How to Read Surface Roughness Symbols? Drawings & ISO Guide

For any machined part, surface smoothness matters. Even a machine part that looks smooth from outside might have valleys and tiny peaks. These small imperfections affect the part’s performance. If a metal part is too rough, it will create friction during its movement, which causes wear. So how does a manufacturer know how smooth each surface should be? That’s where surface roughness symbols come in. But how to read surface roughness symbols? 

 

At first, these symbols look like strange marks with confusing numbers and letters. This is completely normal if you are new to these symbols and drawing. But you need not worry! In this guide, I will cover surface roughness symbols, simple drawing examples, and the ISO standards. So let’s get started. 

 

What Are Surface Roughness Symbols?

 

Surface roughness symbols are special marks used on engineering drawings. They show how smooth or rough a surface should be. These symbols help manufacturers understand the surface finish needed for a particular part. Often, the roughness symbols are denoted by numbers and letters. Those numbers then explain the required level of smoothness. 

 

In simple terms, surface roughness symbols help manufacturers make a metal part’s surface smooth. Suppose you are making a stainless steel shaft for a machine. One part will move inside another part and will definitely create friction. To reduce that friction, the moving part needs to be smoother. However, another part of the same shaft doesn’t need that level of smoothness because it will not move or create friction.

 

That’s where surface roughness symbols will help you. They show these differences on a drawing. As a result, you get a clear idea of which part of the metal surface needs more attention and which does not. But you also might wonder why we cannot simply write “make this surface smooth” on the drawing? 

 

The problem is that the word “smooth” means different things to different people. One manufacturer may consider a surface smooth enough. However, other manufacturers might make it much smoother than necessary. Surface roughness symbols avoid this confusion by giving clear requirements. They make it easier for manufacturers to produce the right surface smoothness. 

 

Main Types of Surface Roughness Symbols You Must Know

 

Now you have a basic idea of what surface roughness symbols are and why they matter. Right? Now it comes to their types. These symbols aren’t all the same. Their different shapes tell you things about how a surface should actually be made. Let’s discuss the main surface roughness symbols and what they mean. But remember, all these symbols follow the traditional ISO 1302 symbol system. 

 

1- Basic Surface Texture Symbol (⌯)

The first and most common symbol is the basic surface texture symbol that looks like check marks. However, its lines have a specific shape. It tells you that a certain surface texture requirement applies to a particular area of a part. For example, a manufacturer is making metal plates. One side of the plate needs a particular surface finish. 

In that case, the manufacturer will place a surface texture symbol near that area of the drawing. This helps the manufacturer pay attention to the surface finish of that part and not overlook it. However, this symbol does not tell you whether material removal is required. To determine that, you will need to look at other numbers and letters. 

 

2- Surface Texture Symbol With a Bar ( √̅ )

This is also a surface texture symbol but with a horizontal bar. That extra bar means material removal is required under the ISO 1302 symbol system. It means that to achieve the required smoothness level, you have to remove some material. Manufacturers remove this extra material through drilling, grinding, and milling. 

For example, a shaft may need to fit inside a bearing. Sometimes its surface is too large to fit. As a result, the manufacturer machines the shaft to remove excess material and produce the required finish. However, this symbol only tells that material removal is important. Still, it does not tell which machining process should be chosen. 

 

3- Surface Texture Symbol With a Circle (√○)

Lastly, there is a surface texture symbol with a circle. Under ISO 1302, this circle means extra material removal is not permitted. That means manufacturers must keep the surface as-is, without any machining process. Instead, the surface retains the condition produced by processes such as casting, forging, or rolling. 

Suppose a brass component comes out of a casting process. It has the required shape and surface condition. Now, if the drawing specifies that material removal is not allowed, the manufacturer must not machine that surface. If they remove material, the surface shape will change. To prevent this mistake, this symbol restricts that action. 

 

How to Read Surface Roughness Symbols on Engineering Drawings? Easier Steps

 

If this is your first time looking at surface roughness symbols on an engineering drawing, they may seem like strange marks. But in reality, these symbols tell the manufacturer how smooth the surface should be. You need not worry! Here is a step-by-step guide to help you read these symbols. 

 

Step 1: Identify the Symbol’s Shape

The first step you have to take is to look at the symbol and try to understand it. Above, I have explained these symbols and their meaning. Check whether the symbol has only a basic mark, a horizontal bar, or a small circle. Imagine you are looking at a drawing of a metal plate. In this drawing, the symbol beside one of these metal plate surfaces has a bar. This means the manufacturer must remove material from that surface. Similarly, if the symbol has a circle instead, material removal is not permitted. 

 

Step 2: Find the Roughness Value

Once you identify the symbol, look at the number. Most probably, this number will be written beside or above the symbol. This number will tell you the required value for the surface roughness parameter. For example, you may see Ra 1.6 µm. Here, 1.6 is the specified roughness value, and µm (micrometer) is the unit of measurement. So, if the roughness value is 1.6 µm, it means manufacturers must meet that roughness level for that part. But values can also be 3.2 µm or higher. The higher the Ra value, the rougher the part will be, and vice versa. 

 

Step 3: Check the Parameter Letter (Rz & Ra)

Now, with the roughness value, pay attention to the letters next to the number. These numbers will be Ra and Rz on the engineering drawing. These two letters mean what kind of surface roughness is being measured. Remember, the number alone isn’t enough to understand the full smoothness requirements. 

So you also have to read these letters to understand what engineers actually want. Imagine you see Ra 1.6 on a drawing. Here, Ra is the measurement type. This Ra means the average size of the small peaks and valleys on a surface profile. The lower the Ra value, the smoother the surface will be (smaller peaks and valleys). 

