A Comprehensive Understanding of surface finish in CNC Machining
Table of Contents
In CNC machining, surface finish, also known as surface texture, refers to the deviations from a perfectly smooth surface that occurs as a result of friction between the cutting tool and the workpiece surface, plastic deformation of the metal surface layer during chip separation, and high-frequency vibration in the manufacturing system. Depending on the machining method and the material of the workpiece, the depth, spacing, shape, and texture of the surface marks left on the machined part can vary.
Surface finish is closely related to the properties of mechanical parts, such as fit, wear resistance, fatigue strength, contact stiffness, vibration, and noise. It significantly impacts the service life and reliability of mechanical products. This article comprehensively describes common information, techniques, and ways to improve surface finish in CNC machined parts.
What is surface finish in CNC Machining?
Surface finish and surface texture are interchangeable terms in CNC machining. Surface finish is a practical concept based on the microscopic geometry of the surface, while surface texture is a visual concept based on human perception. There is a corresponding comparison table between surface finish and surface texture. Surface finish can be measured using mathematical formulas, while surface texture can only be compared to a standard. Therefore, surface finish is considered more scientifically rigorous than surface texture.
Surface glossiness, also known as specular gloss, refers to the strength of light reflected off the surface of an object. If the surface has intense diffuse reflection, it is closer to a mirror effect and has a high glossiness. Conversely, if the diffuse reflection is weak, the glossiness is low. The factors affecting surface glossiness are related to the physical properties of the surface and the chemical properties of the materials used. A gloss meter is required to measure an object’s specular gloss.
Surface finish refers to the small spacing, peaks, and valleys on the machined surface. The distance between two peaks or valleys (wavelength) is small (less than 1mm) and is considered a microscopic geometric error. The smaller the surface finish, the smoother the surface. Surface finish is typically determined by the machining method and other factors, such as friction between the cutting tool and the workpiece surface, plastic deformation of the metal surface layer during chip separation, and high-frequency vibration in the manufacturing system. The depth, spacing, shape, and texture of surface marks left on the machined part can vary depending on the machining method and material of the workpiece.
What are the different ways to express surface finish in machining?
Surface finish, scientifically known as “surface finish,” can be expressed in various ways. The most commonly used method is to represent roughness using the arithmetic average value, Ra. The roughness of metal parts produced by CNC machining can be as low as Ra0.8 (the lower the value, the smoother the surface, and the higher the value, the poorer the quality), but it usually ranges from 1.6. Achieving a roughness of less than 0.8 with just CNC turning is generally impossible, and additional processing is required, such as grinding or other surface treatment processes. It should be noted that the surface finish does not necessarily correlate with the surface’s brightness. A shiny surface does not always indicate good roughness control. Roughness can be evaluated using roughness templates or measuring instruments. Specific surface treatment processes and standards for CNC machining should be determined based on the part’s design parameters.
In a CNC machined part drawing, the symbol for the surface finish should be labeled to indicate the surface characteristics that must be achieved after finishing. There are typically three parameters for surface finish height:
1.The arithmetic average deviation of the profile, Ra: The average arithmetic value of the absolute distances between the points on the profile line in the measurement direction (Y direction) and the reference line within the sampling length.
2.The ten-point height of micro unevenness, Rz: The sum of the average heights of the five highest peaks and the five deepest valleys within the sampling length.
3.The maximum height of the profile, Ry: The distance between the highest peak and the lowest valley within the sampling length.
Ra is the most commonly used parameter in the mechanical manufacturing industry. Ra is calculated using the formula: Ra=1/l ∫t0|Y(x)|dx or approximately Ra=1/n ∑|Yi|. Here, Y represents the distance between the points on the profile line and the reference line (centerline), and l is the sampling length.
Roughness is commonly used to characterize steel plates because they must have a certain degree of roughness before coating to ensure good paint adhesion and prevent peeling.
What Are the Surface Finishing Techniques for CNC Machined Parts?
Due to the subtractive nature of CNC machining, the surface of the machined parts will have tool marks. Therefore, many parts require additional finishing processes to achieve the desired surface characteristics or performance, such as aesthetics, corrosion resistance, wear resistance, or other purposes.
Here, we will discuss the most common metal surface treatments for CNC machining to help you choose the best surface finishing for your application.
(Our standard: Due to the slight visible tool marks left from machining, the standard value of surface finish (Ra) is 3.2μm. The surface finish can be reduced to 0.8μm, 0.4μm, or lower by applying finer machining passes. This will increase the production cost of the parts due to additional processing steps and stricter quality control.)
