What Is Straightness?

At its core, straightness controls how much a feature can deviate from an ideal straight line. It’s all about ensuring that a surface or an axis doesn't "wiggle" beyond a defined tolerance.

Let’s break it down with an example:

You have a part with a specific feature you want to control. On the drawing, you point to a line on the surface using a leader line connected to a feature control frame, and you specify a straightness tolerance – let’s say 0.1. That’s it – you’re telling the manufacturer and quality inspector that any line on that surface, parallel to the direction you pointed at, must stay within a straightness zone of 0.1.

How Is Straightness Measured?

In real life, surfaces are never perfectly straight. So how do we verify straightness?

Imagine measuring points along the surface line. You collect several points, and plot them – that’s your actual surface line. Now, the straightness tolerance tells you this:

All those measured points must lie within a pair of parallel lines that are 0.1 mm apart (your tolerance zone).

These two lines define the straightness zone. You start with the lowest measured point and set the second line 0.1 mm above it. If all the points lie between these lines – the feature passes. If not? It's out of spec.

Here’s what doesn’t matter: whether the line is tilted or at an angle. Straightness has nothing to do with orientation. It's purely about the "waviness" of the line. A tilted but straight line is still acceptable. A wavy line? Not okay.

Avoiding Common Mistakes

Two common misunderstandings often trip people up:

  1. Thinking straightness controls orientation or position: It doesn’t. That’s what parallelism, perpendicularity, or position tolerances are for.

  2. Assuming all directions are controlled: If you specify straightness on a surface, it only applies to lines parallel to the direction you pointed at. Not all directions.

If you want to control straightness in the perpendicular direction as well, you’ll need to apply another straightness tolerance to a different direction.

Straightness of the Center Line

Up until now, we've been talking about straightness applied directly to surfaces. But what if you want to control the axis – say, the centerline of a cylinder?

Here’s how you do it:

  • You put a straightness tolerance on a dimension line (not on the surface), like the diameter of the cylinder.

  • That tells the manufacturer: “I want the axis of this cylindrical feature to be straight within this tolerance.”

Now, the measurement process changes slightly:

  • At each cross-section along the cylinder, the inspector measures the diameter and marks the center point.

  • These center points define the measured centerline.

  • All those points must lie within the tolerance zone.

Controlling Straightness in 3D

There’s a subtle but important distinction here.

If you only specify straightness without a diameter symbol (⌀), you’re only controlling straightness in 2D — like a flat plane.

But to control the entire axis in 3D, use the diameter symbol:

⌀0.1 straightness = A cylindrical tolerance zone with a diameter of 0.1 mm, within which the entire axis must lie.

This is essential for features like shafts or bores, where the entire axis must be straight in all directions.

Real Drawing Example

Imagine you have a part shown in two views. You apply straightness in two directions:

  • In the front view, you point to a vertical line and specify 0.1 straightness. This controls vertical lines on that surface.

  • In the side view, you do the same horizontally.

Now you inspect the part and find it has waves in the vertical direction, but it’s smooth horizontally. In this case, the vertical straightness fails (not acceptable), but the horizontal straightness passes.

Final Exercise

Let’s test your understanding.

You have a cylinder with a diameter of 20 mm. You place a straightness tolerance of ⌀0.1 on the dimension line. You now have two manufactured parts:

  • Cylinder A: The centerline is perfectly straight.

  • Cylinder B: The centerline looks like a banana.

Which one passes the tolerance?

✅ Cylinder A – The axis lies within the cylindrical tolerance zone.

❌ Cylinder B – The axis curves outside the zone. Not acceptable.

Key Takeaways

  • Straightness controls waviness, not tilt or orientation.

  • Use it on surfaces or axes.

  • Add a diameter symbol to control axis straightness in 3D.

  • Evaluate straightness locally: each line on a surface is treated independently.

  • Know what direction you're controlling – and add more tolerances if needed.

Straightness may seem simple, but it’s a powerful tool in your GD&T toolkit. Get this right, and you're one step closer to mastering technical drawing and geometric tolerancing.

Want more? Stay tuned for the next tolerance in the series.