Longer Media Life
- Shorter Finnish Time
- Improves Pre-Plate Finishing
- Reduces Porosity on Plated Parts
4.1 Higher Surface Pressure and Longer Media Life
If operated under suitable conditions, this media can last for a very long time. This is obviously so because it is not a wearing media. It has high hardness due to its composition of relatively though materials. This characteristic is excellent for the various vibratory tumbling applications.
Furthermore, there is a minimal solid waste generation during this process. Steel which is a non-consumable media is not used-up during the entire operation.
4.2 Shorter Finnish Time
Stainless steel tumbling media is comparatively weightier than other materials used for this process. This substantial weight exerts higher pressure to a mass of components in the vibratory, barrel and tumbling finishing equipment. The high surface contact achieved through increased pressure and resistance from steel media works effectively to shorten operation time.
4.3 Improves Pre-Plate Finishing
Parts that appear smooth are actually characterized by micro-imperfections which cause plating problems. Due to the weight of steel media, these minute irregularities are flattened, preparing a surface for more satisfactory plating.
4.4 Reduces Porosity on Plated Parts
When plated parts are finished with steel tumbling media, a compacting action acts downwards and spreads through the entire surface of the softer plate to fill any “pin-point” holes. This eliminates porosity and brings about an increase in the corrosion resistance created by the plating process.
The durability of stainless steel media is thought of as a capital advantage. Finishing’s done with this medium generally lasts longer. Using Stainless steel shot versus plain steel media eliminates the need for rust inhibitors.
5. Success stories in the Application of Stainless Steel Burnishing
Burnishing brass components has become an economical solution for repairing the nicks, scrapes, and scratches developed by manufacturing. Businesses and companies continue to seek better ways of maximizing profit and lowering cost. They have therefore turned to burnish as a means through which they can among other advantages prevent wear, improve shine and extend the lifespan of machine parts and components.
a) brass fitting parts
The brass fitting parts before ball burnishing surface is dull and dark. With ball burnishing in stainless steel ball, the surface gets to shine and clean. This product is ready to deliver to the market.
b) Steel bearing parts
Another example is steel bearing parts. Ball burnishing process not only brightens parts surface but also clean the dirt and remove the oily substance. You could clearly compare the parts before and after processing.
c) Brass stamping parts
The left parts below is brass parts before ball burnishing, and the right part is after ball burnishing. After the tumbling process, burr at the punching hole is flattened. The dirt and oxidation layer is removed as well.
Here are some results of the application of stainless steel burnishing processes
- Commercial burnishing systems have proved useful in extending the lifespan of tableware in restaurant businesses
- Restored beauty and shine in parts and components used in automotive industries
- Improvement in parts used in aeronautic industries
- Repair and restore flatware and hollowware
- Repair and restore metal serving dishes and a host of other applications
In this article, we discussed the concept of burnishing, and two type of burnishing technique including ball burnishing and roller burnishing. In mass finishing, we focus on ball burnishing and its advantages. Stainless steel tumbling media is most commonly used media in ball burnishing process. Later, we list 5 benefits for the process. Last, we give an example of how the ball burnishing process improves metal parts surface conditions.
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