Surface Roughness – Detailed Explanation

Technical drawing is the common language between engineers and technicians a good technical drawing that they can understand them in the same area. While preparing or reading technical drawings or engineering drawings, we need to obey the standards. You will learn how to indicate surface roughness in technical drawings.

Surface roughness is a also very important point to define the surface characteristics of designed parts. There are different symbols and different illustration rules of these values in engineering drawings. 

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What is Surface Roughness?

Surface roughness is a very important surface characteristic that directly defines the quality. In general, there are roughness values from different parts that people design for different applications. For example, for a shaft, these values and tolerances are in the technical drawing. 

Improving the surface roughness value means the improved fracture strength. So, surface characteristics not only depend only on visual characteristics but also on mechanical characteristics. 

To define a surface, there are some kinds of surface texture parameters;

• Roughness: This is the most important surface parameter that defines the little surface deviations from an intended ideal surface structure. You can see above the surface roughness height and width.
• Waviness: This is the bigger version of roughness major manufacturing process deflections causes, vibrations, etc. You can see the waviness spacing and the waviness height illustrations above.
• Flaws: Flaw generally occurs on surfaces because of various and unpredictable reasons. There are lots of types of flaws on surfaces such as craters(you can see the above illustration), cracks, inclusions, etc.
• Lay: These generally occur because of machining processes applied on the surface of workpieces. The application direction of the manufacturing process creates them in general.

Above all, this is a very important parameter that defines the vertical9Lm deviations from nominal and ideal surface structures. And also we can calculate the average roughness with a formulation below;

It is the surface integration of deviations(y) that lie on the length of a workpiece or surface(Lm).

The surface roughness formula calculates the problem like the above illustration. ‘Lm’ is the whole length of the surface. So, you can see the areas that they created with deviations(y) from the nominal surface. We calculate this area with integration and divide this area with ‘Lm2 to find out the average roughness of a surface.

Importance of Surfaces in Manufacturing Methods

The importance of the surface comes from its feature of the parts that touch other parts in the assembly. So we need to define the surface characteristics very well according to the ideal nominal surface. Ideal or nominal surface characteristics are defined, and what kinds of surface features are desired from the related manufacturing process? And we define the part’s surface characteristics that we define according to that nominal characteristics. There are a bunch of reasons why the ‘Surface’ is very important in manufacturing;

• Aesthetics: Aesthetics is very important for marketing the produced product, and customers desire flawless surface characteristics for aesthetic reasons.
• Safety: In terms of strength, superior surface characteristics are very important. A surface defect can be the reason for the crashing of the product because of the stress concentration.
• Friction and wear characteristics: We stated before that the parts in an assembly, touch themselves from their surfaces. So wear and friction characteristics of surfaces must define well according to the application that the part that we use.
• Smooth surface contacts: Surface contacts between different parts will be better if touching surfaces are very smooth in terms of electricity.

Meanings of Surface Roughness Symbols in Technical Drawings

We indicate these values and symbols generally on the surfaces of parts. With the changing surface roughness values, the manufacturing technique and quality change. 

This is the most basic surface texture symbol that we use in the technical drawings. We can use all the manufacturing methods to produce this surface. 

Also, the completed triangle symbol indicated the machining operations are required to obtain this surface texture. 

We add this number that besides the machining operation surface texture symbol means the maximum allowed deepness of the machining in mm or in. 

The meaning of the circle on the surface texture symbol is that any machining operations are prohibited. You must obtain this surface by other manufacturing methods such as casting, forging, powder metallurgy, etc. Extra machining operations are not allowed on these surfaces. 

Surface Roughness Symbol Indication Rules on Technical Drawings

Firstly, there is a sizing restriction of surface roughness symbols on technical drawings according to the ANSI or ASME standards. 

The dimensions of a typical roughness symbol must be like above. 

The placements of these symbols on parts must be straight or rotated 90 degrees to the left. Otherwise is no proper use of the symbol. 

General Elements on a Surface Textures 

Above all, the general illustration of a surface texture is like below. You can see the general elements of a surface texture that they indicated in the technical drawings of parts.

Furthermore, on the surface texture symbol, you need to indicate the surface texture elements like below. 

– The waviness width and the waviness height must lay on the topside of the surface texture symbol 

– You must indicate the roughness width cutoff and roughness widths values right below the surface texture symbol. 

– You must indicate roughness height on the edge of the surface texture symbol.

– The lay symbol must be indicated like above. 

