BASIC ELEMENTS OF ENGINEERING DRAWINGS

Audio: Basic elements of engineering drawings

As we already discussed in Introduction to the Engineering drawing, engineering drawing is a unified language that engineers use to communicate. The rules for creating engineering drawings (communication) are defined by a standards organization (for example, ISO and ASME). Furthermore, we showed you how the usual detailed and assembly drawing looks like. Now we will dig deeper into the basic elements of engineering drawings and explain each of them in more detail.

In the past, engineering drawings were created by hand, and today we use CAD software. For that reason, we will focus more on the elements of engineering drawings applied to the engineering drawing created with the CAD software. Of course, in software like Autodesk fusion 360, you can choose your preferred standard for drawing creation, and most of these rules are already implemented. However, I believe that the experts in their field of work should be familiar with the concepts and practices they use daily. With that being said, let us move forward and learn more about the basic elements of engineering drawing.

Elements of engineering drawings: Font

The font types used on engineering drawings for CAD applications are defined by ISO 3098-5:1997. According to ISO, CA and CB font types should be used for CAD applications. Usually, on the drawing generated in the CAD software, we can use any font that we want, but sometimes different projects and companies will require standardized fonts. In those cases, we can use fonts like ISOCP, ISOCT, ISOCPEUR, ISO3098B, etc.

The uppercase letter height can be 1,8 mm, 2,5 mm, 3,5 mm, 5 mm, 7 mm,10 mm, 14 mm, and 20 mm.

Elements of engineering drawings: Lines

Types of the lines on the engineering drawing are defined by ISO 128-2:2022.

In the table below, you can see line type and their regular use:

CAD software already defines the type of lines based on the preferred standard. But, of course, if you need to adjust the type of lines and thickness, you can change them in software settings.

In the table below, you can see line groups with the paired thickness of wide and narrow lines:

Elements of engineering drawings: Drawing border

The drawing border defines the limit of the formal drawing area. Basically, it means that every content relevant to this drawing must be inside this border. The borders are defined by ISO 5457:1999.

The main elements of a drawing border are:

Trimming mark
Trimmed format
Grid reference border
The frame of the drawing space
Drawing space
Untrimmed format

Trimming marks

The trimming marks are used in cases when untrimmed paper sizes are used. Trimming can be done automatically or by hand. The trimming marks are created on the edges of the standard paper sizes in the form of two rectangles 10x5mm.

The untrimmed sheet sizes are used when, for example, we want to allow printers to “bleed” beyond the edge and give the printer a small amount of space to account for the movement of the paper. Personally, I have created thousands of drawings in my career, and not once have I had to trim the paper or send the drawing to the supplier in paper format.

Trimmed format – paper size

Paper sizes are defined by ISO 216:2007. The engineering drawing should be made on the smallest possible paper size, allowing clarity and resolution. Most used paper sizes are:

Grid reference system

The grid reference system is used to easily locate different elements of the engineering drawing. It is divided into fields, where the vertical fields are defined with letters and the horizontal field with numbers. In that way, it is easy to point out any detail on the drawing.

Let us look at the following example: Imagine that you are in a meeting with your team and want to point out one dimension of the drawing you are unsure about. You could easily say that it is located on the left view, and it measures the distance from the edge of the part to the hole. That is great if your team is composed only of technical members.

The easiest way to point this dimension is to say: “if you would look into the field D2, you can see a 15 mm distance between the edge and the center of the hole.” This drawing is simple, and you might think this is unnecessary. Now imagine an A0 paper sheet with 6 main views, 4 sections, and 10 details. Using the grid reference system makes it much easier to locate different drawing elements without spending too much time looking for them.

The inner line of the drawing border should be moved 20mm from the left edge of the paper and 10 mm from the other three sides. The frame limiting the drawing space should be a continuous line of 0,7mm in width.

The length of the grid reference border field is 50 mm, starting from the middle of the drawing border. The size of the letter should be 3,5mm. The left and right fields (vertical fields) contain letters with capital letters arranged alphabetically, starting from the top. The top and bottom sides (horizontal fields) contain numbers, starting from the left side.

For the A4 paper size, the grid reference border is only shown on the top and the right side.

