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Exploring the Basics of Surface Finish in CNC Machining

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In the process of CNC machining, the smoothness of the processed surface is a very important indicator. Surface finish, defined as the texture or smoothness of a surface, not only affects the aesthetics of a part but also plays a vital role in determining its application performance. Factors such as friction, wear resistance, and even the ability to retain lubricant depend on the accuracy of the surface treatment.

surface roughness

What Is Surface Finish

Machined surface finish refers to the quality of the micro-geometry of the surface of a part during CNC machining. This quality is measured by assessing the smoothness of the surface, the regularity of the machining marks and the surface roughness grade.

Surface finish is one of the key factors affecting the function of parts. It will affect the friction, wear, fatigue strength, sealing, contact stiffness, vibration and noise of the parts.

Machined surface finish is typically measured using the following parameters:

Ra (Roughness Average): This is the most commonly used surface roughness parameter and represents the arithmetic mean height of absolute values along the surface sampled length.

Rz (ten-point height average): This represents the average value from the five highest peaks to the five lowest valleys over the sampling length.

Rmax (maximum height): This is the absolute height from the highest peak to the lowest trough within the sampling length.

Rq (root mean square roughness): This is the root mean square value of the surface height distribution, which gives more information about the peaks and valleys of the surface than Ra.

In addition, there are many other surface finish parameters, such as Rv, Rp, Rsk, etc., which can be used to describe different characteristics of the surface. Surface finish measurements can be performed by contact (e.g. micrometre) or non-contact (e.g. laser scanning) methods. During the design and machining process, the appropriate surface finish level needs to be selected based on the specific application of the part. Generally speaking, higher surface finish levels mean lower roughness but also mean higher machining costs.

Why is surface finish important in engineering manufacturing?

The importance of surface roughness in engineering manufacturing is reflected in many aspects because it directly affects the performance, life and cost of parts.

-Friction characteristics: Surface roughness affects the coefficient of friction and thus the wear rate and lubrication requirements of the part. Smooth surfaces reduce friction and increase mechanical efficiency.

-Sealing performance: For parts that need to be sealed (such as cylinders, valves, etc.), low surface roughness can help improve the sealing effect and prevent leakage.

-Fatigue Strength: Tiny pits and cracks on rough surfaces can serve as starting points for fatigue cracks, reducing the fatigue life of the part.

-Contact stiffness and assembly quality: Surface roughness affects the contact area between parts, which in turn affects the contact stiffness and load distribution during assembly.

-Corrosion resistance: Smoother surfaces have fewer gaps and are less likely to accumulate corrosive media, so they have better corrosion resistance.

-Optical properties: For parts that need to reflect light (such as mirrors), the surface roughness needs to be very low to ensure optical performance.

Surface Finish

Factors Affecting Surface Finish in CNC Machining

Surface finish can be affected by a variety of factors, key factors include:

Processing methods

Different processing methods (such as turning, milling, grinding, polishing, etc.) will result in different surface properties. For example, grinding often results in a smoother surface than turning.

Tool condition

The material, geometry, sharpness, wear status, etc. of the tool will have a significant impact on the surface finish. Dull or worn tools will result in a poor surface finish.

Processing parameters

Including cutting speed, feed rate and depth of cut. Higher cutting speeds may produce smoother surfaces, but if too high, they may also reduce surface quality due to thermal damage to the tool; smaller feed rates and depths of cut can reduce surface roughness.

Workpiece material

Different materials have different processing characteristics, such as hardness, toughness and internal stress state, which will affect the surface finish after processing. Certain materials may be easier to achieve a high finish, while others may be difficult.

Use of coolant

Coolant can reduce the temperature of the cutting zone, provide lubrication, reduce tool wear, and thus improve the smoothness of the machined surface. However, improper use may also lead to surface contamination or corrosion.

Measuring Surface Finish

1. Comparative method

A method of determining the roughness value of the measured surface by comparing it with a roughness sample marked with a certain value.

Features: Easy to measure, used for on-site measurements in workshops, often used for measurements on medium or rough surfaces.

 

2. Light sectioning method

A method of measuring surface roughness using the principle of light section.

Instrument used: light section microscope (double tube microscope)

Suitable for measuring flat or cylindrical surfaces of metal parts processed by turning, milling, planing or other similar methods. Suitable for measuring surfaces with Rz=~80μm and obtaining Ry value at the same time.

 

3. Intervention method

Use the principle of light wave interference.

Instrument used: interference microscope.

Suitable for measuring extremely smooth surfaces, Rz=0.025~0.84m.

 

4. Touch needle method

The method of feeling the cross-sectional profile of microscopic unevenness through the tip of a needle is contact measurement.

Measuring instrument: Electric profile measuring instrument

Measuring range: Ra=0.025~5 μm surface

Features: fast and reliable, easy to operate, easy to realize automatic measurement and microcomputer data processing; the measured surface is easy to scratch.

Surface characteristics, processing methods and application examples of surface roughness

Surface characteristicsRa/μmProcessing methodApplication
Rough SurfacesSlightly visible knife marks≤20Rough turning, rough planing, rough milling, drilling, filing, sawingShaft end faces, chamfers, drilled holes, gear and pulley sides, keyway bottoms
semi-smooth surfaceMicro processing marks≤10Turning, planing, milling, boring, drilling, rough reamingNon-fitting surfaces where bearings and gears are not installed on the shaft, free assembly surfaces of fasteners, undercuts on shafts and holes
Micro processing marks≤5Turning, planing, milling, boring, grinding, drawing, rough scraping, rollingSemi-finished surface, the surface of the box, bracket, cover, sleeve, etc. that is combined with other parts without matching requirements
Can’t see the processing marks clearly≤2.5Turning, planing, milling, boring, grinding, drawing, scraping, pressing, and tooth millingClose to the finished surface, the boring surface of the box for mounting the bearing, the working surface of the gear
Smooth surfaceVisible processing trace direction≤1.25Turning, boring, pulling, scraping, fine reaming, gear grinding, rollingCylindrical pins, tapered pins, mating surfaces with rolling bearings
Micro discrimination of machining trace direction≤0.63Precision reaming, precision boring, grinding and scraping, rollingGuide rail surface of higher precision lathe
Unable to discern the direction of machining traces≤0.32Fine grinding, honing, grinding, super finishingPrecision machine tool spindle taper hole, top conical surface, engine crankshaft, camshaft working surface, high-precision gear tooth surface
Extremely smooth surfaceDark glossy surface≤0.16Fine grinding, grinding, general polishingCylinder liner inner surface, piston pin surface
Bright glossy surface≤0.08Superfine grinding, fine polishing, mirror grindingThe surface of the spindle journal of precision machine tools, the balls of rolling bearings, and the mating surfaces of plungers and plunger sleeves in high-pressure oil pumps
Mirror-like glossy surface≤0.04
Mirror surface≤0.01Mirror grinding, super precision grindingThe working surface of high-precision measuring instruments and measuring blocks, and the metal mirror surface in optical instruments

Conclusion

Surface finish refers to the flatness and smoothness of a material’s surface. It is an important parameter to measure the quality of surface processing.While achieving a precise surface finish can be difficult and potentially more expensive, there’s no need to worry. We have a dedicated team to provide the necessary support to help you overcome these challenges.EASIAHOME can help you achieve the surface finish requirements of your machined parts.

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