Short Answer:

In engineering drawing, a scale is used to represent large objects in a smaller size or small objects in a larger size on paper. Since actual sizes cannot always fit on the drawing sheet, engineers use different types of scales to draw parts proportionally and clearly. A scale ensures that measurements stay accurate, even when the size is changed.

The main types of scales used in engineering drawings are plain scale, diagonal scale, vernier scale, comparative scale, scale of chords, and isometric scale. Each type is used for a specific purpose depending on the drawing requirement and the level of detail needed.

Detailed Explanation:

Types of Scales Used in Engineering Drawing

Scales are an important part of technical drawings. They help engineers, architects, and draftsmen to represent real-life objects that are too big or too small to draw at their actual size. Using a proper scale helps in keeping the drawing readable, clear, and proportional.

A scale is usually written in the format like 1:2 or 1:50, which means “1 unit on drawing = 2 or 50 units in real size.” Engineering standards such as IS:10713 define rules for using scales in India.

There are several types of scales used in engineering drawing:

1. Plain Scale

• This is the simplest and most common type of scale.
• It shows two units only, such as meters and decimeters, or centimeters and millimeters.
• Used when only whole numbers and fractions are needed.
• Example: A scale of 1:100 used for building layouts or room dimensions.

Construction:

• Draw a line, divide it into equal parts, label main units and sub-units on it.

2. Diagonal Scale

• Used when more accuracy is required, such as measuring in three units (e.g., meters, decimeters, centimeters).
• Allows measurement up to second decimal places.
• Very useful in machine drawings and part modeling.

Construction:

• Construct a plain scale, then add a diagonal grid to read finer divisions.

3. Vernier Scale

• Offers even higher accuracy than diagonal scale.
• Used in instruments like vernier calipers and micrometers.
• Helps measure very small values like 0.1 mm or less.

Working:

• A vernier scale is a small scale that slides over a main scale. The reading is taken by matching lines of both scales.

4. Comparative Scale

• Also called comparative or comparative plain scale.
• Used to compare two different units at the same time, like kilometers and miles or inches and centimeters.
• Useful in maps and conversions between units.

Construction:

• Two scales are constructed side by side using different units and the same representative fraction (RF).

5. Scale of Chords

• Special scale used to measure and set angles in degrees.
• Based on the length of chords in a circle.
• Mostly used in geometrical constructions and angle setting tools.

Construction:

• A circle is drawn, and lengths of chords are marked for different angles.

6. Isometric Scale

• Used in isometric projection to maintain the proportion of the drawing.
• Since isometric drawings are not in true length, this scale helps to convert true lengths to isometric lengths.

Construction:

• A 45° line and 30° line are used to project true lengths into isometric scale.

Why Scales Are Important in Engineering Drawing

• Makes large or small objects fit on paper
• Maintains proportional accuracy
• Helps in reading and interpreting technical data
• Allows precise measurement and fabrication
• Required for standardization and communication across design teams

How to Choose the Right Scale

• For small mechanical parts → Use enlarging scales like 2:1, 5:1
• For large structures or layouts → Use reducing scales like 1:10, 1:100
• Always write the scale used on the drawing for clarity.

In modern CAD tools, scales can be set digitally, but understanding manual scale drawing is still essential for engineers and students.

Conclusion

Different types of scales in engineering drawing help in representing real-world objects in a manageable and accurate form. Whether it is a small machine part or a large building layout, using the correct scale ensures that the drawing is clear, proportional, and easy to measure. Understanding how to use plain, diagonal, vernier, and isometric scales is a fundamental skill for every engineer involved in technical drawing or design.