Page 127 - Engineering drawing from first principles using AutoCAD

120 Engineering drawinq [rom first principles .

assembly. It is often referred to as a drive fit. Alternatively, before assembly, the compo-
nent with the hole may be heated until expansion takes place and then the shaft is
inserted; this is often known as a shrink fit. Transition fits occur where there is a small
overlap in the tolerance zones and only light pressure is necessary for assembly, this is
often known as a push fit.

The designer selects the appropriate combinations and is always aware that as increased
precision is required, so costs increase to obtain it. The product designer may apply the
selected tolerances with the nominal dimension and the grade of fit on the design
drawing. A production draughtsman responsible for issuing a production drawing to
the workshop will only need to show the high and low limits of size and should not
expect the craftsman to perform mental arithmetic. There are therefore several ways of
quoting toleranced dimensions on a drawing and the next exercise is given for this
purpose. Assume suitable sizes where other dimensions are not given.

Figure 5.6 shows part of a bearing in its housing at the end of a shaft. The bearing could be
a ball or roller bearing, and in a sectional view there would no need to draw the bearing

3017 29.975 60p6 60.051
CD 29.950 60.032

3017 -0.025 60H7 +0.030
-0.050 -0.000

CD -$

CD

CD ~ j

~29.975 j
CD 3017 ---+-----1--
-. 3ot? ~ 29.975 j
29.950

29.950
(j) 3017 (29.975). OR

29.950

Figure 5.8