Technical Drawing Training Lesson 7: Surface Texture, Ra and Surface Symbols

No machined surface is perfectly flat. Look closely enough and every surface is a landscape of peaks and valleys left by the cutting tool. Surface texture is the drawing’s way of saying how rough that landscape is allowed to be.

It matters because roughness decides how surfaces behave: how they seal, how they wear, how they slide, how well a coating sticks, and how much a fatigue crack likes to start there.

What Ra means

Ra is the arithmetical mean of the absolute deviations of the surface profile from its mean line, measured over a defined sampling length. It is given in micrometres (µm), where 1 µm = 0,001 mm.

So Ra 3,2 means the average departure from the mean line is 3,2 thousandths of a millimetre. Ra 0,8 is four times smoother — and considerably more expensive to produce.

The surface texture symbol

The symbol is a 60° tick whose short arm is half the length of the long one. There are three basic forms:

• Basic symbol — the plain tick. It carries no meaning until something is added to it.
• Bar added — material removal required: the surface must be machined.
• Circle added — material removal not permitted: the surface must stay as cast, forged or rolled.

Information is then written in defined positions around it: the Ra value, a second requirement, the production method or coating, the direction of lay, and the machining allowance.

• The symbol touches the surface — or its extension line — with its point, from outside the material.
• Each surface is marked once.
• If most surfaces share one value, it is given as a general note and only the exceptions are marked individually.
• The symbol is always written so it reads from the bottom or the right of the sheet.

Which process gives which Ra

Reading a roughness value is really reading a process. An Ra of 6,3 can be reached by ordinary turning or milling. An Ra of 0,8 normally needs grinding or reaming. An Ra of 0,1 needs honing or lapping. So the surface symbol quietly tells you how many operations the part will take — and therefore what it costs.

Test yourself

1. What does Ra 1,6 mean in millimetres?
   0,0016 mm average deviation from the mean line.
2. Which is smoother, Ra 0,8 or Ra 3,2?
   Ra 0,8 — the smaller the number, the smoother the surface.
3. What does a circle on the surface symbol mean?
   Material removal is not permitted; the surface stays as produced.
4. Which process would you choose for Ra 0,8?
   Grinding or reaming; ordinary turning will not reliably reach it.
5. How many times is one surface marked?
   Once.
6. Why not specify Ra 0,2 on every surface?
   Because cost rises steeply with smoothness and most surfaces do not need it.
7. Which standard replaced ISO 1302?
   The ISO 21920 series, from December 2021.