Another letter you can see is Rz. It can also be 1.6 Rz. Here, the value is the same, but Rz describes something else. It measures surface profile height using defined peak-and-valley measurements. Simply put, Ra measures the size of valleys and peaks, while Rz measures the height of peaks and valleys. Therefore, Ra 1.6 and Rz 1.6 are not the same requirement, so identify which one your drawing includes. 

 

Step 4: Look for Additional Instructions

Sometimes, a surface roughness symbol contains more than a number and a letter. It may also include extra marks or notes nearby. These extra instructions specify how the surface should be made or what finish is required. For example, a drawing says that the surface of a metal part should be smooth to a certain extent. But do manufacturers have to remove material, or leave it as is, to achieve that smoothness? 

That’s what extra instructions tell. For example, a bar indicates that material should be removed. Similarly, a circle indicates that no material removal is needed. Similarly, in additional instructions, you can also find written notes. A note might demonstrate that the finish or surface must be ground. This means the manufacturer must grind that area to finish it. 

 

Step 5: Check Which Surface the Symbol Actually Applies To

When you look at an engineering drawing, you will see several surfaces on the same part. For each part, the symbol will differ, and the finish requirement will also vary. For example, a metal shaft’s outer surface smoothness requirement will differ from that of its inner surfaces. 

So engineers place a surface roughness symbol with a line pointing toward the shaft’s outer surface. This tells the manufacturer that the requirement applies to that particular surface, not to every part. So your job is to follow this line carefully to see where it points. If the drawing has several roughness symbols, check each one separately. Also don’t ignore written notes or extra instructions. 

 

Surface Roughness Symbols: Some Drawing Examples

 

Now you know how to read surface roughness symbols. But for some people, it might still feel technical. To make things easier, in this section I will discuss real-world examples. You will see how the symbols’ instructions actually work. 

 

Example 1: CNC Turning – Ra 3.2 µm

CNC turning is a process that makes round parts, such as metal shafts, rods, and pins. In this process, the metal part or workpiece moves continuously, and the cutting tool removes material from its outer surface. As a result, you get a finished, smooth part. Now suppose an engineering drawing shows Ra 3.2 µm for the outer surface of an aluminum part. That means the outer surface must meet the specified Ra value of 3.2 micrometers. To achieve it, manufacturers will use the right turning settings and tools. 

 

Example 2: CNC Grinding – Ra 0.8 µm

Grinding is a finishing process that uses a rotating abrasive wheel. The wheel removes rough edges, leaving the part finished and even. For example, consider a metal shaft that must fit accurately inside a machine. Its drawing specifies Ra 0.8 µm. 

You can see that this is a smoother surface requirement than Ra 3.2 µm. Now the manufacturer will use CNC grinding to achieve the required finish. However, grinding is not automatically the choice just because the drawing shows Ra 0.8 µm. The manufacturer still must choose the right method to achieve that smoothness requirement.

 

Example 3: CNC Milling – Ra 6.3 µm

CNC milling is different from turning. In milling, a rotating cutting tool removes material from a workpiece to create flat surfaces, slots, pockets, and other shapes. This means this CNC machining process is not about finishing but creating designs and parts. So often during this process, the machine leaves marks on the metal surface. 

To make them even, often Ra 6.3 µm is required. Suppose a drawing of a metal bracket specifies Ra 6.3 µm on one of its surfaces. Compared to Ra 0.8 µm, Ra 6.3 µm allows a rougher surface finish. No very smooth finish is needed. So manufacturers choose milling tools to meet the requirement. This saves time and money. Why? Because the manufacturer doesn’t need an additional finishing process.

Quick Highlight: The roughness values in these examples are for explanation only. They do not mean that every milling, turning, or grinding operation always needs these Ra values. A roughness value does not automatically require a specific manufacturing process. That’s why you should always follow the engineering drawing and the applicable ISO standard. The reason is that symbol rules may differ between older and newer standards.

 

ISO Standards for Surface Roughness Symbols

 

Imagine you make a metal part. Its surface can be very rough, a little rough, or very smooth. But how will you tell a worker how smooth the surface should be? That’s where engineers draw a special symbol on their drawing. This symbol tells the worker what kind of surface you need. But manufacturers need common rules to understand these symbols. Without them, every manufacturer would use different roughness and smoothness levels. So basically, ISO standards provide these rules. Two standards related to surface roughness symbols are ISO 1302 and ISO 21920. Let’s understand what each means. 

 

• ISO 1302: Traditional Surface Texture Indications

ISO 1302 is an older standard. It explains how to show surface finish instructions on engineering drawings. It helps engineers add a symbol to the drawing to tell the worker what to do with that surface. It includes three common symbols. The first one is the check mark. The second is a surface symbol with a bar. It means the manufacturer must remove material from a specific part to achieve a certain smoothness level. But a symbol with a circle means material removal is not permitted. These marks help workers understand what they can and cannot do to the surface.

 

• ISO 21920: The Newer Surface Texture Series

ISO 21920 is a newer series of standards for specifying and checking surface texture. As you know, technology and engineering practices change over time.  So standards are updated to provide clearer, more consistent ways to describe and measure surfaces. ISO 21920 is just like that. It provides updated rules for explaining the required surface finish. At the same time, it helps check whether the finished part meets those requirements. 

 

Conclusion 

 

Reading surface roughness symbols is very difficult at first. But once you understand the basic symbols, roughness values, and extra marks, things become much easier. In this guide, I discuss the meanings of these symbols in engineering drawings. These symbols tell you how smooth a surface should be and what instructions a worker needs to follow. But to fully understand them, always check the roughness value and read the full symbol before making a part. Also, check which ISO standard the drawing follows.