PRIN. OF DRAFTING AND SHOP DRAWINGS - OD1641 - LESSON 1/TASK 4
interference (negative allowance) that is intended between mating parts.
It
represents the tightest possible fit, or the largest internal member mated with the
smallest external member.
d. Basic Hole Allowance System.
This is the standard method of determining the
size of a hole into which a shaft is to be fitted.
(1) Plus Tolerance. The hole limits are plus only.
The hole can be larger than
the basic dimension, but not smaller.
(2) Minus Tolerance.
The shaft has a minus tolerance only.
It can be smaller
than the basic dimension.
3.
Lines and Sections
a. Lines.
Lines are the language of the draftsman.
He must place a lot of
information in a small space so he uses as few words as possible. The draftsman's
drawings are composed of lines of different construction, each line with a special
meaning.
To learn the appearance and use of these lines is essential to the
understanding of shop drawings. Figure 78 on the following page illustrates each
type of line.
(1) Outline Line.
On a shop drawing, the draftsman uses outline lines.
These
thick, solid lines represent the edges and surfaces that are visible from the angle
at which the view is drawn and make it possible to identify the object.
(2) Hidden Line.
These lines are used to represent lines known to be on the
inside or back surfaces of objects but not visible. To represent these invisible
lines, draftsmen use a medium-weight broken line--made by a series of short dashes
(all the same length) with uniform spacing.
(3) Center Line. Center lines
are used to locate the center of a circle or arc,
and also to divide drawings into
equal or symmetrical parts. They are lightweight,
broken lines of alternate long
and short dashes.
Center lines are aligned to
indicate the travel of a center.
Center line is abbreviated CL.
87
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