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Last month, we concluded with a discussion of line types commonly used in manufacturing or other industries that use engineering drawings. Commonly called an Alphabet of Lines, two widths of lines are commonly used.
WIDE WIDTH LINES
The object line, cutting-plane line and the short break line are the widest of the lines at about .035 in. wide. As we discussed last month, the object line denotes the exterior or surface of the drawn part. A cutting-plane line indicates a location where a part will be explored internally with the use of another drawing, called a section view. Note the letters near the arrows of the cutting-plane line and the indication of the same letters at its representative section "View A-A." Graphic 1 is an example of this drawing method.
Short break lines will also indicate a location of internal part configuration. Commonly called "break-out sections," short break lines are "S" shaped when used with pipe or cylindrical objects or a jagged, torn paper shape when used in small parts that require internal viewing. This shaft is too long to fit on a sheet of paper so it has been shortened with the use of the "S"-shaped break lines, see Graphic 2.
THIN LINES
A dimension line (drawn above with arrows and the dimension of 18 in.) and two extension lines (nonarrowed) indicate the dimension of the shaft. Extension lines can continue from centerlines, explained next.
Centerlines are thin long and short dash lines that indicate the center of circles as seen from the side or top. Examples of centerlines are drawn through these concentric circles:
A leader line (drawn to the inner circle) provides information to the reader. In Graphic 4, the symbol (ø) before the number 4 indicates diameter.
Phantom lines show movement of a part on a drawing. An example is the extension of a spring or the rotation of a hinged part. This drawing technique will provide the necessary space for this movement when the part is assembled or installed. It consists of a long dash and two short dash combinations of thin lines.
DIMENSIONING ON A PRINT
A common component of any print is the dimensions. Dimensions are indexed from the bottom of the drawing (unidirectional) or from the right and bottom of the drawing (aligned). It is easier to read unidirectional dimensions because the numbers are all horizontal on the print. The common practice is to use only one method on a drawing.
All dimensions should be the exact length, width or thickness of the part. Scaling will only change the size of the drawing, not the correct numbers used in dimensioning.
Note: Never measure directly from a drawing. Scaling or reduction from printing will give inaccurate dimensions. Always use the printed dimensions on the print!
The rule is to place all dimensions outside the parts, not over-dimension as to confuse, and be neat and orderly. Some parts require internal dimensioning to properly explain the sizes to the reader. Although acceptable, inside dimensioning may confuse or be difficult to find.
TRY IT YOURSELF!
To all readers and students: To explore the discussion topics above, you can follow along at home or at work. For enrolled students: As part of your homework, draw one object using all the lines from the alphabet of lines and mail to me. Dimension your drawing. Try surfing the Net using phrases or terms from this article. If you find technical information, please let the class know about them on Blackboards Discussion Board or e-mail to me.
References used: Print Reading for Industry by W. Brown and R. Brown