First learn to look at the three-dimensional view, and then look at the assembly diagram. No matter what picture you look at, the most important thing is to find your own perspective first! The specific thing depends on whether you have learned mechanical drawing! When looking at mechanical drawings, you must distinguish the parts from large to small, and finally the thread fit and tolerance fit. From coarse to fine, first the whole and then the parts.
Since the reform and opening up, our country has introduced a lot of foreign equipment, drawings and other technical materials. The mechanical pattern projection methods of many developed countries are different from the projection methods used in our country. In order to better learn the advanced technologies of developed countries, it is necessary to quickly understand foreign mechanical drawings.
1 Overview
In today's world, ISO international standards stipulate that first-angle and third-angle projections are equally valid. Countries have their own emphasis based on their national conditions. Among them, Russia, Ukraine, Germany, Romania, the Czech Republic, Slovakia and Eastern Europe mainly use first-angle projection, while the United States, Japan, France, the United Kingdom, Canada, Switzerland, Australia, the Netherlands and other countries mainly use first-angle projection. Countries such as China and Mexico mainly use third-angle projection. Before liberation, our country also used third-angle projection, but after the founding of New China, it switched to first-angle projection. Third-angle projection is often encountered in imported foreign mechanical drawings and scientific and technological books and periodicals. The ISO international standard specifies projection markings for the first and third corners (Figures 1 and 2). In the title block, there are marking symbols, and the drawing method can be identified based on these symbols. However, some drawings do not have projection marks.
Figure 1 First-angle drawing marking symbols
Figure 2 Third-angle drawing marking symbols
2 Third-angle projection
Space It can be divided into eight areas by the front plane V, the horizontal plane H, and the side plane W, which are the 1st, 2nd, 3rd, 4th, 5th, 6th, 7th, and 8th sub-angles, as shown in Figure 3 shown.
Figure 3
2.1 Projecting the object within the first sub-angle is called the first angle projection, also known as the E method - the European method.
2.2 Projecting an object in the third angle is called third-angle projection, also known as method A - the American method.
Our country uses the third-angle projection method.
Third-angle projection is a method of imagining an object being placed in a transparent glass box, using each side of the glass box as a projection surface, and making an orthographic projection according to the position of the person-surface-object to obtain the figure. As shown in Figure 4 and Figure 5.
Figure 4 Figure 5
2.3 The positions of the six basic views in the third angle projection
The ISO international standard stipulates that the positions of the six basic views in the third angle projection The location is shown in Figure 6.
Figure 6
The above view is obtained by projecting the object onto each projection surface of a closed rectangular (transparent) "projection box".
Each view can be understood as: the actual image of the object that the observer sees when his line of sight is perpendicular to the corresponding projection plane.
The front view is the front projection of the object obtained by the observer imagining that he is in front of the object and moving his eyes point by point, and the line of sight is always perpendicular to an imaginary upright projection surface (transparent) , other views can be obtained in a similar way.
Readers of images should always view the view as a side of the object itself. From the front view, you can see the height and width of the object, as well as the positions of the top, bottom, left and right sides of the object. The top view shows the depth and width of the object.
2.4 Advantages of third-angle projection
(1) The view configuration is better, making it easier to read pictures
The viewing directions are directly reflected between the views, making it easier to read pictures , easy to draw. The left view is on the left and the right view is on the right. The first angle projection sometimes uses "direct view" to make up for unclear expressions.
(2) It is easy to imagine the shape of the object in space.
The left and right views are facing inward, and the top view is facing downwards, so that it is easy to imagine the shape of the object.
(3) Convenient to draw axial views
It is easy to imagine the spatial shape of objects, which is directly helpful for imagining the shape of objects when drawing axial views.
(4) Conducive to expressing part details
Adjacent pictures are arranged nearby and generally do not need to be marked, as shown in Figure 7.
Figure 7
(5) Dimensions and other annotations are relatively concentrated.
3 Comparison of line widths in some major countries (Table 1)
Table 1 Line countries
Kuanjia
Line type
China
United States
Japan
United Kingdom
Thick solid line
b( 0.5～2)
0.032”
0.4～0.8
0.7
Thin solid line
B/ 3(0.17～0.66)
0.016”
Below 0.3
0.3
Introduction to 4 national mechanical drawing standards
4.1 American Standard (ANSI)
The United States only stipulates the use of third-angle projection (occasionally first-angle projection is also used in architectural drawings and structural drawings, but it must be specified).