Standard Lay Symbols for Surface Roughness in Engineering Drawings

According to the ASME Y14.36M-1996 (R2002) standard, the general indications of lay symbols on technical drawings are as below.

This lay symbol indicates parallel lay orientations to tool marks. 

We use this lay symbol to represent the lay orientations on that surface perpendicular to the tool marks. 

We indicate circular lays which can occur with the milling operations. 

Representing the angular lays on indicated surfaces which can occur as knurls. 

We indicate multidirectional random lays. 

Radial lay orientation according to the line represents the surface. 

Indication of Roughness Height

The height of the roughness is the most important thing in surface texture indication of technical drawings. We use some symbols in technical drawings such as Ra and Rz. 

At the indication of the surface roughness height on the surface texture symbol, we use indications such as Ra0.5 or Rz0.3. 

The meaning of the Ra is the average roughness height on the technical drawings. Rz is the maximum roughness height that is allowed on that surface. 

This indication is also valid for the PN-EN ISO 1302: 2004 standards.

Indication of the Processes

Also, there may be some indications about the required processes to be applied to produce indicated surface texture parameters on technical drawings. They make this according to the PN-EN ISO 1302: 2004 standard which does not apply to the ANSI and ASME standards. 

In this indication, the average value of 0.25 surface roughness height that we defined. And we can obtain it with the grinding operations.

In some of the engineering drawings which we obtain according to the PN-EN ISO 1302: 2004 standard, surface roughness symbols that we can write like above on a side of the technical drawing paper. 

We place the general roughness indicator on the outside of the brackets and special surface roughness indicators that we place inside the brackets. We place these indicators also on the technical drawing itself.

How to Measure the Surface Roughness?

The most basic application to asses the surface roughness subjectively is the use of ‘Standard Test Surfaces’. The user asses the surface by touching it which is also called as ‘fingernail’ method, and compares this surface with the standard surfaces. This method is very subjective and not recommended to use professionally.

There is another electronic technique to asses the roughness situation that we do with electronic stylus devices. There is a standard-shaped stylus that traverses along the workpiece and sends the electrical signals that give the topography of the surface which we can show as actual or average surface roughness values by a profilometer device. Profiling devices have a separate flat nominal surfaced plate that compares the data that came from the stylus to asses the surface contour.

The most advanced technology to measure surface roughness is optical technology. The most accurate surface topology information can be obtained from these devices. In general, the reflectance, diffusivity, and light scattering information of measured surfaces are used to assess the surface situation. And there is no direct contact with the workpiece which is a very important feature of these devices. The speed of measurement with these devices is also superior compared to other measurement techniques.

The general methods to measure the roughnesses of workpieces or produced parts can be explained shortly like that.

Surface Characteristics of Some Manufacturing Methods

Generally, we obtain the worst surface roughness values from casting operations. Generally, again, machining operations for a better surface that we apply to the cast parts. In the sand casting of iron, the typical surface tolerance value is 1.3mm (0.050in), which is not acceptable in most applications. After the casting operation of iron, they apply various machining processes to them as a finishing operation.

For example in general turning operations, the surface tolerance value is typically around 0.05mm(0.002in) which is a much more advanced and acceptable surface tolerance in most applications.

But the plastic molding tolerance values are much more acceptable if we compare with sand casting. For instance molding of Polystyrene has surface tolerance values of around 0.015mm(0.006in) which is a more acceptable value.

If the surface tolerance values that we expect to be better from a manufacturing process, we can apply some finishing abrasive processes at the end of the process. With these abrasive processes such as grinding, honing, etc. we can obtain much better surface for typical applications in that surface characteristics are very important. For example in grinding operations, we can obtain surface tolerance values around 0.008mm(0.0003in). These kinds of surface roughness values are very superior and we can obtain for specific tasks.

In manufacturing, there are also non-traditional methods of production that we can obtain the desired high quality surface roughness tolerances. Around these non-traditional manufacturing methods, there are kinds of electric discharge, chemical machining, etc. For example around 0.08mm(0.003in), we can obtain surface roughness tolerance values with the laser beam cutting method. These kinds of tolerance values can be appropriate for different applications.

Conclusion

These are general information about the surface roughness and surface texture indications on technical drawings and engineering drawings. 

As you understand, surface roughness value is very important in engineering. There are different kinds of symbols that we use to define surface roughness and surface texture values on technical drawings.

Also, there are different methods to measure these values.

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FAQs About Surface Roughness