Flat pattern calculator for bent sheet parts: blank length from sizes, radius and K

Flat pattern calculator

Enter the sheet thickness and the radius, choose the number of bends and enter the flange sizes as they stand on the drawing. For a quick start you can take a profile example and replace its sizes with yours.

How to use it

• thickness and metal: the metal affects only the mass of the blank, the K-factor in the «auto» mode is given for steel;
• radius: either the inside radius from the drawing or, for air bending, the radius from the die opening (about 0.16 of the opening);
• sizes: choose how they are dimensioned — to the outer corners, to the inner corners or as lengths of the straight parts; the flanges are counted in order from the first to the last;
• angles: the bend angle α is how far the flange turns, 90° for a right angle; you can enter the inside angle between the flanges β, then α = 180° − β;
• K-factor: «auto» is taken from the radius to thickness ratio, and if you know K for your tool and material, enter it.

What the result shows

The main number is the flat pattern length. Next to it is the interval that results if K differs from the one used by 0.05: this is the visible part of the uncertainty, which depends on the number and angles of the bends.

Below are two tables: for each flange the drawing size and the straight part, for each bend the angle, arc length, corner setback and the correction to the sum of the sizes. If you give the blank width, the calculator adds the area and the mass of one blank.

The calculator warns when the straight part of a flange comes out negative (the flange is shorter than the bend zone), when the radius is smaller than the thickness, when the angle is above 150° and, in the air bending mode, when an end flange is shorter than the minimum for the chosen die opening.

Examples

All examples are for outside sizes, right angles and the automatic K. The last column shows how much shorter the flat pattern is than the plain sum of the sizes: this is the difference that is lost when the sum is not corrected.

How it is calculated

The flat pattern equals the sum of the straight parts and the arc lengths along the neutral layer: BA = π · α/180 · (r + K · s). For outside sizes the straight part of each flange is shorter than the drawing size by the corner setback (r + s) · tan(α/2) at each end. All the formulas and a step-by-step example are in the article Flat pattern of a bent part: blank length, K-factor and bend deduction.

For air bending the calculator takes the radius as 0.16 of the die opening and the smallest flange as 0.7 of the opening. These are practical guides that depend on the material and the tool. The force for the chosen die is calculated by the bending force calculator.

What the calculator does not take into account

• springback: it changes the angle after the load is removed and is corrected by setting the press, not by the length of the flat pattern;
• rolling direction, edge condition and the real strength of the batch: they shift K;
• bottoming, hemming (an angle above 150°) and complex sections with a variable radius;
• tolerances on sizes and angles: they are set by the drawing and the order conditions.

What to send for an exact calculation

For an order we confirm the flat pattern and the radius against the material and the drawing. It is enough to send the steel grade and thickness, a drawing or sketch with the sizes and angles, the inside bend radius or «the minimum possible», the bend line length and the number of parts. If there is no drawing yet, KM and KMD design will help, and bending, cutting and welding are done within steel fabrication.