A cutting plane line tells you where to slice an object in your head and which way to look afterwards. Read three things: the position, the arrows and the letters.
This lesson is in sections and spatial reasoning. The next lesson, representing the visible section of a solid, builds on it.
How do I read a cutting plane line?
Take the three parts in order.
- Position: where does the line cross the top view? That sets what material is sliced.
- Arrows: they point in the direction of sight. The material behind the arrows, on the viewer’s side of the plane, is imagined removed.
- Letters: they match the plane to its section view, labelled A-A beneath it.
Worked example: a drilled block
An original block is 60 mm wide, 40 mm deep and 30 mm high. A vertical hole of diameter 20 mm goes through it, centred 30 mm from the left and 20 mm from the front. In the top view, a plane A-A runs left to right through the centre of the hole, with arrows pointing towards the front. So you look from the back, and the back half of the block is imagined removed.
The plane is 20 mm from the front. The hole spans from 10 mm to 30 mm from the front, so the plane passes through its middle.
Predict the section view. The cut face is a rectangle 60 mm wide and 30 mm high. The hole is empty, so the middle 20 mm of that width, from 20 to 40 mm across, is not hatched.
The hatched parts are two strips, each (60 − 20) ÷ 2 = 20 mm wide, at the left and right, each 30 mm high.
The mistake: hatching the whole face
The habit is to hatch the full rectangle because it is the whole cut face. That ignores what the plane actually crosses.
| Step | Wrong | Right |
|---|---|---|
| Cut face | Hatch all 60 × 30 | Hatch only where material is cut |
| The hole | Hatched over | Left clear, sides drawn as two vertical lines |
| Strips | One block of hatching | Two strips of 20 mm each |
A plane that misses the hole
Move the plane to 35 mm from the front on the same block. The hole spans 10 to 30 mm from the front, so the plane stops 5 mm behind its edge, and cuts only solid material.
The cut face is now a complete 60 × 30 rectangle, fully hatched. Nothing in the cut face shows the hole, because the plane never crossed it. The position of the plane decides what the section shows.
Check yourself
Use the same block. Plane B-B runs left to right, 25 mm from the front. Does it cut the hole? Describe the hatching.
Answer
The hole spans 10 to 30 mm from the front, and 25 mm lies inside that range, so the plane cuts the hole.
The plane is 5 mm from the hole’s centre line, so the hole appears narrower than 20 mm at this plane. Its width there is 2 × √(10² − 5²) = 2 × √75 ≈ 17.3 mm. The cut face is hatched on both sides of that gap, and the gap shows the two sides of the hole as vertical lines.
What to study next
Go on to representing the visible section of a simple solid and then try the sections practice set.
If you want a teacher to give you a new object and move the plane for you to predict, see online one-to-one Engineering Drawing tuition.