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Concise graphic method of process dimension

in the process of machining, the principle of dimension chain is often used to solve the process dimension and its tolerance. However, when there are many parts processing procedures, the process benchmark and design benchmark do not coincide, and the positioning benchmark needs to be converted many times during processing, it is not easy to establish the dimension chain quickly and accurately. At this time, most people will use the "graphic method". Because the representation of process dimension and the position of machining allowance in the traditional graphic method are not drawn according to the actual machining conditions, the concept of process dimension is unclear and the relationship between dimensions is chaotic, which brings certain difficulties to the establishment of dimension chain. Aiming at the problems existing in the traditional graphic method, this paper puts forward a "concise process dimension graphic method" which is intuitive, clear and truly reflects the dimensional connection in the processing process. Now it is introduced with examples as follows

example: the schematic diagram of a part is shown in Figure 1. Its machining process is composed of process 05 (Figure 2a) and process 10 (Figure 2b). Try to find the process dimensions H1, H2, H3, H4 and their limit deviations. (Note: the machining allowance of each surface is 1.5m)

Figure 1 part diagram

(a) process 05 (b) process 10

Figure 2 process diagram

1 steps of establishing process dimension chain with concise process dimension diagram

1.1 preparatory work

(a) draw a sketch of parts by hand and mark the design dimension above the sketch. (as shown in Figure 3)

Figure 3 illustrates the establishment of

(b) analyze the part processing process and understand the processing times of each surface. In this example, the surfaces 1, 2, 3 and 4 are processed once respectively

(c) add allowance on each machined surface of the part drawing to convert the part drawing into a rough sketch

note: A. several layers of allowance should be added to each surface after several times of machining: B. in order to make the diagram clear, the allowance should be appropriately enlarged

the sketch of hand damage after the conversion of this part drawing is shown in Figure 3

1.2 graphically represent the processing process

(a) lead vertical lines downward from each surface to be processed, representing the blank surface and the processing surface of each process respectively

(b) strictly follow the processing sequence, from top to bottom, and use the specified symbols to indicate the processing process

if it is used to represent positioning datum, use "·" to represent process datum, use to represent machined surface, use to represent process dimension, use to represent machining allowance, etc

the graphical establishment of this example is shown in Figure 3

note: the process benchmark, process size, machining allowance, etc. in the diagram must be drawn according to the actual machining position, and the arrow should point to the machined surface of each process

this step uses the specified symbols to "assemble" the different processes of the whole processing process on the same diagram, which intuitively and clearly shows the real changes of the dimensions of each processing surface and the internal relationship between the dimensions in the process of parts changing from the soft and hard billets of the tested material to finished products by comparing the size of local plastic deformation on the surface of the raw material, forming the "dimension connection diagram" of the process. This is the key to establish the process dimension chain, so be careful

1.3 establish the dimension chain from the diagram

(a) determine the closed ring

there are two types of closed rings in the machining process:

(1) the indirectly obtained design dimension is the closed ring

(2) the machining allowance (except by spark grinding) is the closed ring

since the closed ring must be formed indirectly, press the "P clear/1" key to draw the closed ring at the bottom of the whole diagram, represented by. In this example, the design size 3. What we get directly is the constituent ring; The design size is 8+0.3 and 3 is obtained indirectly, which is a closed ring; The remaining Z1, Z2, Z3 and Z4 are closed rings

(b) find the constituent rings from the closed rings

find the relevant constituent rings in turn from the closed rings obtained above to form a dimension chain

the following formula can be used to find the constituent ring:

look up from both ends of the closed ring, turn around when encountering an arrow, and continue to look up when encountering "·". Until it intersects

Description:

(1) encountering an arrow means that the size of this process has an impact on the closed ring, which should be counted in the dimension chain, so it should turn; Meeting "·" means that it is still the same benchmark, so it should continue to rise instead of turning

(2) a closed loop can only establish a dimension chain, and a dimension chain can only solve an unknown quantity. Therefore, in addition to the directly obtained process dimension, how many unknown quantities should be established

(c) list the found dimension chains separately to form a dimension chain diagram

the dimension chain diagram related to the process size formed in this example is shown in Figure 4 (a), (b), (c)

(a) (b) (c)

Figure 4 dimension chain diagram

2 solution of process dimension chain

the purpose of establishing process dimension chain is to solve the process dimension and its tolerance, and check the size of machining allowance, which can be solved by using the dimension chain calculation formula. But we should pay attention to the following problems:

(a) treatment when the tolerance of the constituent ring exceeds the tolerance

when calculating the dimension chain, sometimes the sum of the tolerances of the constituent ring is greater than the tolerance of the closed ring, which is not allowed. At this time, the machining accuracy of the corresponding process should be improved and the tolerance of the constituent ring should be compressed

in this example, the dimension h3=300.3 is the design dimension obtained directly, because its tolerance is equal to the closed ring 8+0.3 and 3, so it should be compressed. In this example, h3= 15, get h2=14+0.15, h4= 30。

(b) treatment of common component ring

common component ring should meet the tolerance and deviation requirements of its multiple dimension chains at the same time, and its tolerance and deviation should be determined by the dimension chain with high accuracy requirements

(c) treatment of coordination ring

when there are multiple unknown process sizes in a dimension chain, it is necessary to select a constituent ring as the coordination ring, and its limit deviation cannot be determined in advance, but can only be determined through dimension chain calculation when the limit deviation of other cycles has been determined, so as to meet the requirements of the limit deviation of the closed cycle

(d) treatment of component rings irrelevant to the design dimension

for dimensions irrelevant to the design dimension, the tolerance can be determined according to the economic accuracy level of this processing procedure, and the deviation can be marked in the principle of entering the body

in this example, the size H1 is not directly related to the design size, and its basic size h1=h3+z3=31 5mm, the economic accuracy grade is it12, the tolerance t=0.25mm, and the deviation is marked according to the principle of entering the body, then h1=31 25mm。

3 conclusion

in this graphical method, the representation of process size and allowance is completely consistent with the actual processing situation, which is very intuitive and easy to promote

the trilogy of establishing a dimension chain is to establish and test a long cycle, expensive diagrams, determine the closed ring, and find the constituent ring. The essence of establishing diagrams is to use "specified symbols" to represent the processing process; The key to determine the closed ring is to find out the design size of "indirectly obtained"; Remember the "pithy formula" in the text, and you can quickly find 10 Input power: 1 kW to form a ring, and then establish a dimension chain

this graphic method is applicable to the establishment of any complex process dimension chain, with high efficiency and good effect, and is of great popularization and practical value. (end)

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