Tolerances and allowance for die forgings
In the process of die forging, the surface quality of forgings is not good because of the oxidation and decarbonization of the surface of billets at high temperature, and even other defects are produced. The phenomenon of misalignment of the upper and lower die is unavoidable, resulting in the deviation of forging size. All these reasons make the forging not only have the machining allowance, but also have to specify the appropriate size tolerance of the forging.
The machined allowance should be given to the parts that need to be machined on the forging. In addition, some important load-bearing parts require 100% sampling test or necessary process residue for inspection and machining positioning.
In the actual die forging production, due to the influence of various technological factors, the actual size of the forging can not be equal to the nominal size, whether in the direction of height or horizontal dimension will be deviated, so the forging provisions allow a certain range of deviation. The part whose maximum size is larger than the nominal size of the forging is positive tolerance, and the minimum size of the forging is smaller than the nominal size of the forging is negative tolerance, as shown in the figure.
According to the principles stipulated in JB3834-85 standard, the following factors should be taken into account in formulating tolerances and machining allowances for forgings:
1) Tolerance grade: Tolerance is divided into two levels, common tolerance refers to the precision tolerance that can be achieved according to the general forging method; precision tolerance has a higher dimensional accuracy, suitable for precision forgings.
2) Forging quality: Forging quality is calculated according to the nominal size of the forging drawing, and tolerances and allowances are determined according to the quality table.
3) Form Complexity Factor of Forging: Form Complexity Factor S is the ratio of the forging mass m to the mass MC of the corresponding outer contour of the forging, that is, S = m forging / mc.
4) Dividing surface shape: Dividing surface shape can be divided into two types: one is flat dividing surface and symmetrical bending dividing surface; the other is asymmetrical bending dividing surface.
5) forgings material factor: forgings material coefficient is divided into two levels: M1 and M2. M1 is a steel with a maximum carbon content of less than 0.65% or an alloy steel with a maximum total content of less than 3.0%. M2 is an alloy steel whose maximum carbon content is greater than or equal to 0.65% of steel or whose maximum total content of alloy elements is greater than or equal to 3.0%.
6) machining accuracy of parts: JB3834 - 85 is applicable to parts whose surface roughness is greater than Ra1.6. If the surface roughness is less than Ra1.6, the remaining amount should be appropriately increased.