Determination of process size and tolerance when benchmarks coincide

1. Determine the machining allowance of each processing procedure

2. Starting from the last process, that is, from the design size to the first processing process, adding the margin of each process step by step, the basic size of each process (including the size of the blank) is obtained.

3. Except for the final process, the remaining processes shall determine the process dimensional tolerances according to the processing accuracy of their respective processing methods.

4. Mark the size of the process according to the principle of entering the body.

 

 

A. Process dimension chain

Dimension chain definition:

Dimension chain: Connect the interrelated dimensions in a certain order into a closed figure connected end to end.

Process dimension chain: a dimension chain of related dimensions formed by a single part in the process.

Composition of dimensional chain

1. Ring: each size A1, A2, A3 composing the size chain

2. Closed ring: size A2 obtained indirectly during processing.

3. Forming ring: the size A1 and A3 obtained directly in the process of processing.

Increased ring: The remaining components of the ring remain unchanged, and the increase of this ring increases the size of the closed ring.

Reduced ring: The remaining components of the ring remain unchanged, and the increase of this ring reduces the number of closed rings.

Specific judgment: Choose a direction for the closed ring, make a turn in this direction, add a direction to each ring, the one with the same direction as the closed ring is a decreasing ring, and the opposite is an increasing ring.

Features:

1. The inch chain must be closed

2. The dimensional chain has only one closed loop

3. The accuracy of the closed ring is lower than that of the composed ring

4. The closed ring changes with the composition of the ring

practice:

1. Find the closed loop

2. Starting from the closed loop, draw the constituent loops in sequence according to the relationship on the surface of the workpiece, until the size returns to the closed loop to form a closed pattern, and the number of constituent loops forming the dimension chain should be the least.

3. The principle of connection is to determine the increase and decrease of the ring.

B. Basic calculation of dimensional chain

1. Each dimension in the dimensional chain diagram is called a chain link, and all the chain links constitute a dimensional chain.

2. In the dimensional chain, the size that can be artificially controlled or directly obtained is called the constituent ring.

3. The size that is obtained indirectly in the size chain, and the size that is naturally formed when other sizes appear is called a closed ring (terminal ring). The increase or decrease of each component ring will change the closed ring, and there is only one closed ring in a dimensional chain.

4. If a certain component ring increases while other components remain unchanged, the closed ring will increase accordingly, so this component ring is an increasing ring. (The direction opposite to the closed ring is the increasing ring.)

5. If a certain component ring increases while other components remain unchanged, the closed ring will decrease accordingly, so this component ring is a subtractive ring. (The one that is consistent with the direction of the closed ring is a subtractive ring.)

 

 

 

C. Calculation formula

The basic size of the closed ring = the algebra of the basic size of each component ring and-the sum of the basic size of the increasing ring-the sum of the basic size of the subtracting ring.

The maximum limit size of the closed loop = the maximum limit size of all increased loops-the minimum limit scale of all subtracted loops.

The minimum limit size of the closed loop = the minimum limit size of all added loops-the maximum limit size of all subtracted loops.

The upper deviation of the closed loop = the sum of the upper deviations of all the increased loops-the sum of the lower deviations of all the decreased loops

The lower deviation of the closed loop = the sum of the lower deviations of all the increased loops-the sum of the upper deviations of all the decreased loops

Tolerance of the closed ring = the maximum limit size of the closed ring-the minimum limit size of the closed ring

= Upper deviation of closed loop-lower deviation of closed loop

= Sum of tolerances of increasing ring + Sum of tolerances of decreasing ring

= Sum of tolerances of all components of the ring

Deviation determination: generally based on the principle of ‘into the body’

1) When the component ring is a containment surface, the lower deviation is 0, and the upper deviation = tolerance

2) When the component ring is the contained surface, the upper deviation is 0, and the lower deviation = tolerance