The process dimension chain is composed of a series of rings. One size of plum in the process dimension chain is called the “ring” of the dimension chain.

There should be at least three loops in each dimension chain. The loops are divided into closed loops and constituent loops.

1. Closed loop

In the process of part processing or machine assembly. The final size that is naturally formed (or obtained indirectly) is called a closed ring. In the dimensional chain shown in Figure 2-13 (b”, if the dimensions L1 and L2 are directly guaranteed during processing, then L0 is the indirectly obtained size, which is the closed loop of the dimensional chain. In a dimensional chain, There can only be one closed ring, and its size changes with other dimensions. The dimensional tolerance of the closed ring should be the largest in the dimensional chain.

2. Constituent ring All the other rings except the closed ring in the process dimension chain are called the constituent ring. In the dimensional chain shown in Figure 2-13 (b), if La is a closed ring, then the L1 and L2 dimensions are all components of the dimensional chain.

According to the different influences of the composition ring on the closed ring, the composition ring can be divided into an increase ring and a decrease ring.

(1) Increase the ring. Under the condition that other constituent rings remain unchanged, when a certain constituent ring increases or decreases, the closed ring will increase or decrease accordingly, and this constituent ring is called an increased ring. Increased ring is represented by the letter L plus “one”. Figure 2-13 (b) Dimension chain

The B2 in is the increase ring, which can be expressed as L2.

(2) Reduce the ring. Under the condition that the other constituent rings remain unchanged, when a certain constituent ring increases or decreases, the closed ring will decrease or increase accordingly. This constituent ring is called a reduced ring. The subtraction ring is indicated by adding “-” to the word. As shown in Figure 2-13, L1 in the dimension chain is a subtracted ring, which can be expressed as L1.

(3) The method of discriminating between increasing and decreasing rings. In order to quickly distinguish the increasing and decreasing loops in the composition ring, on the process dimension chain diagram, first arbitrarily set a direction for the closed loop and draw an arrow, and then circle the dimension chain loop in this direction, and give each composition ring in turn. Draw a direction arrow. Any component ring whose arrow direction is opposite to the arrow direction of the closed ring is an increasing ring, and the component ring whose arrow direction is the same as the arrow direction of the closed ring is a decreasing ring. In the dimensional chain shown in Figure 2-13 (b), this method is used to determine whether the constituent ring is increasing or decreasing. In the figure, the arrows of L2 and L0 are in the opposite direction, and L2 is the increasing ring: L1 and L0 have opposite directions, and L1 It is a subtractive ring.

# The composition of the process dimension chain of CNC machine tools

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