Mechanical Drawing

Mechanical drawing is a discipline that uses drawings to accurately represent the structure, size, working principle and technical requirements of machinery.

Drawings are composed of graphics, symbols, text, and numbers. They are technical documents that express design intent, manufacturing requirements, and exchange experience. They are often referred to as the language of engineering circles.

In 1799, French scholar Monge published the work “Descriptive Geometry”. Since then, the figures in the mechanical drawings have been drawn strictly in accordance with the projection theory of descriptive geometry.

In order to enable people to have a consistent understanding of the format, text, graphics, simplification of graphics and the meaning of symbols involved in the drawings, a unified specification was gradually developed and developed into a mechanical drawing standard.

Countries generally have their own national standards, and there are international standards established by the International Organization for Standardization.

The items specified in the mechanical drawing standards include: drawing size and format, proportions, fonts and lines, etc. The size of the standard format of the drawing and the corresponding size of the frame in the drawing are specified in the drawing format and format.

Proportion refers to the ratio of the length of the dimension in the drawing to the actual size of the machine. Except for drawing at a 1:1 ratio, only the reduced and enlarged ratios specified in the standard are allowed to be used for drawing.

Mechanical drawings mainly include parts drawings and assembly drawings, in addition to layout drawings, schematic diagrams, and shaft drawings.

The part drawing expresses the shape and size of the part and the technical requirements for manufacturing and inspection of the part; the assembly drawing expresses the assembly relationship and working principle between the parts and components in the machine; the layout drawing expresses the position of the mechanical equipment in the plant; the schematic drawing expresses the mechanical The working principle, such as the mechanism motion diagram that expresses the principle of mechanical transmission, the pipeline diagram that expresses the liquid or gas transmission line, etc.

The mechanical components in the schematic diagram are represented by symbols. Axis drawing is a kind of three-dimensional drawing, which is intuitive and a commonly used auxiliary drawing.

Graphs that express the shape of a mechanical structure, commonly used are views, cross-sectional views, and cross-sectional views.

The view is the figure obtained by projecting the machine to the projection surface according to the orthographic projection method. According to the projection direction and the position of the corresponding projection surface, the views are divided into front view, top view and left view.

The view is mainly used to express the external shape of the machine. Invisible contour lines in the figure are indicated by dashed lines.
When the machine is projected onto the projection surface, there are two relative positions between the observer, the machine and the projection surface. When the machine is located between the projection surface and the observer, it is called the first angle projection method.

When the projection surface is between the machine and the observer, it is called the third angle projection method. Both projection methods can express the shape of the machine perfectly. The Chinese national standard stipulates that the first angle projection method is adopted.

The cross-sectional view is an imaginary cut-off surface to cut the device, remove the part between the observer and the cut-off surface, and project the remaining part to the projection surface to obtain the figure. The sectional view is mainly used to express the internal structure of the machine. The cross-sectional view only draws the figure of the cut surface. The cross-sectional view is often used to express the cross-sectional shape of a rod-shaped structure.

For some cumbersome structures in the drawing, in order to improve the drawing efficiency, it is allowed to be simplified and drawn. The simplified drawing method is called the simplified drawing method.

The mechanical drawing standards have independent standards for the drawing method of structures or parts such as threads, gears, splines and springs.

The drawings are drawn in appropriate proportions according to the structural shape and size of the machine parts. The dimensions of the parts in the drawings indicate the range to be measured with dimensional lines, boundary lines and arrows, and numbers indicate the size. In the mechanical drawing, the unit of the number is specified as millimeter, but it does not need to be noted.

For dimensions such as diameter, radius, taper, slope, and arc length, add symbols before the numbers for explanation.

When manufacturing machine parts, it must be processed according to the size numbers marked in the drawings, and it is not allowed to directly measure the size of the graphics from the drawings.

The technical conditions that must be met in mechanical manufacturing, such as tolerances and fits, shape and position tolerances, surface roughness, materials and heat treatment requirements, etc., should be marked with symbols, words and numbers in the drawings in accordance with the mechanical drawing standards.

Before the 20th century, the drawings were drawn by hand using common drawing tools. At the beginning of the 20th century, a drawing machine with mechanical structure appeared, which improved the efficiency of drawing.

Computer graphics appeared in the second half of the 20th century. The patterns that need to be drawn were compiled into a program and input into an electronic computer. The computer converted it into graphic information and sent it to a plotter to draw the pattern, or sent it to a computer-controlled automatic machine tool for processing.

The design generally needs to be drawn as a transparent base map, which is used to wash out the blueprint or ammonia to smoke out the purple map.

In the middle of the 20th century, an electrostatic copier appeared, which can directly copy the original pattern and enlarge or reduce the picture. Using this new technology can save the drawing process.

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