What is mechanical drawing?

A mechanical drawing is described as a technical drawing created by engineers following standard layout conventions, terminology, interpretation, and many other factors. It is also known as an engineering drawing.

The function of mechanical drawing is to accurately and thoroughly identify all the machine component's geometric features. It contains various orthographic views to convey all the product information to the manufacturer to produce that product.

Types of mechanical or engineering drawing

Following are the two types of mechanical or engineering drawing:

• Manual drawing
• Computer aided drawing (CAD)

Manual drawing

In the manual drawing, the drawings are made by hand with the help of drawing instruments like pencil, sheet, ruler, protractor, and others. The manual drawing is easily misread because of ambiguity and drawing errors.

Following are the major types of manual drawings:

• Machine drawing
• Production drawing
• Part drawing
• Assembly drawing

Machine drawing

Machine drawing is a drawing consisting of several orthographic views pertaining to machine components. The various dimensions and shapes are completely defined in this drawing. The representation of machine drawing is shown as:

Production drawing

Production drawing is a drawing that represents all the dimensions, limits, and tolerances along with their finishing processes like heat treatment, surface finish, drilling, and many others. It also shows the total number of parts in the assembly, the material used, etc. The representation of production drawing is shown as:

Part drawing

Part drawing is a detailed drawing of the component that helps in its manufacturing. The part drawing is based on orthographic projections. The representation of the part drawing is shown as:

Assembly drawing

Assembly drawing is a drawing that shows various parts of a machine in their working locations. It contains dimensions, number of parts, weight, material quantity, and many others.

Drawing standards for manual drawing

Following are the various important manual drawing standards:

• Sheet size
• Title block
• Scale
• Drawing lines
• Dimensioning

Sheet size

Sheet size is described as the dimension of a sheet on which a mechanical or engineering drawing is performed.

The tabular form of standard preferred drawing sheet sizes is shown as:

For reference, size A0 has a surface area of 

or

 and 

.

The relation between the different sheet sizes is shown as:

Title block

The title block is a block situated at a corner of the drawing sheet and gives the following information:

• Title of drawing
• Method of projection
• Sheet number
• Scale
• Name of organization
• Initials of staff drawn, checked, and approved.

The diagrammatical representation of a drawing sheet layout is shown as:

Scale

Scale in the mechanical drawing is used to represent the drawing size as per the object's true size. The various types of scales used in mechanical or engineering drawing are:

• Full scale (1:1)
• Reduced scale (1:X)
• Enlarged scale (X:1)

Drawing lines

Following are the various types of lines used in mechanical or engineering drawing along with their representations and applications:

Dimensioning

The dimensioning is an expression that represents the distance between the surfaces, location of the hole, type of material, surface finish, and many others over a drawing by using lines, symbols, figures, etc.

Following are the general principles of dimensioning:

• The dimensions should be placed as far as outside the object.
• The dimensions should be taken from visible outlines and not from hidden lines.
• Dimensioning to the center line should be avoided if it passes through the center of the hole.
• Each feature should be dimensioned once in drawing.
• Dimensions should be placed on the view or section that is clearer to the corresponding feature.
• The same unit should be used for all dimensions in the drawing.
• Minimum dimensions should be placed to define a complete assembly.

Elements of dimensioning

Following are the various elements of dimensioning:

• Dimension line
• Extension line
• Leader line
• Arrowhead

Dimension line

Dimension line is represented as a thin continuous line having arrowheads at ends joining the outlines or extension lines.

Extension line

An extension line is also a thin line drawn outside the outline and along with an outline. A gap of 1 mm should be present between the extension line and outline.

Leader line

One end of the leader line ends either in an arrow or a dot. The arrowhead touches the outline, and the dot is placed within the outline of the object. The other end of the leader line finishes as a horizontal line.

Arrowhead

An arrowhead is placed at the ends of the dimension line. The length of the arrowhead should be three times its width, and the triangle of the arrow should be filled.

The diagrammatical representation for the elements of dimensioning is shown as:

Methods of indicating dimensions

The dimension on the drawings in characters of sufficient size ensures the total liability. Dimensions should be placed so that it is not crossed or separated from any other drawing line. Only one method of indicating dimensions should be used in any one drawing.

