these the same length as the first; then slide the set square forward until it reaches the ends of the vertical sides, and lines drawn therefrom will complete the hexagon. The oblique projectors are at an angle of 450.
A Trussed Partition, such as is used to divide large and lofty buildings into apartments, is shown in oblique projection on page 104, and a portion of the head of an opening in a similar partition is shown below. The former is projected from the elevation at an angle of 30° the latter
at 450. The opening shows linings to receive a door with portion of the architrave grounds attached to indicate method of fixing them. The cross or counterlaths C.C.C. are fixed to the face of the framing, to provide a space behind the ordinary laths to receive the key of the plaster.
The elevation in both cases should be completed in the usual manner before setting them into oblique.
Diminished Oblique Projection.—The author woidd distinguish by the above term a method of placing objects in oblique projection that he has used in his classes for some years with success. The view obtained is clear and
I of) AN OBLIQUE SCALE
convincing, and docs not offend one's sense of proportion, as do drawings by the common method. Within well-defined iirnits it gives very rational graphs of polygonal and other than rectangular figures, which are difficult of representation by other methods, and it has the advantage that the oblique sides are diminished in a definite and constant ratio to the parallel sides. Moreover, no scale need be used to read the dimensions ; all that is necessary to discover the dimensions of any part is to project a parallel from it to
the ground or picture line, when it is at once seen in its true size, or to the same scale that the parallel portion is drawn. The method will be made clear in the description of the examples. If, however, it is preferred to use a scale for tak;ng the dimensions, one can easily be constructed as shown above. Thus, lay down the real scale, or a full-size measure, horizontally and from its left extremity draw a, line at an angle of 450 (or whatever angle it is intended to use), and upon this line draw projectors at reverse angle, from each division upon the real scale, and a scale conducted as shown will indicate the real dimensions when it is applied to the oblique drawing.
Diminished Oblique Projection Examples
Figs, i and 2. A Ship's Grating. Fig. 3. Details of Joints. Fig. 4. Construction of Pentagon. Fig. 5. Projection of a Pentagonal Prism
I of? A SHIP'S GRATING
Fig. x, page 107, is the plan in orthographic projection of a wood grating or ventilator used for covering openings in floors, decks, etc. Fig. 2 is a view of the same object placed in oblique projection by the diminishing method.
The front edge of the frame is drawn resting on the ground, and the ground line is produced to the left indefinitely. Project the two ends of the frame at angles of 45" with the ground line, then set off on the ground line to the left of angle a', the point b', equal to the width of the frame a-b, Fig. 1. It is perhaps unnecessary to say that this plan is only drawn for the purpose of explaining the method clearly, and that the view could be made equally well from written data. Having obtained point b', project a line from it at an angle of 45°. intersecting the first drawn projector at point b"; this locates the back edge of the frame, and defines its width. Complete the outline of frame. Next set off the widths of the bars and spaces along the top edge as shown, also along the ground line for the end, as indicated by the figures 1 to 8; project these respectively parallel with the ends and the line V~b") by aid of the set square of 450, and carry the latter vertically to the upper surface, when the completion of the grid will be a simple matter. The intersecting angles are to be shown as in the drawing, and a little shading introduced to give the effect of solidity; the light is supposed to come from the right hand.
Details of the Joints are shown in Fig. 3 by the same method, but the construction lines are removed. The piece A is shown beneath B, instead of above it, for want of space; otherwise it is relatively in the correct position. An angle of 453 is generally the best for this method of projection, but any other convenient angle may be used, if it is remembered that the diminishing projectors must always be at a similar angle reversed.
Pentagons-- -The application of the method for projecting polygonal figures is shown in Fig. 5. It may be useful first to give a method ol constructing a pentagon geometrically. Let A-B, Fig. 4. be the given side of the
Fig. i. Shuttering and Forms for Reinforced Concrete Work. Fig. 2. A Light Clamping Frame
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