All the ins and outs of the flat blank for a pressed metal part

In the production of pressed metal parts, reference is often made to the ‘flat blank’. We tell you more about this in this blog post. We discuss the following questions:

1.  What is the flat blank for a pressed metal part?
2. How do you calculate the size of the flat blank?
3. The flat blank and Sophia®

1. What is the flat blank for a pressed metal part?

The flat blank is your pressed part folded out into a flat plate. 

Technical draughtsmen usually create parts in their software as 3D objects. However, many of these parts have to be cut from a flat plate. That requires the details of the flat blank from which the part is formed.

But producing a flat blank is not as simple as it sounds. 

That's because bends in the surface change the effective length of the metal. After all, when metal is bent, the outside is stretched and the inside is compressed. This also shifts the neutral axis inward. You can read more about this in our blog post on the bending radius.

In general, this means that the overall length of a 3D object is greater than the length of the plate in unfolded form. Let's take the pressed part shown below as an example.

If we add the lengths together, we arrive at 200 mm. But when we create the drawing of the flat blank, the total plate length is about 198 mm. A bending loss of ±0.5 mm arises on both sides of each bending line, resulting in this flat blank:

This is because the metal in the radii stretches during bending.

What if you don't allow for this? Then the dimensions of your flat blank will be incorrect and the final product will no longer fit together properly.

Formulas are available for calculating the size of the flat blank to avoid this type of error.

2. How do you calculate the size of the flat blank?

There are two ways to calculate the size of the flat blank. This can be done with the:

1. Bending Allowance
2. Bending Deduction

When using the bending allowance (BA), you look at the lengths of the flanges to the radius and calculate what length the radius needs to add to the lengths.

When using the bending deduction (BD), you look at the complete lengths of the flanges, calculate what length the radius has, which you then have to deduct from the total.

NOTE: Although the results of these theoretical formulas provide an excellent guideline, they can still differ from the values needed in practice. So use the formulas as a guide. 

Formula for the bending allowance

Below is the formula for the bending allowance for a single bend.

BA = θ × (R + K × t)

Where:

• BA = Bending allowance (in mm)
• θ = Bend angle in radians ((bend angle in degrees × π) / 180))
• R = Inner radius of the bend (in mm)
• t = Sheet thickness (in mm)
• K = K-factor (ratio between the neutral axis and the thickness)

Have you calculated the BA? Now you can calculate the total flat blank as follows:

Flat blank = L1 + L2 + BA

In this calculation, L1 and L2 are the lengths of the flanges up to where the radius begins.

Formula for bending deduction

Below is the formula for the bending deduction for a single bend.

BD = 2 × (t + R) × tan (θ / 2)

Where:

• BD = Bending Deduction (in mm)
• θ = Bend angle in degrees
• R = Inner radius of the bend (in mm)
• t = Sheet thickness (in mm)

Have you calculated the BD? Now you can calculate the total flat blank as follows:

Flat blank = L1 + L2 - BD

In this calculation, L1 and L2 are the total lengths of the flanges.

When is which formula used?

While the choice of formula may be a matter of preference, there are several situations in which one formula is generally used more often than the other.

They are summarized below:

3. The flat blank and Sophia®

At 247TailorSteel, we laser-cut your parts from a flat plate. Are you ordering a pressed part via Sophia®? Then our software automatically calculates the required flat blank size. It does this based on your selected material, the thickness and the angles as drawn. We retrieve practice-based values such as the K-factor, bending radius, bending allowance and so on from our own database.

Would you like to order only a flat part from us and perform the bending operations yourself? Then you can use the formulas above to calculate the flat blank size. Please note, though, that the results of these formulas can always differ from the values you need in practice. So use the formulas as a guide.

Do you have any questions about the flat blank?

Please contact our Customer Service. We will be happy to assist you!