42 Basic Knowledge Of Mechanical Drawings

  1. The paper format can be divided into 5 types according to size, and the drawing format codes are A0, A1, A2, A3 and A4 respectively. There must be a title bar at the lower right corner of the frame, and the text direction in the title bar is consistent with the viewing direction.
  2. The types of drawing lines include thick solid line, thin solid line, wavy line, double broken line, dotted line, fine dotted line, thick dotted line, double dotted line and so on.
  3. In the drawing, the visible contour line of the machine part is drawn with thick solid lines, the invisible contour line is drawn with dotted lines, the dimension line and boundary line are drawn with thin solid lines, and the symmetrical center line and axis are drawn with thin dotted lines. The width of dotted lines, thin solid lines and thin dotted lines is about 1/3 of that of thick solid lines.
  4. Scale refers to the ratio of the graphic size in the drawing to the physical size.
  5. Scale 1:2 refers to that the size of the physical object is twice the size of the figure, which belongs to the reduction scale.
  6. Scale 2:1 refers to that the size of the figure is twice the size of the physical object, which belongs to the amplification scale.
  7. When drawing, try to use the scale of the original value scale. If necessary, you can also use the zoom in or zoom out scale, where 1:2 is the zoom out scale and 2:1 is the zoom in scale. No matter which scale is adopted, the actual dimensions of the parts shall be marked on the drawing.
  8. The Chinese characters, numbers and letters written in the drawings must be neat in font, clear in strokes, evenly spaced and neatly arranged. The Chinese characters should be written in long imitation Song typeface.
  9. The three elements of dimensioning are dimension limit, dimension line and dimension number.
  10. Symbol in dimension: R refers to circle radius, ф Indicates the diameter of the circle, s ф Indicates the diameter of the ball.
  11. The size on the drawing is the actual size of the part. When the size is in mm, there is no need to mark the code or name.
  12. For standard horizontal dimension, the prefix direction of dimension number shall be upward, and for vertical dimension, the prefix direction of dimension number shall be left. The dimension numbers of angles are written in horizontal position. Any drawing line must be disconnected when it passes through the dimension number.
  13. Inclination refers to the inclination of the oblique line to the horizontal line, which is indicated by the symbol ∠. When marking, the inclination direction of the symbol should be consistent with the inclination direction of the marked inclination. The taper direction marked is consistent.
  14. The symbol “∠ 1:10” indicates the slope of 1:10, and the symbol “1:5” indicates the taper of 1:5.
  15. Line segments in plane graphics can be divided into known line segments, intermediate line segments, and connecting line segments. Their drawing sequence should be to draw the known line segment first, then the middle line segment, and finally the connecting line segment.
  16. The line segment with known fixed size and positioning size is called known line segment, the line segment with fixed size but incomplete positioning size is called intermediate line segment, and the line segment with only fixed size but no positioning size is called connecting line segment.
  17. The projection plane where the main view is located is called the orthographic projection plane, which is referred to as the front side for short, and is represented by the letter v. The projection plane where the top view is located is called the horizontal projection plane, referred to as the horizontal plane, which is represented by the letter H. The projection plane where the left view is located is called the side projection plane, which is called the side for short, and is represented by the letter W.
  18. The projection law of the three views is that the main view is the same length as the top view, the main view is the same height as the left view, and the top view is the same width as the left view.CAD drawing
  19. The parts have dimensions in three directions: length, width and height. The main view can reflect the length and height of the parts, the top view can only reflect the length and width of the parts, and the left view can only reflect the height and width of the parts.
  20. The parts have six directions: up, down, left, right, front and back. In the main view, only the up, down, left and right directions of the parts can be reflected: in the top view, only the front, rear, left and right directions of the parts can be reflected: in the left view, only the up, down, front and rear directions of the parts can be reflected.
  21. There are three basic views. Their names are main view, top view and left view.
  22. In addition to the basic view, there are three views: bottom view, right view and rear view.
  23. According to the size of the cutting range, the section view can be divided into full section view, half section view and partial section view.
  24. The sectioning methods of sectional view can be divided into five types: full sectioning, half sectioning, partial sectioning, stepped sectioning and combined sectioning.
  25. The marking of the section view includes three parts: ① the symbol indicating the position of the section plane (section line), and the letters are marked at both ends; ② the arrow indicating the projection direction; ③ the arrow indicating the projection direction is marked above the section view“ ×——×” word.
  26. Omit all marked sectional views, indicating that its cutting plane is drawn after cutting through the symmetrical plane of the part.
  27. The section drawing is used to express the internal shape of parts. The section can be divided into solid part and hollow part.
  28. The difference between the removed section and the coincident section is: the removed section – the section drawn outside the view outline, and the coincident section – the section drawn inside the view outline.
  29. The graphics in the drawing can only express the structural shape of the parts, and the real size of the parts should be based on the dimensions marked on the drawing.
  30. The numbers marked with dimensions are called dimension basis. There is at least one dimension datum in each of the three directions of length, width and height of machine parts.
  31. The five elements of thread are thread profile, diameter, pitch, lead, number of threads and direction of rotation.
  32. Only when the tooth shape, diameter, pitch, number of lines and direction of rotation of the inner and outer rib are consistent, can they screw into each other.
  33. The thread whose profile, diameter and pitch comply with the national standards is called standard thread. The thread whose profile does not comply with the national standards is called non-standard thread. The thread whose profile complies with the national standards, but whose diameter and pitch do not comply with the national standards is called special thread.
  34. The specified drawing method of external thread is: the major diameter is represented by D, the minor diameter is represented by D1, and the termination line is represented by a thick solid line.
  35. In the sectional view, the major diameter of the internal thread is represented by D, the minor diameter is represented by D1, and the termination line is represented by a thick solid line. The major diameter, minor diameter and termination line of invisible threaded holes are represented by thick solid lines.
  36. Common thread connection forms include bolt connection, double headed stud connection and screw connection.
  37. The types of common keys include ordinary flat keys, semicircular keys, hook wedge keys and splines.
  38. Cylindrical gears can be divided into straight teeth, helical teeth and herringbone teeth according to the direction of gears.
  39. The specified drawing method of the gear tooth part is: the tooth top circle is drawn with thick solid lines, the indexing circle is drawn with fine dotted lines, and the tooth root circle is drawn with thin solid lines, which can also be omitted. In the sectional view, the root circle is drawn with thick solid lines.
  40. When all surfaces of the part have the same surface roughness requirements, it can be uniformly marked in the upper right corner of the drawing. When most of the roughness of the part surface are the same, the same roughness code can be marked in the upper right corner, and the other two words can be marked in the front.
  41. A complete assembly drawing should have the following four parts: ① a group of views, ② necessary dimensions, ③ technical requirements, ④ part serial number and detail list.
  42. The size categories in the assembly drawing include ① specification size ② assembly size ③ installation size ④ overall size ⑤ other important dimensions.

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