Introduction
Now we begin our discussion of the ABC’s of engineering drawings – literally. Lines, letters, and numbers are what engineering drawings are made of. In the United States, the predominate standard is ASME Y14.2M – Line Conventions and Lettering. Adherence to a standard guarantees consistency and readability between companies or agencies. Students should understand the standard use of lines and how to produce standard text for use in letters and numbers.
Lettering
Above all else, an engineering documentation must be clear. All text on engineering drawings should be simple and readable. Most older texts (and the ANSI Standard) refer to the standard style of lettering as single-stroke Gothic. This term is somewhat archaic today, but what it means is simply a style of text that does not have serifs or ornaments of any kind. The style of text traditionally used on drawings closely resembles the Helvetica or Arial typeface. When drawing these letters by hand, do not reverse the pencil and draw back across an existing line (this is what single-stroke implies).
There is still value to learning how to letter manually. Even today, engineers and technicians often need to document information manually. Taking time to be careful about the information you are trying to convey is a hallmark of a professional attitude. Engineering journals, purchase requests, even simple notes to other people are examples of documentation that should be lettered carefully. If a student practices enough, these documents can be lettered easily and quickly. All text on student work should be lettered carefully.
Below is a guide to lettering upper-case in the traditional sans-serif (without serifs) style. The letters are laid out on an eight unit tall grid; notice that the width of the letters varies. Also note that while many letters have elements that align with the middle line, some do not.
The number characters are defined in a similar way. Note that shape of many of the numbers may not be the same as what you are accustomed to writing. It is especially important that the number characters are made as they are shown below.
Guide Lines
In order to letter by hand properly, the height and position of the letters must be controlled. Guidelines are very light lines laid out with a 6H or harder pencil to give the drafter. Lettering without guidelines can only be done if a lettering template is used. Even so, a bottom guideline may still be required to help align the letters. There are tools available to speed the process of drawing guidelines. The Ames® lettering template is of the most popular.
Be sure not to just draw the guide lines, but to actually use them. It is a common problem for students to omit guidelines or for their letters to drift away from them.
Letter Height
Whether a drawing is done in CAD or by hand, the letters must be large enough to be read and reproduced clearly. The minimum height of letters on a printed drawing is .12 inches (about 3.0 mm). The letters for the title block on large (D size and above) is .24 inches (about 6.0 mm). Letters that are used to indicate section views should be .24 inches tall. The only text that is allowed to be smaller is the text used to label the title block. This text can be .10 inches (a little bigger than 2.5 mm).
Letter Spacing
There should be adequate space between the letters, but not too much. Try to maintain an equal amount of space between the letters. Note that this does not mean to space the letters equally. If the letters are spaced an exact distance apart, the effect is to make the spacing look inconsistent. The shapes of the letters themselves influences how closely they should be placed together. For example, the letter “A” would be placed closer to the letter “T” than the letter “M” would be to the letter “I.” It would be incorrect to space each letter an equal distance from one another as this would make the text hard to read.
Properly spaced letters have an approximately equal amount of empty area between them. Some letter combinations (such as L and Y in the example below) actually overlap.
Like the spacing between letters the spacing between rows can be too large or too small. There should be a minimum of half a letter height between rows of text. If the spacing is greater than the height of the letters, the meaning of the spacing becomes ambiguous. If two rows of text are not meant to be read together, be sure there is greater than 2 text heights between them. As a rule of thumb, use 75% of the letter height for the space between rows.
Other Options
You may have noticed that all of the sample lettering has been in upper case so far. That is because all lettering on a drawing should be done in upper case. There are very few exceptions to this rule. Some companies prefer that the letters be slanted like italic letters. This is an option, but is not covered in this text in the interest of simplicity. When revising an existing drawing, however, personal preference is not an option. You must letter in the same style in which the original drawing was done.
Line Conventions
A fundamental part of the language of engineering graphics the meaning conveyed by different types of lines. The thickness and form of a line, as well as its relationship to other lines communicates important information. Know all of the line types by heart, they are fundamental to your understanding of a drawing. All lines must be clear, sharp, and opaque; they should have definite start and end points. Lines should be of two thicknesses: thin or thick. Thin lines should be a minimum of .3 mm thick on the printed drawing. Thick lines should be twice as thick as the thin lines.
