Surface finishing refers to a wide range of industrial processes to obtain certain required properties on the surfaces of manufactured products. Surface finishing is performed by mechanical, chemical, or electrical means and they work by removing, altering, adding, or reshaping the material. The final looks and durability of a product are decided by the surface finish of that part. Surface finishing is applicable to all useful materials like metals, plastics, ceramics, concretes, or any other engineering materials. Surface finish is also known as surface texture or surface topography that represents characteristics of the surface. The main purposes behind the surface finishing process are:
- Elimination of surface defects, remove burrs, and other surface flaws.
- Increased corrosion resistance.
- Increased resistance to chemicals.
- Helping with the adhesion or wettability of paint and other coatings.
- Increased strength, hardness, and resistance to wear.
- Decreased the frictional effects.
- Improved electrical conductivity and added surface electrical conduction.
- Improved chemical resistance.
- Enhanced aesthetics or appearance.
From the above purposes, it is quite clear that Surface finishing is an essential part of all manufacturing processes. Basically. the surface finish of a component is characterized by surface roughness, surface waviness, and lay (Refer to Fig. 1).
- The surface roughness is a measure of the finely spaced irregularities on the surface.
- Surface waviness refers to the warped or deflected surfaces due to coarser irregularities, and
- Lay signifies the direction that the predominant surface pattern takes.
Types of Surface Finishing
Depending on how the manufactured surface if affected by the surface finishing operation, they are categorized into two classes:
- Removing or Reshaping surface finishing; and
- Adding or Altering surface finishing.
In each of the above group, there are various surface finishing methods that can be employed depending on the required surface texture.
Removing or Reshaping Surface Finishing Processes
As the name suggests, the surface finishing processes in this category remove layers from the product surface. Major surface finishing technology processes of this group are:
- Abrasive blasting
- Chemical-mechanical planarization (CMP)
- Flame polishing
- Gas cluster ion beam
- Industrial etching
- Laser ablation
- Laser engraving
- Magnetic field-assisted finishing
- Mass finishing processes
- Tumble finishing
- Vibratory finishing
- Shot peening
- Laser peening
Adding or Altering Surface Finishing Processes
In this group of surface finishing technology processes, metal is added or altered on the surface of the product. The processes that fall under this category are:
- Case hardening
- Ceramic glaze
- Corona treatment
- Diffusion processes:
- Electroless plating
- Passivation/Conversion coating
- Chromate conversion coating
- Phosphate conversion coating
- Plasma electrolytic oxidation
- Shot peening
- Laser peening
- Plasma spraying
- Powder coating
- Thin-film deposition
- Chemical vapor deposition (CVD)
- Electrophoretic deposition (EPD)
- Mechanical plating
- Sputter deposition
- Physical vapor deposition (PVD)
- Vacuum plating
- Vitreous enamel
Unit of Surface Finish/Surface Roughness
The surface finish is quantified by a unit called surface roughness. Surface roughness provides the deviation from the ideal surface form and represents the surface smoothness. Surface roughness is very important in engineering applications. Rough surfaces wear out easily and have greater friction coefficients.
The surface roughness of a surface finishing process is usually provided by average roughness (Ra) or root mean square roughness (Rq). The most common measure of surface roughness is Ra and widely used in engineering applications. A smaller value of Ra represents a smoother surface finish. Surface Roughness Ra provides the arithmetic mean deviation in micro-meters or micro-inches. The Ra surface finishing chart is also one of the most used for absolute values. In a normal sense, whenever machinists talk about “surface finish,” they usually refer to surface roughness.
Measuring Surface Roughness
Various surface roughness measurement methods are available to measure surface finishing for industrially manufactured products. The measurement systems are:
- Direct measurement methods using a stylus.
- Non-contact methods using optical instruments having optical probes, microscopes, or sound waves.
- Comparison methods using visual senses to compare results.
- In-process methods using electromagnetic energy.
All the above-mentioned systems fall into any one of the below-mentioned surface finishing measurement techniques:
- Profiling techniques using a high-resolution probe.
- Area techniques measuring a finite area of the surface, and
- Microscopy technique measuring contrasts.
Surface Finishing Chart
A surface finishing chart is a reference table used in organizations for quality control purposes. Surface finish charts provide guidelines for measuring standard surface finishes like
- Parameters used.
- Typical roughness ranges.
- Conversion from one unit to another (American to Metric Systems).
The following table (table-1) provides a typical surface finish chart as sample.
|Average Surface Roughness, Ra (micro-meters)||Ra (micro-inches)||Root Mean Square Surface Roughness, RMS (micro-inches)||Center Line Average, CLA (N)||Roughness Total, Rt (microns)||ISO Scale Number, N||Cut-off Length |
required for sample (inches)
Surface Finishing Symbol
Surface Finishing Symbols are used to graphically represent the surface finish requirements for machined and structural parts. The philosophy of surface finishing symbols are taken from ISO 1302. The pictorial representation that is defined in ISO 1302 are provided in Fig. 2 below:
The various required information like the surface roughness value, cutoff value, sampling length, machining method, crease direction symbol, surface waviness, etc. are displayed along with the above-mentioned surface finishing symbols. All these data are required during the surface finish process. Fig. 3 below explains the information along with the surface finish symbol.
Surface Finishing Standards
The common surface finishing standards are:
- ISO 1302
- ASME Y14.36M