Reading Blueprints & Technical Drawings

A technical drawing, also called a blueprint or print, tells you what someone wants built without them standing next to you. A building, a pipe run, a duct, a machine part and a wiring layout all look different on paper, but they share a common set of rules. Learn those rules once, and any new print becomes easier to read. Misread them, and you can frame a wall in the wrong spot or cut a part to the wrong size, and then everyone after you has to work around the error.

What You Will Be Able to Do

• Pull the key facts from a title block and check the revision.
• Tell what each common line type means.
• Build a picture of an object from its flat views.
• Read a section view.
• Use written dimensions and scale correctly.
• Follow a repeatable order when you get a new print.

Start With the Title Block

Read the title block before you look at anything else. It usually sits in the lower right corner, and on most prints it gives you:

• Title and number: what the drawing is. Confirm it is the print for your job.
• Revision: a letter or number showing the version.
• Date, drawn by and approved by: who made it, when, and who to ask.
• Scale: how the drawing size compares to the real size.
• Units: inches, feet and inches, or millimeters.
• Sheet: such as "2 of 7", so you know how many pages make up the set.

Nearby you will find a revision block listing each change. Always check that you are working from the current revision. An old print can be perfectly drawn and still wrong.

Line Types

Lines work like an alphabet. Each style has a meaning.

How Views Work

A print shows a solid object using flat views, each looked at straight on. This method is called orthographic projection. Most objects need two or three views.

• Front view: shows the most about the shape.
• Top view: looks down, and in the US sits above the front view.
• Right side view: sits to the right of the front view.

The views line up with each other. The height in the front view matches the height in the side view. The width in the front view matches the width in the top view. The depth in the top view matches the depth in the side view.

Worked example: A block has a hole drilled down from the top.

1. In the top view, you see a circle, with center lines crossing it.
2. In the front view, the hole is behind the surface, so it shows as two dashed vertical lines with a center line between them.
3. Trace straight down from the circle's edges in the top view, and you land on those dashed lines. This shows they are the same feature.

Put a finger on a feature in one view, then trace it across to the next. If you cannot find it, one view is hiding it or you are reading the view wrong.

A surface on a slant looks squashed in those views, so its true shape is hard to read. An auxiliary view solves that by looking straight at the slanted surface.

Some prints add a 3D-looking picture, called an isometric or pictorial view. It shows several faces at once. Use it to get the overall shape, but do not take sizes from it.

Section Views

A section view shows the inside of something, as if it were sliced open.

1. Find the cutting plane line. It runs through the object, with arrows and letters at each end, such as A and A.
2. The arrows show which way you are looking.
3. Find the view labeled SECTION A-A.
4. Hatching shows where material was cut. Open areas show the hollow parts.

A section can cut through a wall to show its layers, or a part to show a bore. Different hatch patterns can indicate different materials.

Dimensions and Scale

Dimensions tell you the size. Extension lines mark where the size begins and ends, and the dimension line runs between them with the number on or near it. Dimensions can be given as:

• Feet and inches, such as 12'-6".
• Decimal inches, such as 2.750.
• Millimeters, such as 70.
• A diameter, marked with a circle and slash symbol or "DIA", or a radius, marked R.

Parentheses or REF mean a reference dimension, which is there for information only.

Scale tells you how the drawing compares to the real thing. Scale can be written as a ratio, such as 1:2 for half size, or as an equation, such as 1/4" = 1'-0".

Worked example: A drawing at 1:4 shows a line 3 inches long. The real length is 3 x 4 = 12 inches.

Warning: Do not measure a print with a ruler to get a size to build from. Copies shrink and stretch, and drawings are not always drawn exactly to scale. Written dimensions rule. If a size you need is missing or two sizes conflict, ask.

Notes, Symbols and Abbreviations

A lot of information is not in the views.

• General notes apply to the whole drawing.
• Local notes use a leader line to point at one spot.
• Symbols stand in for common items, such as a weld, a valve or an outlet. A legend or key lists them.
• Abbreviations save space. Use the list in the set. The same letters can mean different things in different trades.

Read every note. Many errors come from skipping a line that changed the whole job.

A Reading Order That Works

1. Read the title block and confirm the print, the revision and the units.
2. Scan all the sheets or views to get the big picture.
3. Read the general notes.
4. Check the legend for symbols.
5. Trace your part of the job through every view that shows it.
6. Find the dimensions you need and work out anything that is not given directly.
7. Mark your questions, then ask them before you start.

Common Mistakes

Check Your Understanding

1. You see dashed lines on a front view. What do they mean?

An edge or feature hidden behind the surface you are looking at.

2. On a drawing at 1:2, a dimension says 10.00 but your ruler measures 4.9 on the page. What is the real size?

10.00. The written dimension is the size. The ruler reading only tells you the print is not exactly to scale.

3. Your print is revision B, and your supervisor's copy is revision C. What do you do?

Get the revision C print and read the revision block to see what changed. Stop any work that may be affected.

4. A section view is labeled SECTION B-B. How do you find out where it was cut?

Find the cutting plane line marked B on the other views. The arrows show which way you are looking.

Key Takeaways

• Read the title block first, and confirm the revision.
• Line types are a language: solid for visible, dashed for hidden, center lines for centers.
• Views line up with each other, so trace features from view to view.
• Section views show the inside, and hatching marks cut surfaces.
• Written dimensions beat anything you measure on the page.
• Read every note, and ask before you guess.