TD6/1: OBLIQUE PROJECTION

OBLIQUE PROJECTION

This is another method of pictorial drawing. It is simpler than isometric but it does not present so realistic a picture.

Figure below shows a shaped block drawn in oblique projection.

There are three drawings of the same block in it . They all show the front face of the block drawn in the plane of the paper and the side and top faces receding at 30 °, 45 °and 60 °on the three drawings. An oblique line is one which is neither vertical nor horizontal, and the receding lines in oblique projection can be at any angle other than 0 ° or 90 ° as long as they remain parallel in any one drawing. In practice, it is usual to keep to the set square angles and, of the three to choose from, 45 ° is the most widely used.

If you check the measurements on the oblique drawings with those on the isometric sketch, you will find that the measurements on the front and oblique faces are all true lengths. This gives rise to a distorted effect. The drawings of the block in the oblique view appear to be out of proportion, particularly when compared with the isometric view

If you check the measurements on the oblique drawings with those on the isometric sketch, you will find that the measurements on the front and oblique faces are all true lengths. This gives rise to a distorted effect. The drawings of the block in the oblique view appear to be out of proportion, particularly when compared with the isometric view.

The oblique lengths have been altered. The degree of alteration has been determined by the oblique angle.

An oblique angle of 60 °causes a large distortion and the oblique length is thus altered to ^1/₃ the true length. An oblique angle of 30 ° causes less distortion and the oblique length is only altered to ²/₃ the true length.

At 45 °the true length is reduced by half. These alterations need not be rigidly adhered to. The ones illustrated are chosen because they produce a reasonably true to life picture of the block, but a complicated component might have to be drawn with no reduction at all in order to show all the details clearly.

If an oblique drawing is made without any reduction in oblique length, this is Sometimes known as ‘cavalier projection ’ . If a reduction in oblique length is made, this is sometimes known as ‘ cabinet projection ’. If you were now asked to draw an object in oblique projection, you would probably be very confused when trying to decide which angle to choose and what reduction to make on the oblique lines. If you are asked to produce an oblique drawing, draw at an oblique angle of 45 ° and reduce all your oblique dimensions by half,unless you are given other specific instructions.

 Circles and Curves in Oblique Projection

Oblique projection has one very big advantage over isometric projection. Since the front faces drawn in the plane of the paper, any circles on this face are true circles and not ellipses as was the case with isometric projection.

Figure 6.3 shows an oblique drawing of a bolt. If the bolt had been drawn in isometric projection, it would have been a long and tedious drawing to make.

There are occasions when there are curves or circles on the oblique faces. When this arises, they may be drawn using the ordinate method that was used for circles on isometric drawings. If the oblique length has been scaled down, then the ordinates on the oblique lengths must be scaled down in the same proportions. Figure 6.4 shows an example of this.

In this case, the oblique angle is 45 ° and the oblique scale is ¹/ ₂ normal size. The normal 6 mm ordinates are reduced to 3 mm on the oblique faces and the 3 mm ordinates are reduced to 1.5 mm.

It is also worth noting that the ordinates are spaced along a 45 °line. This must always be done in oblique projection in order to scale the distances between the ordinates on the oblique view to half those on the plane view. The advantage of oblique projection over other pictorial projections is that circles drawn on the front face are not distorted.

Unfortunately, examiners usually insist that circles are drawn on the oblique faces, as in Fig. 6.4 . However, if you are free of the influence of an examiner and wish to draw a component in oblique projection, it is obviously good sense to ensure that the face with the most circles or curves is the front face. Figure 6.5 shows a small stepped pulley drawn twice in oblique projection. It is obvious that the drawing on the left is easier to draw than the one on the right

Exercise 6 (All questions originally set in imperial units.) 1. Figure 1 shows two views of a small casting. Draw, full size, an oblique projection of the casting with face A towards you.

Figure 1 2. Figure 2 shows two views of a cast iron hinge block. Make an oblique drawing of this object, with face A towards you, omitting hidden detail.

6. Two views of a casting are shown in Fig. 6 . Draw (a) the given views, and (b) an oblique pictorial view, looking in the direction shown by arrow L, using cabinet projection, that is with the third dimension at 45 ° and drawn half-size.