1. Projection principle and three views
1. Basic knowledge of projection
1) Center projection: The light source is emitted from the center of the projection.
2) Orthographic projection: When the projected rays are parallel to each other and perpendicular to the projection surface, the projection obtained by the object on the projection surface is called positive projection.
2. Three views
The view composed of the three main projections of the object, the orthographic projection drawing (the main view, the top view, the left view, the three basic views) that can correctly reflect the length, width and height of the object is three views, which is a conventional abstract expression of the geometric shape of the object in the engineering world.
1) Main view
You want to select the direction that most clearly reflects the characteristics of the object as the direction of projection for the main view.
2) Top view
3) Left view
4) Unfold the projection surface to summarize
The projection rules are:
a. Main and top view: long alignment
b. Main and left views: high and flush
c. Top and left view: equal width
It can be summarized simply: the length is right, the height is flush, and the width is equal.
5) Projection properties of planes and straight lines on objects
a. Tilt with the projection surface - shrinkage
b. Perpendicular to the projection surface – agglomeration
c. Parallel to the projection plane – authenticity
Second, the various expressions of parts
There are several ways to express the shape of a part:
1. Basic view
The basic view includes the main view, top view, left view, right view, bottom view, and rear view.
The basic view is placed on a sheet, if it is placed according to the unfolded position of the basic view, the name of each view is not marked, otherwise the view name "X direction" should be marked above the view, and the projection direction should be indicated with an arrow near the corresponding view, and the same letter should be noted.
2. Oblique view
When the surface of the part is inclined to the projection surface, an auxiliary projection surface parallel to the surface of the part is attached and the view obtained by projection is called oblique view.
3. Partial view
The view obtained by projecting a part of a part onto the basic projection surface is called a partial view.
4. Rotate the view
When the slanted part of a part has a distinct axis of rotation, you can imagine that the slanted part is rotated parallel to a selected basic projection plane and then projected onto that projection plane.
5. Section view
The internal structure of the part is complex, and the dotted lines in the view will also increase, which is not conducive to seeing the picture.
1) Semi-sectional view: used to express the shape and internal structure of the object with a symmetrical plane (delimited by a symmetrical line)
2) Partial cross-sectional view: It is used to express the local inner shape of the object and retain most of the shape of the object (local section)
3) Step section: several parallel section planes for step sectioning
6. Coincident profiles
It is used to express the cross-sectional shape of the local structure of an object.
7. Partial enlargement
3. Basic steps of cartography
1. Determine the drawing sheet and drawing scale (product size and structure);
2. Make a good frame according to the specified size requirements;
3. View planning: according to the size of the product and the complexity of the structure, plan the distribution of views, especially the number and distribution of cross-sectional views, the location of technical requirements, and leave space for the title bar;
4. Product analysis and drawing;
5. Dimensioning.