How To Determine The Machining Margin And Tolerance Of The Forging

Ordinary mold forging method is difficult to meet the requirements of mechanical parts, generally speaking, there are two aspects of the problem, that is;

1. Forgings go the way.

Due to under-pressing, forging wear, up-and-down die misaling, blank volume changes and final forging temperature fluctuations, so that the shape of the forging changes, the size fluctuates within a certain range;

2. Surface quality is not easy to ensure.

Due to forging skin oxidation and decarbonization, as well as the evaporation or contamination of alloy elements, surface cracks occur from time to time, surface roughness can not meet the requirements of the part map, so that the surface quality of forgings is far lower than the surface quality of machined parts forgings.

It is these two reasons that make forging design, should be added a layer of metal coating the outer layer of the part, that is, the margin, but also to specify an appropriate tolerance to ensure that the error of the forging falls within the margin range.

Any surface on the forging that requires machining should be given a processing balance. In addition, for important components, 100% sampling test or in order to test and mechanical positioning needs, but also to consider the necessary process remaining blocks. The size of the machining balance is related to the shape complexity, dimensional accuracy, surface roughness, forging material and forging equipment of the part. Too large a machining surplus will increase the amount of cutting and metal loss, and insufficient machining capacity will increase the scrap rate of forgings.

Due to various process factors, the actual size of the forging can not be equal to the nominal size, both the height direction and the horizontal direction will be biased, so the forgings should be required to specify the allowable dimensional deviation range, which is necessary to control the working life of the forging and forging 裣 inspection. Forging dimensional tolerances are unsysysxual, i.e. positive tolerances are larger than negative tolerances. This is due to the height direction affecting the size deviation of the root cause is insufficient forging, and the bottom of the groove wear and die surface pressure caused by dimensional changes are secondary. Dimensional tolerances in the horizontal direction are also positive tolerances greater than negative tolerances, which is an unavoidable phenomenon considering groove wear, forging misalovation, and are all factors affecting the increase in size. Further negative tolerance refers to the lower limit of forging size, should not be too large, and the size of positive tolerance will not lead to forging scrap;

There are two main ways to determine the mechanical margin and tolerance size of forgings: one is to determine the tonnage size of the forging hammer, and the other is to check the table by the size of the forging shape.

It should be pointed out that the processing margin and forging size tolerance standards used in the production of forgings by various departments and factories are not uniform, departments have ministerial standards, and some specialized factories have factory-issued standards.

The difference between die forgings can be divided into:

(1) Dimensional tolerances, including length, width, thickness, center distance, angle, die-forged slope, arc radius and fillet radius.

(2) Shape position tolerances, including straightness, flatness, coaxiality of deep hole shafts, misalter, shear end deformation and rod deformation, etc.

(3) Surface technical factor tolerances, including depth, size of shearing burrs, top bar indentation depth and surface roughness. Tolerances should not be superimposed on each other.