When performing technical drawings, along with the outline of objects and parts in orthogonal projections, you need to have more visual images. For this, the method of axonometric construction is used.
It consists in the following: the object, together with the coordinate system (mutually perpendicular to the X, Y, Z axis) with which it is connected in space, is projected in parallel onto the plane of axonometric projections (or the picture plane). The projection of an object onto this plane is called axonometry. It also displays the coordinate axes, called axonometric axes. It should be borne in mind that the projection direction must be chosen so that there is no coincidence with any of the coordinate axes.
Axonometric projections with perpendicular projection are called rectangular or orthogonal, and with non-perpendicular projection - oblique.
Typically, an object is positioned so that three sides of the object can be seen on the picture surface: lower, upper, front, left, or right. Axonometric drawing should give the most complete picture of this subject, its size and structural form. Its main advantage is visibility.
All axonometric projections can be classified as follows:
1. Isometric. In this case, all three coordinate axes have the same slope, and all dimensions in the direction of the three axes have the same decrease.
2. Dimetric. Two coordinate axes in these projections have the same slope, and the third - the other. Accordingly, in the first two axes the reduction in size will be the same, and in the third - different.
3. Trimetric. All three axes in this case have a different slope and the size reduction is different for all three axes.
By positioning the depicted object in different ways on the surface and choosing a different direction of the rays, you can get a variety of axonometric projections of the same object.
Most often, when constructing drawings, an isometric rectangular projection is used, in which the size reduction factor is the same along all axes, and the angles between the coordinate axes of the axonometry are 120 degrees. It is obtained in a parallel way.
Almost everyone who studied engineering graphics and drawing had to build axonometric projections. In practice, the wide selection of these drawings is greatly limited by the requirements of low labor intensity and visibility. GOST 2.317-69 recommends using axonometric projections, which allow for a good image perception, while having a good combination of distortion factors and simple construction. These include five types: three oblique (frontal, horizontal isometry, frontal dimetry) and two rectangular (dimetry and isometry). Of course, they cannot replace the projection drawing, but can only supplement. The construction of axonometry is very time-consuming, it is used when the drawing does not give a complete picture of the shape of the depicted part or the whole object.