Centering marks

The centering marks are used for positioning the engineering drawings when the engineering drawings are reproduced or microfilmed. These marks should be created in the center of the reference field, and they should be created with thick 0.7mm continuous lines, 5mm from the frame of the drawing area. I believe that most of us will never use microfilms.

Elements of engineering drawings: Title block

The title block is one of the most essential elements of engineering drawings that contains all the relevant information needed to identify the drawing, e.g., part number, part name, drawing owner, designer name, etc. Furthermore, a title block defines other relevant information, e.g., material, standard, perspective type, scale, page number, etc. The title block is defined by ISO 7200:2004.

Data fields in the title block can be divided into two groups: mandatory “M” and optional “O”:

Other data fields are presented outside the title block only when used, for example, scale, projection symbol, general tolerances, and surface texture requirements.

1. Applicable standards

It is common to note on the drawing to define applicable standards. Usually, it is standard praxis to define applicable tolerancing standards on the drawing. For general tolerances, ISO 2768-1:1989 is generally used. This standard specifies general dimensional tolerances, and if the dimensional tolerance is exactly as in the standard, it is not necessary to define it on the drawing.

When referring to the standard, it is also necessary to state the desired class; e.g., ISO 2768 – mk refers to the medium class. To learn more about the general tolerances, check Engineering tolerances – Introduction.

Many different standards can be defined on the drawing. For example, you can define surface texture according to ISO 21920; or indication and dimensioning of undefined edges according to ISO 13715. Which standard you should put on your drawing depends on the industry and manufacturing process you are using.

2. Projection symbol

The projection symbols are defined by ISO 5456-2:1996. They describe what type of projection was used: first-angle projection or third-angle projection.

3. Units

Considering that we have an international and imperial system of units, it is desirable to define applicable units on the drawing.

4. Scale

Imagine that we must create a drawing of the car. It would be challenging to draw a complete car on one of the standard paper formats in his actual dimensions. In case when we have big objects (or small ones), we scale them to fit on the drawing. The scale represents the ratio between the same sizes in the drawing and reality.

The natural scale would be displayed on the drawing in his actual size, and it would be marked 1:1. That means that one millimeter on the drawing is equal to one in reality.

If we would like to enlarge the object two times, we would use a scale of 2:1. That would indicate that two millimeters on the drawing are equal to one millimeter in reality.

If we would like to reduce part for five-time, we would use a scale of 1:5. This would mean that one millimeter on the drawing is equal to the five millimeters in reality. The scales are defined by ISO 5455:1979.

5. Legal owner

In this field, usually, the company name is displayed. It could be the company’s name, initials, or logo. The remark is added with the company name to protect the drawing from freely being shared with the competition. For example: “This document is the property of the Newtonian world, and as such, it is strictly confidential. The document is supplied with the understanding that it will not be disclosed to third parties without the prior consent of Newtonian world.”

6. Material

In order to manufacture a part, we need a raw material that will be shaped into the desired form. In the title block, we can define which material type and color (or finishing) we want to attribute to this part.

7. Revision index

In an ideal world, the mechanical design engineer will design the part, create a drawing, and send it to a supplier. After that, the manufactured part will arrive and perfectly fit into the assembly.

In the real world, the last step is not necessarily true. Usually, the mechanical designer first will create a prototype. Every individual part will first be inspected on its form, then assembled, and its function will be inspected. If individual parts are not designed correctly, or the assembly is not designed correctly, the mechanical design engineer has to introduce changes to the drawing.

For the proper drawing revision tracking, ECR/ECN/ECO exists. We are marking the revision using letters A-Z, AA, AB, etc. We can alternatively use numbers 1, 2, 3… These numbers indicate that the original design has been updated somehow. If the part is significantly changed, so it is no longer interchangeable with the older versions, assigning a new number is better than creating a new revision level.

The assembly drawing defines different components with the ballon containing an identification number. For this reason, I prefer to use a letter for marking the revision index.

Some companies prefer “revision clouds” to highlight the areas where change was applied since the previous drawing revision.

8. Responsible department

The responsible department refers to the internal department inside the company. This can be a department name or department code.

9. Creator

The creator refers to the name of the person that created the drawing.

