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Burnishing is a surface modification process that produces a very smooth surface finish by the planetary rotation of a tool over a bored or turned surface. The tool may consist of one or more ball or roller. This process does not involve the removal of material from the workpieces. All machined or other processed metal surfaces consist of a series of peaks and valleys which constitute the surface irregularities. The force applied by the burnishing tool forces the material from the peaks to flow into the valleys. This reduces the height of the peaks and depth of the valleys, thereby reducing the surface roughness. This is shown in figure
Burnishing is a superfinishing process of obtaining a very fine surface finish having a grainless appearance on metal objects. This process is used on various flat, cylindrical, or conical surfaces. It removes scratches and tool marks on the surface.
Burnishing Definition:- Burnishing operation is the process of getting a smooth and shiny surface by contact and rubbing of the surface against the walls of the hard tools. It is a finishing and strengthening the process. Burnishing is basically a cold surface plastic deformation process.
Principle of burnishing operation
Burnishing is a versatile process that improves the surface finish and dimensions of the turned parts, without the usage of extensive tooling. A conventional lathe, on which the workpieces were turned, can be used for burnishing, thereby eliminating the time and effort for remounting the workpiece. The tool used for burnishing consists of one or more ball or roller, held in a casing. This tool can be mounted on the tool post of the lathe. When the tool is made to come in contact with the rotating workpiece, the friction force rotates the balls or rollers of the tool, in a planetary motion.
The burnishing process is considered as a cold working process because the surface of the workpiece is subjected to severe stress due to the planetary motion between the tool & workpiece and the pressure applied by the tool. When this stress exceeds the yield strength of the material, it results in the plastic flow of the material from the peaks of the surface irregularities into the valleys, thereby reducing the surface roughness. This also induces thermally stable and long-lasting compressive residual stresses.
Types of Burnishing Process
The burnishing process can be typically classified into two categories as follows:
1) Based on deformation element
a) Ball burnishing
b) Roller burnishing
2) Based on the motion of the tool, on the surface
a) Normal or ordinary
3. Classification based on the tool:
The burnishing process can be broadly classified into two types based on the geometry of the tool. They are
1. Ball burnishing
2. Roller burnishing
In this type of burnishing, the tool consists of one or more spherical balls, supported in shank by the hydraulic pressure of the fluid or spring and the reactive force of the workpiece. The schematic of ball burnishing is shown in the figure . The fluid is circulated constantly, using a hydraulic pump, through the recesses around the ball to keep it in contact with the workpiece. When the tool is fed along with the workpiece, the ball is pressed against the workpiece, resulting in the burnishing operation. The force of burnishing can be controlled by varying the hydraulic pressure of the fluid. In some ball burnishing tools, the hydraulic fluid will be replaced by a spring to control the positioning of the ball and the force applied on the workpiece.
Roller burnishing, as the name suggests, employs a tool with single or multiple rollers. For multiple rollers tools, the rollers are present around the circumference of a supporting shank. The figure shows the schematic of a burnishing operation with a single roller burnishing tool. The shank will be connected to the machine, which can be a drilling machine or milling machine or even a lathe. When the tool is made to come in contact with the workpiece, the rollers around the shank also rotate, resulting in the burnishing of the workpiece.
Advantages and disadvantages of Burnishing :
Every manufacturing process has its own merits and demerits, which control its applicability in the industries. Burnishing also has merits and demerits, which are discussed in this section.
Advantages of Burnishing Process :
1. Accurate size:
Parts can be produced by burnishing with high control over the dimensions. Hence very close tolerances can be achieved
2. Superfine surface finish:
Very smooth surfaces finish, as high as 0.05 μm Ra is possible with burnishing. The surface finish obtained is comparable with any of the other conventional surface finish operations like grinding, peening, etc.
3. Improves physical properties:
Burnishing produces hard, wear, and corrosion-resistant surface because of the cold rolling. It also induces compressive stresses which increase the fatigue life of the components that are subjected to cyclic loads.
4. More economical:
Burnishing eliminates grinding and honing, which are expensive and time-consuming processes. Burnishing can be done on any standard lathe or drill machine, which eliminates initial investment. Skilled labor is not needed for this process. Any worker who has experience in operating a lathe or drilling machine can work on the burnishing process.
5. Saves time:
Workpiece loaded on a lathe, milling, or drilling machine need not be re-mounted for burnishing. The previous tool can be replaced with a burnishing tool and the process can be done on the same mount. Thus the cycle time is reduced, which increases the throughput.
6. Adjustable settings:
Most of the commercially available burnishing tools have adjustable settings, which increases their scope of work. This reduces the cost when any change takes place in the component design.
7. Replaceable wear parts:
Wear parts such as roller, balls, guide rollers, etc can be easily replaced, which helps in prolonging the tool life with less maintenance cost.
8. A wide variety of workpieces geometries, like flat, tapered, cylindrical, free-form surfaces can be processed by burnishing.
Disadvantages of Burnishing process:
1. The initial cost of the burnishing tool is high.
2. Burnishing cannot be applied on miniature workpieces.
3. Components with thinner walls, which do not have enough strength, cannot be burnished, because the forces applied during burnishing are generally high.
4. Burnishing of intricate shapes and contours requires dedicated tools and highly skilled workmen. If the design or shapes of the contours change, a new set of tools has to be designed and manufactured. This increases the cost and time.
Application Of Burnishing :
- Hydraulic system components,
- Fillets on the shaft.
- Home Appliances
- Motors and Pump Industry
Flat, cylindrical, or conical surfaces (both internal and external ) are burnished with hardened steel or cemented carbide rollers or with steel balls mounted in a holder, fig. Fillets and grooves are burnished by rollers rounded to a radius, Fig. Hole burnishing is performed with multi- roller tools on drill presses, turret lathes, horizontal borers, unit built machines, and automatic lathes.
Difference Between Burnishing and Broaching.
Sr. No. Broaching Burnishing 1. Broaching is material removing finishing process by using multi point broach tool. It is a Process of superfinishing, in which mirror-like/glazed smooth surface is produced, by using ball or roller type of tool. 2. It is a machining process It is rubbing process no metal removal 3. Tools have small and multiple teeth. Smooth rollers/balls act as tool 4. Pulling or pushing force is required to remove the material. Pressure is required to press the balls or rollers. 5. Broaching components are- Bearing caps, cylinder blocks, connecting rods etc. Burnishing components are- Cam & followers, matting parts of engine, aesthetical components etc.
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