Roll Form Tool Design

Chapter 1: Bend Allowances

MATERIAL DISTRIBUTION-

Fundamental to the design process for roll form dies is the necessity to define the material distribution in the product, which is to say, the sum of the lengths of material in all of the bends in the product, plus the sum of all the straight lengths. This total is the developed width. Furthermore, it is necessary to determine the material distribution in each and every forming station in the tool set. The methods now in use to calculate the straight lengths of material needed to form the bend are approximations which equate that straight length to the length of the neutral axis in the bend. See Fig. 1.1 below.


Fig. 1.1

All material fibers on the inside of the neutral axis (shaded area) are in varying degrees of compression, and all those on the outside are in varying degrees of tension. By definition, the stress on the neutral fiber is zero. The location, then, of the neutral axis is that position where the tension and compression stresses are in balance. The value of the stress in either tension or compression increases in proportion to the distance from the neutral axis. In general, the methods used by most roll form tool designers are based on the notion that the neutral axis is at the midpoint of the material only for very large inside radii, and that the neutral axis moves inward as the inside radius gets smaller. The bend allowance is calculated as follows:


Where

  • B. A=bend allowance...

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