What metal’s surface finish and surface roughness?
Different molds have different processing requirements for accuracy. The mold factory will clarify the surface roughness or finish of the workpiece when purchasing materials from the mold steel material supplier. Then, what surface finish? What is surface roughness?
What surface finish?
Surface gloss is the strength of the diffuse reflection of light on the surface of the object. From the naked eye, if the surface diffuse reflection is strong, it is closer to the mirror effect, and the gloss is high. On the contrary, the surface diffuse reflection is weak, and the gloss is low. Gloss is also called mirror gloss. The influencing factors of surface gloss are related to the physical properties of the surface and the chemical properties of the materials used on the surface. The method of detecting the surface glossiness of an object requires the use of a surface gloss meter.
What surface roughness?
In mechanics, roughness refers to the microscopic geometric characteristics of the small spacing and peaks and valleys on the machined surface. It is one of the problems of interchangeability research. Surface roughness is generally formed by the processing method used and other factors, such as the friction between the tool and the surface of the metal part during the processing, the plastic deformation of the surface layer metal when the chips are separated, and the high-frequency vibration in the process system. Due to the difference in processing methods and workpiece materials, the depth, density, shape and texture of the traces left on the processed surface are different. Surface roughness is closely related to the matching properties, wear resistance, fatigue strength, contact stiffness, vibration and noise of mechanical parts, and has an important impact on the service life and reliability of mechanical products.
After processing, the surface of the metal part looks very smooth, but it is uneven after magnification. Surface roughness refers to the micro-geometric features composed of small spacing and tiny peaks and valleys on the surface of the processed part, which is generally formed by the processing method adopted or other factors. The function of the surface of the metal part is different, and the required surface roughness parameter value is also different. The surface roughness symbol should be marked on the metal part drawing to illustrate the surface characteristics that must be achieved after the surface is completed.
No matter which processing method is used, the surface of the metal part is not absolutely flat and smooth. When observed under a microscope (or a magnifying glass), no microscopic uneven peaks and valleys can be seen. This kind of microscopic unevenness on the surface is generally formed by various factors such as the movement and friction between the tool and the metal part, the vibration of the machine tool and the plastic deformation of the metal part. The micro-geometric features composed of such small spacing and peaks and valleys on the surface are called surface roughness. Surface roughness is a technical index for evaluating the surface quality of parts. It must not affect the matching properties, wear resistance, corrosion resistance, contact stiffness, fatigue strength, sealing properties and appearance of the metal parts. Therefore, according to the functional requirements of the metal parts, the surface roughness of the metal parts should be specified on the drawing.
Surface roughness refers to the small spacing and small peaks and valleys of the processed surface. The distance (wave distance) between its two crests or two troughs is very small (below 1mm), which is a microscopic geometric error. The smaller the surface roughness, the smoother the surface.
Surface roughness is generally formed by the processing method used and other factors, such as the friction between the tool and the surface of the metal part during the processing, the plastic deformation of the surface layer metal when the chips are separated, and the high-frequency vibration in the process system. Due to the difference in processing methods and workpiece materials, the depth, density, shape and texture of the traces left on the processed surface are different.
Surface roughness is closely related to the matching properties, wear resistance, fatigue strength, contact stiffness, vibration and noise of mechanical parts, and has an important impact on the service life and reliability of mechanical products. General labeling uses Ra.
Causes of surface roughness:
- 1) Tool marks during processing;
- 2) Plastic deformation during cutting separation;
- 3) The friction between the tool and the machined surface;
- 4) High-frequency vibration of the process system.
The influence of surface roughness on parts is mainly manifested in the following aspects:
- The rougher the surface that affects wear resistance, the smaller the effective contact area between the mating surfaces, the greater the pressure, the greater the frictional resistance, and the faster the wear.
- Affecting the stability of the fit. For clearance fits, the rougher the surface, the easier it is to wear, which will gradually increase the clearance during work; for interference fits, the microscopic peaks are flattened during assembly, reducing the actual Effective interference reduces the connection strength.
- Affecting fatigue strength There are large wave troughs on the surface of rough parts. Like sharp notches and cracks, they are very sensitive to stress concentration, thereby affecting the fatigue strength of the metal parts.
- The surface of rough parts that affects the corrosion resistance is likely to cause corrosive gas or liquid to penetrate into the inner metal layer through the microscopic valleys on the surface, causing surface corrosion.
- Rough surfaces that affect the sealing performance cannot be tightly attached, and gas or liquid leaks through the gaps between the contact surfaces.
- Affecting the contact stiffness The contact stiffness is the ability of the joint surface of the metal part to resist contact deformation under the action of external force. The stiffness of the machine largely depends on the contact stiffness between the metal parts.
- The surface roughness of the measured surface of the metal part and the measuring surface of the measuring tool will directly affect the accuracy of the measurement, especially in the precision measurement.
- In addition, the surface roughness will have varying degrees of influence on the coating, thermal conductivity and contact resistance, reflectivity and radiation performance of the metal parts, the resistance of liquid and gas flow, and the flow of current on the conductor surface.
