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Causes of Surface Roughness | Factors Affecting Surface Roughness

What is Surface finish ( Surface Texture)?

Surface finish is also known as surface texture or surface topography, is the nature of a surface. It comprises the small local deviations of a surface from the perfectly flat ideal (a true plane).

When a part is machined, a particle is detached by the process, leaving on the component a scratch which is a minute groove. The formation of these grooves by the tool as it passes across the part produces the surface finish. Within each groove, the texture is determined by the way the material is detached from the solid material.

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If the tool is perfectly set up and guided, then the particles will be of equal size and depth and the part will form a flat plane. If this is not the case, then the component will form an undulating surface.

What are The Factors that Causes of Surface Roughness?

The factors that causes of surface roughness in metrology.

  • Feed marks of cutting tools.
  • Chatter marks on the workpiece due to vibrations caused during the manufacturing operation.
  • Irregularities on the surface due to rupture of workpiece material during metal cutting operation.
  • Surface variations caused due to deformation of workpiece under the action of cutting forces.
  • Irregularities in the machine tool itself such as lack of straightness of guide ways.

Why is Surface finish - Surface Roughness or Waviness Important?

The prime purpose of assessing surface finish is that of ensuring that the part is “fit for purpose” and that it will meet or exceed its stated performance criteria and life expectancy.

Surface roughness, often shortened to roughness, is a component of surface texture. It is quantified by the deviations in the direction of the normal vector of a real surface from its ideal form. If these deviations are large, the surface is rough; if they are small, the surface is smooth.

From this point of view the designer and inspector must work together to ensure that the ideal filter and roughness parameters are chosen based on the desired performance characteristics. The designer must clearly articulate the key features of the surface which are “ideal” in terms of ensuring good performance (e.g. sufficient deep narrow grooves for oil retention on a cylinder liner).

In addition, those features deemed “unacceptable” must also be articulated (e.g. deep narrow grooves would be a disaster on fillet radii of crankshafts due to high stresses and are the likely points of crack propagation and failure emanating from the deep narrow groove).

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How to Improve or Reduce the Surface Finish (Surface Roughness)?

Ideally a good surface finish specification will use filters and parameters that will clearly identify unacceptable defects as well as confirm that the surface is generally acceptable. This may often mean the use of averaging roughness parameters (e.g. Ra, Rq, Rpm etc.) for confirmation of general surface quality and the use of peak parameters (e.g. Rp, Rv, Rt etc.) for the identification of potential defects.

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When choosing the filter, a similar choice needs to be taken by the process control engineer. Some parameter/filter combinations may act as a good control of the primary condition of the machining process. It may be possible to detect degradation in cutting tool and/or lubricant just by observing the change in roughness. Similarly, it may be possible to observe changes in the basic condition of the machine tool itself by measuring a component’s waviness.

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It is often good practice when designing a new part to write down the “favourable” surface finish features as well as those features which might be deemed “unfavourable”. If possible, draw a sketch of the profile feature(s) in each category. From these sketches determine if the features of interest are closely spaced (in which case a surface roughness filter would be used) or widely spaced (in which case a waviness filter would be used).

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