Burnishing Process – Principle , Advantages and Disadvantages

Introduction to burnishing
Burnishing is a surface modification process which 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 work pieces. 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, there by reducing the surface roughness. This is shown in figure

Read also : What Is Honing Machine Process used For Super Surface Finishing

basic of operation of burnishing
basic of operation of burnishing

Principle of burnishing operation

Burnishing is a versatile process that improves the surface finish and dimensions of the turned parts, with out usage of extensive tooling. A conventional lathe, on which the work pieces were turned, can be used for burnishing, there by eliminating the time and effort for remounting the work piece. 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 work piece, the friction force rotates the balls or rollers of the tool, in a planetary motion.
Burnishing process is considered as a cold working process, because the surface of the work piece is subjected to severe stress due to the planetary motion between the tool & work piece 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, there by reducing the surface roughness. This also induces thermally stable and long lasting compressive residual stresses.

Classification based on tool:

Burnishing process can be broadly classified into two types based on the geometry of the tool. They are
1. Ball burnishing
2. Roller burnishing

Ball 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 a spring and the reactive force of the work piece. Schematic of ball burnishing is shown in figure 1.3. Fluid is circulated constantly, using a hydraulic pump, through the recesses around the ball to keep it in contact with the work piece. When the tool is fed along the work piece, the ball is pressed against the work piece, 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.

ball burnishing process
ball burnishing process

Roller Burnishing: Roller burnishing, as the name suggests, employs a tool with single or multiple rollers. For a multiple rollers tools, the rollers are present around the circumference of a supporting shank. Figure 1.4 shows the schematic of burnishing operation with 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 work piece, the rollers around the shank also rotate, resulting in the burnishing of the work piece.

roller burnishing process
roller burnishing process

Advantages and disadvantages of Burnishing :

Every manufacturing process has its own merits and demerits, which control their applicability in the industries. Burnishing also has merits and demerits, which are discussed in this section.

Advantages:

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 lathe or drilling machine can work on burnishing process.
5. Saves time: Work piece loaded on a lathe, milling or drilling machine need not be re-mounted for burnishing. The previous tool can be replaced with burnishing tool and 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. Wide variety of work pieces geometries, like flat, tapered, cylindrical, free-form surfaces can be processed by burnishing.

Disadvantages:
1. The initial cost of the burnishing tool is high.
2. Burnishing cannot be applied on miniature work pieces.
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 require dedicated tools and high skilled workmen. If the design or shapes of the contours change, new set of tools have to be designed and manufactured. This increases the cost and time.

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