Presence of scallop marks or feed marks on machined surface gives rise to surface roughness. Roughness of the surfaces produced by conventional machining processes usually varies within 10 – 200µm. A reasonable estimation of surface roughness can be carried out using few machining parameters and certain tool angles. However, these estimated values do not take care of many influencing parameters that can affect surface quality during machining. For example, vibration, tool wear, built up edge (BUE) formation, machining environment, cutting temperature, etc. Thus it is indispensably necessary to directly measure surface roughness using proper instruments to know the actual roughness of a machined surface.

Instruments for measuring surface roughness

Profilometer—Roughness of a solid surface can be quantified using this instrument. There are several types of profilometers, such as contact type or non-contact (optical) type, 2-D or 3-D profilometers (e.g., Taylor Hobson Talysurf), etc. It can measure surface topography including roughness, waviness, texture, and even irregularity. Contact type profilometer utilizes a tipped stylus that is moved over the concerned surface for a specific distance and the deflection of stylus is recorded, which actually gives surface roughness value. These instruments directly display the roughness value, so no further calculations are required; however, physical contact between stylus and surface is mandatory. Non-contact type profilometer utilizes a small beam of light and sensors to detect reflection and refraction. It does not require physical contact and thus is suitable in few situations.

Microscope—Electronic and optical microscopes can also be used for characterizing machined surface. Scanning Electron Microscope (SEM) and Transmission Electron Microscope (TEM) are common examples in this group. Apart from surface topography, such instruments can provide information about burrs, dimensional inaccuracy, etc. However, microscopes are useful for approximate assessment of surface roughness.

How to express surface roughness values?

Unlike surface finish, surface roughness is a measurable parameter and it can be quantitatively expressed with the help of value and corresponding unit. Commonly roughness of machined surface is expressed in micro-meter (µm) and sometime nano-meter (nm). While measuring roughness, elevation values of few points (with respect to a mean line) over a specified length or area are noted down. All such noted values are manipulated to express roughness in a standard manner. Usually it is expressed in any of the following four indices.

• Centre line average roughness (R_a)
• Root means square value of roughness (R_rms)
• 10-point average roughness (R₁₀)
• Maximum peak to valley roughness (R_max).

References

• Book: Surface Integrity in Machining by J. P. Davim (Springer).
• Book: Machining and Machine Tools by A. B. Chattopadhyay (Wiley).
• Book: Metal Cutting: Theory And Practice by A. Bhattacharya (New Central Book Agency).