Processing method comparison
| Surface features | Surface roughness (Ra) value | Examples of processing methods |
| Obviously visible knife marks | Ra100,Ra50,Ra25 | Rough turning, rough planing, rough milling, drilling |
| Slightly seen knife marks | Ra12,5,Ra6,3,Ra3,2 | Fine turning, fine planing, fine milling, rough reaming, rough grinding |
| No processing traces can be seen, micro-discussion of processing direction | Ra1,6,Ra0,8,Ra0,4 | Fine turning, fine grinding, fine hinge, grinding |
| Dark glossy side | Ra0,2,Ra0,1,Ra0,05 | Grinding, honing, super fine grinding, polishing |
The surface roughness that can be achieved by different processing methods and different materials
| Processing method | Material | Ra parameter value range μm | ||||||||||||
| 0.012 | 0.025 | 0.05 | 0.1 | 0.2 | 0.4 | 0.8 | 1.6 | 3.2 | 6.3 | 12.5 | 25 | |||
| Injection molding | Thermoplastic | PMMA(Plexiglass) | 0.025 | 0.05 | 0.1 | 0.2 | 0.4 | 0.8 | 1.6 | |||||
| ABS | 0.025 | 0.05 | 0.1 | 0.2 | 0.4 | 0.8 | 1.6 | |||||||
| AS | 0.025 | 0.05 | 0.1 | 0.2 | 0.4 | 0.8 | 1.6 | |||||||
| Polycarbonate（PC） | 0.05 | 0.1 | 0.2 | 0.4 | 0.8 | 1.6 | ||||||||
| Polystyrene（PS） | 0.05 | 0.1 | 0.2 | 0.4 | 0.8 | 1.6 | 3.2 | |||||||
| Polypropylene（PP） | 0.1 | 0.2 | 0.4 | 0.8 | 1.6 | |||||||||
| Nylon | 0.1 | 0.2 | 0.4 | 0.8 | 1.6 | |||||||||
| Polyethylene（PE） | 0.1 | 0.2 | 0.4 | 0.8 | 1.6 | 3.2 | 6.3 | |||||||
| Polyoxymethylene（POM） | 0.05 | 0.1 | 0.2 | 0.4 | 0.8 | 1.6 | 3.2 | |||||||
| Polysulfone（PSU） | 0.2 | 0.4 | 0.8 | 1.6 | 3.2 | |||||||||
| Polyvinyl chloride（PVC） | 0.2 | 0.4 | 0.8 | 1.6 | 3.2 | |||||||||
| Polyphenylene ether（PPO） | 0.2 | 0.4 | 0.8 | 1.6 | 3.2 | |||||||||
| Chlorinated Polyether | 0.2 | 0.4 | 0.8 | 1.6 | 3.2 | |||||||||
| PBT | 0.2 | 0.4 | 0.8 | 1.6 | 3.2 | |||||||||
| Thermoset plastic | Aminoplast | 0.2 | 0.4 | 0.8 | 1.6 | 3.2 | ||||||||
| Phenolic plastic | 0.2 | 0.4 | 0.8 | 1.6 | 3.2 | |||||||||
| Silicone plastic | 0.2 | 0.4 | 0.8 | 1.6 | 3.2 | |||||||||
| Pressing and extrusion molding | Aminoplast | 0.2 | 0.4 | 0.8 | 1.6 | 3.2 | ||||||||
| Phenolic plastic | 0.2 | 0.4 | 0.8 | 1.6 | 3.2 | |||||||||
| Pyramine plastic | 0.1 | 0.2 | 0.4 | 0.8 | ||||||||||
| Silicone plastic | 0.2 | 0.4 | 0.8 | |||||||||||
| DAP | 0.4 | 0.8 | 1.6 | 3.2 | ||||||||||
| Unsaturated polyester | 0.4 | 0.8 | 1.6 | 3.2 | ||||||||||
| Epoxy plastic | 0.2 | 0.4 | 0.8 | 1.6 | 3.2 | |||||||||
| Machining | Plexiglass | 0.025 | 0.05 | 0.1 | 0.2 | 0.4 | 0.8 | 1.6 | 3.2 | 6.3 | ||||
| Nylon | 1.6 | 3.2 | 6.3 | 12.5 | ||||||||||
| Teflon(PTFE) | 0.8 | 1.6 | 3.2 | 6.3 | 12.5 | |||||||||
| Polyvinyl chloride | 1.6 | 3.2 | 6.3 | 12.5 | ||||||||||
| Reinforced plastic | 1.6 | 3.2 | 6.3 | 12.5 | 25 | |||||||||
What is the difference between surface finish and surface roughness?
The processing surface has the characteristics of microscopic geometric shape composed of small spacing and peaks and valleys. It is one of the problems of interchangeability research. Surface roughness is generally formed by the processing method used and other factors, such as the friction between the tool and the surface of the metal part during the processing, the plastic deformation of the surface layer metal when the chips are separated, and the high-frequency vibration in the process system. Due to the difference in processing methods and workpiece materials, the depth, density, shape and texture of the traces left on the processed surface are different. Surface roughness is closely related to the matching properties, wear resistance, fatigue strength, contact stiffness, vibration and noise of mechanical parts, and has an important impact on the service life and reliability of mechanical products. The general label uses Ra.
Ra (arithmetic mean deviation of contour): the arithmetic mean of the absolute value of the contour deviation within the sampling length L.
Surface finish Another term for surface roughness. The surface finish is put forward according to the human visual point of view, and the surface roughness is put forward according to the actual surface micro-geometry. After the 1980s, thinking that it was in line with international standards (ISO), China adopted surface roughness and abolished surface finish. After the national standards for surface roughness GB3505-83 and GB1031-83 were promulgated, surface finishes were no longer adopted.
In fact, the two are mainly named differently, and there is a corresponding comparison table. Roughness has a calculation formula for measurement, and the finish can only be compared with a model gauge. Relatively speaking, the roughness is more scientific and rigorous.
Source: China Metal Flanges Manufacturer – Yaang Pipe Industry (www.epowermetals.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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