10. Date of issue

The date of the issue represents the date when the part was officially released for the first time. Every revision after the original design should also have a date of release.

11. Approval person

The approval person is the name of the person who approved the document.

12. Document type

The document type states what type of document (in our case, drawing) we are looking at:

Component drawing,
Assembly drawing,
Tabular drawing,
Fabrication drawing,
Outline drawing,
Other industry/company-related drawings.

You can read more about different document types here: Technical product documentation.

13. Document status

Depending on the internal documentation management procedure in the company, a document can be at different stages of product development in different statuses (draft, approved, reviewed, obsolete, etc.).

14. Identification number

The identification number or the part number is the main mean of identifying the part or assembly. Inside the company, no part or assembly should have the same part number, i.e., every part number should be unique. Different companies use different methods of assigning part numbers. Some companies use software for automatic assigning, like PDM; others do it manually, define it by product family, etc. The naming scheme is usually defined inside the company. The identification/part number should not exceed 16 characters.

15. Title or the part name

The title or the part name is usually given for convenience and is linked to the part’s function or appearance.

For example, welding flange, plate, main holder plate, etc. Although the different parts can have the same part name, unique part numbers (identification numbers) will still differentiate them.

Supplementary title

The supplementary title is used to add more information to the part. For example, we can use the product family name or the assembly name for which this part is used.

16. Paper size

The paper size refers to the paper size we used for drawing creation.

17. Sheet number

In reality, some parts are fairly complex to fit on just one sheet. Depending on the paper size used for drawing creation, a drawing can contain many sheets. On the drawing, the current sheet number should be displayed.

Number of sheets

The number of sheets represents the final number of drawing sheets.

Elements of engineering drawings: Notes

The notes specify other equally important information needed for complete drawing specifications. For example, overall surface finish or surface finish specified for a specific surface, material (in this case, in the Title block, there should be written “see notes”), color, reference to CAD model, etc. They are usually listed numerically, each requirement listed individually. The notes are usually listed together on the same part of the drawing. We can say that the notes are added whenever we want something done to the part, and we do not have another way to specify it.

Examples:

No sharp edges,
All internal edges to be free of burs,
Part to be cleaned and degreased when completed.

If the notes are not general in nature, we can use “Flag notes.” The flag notes are used to apply indications on specific geometry or dimension. The choice of symbol can be specific to the company or the industry.

Elements of engineering drawings: Part lists or Bill-of-Materials (BOM)

The bill of materials lists the subassemblies, parts, and materials required for building the assembly. The parts list or BOM can be added directly to the drawing or provided with the assembly drawing as a separate list. The difference between the parts list and BOM is that the parts list defines only one structural level.

Data fields generally defined in the BOM are:

Item number (position) defines the part position in the BOM. Each part is identified with a balloon containing the corresponding number. For example, item number one will have the corresponding balloon with the number one inside it. A leader with the arrowhead end is used when pointing at the outline of a part. A leader line with the dot end is used when it points to the surface of a part.

Identification number or part number
Quantity – how many of these particular parts are used in this drawing
Information about the suppliers for the purchase parts
Material identification – the material used to produce the part
Weight
Overall dimensions
Revision index
Type etc.

The Part lists or BOM can contain some of this or all this information. Some companies do not place part lists or BOMs on the assembly drawing; they create it on a separate sheet. This depends on the company standards.

Closing words

In your engineering praxis, you will encounter engineering drawings on a daily basis. The elements listed here will be defined (right or wrong) on each.

As a real expert, you want to ensure that you communicate the right message every time someone checks your drawing. You are not only communicating design details on your drawing; you also communicate your expertise and your company brand.

Considering the advances in CAD software, you do not need to create and/or implement these elements every time you create a drawing. In this case, you will create drawing templates that you will use whenever you start drawing. Therefore, understand these elements and implement them correctly in your drawing templates.

Now you have an excellent overview of the basic elements of the engineering drawing. However, I suggest you go through the text once more and identify areas you think need more understanding and clarity. Then, once you have identified those areas, start building up your knowledge in those areas.

To make it easier for you to find related posts, check the “Further reading” chapter below. Do you have any questions or need something to be clarified better? Leave a comment below, and I will give my best to adjust the post accordingly.

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