- Type II anodizing,also known as “standard” or “decorative” anodizing, can produce a coating thickness of up to 25 microns. The typical coating thickness depends on the color. For parts dyed black, the thickness can be between 8-12 microns, while for transparent (undyed) parts, the thickness can be between 4-8 microns.
- Type III anodizing (hard coating), or “hard coat” anodizing, can produce a coating thickness of up to 125 microns. Unless otherwise specified, the typical thickness of Type III anodized coatings is 50 microns.
- Spray painting: CNC service companies use spray painting during the production of large CNC precision machined parts. Spray painting treatment prevents CNC machined parts from rusting, such as household items, electrical cabinets, handicrafts, etc.
- Plating is the most commonly used finishing technique for CNC machined parts. The surface of CNC machined parts is plated using modern technology to ensure that it does not rust or peel off during long-term use. Standard plating processes include screws, stamping parts, batteries, automotive parts, small fittings, etc.
- Polishing: Use polishing materials such as wax or copper paste to hand-polish machined parts with sharp edges to prevent injury during use.
- Brushing: Drawing some strokes on the surface of aluminum, steel parts, or other products to improve their feel. Plastic wire drawing usually involves spraying a thick layer of paint and then processing it through this method. However, the effect is worse than that of metal CNC machined parts, and the texture is not as good as that of metal parts.
- Sand blasting: There is often a lot of residual plastic in the CNC machined parts, and manual operation can be very difficult, especially when dealing with holes or buckles. Therefore, sandblasting machines are used to remove the residual plastic. Aluminum CNC machined, or other metal parts can also be covered with knife lines on the surface. Some customers also request sandblasting and oxidation to give the machine a sturdy, granular metal texture. The sand used is fine glass beads, distinguished by patterns (e.g., 60 grit for coarse sand, 200 grit for fine apple sand).
- Oil spraying: We spray oil on our products according to customer requirements. Our site is a dust-free oil injection room that uses the best oil. (Injection molding can be sprayed with rubber oil, glossy varnish, light semi-matte, matte, baked paint, and different paint colors.)
- Powder coating is a process of spraying powder onto a part’s surface under static electricity. The powder coating process is similar to spray paint, but the “paint” is a dry powder instead of liquid. The powder adheres evenly to the surface of the aluminum machined part. It is then baked in an oven to form a durable, wear-resistant, and corrosion-resistant coating that is more durable than standard spray methods. Various colors can be used to achieve the desired aesthetics of the part. Powder coatings are superior to wet coatings in terms of mechanical strength, adhesion, corrosion resistance, and aging resistance. They can achieve a 100% high utilization rate and are environmentally friendly.
The above is the surface finish processes commonly used in CNC machining factories. In addition to these, there are many other surface finish processes. The specific process used is generally based on the different requirements of each part and the use of different surface treatment processes to achieve the design requirements of the parts. If you want to learn more about the surface finish processes for CNC machined parts, please contact us.
Primary Methods to Improve surface finish in CNC Machined Parts.
Normally, the surface finish of parts machined by a CNC machining center is quite good. However, sometimes it is not ideal and does not meet the desired surface finish. This is mainly due to equipment settings and environmental factors. To ensure surface quality, the following points should be noted:
- The cutting edge of the tool must be smooth and rounded. Tools with nicks are not suitable.
- Use appropriate cutting depths. In principle, the cutting depth should not be less than the radius of the tooltip arc (in reality, this is not always the case, but this standard can be used).
- Use appropriate cutting speeds, which means the rotation speed should be appropriate. The tool life will be short if the rotation speed is too high. If it is too low, chip lumps will occur, resulting in extremely poor surface quality. When the rotation speed is too low, there will be no chip lumps, but efficiency will be too low. Have you ever noticed that the surface of a machined part is smooth on the outside, rough on the inside, and smooth again on the very inside? The rough area is where the chip lumps occur, and to machine parts well, it is necessary to avoid the area where chip lumps occur.
- Use appropriate feed rates. High feed rates lead to high efficiency, while low feed rates lead to smooth surfaces. However, the further reduction has no meaning when the feed rate is reduced to a certain level. Precision machining usually recommends a feed rate of around 0.1mm per rotation.
- Use appropriate cooling conditions. Appropriate cooling fluids can improve surface quality somewhat (but not always).
Conclusion
Here is some basic knowledge about surface treatment in CNC machining, which we have summarized from years of experience in machining mechanical parts. Our goal is to provide customers with parts that are practical and suitable for their needs and to create more miracles to the best of our abilities. This is currently one of our top priorities. For CNC machining engineers, mastering this technology requires a thorough understanding of all aspects. Please contact us if you need a factory that provides CNC parts machining and surface treatment services.