Following are the various methods of indicating dimensions in manual mechanical drawings:

• Aligned system
• Unidirectional system

Aligned system

In an aligned system, the dimensions should be set parallel to dimension lines and at the center, above and clear-off the dimension line. The dimensions written are to be read from the bottom or right side of the drawing. The diagrammatical representation of the aligned system is shown as:

Unidirectional system

In the unidirectional system, the dimension lines are interrupted near the centre for insertion of the dimension. The dimensions should be indicated in the drawing to read from the bottom of the drawing only. The diagrammatical representation of the unidirectional system is shown as:

Computer aided drawing (CAD)

Computer-aided drawing is the creation of a sketch or drawing on a computer to assist engineers in design and drafting. It is also termed computer-aided drafting.

Following are the benefits of computer-aided drawing:

• Improves drafting productivity.
• Takes less time for drawing.
• Improves the accuracy of drawing.
• Changes in the drawing are easy.
• Low wastage.
• Reduction in required human resources.
• Minimum transcription errors.
• Quickly produce orthographic projections with tolerances.
• Printing can be done up to any scale.
• Hatching of all sections with different filling patterns.

Limitations of computer aided drawing

Following are the various limitations of computer-aided drawing:

• Computer-aided drawing requires a computer having sufficient specifications.
• Skill and judgment are required to prepare the drawing.
• Huge initial investment.

Common Mistakes

Following are the common mistakes performed by students:

• Sometimes, students forget the differences between mechanical drawing and simple drawing.
• Sometimes, students get confused between part drawing and assembly drawing.
• Sometimes, students are confused between various drawing lines and their applications.
• Sometimes, students forget the benefits of CAD over manual mechanical drawings.

Context and Applications

The topic of mechanical drawings is significant in various courses and professional exams of undergraduate, graduate, postgraduate, doctorate levels. For example:

• Bachelor of Technology in Mechanical Engineering
• Bachelor of Technology in Automobile Engineering
• Bachelor of Technology in Production Engineering
• Master of Technology in Machine Design
• Doctor of Philosophy in Machine Design

Related Concepts

• Engineering drawing
• Computer-aided drafting (CAD)
• Computer-aided machining (CAM)
• Manufacturing processes
• Production of machine elements or parts

Practice Problems

Q1. Production drawing is a type of ____.

1. Manual mechanical drawing
2. Computer-aided drawing
3. Simple drawing
4. None of these

Correct option- (a)

Explanation: The manual drawings are made by hand with the help of drawing instruments like pencil, sheet, ruler, protractor, and others. The manual drawings are commonly classified as machine drawing, production drawing, part drawing, and assembly drawing.

Q2. A single A2 sheet is equivalent to how many A4 sheets?

1. 2
2. 4
3. 6
4. 8

Correct option- (b)

Explanation: A combination of two A4 sheets makes a single A3 sheet, and a combination of two A3 sheets makes one A2 sheet. Hence, one A2 sheet is equivalent to four A4 sheets.

Q3. Which of the following is an application of continuous thin straight with zigzag line?

1. Trajectories
2. Hidden outlines
3. Center lines
4. Long break line

Correct option- (d)

Explanation: Continuous thin straight with zigzag line is applicable for long breaks lines. In contrast, dashed thick or thin lines are used for hidden outlines.

Q4. The dimension lines are interrupted near the center for insertion of the dimension in which system?

1. Both aligned and unidirectional system
2. Aligned system
3. Unidirectional system
4. None of these

Correct option- (c)

Explanation: In the unidirectional system, the dimension lines are interrupted near the center for insertion of the dimension. The dimensions are indicated in the drawing to read from the bottom of the drawing only.

Q5. Which of the following is a limitation of computer-aided drawing (CAD)?

1. Scaling is not possible
2. Less productive
3. High initial cost
4. Less accurate

Correct option- (c)

Explanation: Computer-aided drawing (CAD) is the creation of a sketch or drawing on a computer to assist engineers in design and drafting. CAD requires computers and various software that increase the system's initial cost.