Bend Layout and Forming

 

Radius

 

Simple rules govern the procedures which allow us to bend sheets of metal into complex shapes without cracking.  One basic rule involves the minimum radius around which bends are made.    In the following chart, we illustrate the recommended bend radius for one of our most common materials, 2024 T3 in its various thicknesses.

 

	0.016”	0.032”	0.064”	0.128”	0.182	0.258”
2024 T3	1.5T – 3T	2T – 4T	3T – 5T	4T – 6T	4T – 6T	5T – 7T

 

For example, in aviation we commonly use .032” 2024 T3 material for aircraft skins, stiffeners and stringers.  According to the chart, our recommended band radius is 2 to 4 times the thickness of the material.  This would require us to use a nose bar equivalent to .064” to .096” for our forming purposes.  Any sharper of a bend would invite cracking in the material. 

 

With this in mind, our definition of Minimum Bend Radius (MBR) would state that an MBR is the smallest radius around which we can bend sheet metal without danger of damage.

 

Bend Allowance

 

When a piece of material is formed around the nose bar of a bending brake, the inside of the material (next to the nose bar) becomes compressed, and the outer surface is placed in tension.  The actual amount of material used in the bend, or the rounded portions of the material only, is the Bend Allowance.  Each end of the bend allowance is bordered by the Bend Tangent Lines.

 

When we are calculating the amount of material required to form a bend, we should consider each of three parts of a bend.  There are two “Flat Leg” sections on either side of the bend, and in between is the “Bend Allowance” portion.  The points at which the Bend Allowance meets the Flat Leg section becomes the Bend Tangent Line, or the point at which the bend straightens out to become flat.

 

 

 

A common formula for calculating the amount of material consumed in the bend is:

            Bend Allowance = 2 x 3.14 (R + T/2) / 360

 

This allows us to calculate the amount per degree of bend.

 

In another form, if our bend is required to be 90°, the formula can be simplified to:

 

            Bend Allowance = 2 x 3.14 (R + T/2) / 4

 

In both these formulae, R = Radius, and T = Thickness of material.  We have used the figure 3.14 instead of the greek symbol pi.

 

 

An alternate to using this formula is to use a bend allowance chart which gives pre-calculated quantities.  The chart below shows two figures for each thickness/radius combination.  The upper figure shows bend allowance for a 90°bend, and the other is a quantity for each degree of bend.

 

 

Bend Allowance charts are calculated using an imaginary line called the Neutral Axis, which runs approximately halfway through the material between the inner and outer surfaces.  The chart shows bend allowances for common bend radii, and common thicknesses of material.

 

Setback and layout

 

When using a Cornice or leaf bending brake, we need to calculate the difference between the Mold Point, and the Bend Allowance.  This is done using the Setback.

 

 

 

Often, on diagrams and blueprints, we are given mold points.  From these specifications, we can calculate the difference between the bend allowance and the flat layout of the material.  If a diagram calls out that we need an L-shaped part built from 2024 T3 .040” with two flat legs measuring 2 inches each, and indicates a bend between the two using a radius of 1/8”, we can calculate the amount of material required as follows;

 

Each flat leg of two inches is laid out in a strip.

 

Calculating the Setback is easy.  Use:

 

            Setback = R + T   (Radius plus Thickness)

 

We know our thickness is .040”.  We know our radius is 1/8” or .125”.   Adding the two gives us:

 

            Setback = .040 + .125 = .165”

            Setback = .165”

 

Deduct the Setback from one side of the centerline of our strip, as follows;

 

 

 

Our new flat leg dimension is now 1.835”.  From this line, we can now add in our Bend Allowance.  Using either the chart or formula, figure bend allowance based on our .040” material bent around a 1/8” radius, at a 90° angle.  The chart quotes this to be 0.224”.

 

Layout a portion of your strip dimensioned to 0.224” as below:

 

The two ends of the Bend Allowance are called the Bend Tangent Lines.  The final 2-inch leg will now be adjust to reflect the shorter distance around the nose bar than it would have used if it went all the way to the mold point.  Deduct the setback from the remaining 2-inch leg:

 

 

For sake of cleanliness, we have removed the centerline from the diagram.  Now, all that is left to do is add up the portions of the material to obtain a total amount of metal required for this part.

 

We add:

 

            1.835 + 0.224 + 1.835 = 3.894

 

This represents a “savings” of a bit over .100”  from the mold point layout given in the diagram. 

 

If required, we can continue on to perform second, third or more bends, but make sure to subtract an additional setback for flat sections that are found between 2 bends.

Before we leave this topic, it should be noted that bends other than 90° are calculated using a K-factor chart. 

 

 

The chart specifies the formula for Setback as:

 

            Setback = K ( R + T )

 

Note that the above layouts should use only non-leaded pencils, and not scribes or awls to mark lines.  In parts which require two bends, the center flat section between the bends will require TWO setbacks to be deducted.

 

In positioning the materials into the bending brake, it is handy to mark a line on your material called a Sight Line.  The sight line is exactly the same dimension as the radius, and is marked that amount from the Bend Tangent Line.  In our case, the sight line would be exactly 0.125” from the outermost BTL.  Either BTL can be selected, depending on which direction will be inserted under the nose bar of the bending brake. 

 

In other words, select which direction the material will be inserted into the brake.  Find the BTL closes to that edge.  Mark a sight line of 0.125” from the BTL towards the center of the bend, and view this sight line from directly above the bending brake.

 

 

Clamp the metal into the bending brake, and slowly bring the bending leaf a few degrees beyond 90° (to allow for springback).  Release the brake clamp, and your bend is complete.  Continue with any other bends required on that part.  Check your material to ensure conformance with any tolerances quoted on the diagram.