Mosi Li
Mosi Li

Director Project Engineer at 3Q Machining - China top CNC Machining Custom Part Manufacturer

What is surface roughness? The question pop up all the time when engineers edit and review a CAD drawing, Today we would like to introduce all details for everyone to better understanding of surface roughness, hence engineers can indicate roughness on the CAD drawing appropriately in order to machined parts can be manufactured smoothly to meet the product design requirements.

Surface Roughness

Surface roughness refers to the small spacing and small peaks and valleys of the machined part’s surface. The distance (wave distance) between its two crests or two troughs is very small (under 1 mm in generally), which is a microscopic geometric shape error. The smaller of the surface roughness, means the smoother of the surface.

There will be corresponding surface quality requirements in the mechanical drawings, and the surface quality after processing directly affects the physical, chemical and mechanical properties of the machining parts.

Surface Roughness 

                                 Surface Roughness indicated on the CAD Drawing

 

FAQ: surface roughness

  • 1) Tool marks during processing;
  • 2) Deformation during cutting and separation;
  • 3) The friction between the tool and the surface of the machined part;
  • 4) High-frequency vibration of the processing system.

Surface Roughness can be measured by manual comparison against a “surface roughness comparator” (a sample of known surface roughness), but more generally a surface profile measurement is made with a profilometer. These can be of the contact variety (typically a diamond stylus) or optical (e.g.: a white light interferometer or laser scanning confocal microscope).

How surface roughness is measured?                                    How surface roughness is measured?

The function of the surface finish of the part is different, and the required surface roughness parameter value is also different. The surface roughness symbol should be marked on the part drawing to illustrate the surface characteristics that must be achieved after the surface is completed.

Roughness is generally represented by three codes of Ra, Rz, and Ry plus a number. Among them, Ra is widely used in the CAD Drawings.

1. Contour arithmetic mean deviation (Ra)

Within the sampling length, the arithmetic mean of the absolute value of the distance between the point on the contour line in the measurement direction (Y direction) and the reference line.

surface roughness measurements Ra 

2. Ten-point height of microscopic unevenness (Rz)

Refers to the sum of the average value of the 5 maximum profile peak heights and the 5 maximum profile valley depths within the sampling length.

3. Maximum profile height (Ry)

Within the sampling length, the distance between the top line of the highest peak and the bottom line of the lowest valley of the profile.

 

Why Surface Roughness is Important for Machined Parts?

Surface roughness is closely related to the mating 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 machined part products.

The performance, reliability and life of the product depend to a large extent on the surface quality of the main parts. Generally speaking, the surface quality requirements of important or critical machined parts are higher than those of ordinary parts. This is because parts with good surface quality will greatly improve their wear resistance, corrosion resistance and fatigue damage resistance.

The surface finishing has 6 impacts for a machined parts:

  • Improve corrosion and chemical resistance effectively. 
  • Get a specific texture or appearance to have outstanding looks for a product.
  • Cover or eliminate surface defects.
  • Improve mating properties, wear resistance, fatigue strength, contact stiffness, vibration and noise of mechanical parts
  • Improves conductivity and adds surface electrical conductions.
  • Increases product’s strength to have better reliability.

surface finish chart / Surface Roughness Chart

The selection of surface roughness parameter should not only meet the functional requirements of the surface of the part, but also consider economic rationality.

For specific selection, you can refer to the existing drawings of similar parts and use the analogy method to determine. Under the premise of meeting the functional requirements of the parts, a rougher surface should be selected as far as possible to reduce the processing cost.

Generally speaking, the working surface, mating surface, sealing surface, friction surface with high moving speed and high unit pressure, etc., have high requirements for the smoothness of the surface finish, and the roughness parameter should be smaller. For non-working surfaces, non-matching surfaces, and surfaces finish with low dimensional accuracy, the parameter value should be the relationship between the parameter Ra value and the processing method and its application examples, which can be used as a reference when selecting.

surface finish chart surface roughness chart surface finish chart surface roughness chart

Surface roughness is one of the most important factors when it comes to producing a high-quality finished product.

There are various surface finishes in the industry that are commonly used, but they all have their advantages and disadvantages. You can use a surface finish chart to see which one is suited for your specific application.

How to choose a surface finish?

The general principle is to select a larger surface roughness value under the premise of ensuring that the technical requirements are meet. When making specific choices, you can refer to the following principles to choose a surface finish:

  • (1) The surface roughness value of the working surface is smaller than that of the non-working surface.
  • (2) The surface roughness value of the friction surface is smaller than that of the non-friction surface. The higher the friction speed of the friction surface, the greater the unit pressure it receives, and the higher it should be; the rolling friction surface requires a smaller roughness value than the sliding friction surface.
  • (3) For clearance fits, the smaller of the fit clearance, the smaller of the roughness value; for interference fits, in order to ensure the firmness and reliability of the connection strength, the greater the load, the smaller of the roughness value is required. Generally, the roughness value of clearance fit is smaller than that of interference fit.
  • (4) The surface roughness of the mating surface should be equivalent to its dimensional accuracy requirements. When the matching properties are the same, the smaller size of the part, the smaller of the roughness value; the same accuracy level, the smaller size has a smaller roughness value than the large size, and the shaft ratio hole has a smaller roughness value (especially the accuracy of IT8~IT5) .
  • (5) The surface roughness value of the surface subjected to periodic load, the internal fillet and the recess where stress concentration may occur, should be small.

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