Although it was not always the case, most CAD systems today can produce lines in varying thicknesses. Be sure to understand how to take advantage of this functionality with the system you will be using.
Visible Lines
Also called “object” lines, visible lines denote physical features that are visible in the view in which they appear. For instance, a cylinder shown below is shown in two views. In one view, the cylinder looks like a circle, in the other it appears as a rectangle. The visible lines show the contour of the curved surface and the edge of the flat surface.
Visible lines are thick.
Hidden Lines
Hidden lines show edges and contours of important features that are obscured by the geometry of the part. They are drawn as short dashes that are equally spaced. Hidden lines will always begin and end with a dash. Sometimes the length of the dash will need to be adjusted to show a break, but the overall appearance of the dashes should be consistent on the drawing.
Hidden lines are not absolutely necessary for every feature and can be omitted if it makes the drawing more clear. If you do omit hidden lines, it is a good idea to add a note indicating this.
Hidden lines are thin.
Center Lines
Center lines show the axis of a hole or a cylinder. Center lines start and end with long segments with a short dash in the middle. They should extend beyond the edge of the feature they are placed on a short, but distinct amount.
They can be used to show the position of more than one hole or cylindrical element by extending from object to object. If they are very small and it is not confusing, they do not have to be broken.
Center lines should not start or stop where the end lines up with another line, and they should not extend from one view to another.
Dimension & Extension Lines
Dimension lines serve to illustrate the orientation and extent of specified length or size. If there is room for the line and the arrowhead, it is preferred that dimension lines be placed between the extension lines.
Extension lines are used to clarify the points at which a dimension begins and ends.
Extension lines are also used to extend the surface of an object to show where a theoretical intersection is located.
Both dimension and extension lines are thin.
Leader Lines
Leaders are used to indicate information about hole diameters, radii, and other information that occurs as a specific location or on a particular surface on the drawing. Leaders are drawn as straight lines, but they must be drawn at an angle other than horizontal or vertical. They should have a level “elbow” at the end where the note is located. Leaders with arrowheads must terminate on an object line. Leaders should not be unnecessarily long, and they should not cross each other. They should be drawn at an angle that is different from the surrounding object lines. The angle between the elbow and the leader line must be large enough to be obvious. Leaders can be ended with a round dot (approximately 1.5mm in diameter) if they are intended to indicate an area on a surface.
Arrowheads
Arrowheads are used at the end of dimension lines and leaders. They are also used to indicate the direction of a cutting or viewing plane line. A filled arrowhead with 3:1 proportions is the preferred style. Other styles are allowed, but they are discouraged.
Symmetry Lines
Often it is convenient to only dray half of a symmetrical part. This saves space and is often easier to read than t drawing of the full view.
Cutting Plane and Viewing Plane Lines
Cutting plane lines are interpreted as actually slicing the object in two. The sliced object is then shown in a section view. The position of the observer is shown by the direction arrows.
Viewing plane lines are similar to section lines in that they indicate the point of view of the observer relative to a particular view. However, unlike section lines, viewing plane lines to not “cut” the part.
Some companies will choose to use different line types for section views and for cutting views. Be sure to conform to the standard you are working under.
Section Lines
In a section view, the object is shown as if it were cut along the cutting plane line. To illustrate where solid material be exposed by such a cut, section lining is used. The most general form of section lining consists of parallel thin lines drawn at an angle.
Section lines are thin. Do not confuse section lines with cutting plane lines.
Phantom Lines
Phantom lines have several uses on a drawing. Phantom lines can be used to show how other parts are positioned relative to the part shown. In the illustration below, a cylinder is placed on a v-block to be inspected. The position of the cylinder relative to the v-block is illustrated using phantom lines.
Phantom lines can show alternate positions of moving parts.
Phantom lines are used to show repeated detail. For instance, a spring or threaded part may consist of a lot of detail that is not necessary to understand the form of the part.
Stitch Lines
Chain Lines
lines are used on drawings
- Chain Line
- Short Break Line
- Section line
- Hidden Line
- Cutting Plane Line
- Visible Line
- Center Line
- Long Break Line
- Phantom Line
- Extension Line
- Dimension Line
