Oblique projection is enother method of pictorial drawing. It is simpler than isometric but it does not present so realistic a picture.
Fig. 6/1 shows a shaped block drawn in oblique projection
There are three drawings of the same block in Fig. 6/1. 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*. 49» and 6C 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 CP 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 tho isometric view.
Fig. 6/2 show« how we attempt to overcome this distortion.
The oblique lengths hove 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 i x the true length. 3CT causes less distortion and the oblique length is only altered to | x the true length. At 450 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 esked to draw an object in oblique projection, you would probably be very confused when trying to decxle which angle to choose end what reduction to make on the oblique lines. If you are asked to produce an oblique drawing, drew at an oblique angla of 45P andraduea all your oblique dimensioni by half, unlets you are given other specific instructions.
Circles end curves in Oblique Projection Oblique projection has one very big advantage over isometric projection. Since the front face is drawn in the ptene of the paper, any circles on this face are true circles and not ellipses as was the case with isometric projection Fig. 6/3 shows an oblique drawing ol a bolt. If the bolt had been drawn in isometric, it would have been a long and tedious drawing to make.
There «re occasions when there are curves or circles on the oblique (aces. When this erises. 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 ordinate* on the oblique lengths must be scaled down in the same proportions. Fig. 6/4 shows an example of this.
In this case, the oblique angle is 45» and the oblique scale is | normal sixe. The normal 6 mm ordinate* are reduced to 3 mm on the oblique faces and the 3 mm ordinate» ere reduced to 1.5 mm
It is also worth noting that the ordinates ere 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 pic-tonal 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 sch
Fig. 6/5 shows a smalt 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.
Fig. 6/5 shows a smalt 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.
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Exercises 6 (All questions originally set in Imperial units) 1. Fig. 1 shows two views ol a small casting. Draw, full size, an oblique projection of the casting with face A towards you.
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