What Lies Inside the Object

Hidden Details and Sections

It is the rule rather than the exception that engineering components are literally 'holes surrounded by metal' and the shape of these holes is vital to the operation of the machine. We need a means of describing and dimensioning these holes. There are two methods; one is to show the hidden detail, using dotted lines (see Fig. 5) to distinguish between this and the outline and the other is to draw a sectional view, again using some convention of lines to indicate the fact. Let us deal with these in turn.

Hidden details

The first rule is that the main views should be chosen so that the minimum of hidden detail drawing is needed. Thus a part which is hollow at the back might be drawn with a rear elevation instead of the usual front as the main view. A plethora of dotted lines on a drawing can be very confusing. Fig. 28 is a case in point; the detail of the square flange would be better shown by offering a partial view in the direction of the arrow (which I have added)

'X'. The other detail required was clear from the other views on the full drawing.

Whether dotted lines should be used to indicate the presence of hidden edges in one view which are clear in another is a debatable point. You are making the drawing for one purpose only - to convey information. If the addition of dotted lines in one view to show detail which is clear in another would make the drawing easier to follow, then they should be included. If such dotted lines will not clutter up the drawing and make it hard to read, then they may be included. But if the drawing is already full of detail, and especially if it carries many dimensions, it is best to omit hidden detail on that view, and, if need be, draw an extra one to show the detail.


(1) Where a dotted line meets a full line or another dotted line the 'dash' should touch - Fig. 29 (a) to (c).

(2) Where a dotted line crosses a full line the latter should lie in the gap between dashes. But if two dotted lines cross, one should show a dash in the gaps on the other, Fig. 29d.

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