Surface Finish: Its Importance and Controlling Factors
A good surface finish can improve:
• Functional Performance
• Durability & Wear Resistance
• Adhesion of Coating
• Reduction in friction and Heat generation
• Improved conductivity & Heat Dissipation
• Control of light reflection & scattering essential for optical applications
• Eliminates surface defects
• High Strength & Wear resistance
• Enhanced flow or retention of lubricant
Surface finish refers to the process of changing a metal’s surface by adding, removing, or reshaping. Surface finish is characterized by surface roughness, waviness, and lay.
Surface roughness is a measure of total spaced irregularities on the surface. It is a quantifiable term,- Ra,. It is measured in microns. It is the measure of peak- to- valley height in surface undulations measured about the mean value. Ra is referred to as Surface Roughness average.
Waviness refers to the warped surfaces whose spacing or pitch is greater than that of surface roughness. Lay refers to the direction the predominant surface pattern takes.
Important parameters of surface grinding to achieve good surface finish:
• Grinding wheel specifications
• Grinding wheel grade
• Increasing grinding wheel speed
• Reduction in depth of cut
• Reduction in table feed rate
• Dressing conditions
• Spark out passes
Parameters with respect to dressing conditions are addressed below, while other parameters mentioned above can be optimized in machine control.
Dressing Conditions For Wheel
Dressing conditions for the wheel can be manipulated to help improve surface finish. This is accomplished by slowing down the dress traverse speed and/or reducing the depth of cut. This makes the sharpening process less aggressive and results in a smoother wheel face instead of opening up the wheel face with sharp grits.
Reducing the traverse speed will reduce the dressing lead per rotation of the grinding wheel and increases the dressing overlap ratio. An increase in the dressing overlap ratio creates a fine topography on the wheel face, which makes the wheel face dull and closed, resulting in a finer surface finish.
However, recommended values of overlap ratio have to be adhered to, as a very high ratio can close the wheel face too much, causing higher grinding power and thermal damage.
The following are the recommended ratios:
• Rough Grind: 2-4
• Medium Grind: 5-9
• Finish Grind: 10-22
The overlap ratio can be defined as the number of times any one point on the grinding wheel face will contact the dresser face as the dresser/wheel moves across. The overlap ratio determines the surface condition of the wheel face.
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