In sheet metal fabrication, the bend allowance is a critically important concept. To correctly calculate the bend allowance, it is essential to specify which lower die should be used for materials of different thicknesses. Designers and process engineers must reach a consensus with bending operators, clearly defining the appropriate lower die size for different materials and thicknesses, and adjust the bend allowance accordingly.

Typically, the width of the lower die should be 6–8 times the material thickness. In special cases, this must be clearly noted on the drawings to avoid errors due to miscommunication.

I. 90-Degree Bending

For 90-degree bends, the company has specified corresponding lower die sizes and bend deduction values for different material thicknesses. Designers and process engineers must strictly adhere to these standards. Please refer to the relevant specification table for specific values.

II. Non-90-Degree Bending

For bends that are not 90 degrees and have conventional inner radii (inner R), the company uniformly stipulates:

In drawings, treat the inner R as 0.1mm.

Select the lower die according to the normal specifications corresponding to the material thickness.

Represent the bend allowance using the K-factor.

Currently, for materials with a thickness T < 4.0mm, the company's empirical K-factor value is 0.23, which is widely recognized by bending operators.

III. Hem Bending

Hemming is a commonly used bending method in sheet metal, and it also employs the K-factor for calculation. Note: For materials with T > 2.0mm, hemming is not recommended to avoid material cracking.

IV. Special Inner R Bending (e.g., Customer-Specified Inner Radii)

If the customer specifies the inner radius of the bend, the situation becomes more complex. If the company lacks relevant process records, the most accurate approach is:

Conduct on-site trial bending and measurements to obtain actual data.

Designers and process engineers must clarify with bending operators: material thickness, inner R value, and lower die specifications.

The final determined bend deduction value or K-factor must be clearly noted on the engineering drawings.

Generally, when the inner R > 5 times the material thickness (R > 5T), if precision requirements are not high, a K-factor = 0.5 can be used for flat pattern development. However, for high-precision requirements, it is still recommended to determine the final value through multiple trials.

Summary:

Standardizing the use of lower dies and unifying bend allowances are key to ensuring the quality of sheet metal bending. Design, process, and manufacturing departments should maintain good communication. All special requirements should be clearly noted on the drawings to avoid errors caused by inconsistent standards.

If there are any doubts, coordinate with bending operators promptly and, if necessary, determine parameters through trials to ensure product accuracy.