Читать книгу Basic Benchwork for Home Machinists - Les Oldridge - Страница 9

A fitter working at a bench will normally be required to produce components to the dimensions and outline as shown on an engineering drawing so, quite obviously, he must be able to read drawings of this kind. Although it is not intended to study this complex subject at any great depth, certain fundamentals will be explained, sufficiently enough it is hoped, to allow the hobbyist to interpret drawings that he has to work from.

While the main details of a simple part can be shown in a pictorial view, for sufficient information to be available to construct the average component, several related views showing the front, sides, top, and/or bottom are required. For example, the single drawing shown at the top of Fig. 3.1 could be an illustration of any of the shapes shown below.

For all the details required to make all but the simplest of components, drawings using ORTHOGRAPHIC PROJECTION are needed. There are two systems: FIRST ANGLE, widely practiced in the UK and in North America, and THIRD ANGLE, mainly used in North America but occasionally in Britain.

Orthographic projection results when the outline of an object is projected, at right angles, on to a flat surface known as a plane. See Fig. 3.2 where lines have been projected from a cube, downwards onto a flat surface.

Fig. 3.1 View (a) could be any of the other shapes viewed from above.

Fig. 3.2 Perpendicular orthographic projection.

The two systems of first and third angle take their names from the first and third quadrants of a circle (see Fig. 3.3). Here four open ended “boxes” are shown around the quadrants of a circle. Objects to be drawn are imagined to be placed in one of these boxes and their outlines projected on to the “walls” of the box. First and third angle projections have similar merits; only local preference makes one more popular than the other.

Fig. 3.3 Principal planes of projection.

Fig. 3.4 First angle projection onto vertical and horizontal planes.

Fig. 3.5 Viewing for first angle projection onto auxiliary vertical plane.

Fig. 3.6 When “box” is opened out, drawing in first angle orthographic projection will look like this.

Fig. 3.7 The same component as the previous, now drawn in third angle projection.

FIRST ANGLE ORTHOGRAPHIC PROJECTION. In Fig. 3.4, perpendicular lines have been projected from an object onto the horizontal and vertical planes in a first angle situation. If the horizontal plane is swung down to the vertical plane, two orthographic views are obtained, an elevation and a plan. A third outline can be viewed and projected on to an auxiliary vertical plane (AVP) as shown in Fig. 3.5. If this plane is swung through 90°, so that it is in line with the other vertical view, the three together give a plan and two elevations of the object, as shown in Fig. 3.6. Generally, these three views will give sufficient detail for a component to be made, but there is no reason why other views (up to six) cannot be projected in the same way to disclose further details.

THIRD ANGLE ORTHOGRAPHIC PROJECTION. Going back to Fig. 3.3, if lines are projected from an object placed in the third angle position onto the horizontal and vertical planes, outlines will appear exactly as viewed. If the sides of the box are then straightened out, as was done in the first angle exercise, then two elevations and a plan will result, as shown in Fig. 3.7. It will be seen that the plan view now appears in the top right section and the two elevations in the lower half of the paper.

Both systems of projection have their own international symbols by which their use can be recognized. The symbols are similar to two views of the frustum of a cone (see Fig. 3.8a and b).

CONVENTIONAL SYMBOLS. In more leisurely times, when labor was cheap, engineering drawings were completed in great detail, every thread on every bolt was shown, every rivet was drawn, but now drawings are very much simplified to save the draftsman’s time. Conventional symbols are used for common features such as bolts, studs, and equally spaced holes. Those which are likely to concern benchworkers appear in Fig. 3.9.

HIDDEN DETAILS are shown by short dashes. Fig. 3.10a shows holes in a bar indicated in this way. This sketch also shows how, when a component is too long to accommodate on the paper but is of uniform section, an artificial break is indicated.

Fig. 3.8a Symbol for first angle projection drawing.

Fig. 3.8b Third angle projection drawing symbol.

Fig. 3.9 Conventional symbols.

Fig. 3.10.

SECTIONED VIEWS are views in which it is imagined that the components have been cut through to show their internal construction. Such views are indicated by diagonal lines called hatching lines. Fig. 3.10b shows a connecting rod small end showing the bush, probably made of phosphor-bronze, and the hole for lubricating purposes. The hatching lines are generally at 45° to the centerline of the component. On sectioned features that are adjacent to one another, the hatching lines are drawn at opposite angles.

Sometimes it is necessary for a drawing to contain a special sectional view of a component to make its shape abundantly clear; several examples are given in Fig. 3.11. Illustrations (b) and (c) show sectional views where the direction of viewing is shown by arrows. There are several examples of centerlines in these drawings and it will be seen that they consist of a chain line, which is a long thin line followed by a short one.

Fig. 3.11 Various sectional views.

Items to be riveted in position after screwing item 2 in position; allowing item 3 to turn freely on item 2

Fig. 3.12 Assembly drawing of a C-clamp.

ITEM NUMBER	DESCRIPTION
1	FRAME
2	SCREW
3	HANDLE
4	FERRULE
5	PAD

Usually a component is shown in an ASSEMBLY DRAWING, where it is depicted with all its pieces assembled, and a DETAILED DRAWING where each single part is shown. An assembly drawing of a C-clamp is shown in Fig. 3.12 and a detailed drawing of the same component in Fig. 3.13.

Fig. 3.13 Detail drawing of C-clamp. All items in CRS.