There are two forms of view layout. The first one is the same as the ISO international standard, as shown in Figure 6. The second type is shown in Figure 8.
Figure 8
American standard dimensioning method: Dimensions in American drawings are rarely in mm, and are generally in inches (1 inch = 25.4mm). Originally they were in fractional form. Indicates the number of inches, such as 9/16 inches, etc. After 1966, it was changed to decimal and written in decimal form. When the value is less than 1, do not write 0 before the decimal point. It is recommended to write the numbers horizontally. For the upper and lower deviation of the tolerance size, attention should be paid to keeping the same number of decimal places as the basic size. For example, if the size is more than 6 feet, feet and inches symbols should be noted, such as "12'7".
( 1) Diameter, radius, and spherical codes
① When the view is clearly reflected as a circle, the diameter code DIA (diameter) or D is not noted, as shown in Figure 9; when there is only a non-circular view, the size number is added after Note the diameter code DIA or D, as shown in Figure 10
Figure 9
Figure 10
② Do not add the radius code R (radius) after the radius size number. , when the radius dimension is marked in a view that does not reflect the radius and arc real shape, it is required to add the code TRUER (TRUE RADIUS) (real R) after the radius dimension number. The spherical code is required to add the code SPHER DIA (ball) after the dimension number. Diameter) or SPHERR (SPHER RADIUS).
(2) Chord length (CHORD) and arc length (ARC) are shown in Figure 11. 11
(3) Chamfer CHAM (CHAMBER) as shown in Figure 12
Figure 12
(4) Counterbore as shown in Figure 13
p>
Figure 13
(5) The keyway is as shown in Figure 14
Figure 14
(6) The thread mark is as shown in Figure 15. >
Figure 15
4.2 Japanese Standard (JIS)
The drawing representation method is close to that of the United States. The third-angle projection drawing method is generally used. In principle, the same drawing cannot be mixed with the first Angle and third angle drawing methods, the two drawing methods can be partially mixed when necessary, but the projection direction of the other drawing method must be indicated by an arrow
Japanese standard dimensioning method:
p>(1) Diameter, radius, square, and spherical codes
When there is a diameter, radius, square or spherical diameter or radius in the figure, add "φ" or "R" before the size number ", "□", when the graphics are clear, φ, R, □ can be omitted.
(2) Chamfer
Generally the same as in my country, for 45° chamfer, It can be represented by the letter "C", as shown in Figure 16, C2 is equivalent to 2×45°, and C3 is equivalent to 3×45°.
Figure 16
(3) When the plate thickness is not shown, the letter "t" can be added, such as t10 in Figure 16, which is equivalent to δ=10 in my country.
(4) Use "P" to indicate the rivet hole spacing, such as: P=100, P=98 (indicating that the hole spacing is 100 or 98).
(5) The hole size number can indicate other contents.
① Blind hole, as shown in Figure 17. Instructions on the processing method are usually marked after the size number, such as 深サ (indicating depth), キソ (indicating drilling), etc.
②The thread drawing method is similar to that in my country, and its marking form is shown in Figure 18.
Figure 17
Figure 18
4.3 British Standard (BS)
The view expression method is basically the same as the ISO international standard, and the dimension labeling The method is basically the same as our country's national standard (GB), the unit is also mm, and there is a gap (about 1mm) between the size lead-out and the outline. In section views, some draw section lines and some do not draw section lines.
4.4 French Standard (NF)
The view expression method is basically the same as the ISO international standard, and the dimensioning is basically the same as our country's dimensioning.
4.5 German (mainly refers to the former Federal Republic of Germany) standard (DIN)
The view representation method is basically the same as the ISO international standard. The dimension line in the view projected as a circle has only one arrow, and the size Add "Φ" at the end; do not add "Φ" if there are two arrows. All other dimensioning methods can be understood.
4.6 Russian and other CIS national standards (ГОСТ)
View expressions and dimensioning are basically the same as those in our country.
Others such as Canadian standards (CSA), Polish standards (PN) and ISO national standards are also similar.