A section reveals exactly what the cutting plane crosses, and nothing else. To explain a section, name the plane, then list the features it passes through.
This lesson belongs to sections and spatial reasoning. It assumes you can already represent the visible section of a solid.
What can a section tell you?
It tells you three things: whether a feature is hollow, how deep it goes and how thick the wall around it is. All three are measured at the plane, in the direction you look.
It cannot tell you about features behind the plane that the plane does not touch.
Worked example: two holes, one plane
An original block is 50 mm wide, 30 mm deep and 30 mm high. Hole P has diameter 10 mm, is blind, and is drilled from the top to a depth of 20 mm. Its centre is 15 mm from the left and 15 mm from the front.
Hole Q has diameter 10 mm, goes through, and is centred 35 mm from the left and 25 mm from the front. Plane A-A runs left to right, 15 mm from the front.
Hole P. The plane passes through its centre. The section shows a gap 10 mm wide, from 10 to 20 mm across, running from the top down 20 mm. Below it, 30 − 20 = 10 mm of hatched material proves the hole is blind.
Hole Q. It spans from 20 to 30 mm from the front. The plane at 15 mm stops 5 mm short of its near edge, so it never cuts Q. The section shows solid, fully hatched material where Q sits.
A correct explanation says: the section reveals the depth of hole P and the 10 mm base under it, and does not reveal hole Q.
The mistake: describing the whole object
A student who knows the block reads the two holes from the views and writes that the section shows both. That answers a different question, about the object, not about the slice.
| Feature | Wrong claim | Correct claim |
|---|---|---|
| Hole P | Shown | Shown: 20 mm deep, blind, 10 mm base |
| Hole Q | Shown as a through hole | Not shown: the plane misses it |
| Wall thickness | Not mentioned | 10 mm of wall left of P at the plane |
Wall thickness at P: the hole spans 10 to 20 mm from the left, so the wall on the left is 10 mm and on the right the block continues for 50 − 20 = 30 mm.
What if you want to see both?
A single straight plane only sees what is on it. To show both holes, a second plane is needed, for example one 25 mm from the front, which would cut Q through its centre. Two sections then reveal two different features.
Check yourself
Use the same block. Plane B-B runs left to right, 25 mm from the front. What does it reveal?
Answer
It passes through the centre of hole Q. Across the block, Q spans 35 − 5 = 30 mm to 35 + 5 = 40 mm from the left. The section shows a 10 mm gap through the full 30 mm height, so Q is a through hole.
Hole P spans 10 to 20 mm from the front, and 25 mm is beyond it, so B-B does not reveal P.
What to study next
Practise spotting errors in comparing an inconsistent section with its corrected diagram and test yourself on the sections practice set.
If you want a teacher to train your explanation wording on new objects, see online one-to-one Engineering Drawing tuition.