K Factor in BricsCAD Mechanical Sheet Metal Explained

Quick answer: Bending a metal sheet with a press brake deforms its material, causing the overall length to differ between its folded and unfolded states. BricsCAD accounts for this automatically when unfolding a sheet metal part, using either a simplified approach (K-factor) or a more advanced one.

The Neutral Surface Concept

The calculation assumes an internal surface within the sheet metal part that isn’t deformed when bent, one that’s equidistant from one side of the bend, local to each individual bend, and in simple cases propagatable across the whole part. This surface is assumed not to stretch during bending. Two variables matter here: T (the sheet metal part’s thickness) and R (the bend’s internal radius).

The K-Factor Formula

The K-factor (k) is the ratio of the neutral surface’s location to the material thickness, so the neutral surface sits at a distance kT from the bend’s internal surface, and its bend radius equals R + kT. This is a simple geometric calculation, it doesn’t account for forming stresses or other unknown factors, and depends heavily on material type, bending type, and tools used. It typically falls between 0.3 and 0.5.

The default K-Factor for a bend radius equal to the material thickness (R = T) is 0.27324, confirming the exact same default covered on this hub’s Sheet Metal Global and Feature Properties page. SMDEFAULTKFACTOR sets this default value.

Bend Allowance and Bend Deduction Formulas

In the unfolded state, the flanges (L1 and L2) don’t stretch; the bent portion is the Bend Allowance (BA), the length of the unfolded neutral surface:

BA = BendAngle × (R + kT)

Since measuring K-factor or Bend Allowance directly is difficult in practice, the tangential Bend Deduction (BD) can be calculated instead:

BD = B1 + B2 - Unfolded Length = B1 + B2 - (L1 + L2 + BA)

For simple cases, override the K-Factor directly in the Mechanical Browser; for maximum precision, supply a bend table of tangential bend deductions instead. Every sheet metal part can either specify its own K-factor or use the default.

Changing the K-Factor for a Sheet Metal Part

Select the root node in the Mechanical Browser and type a value into the K-Factor field, within the range [0, 1], since the neutral surface must sit inside the part. This value is specifically defined for a bend radius equal to the material thickness (R/T = 1); for other bend radii, BricsCAD interpolates using a technique proven valid for industrial sheet metal applications, better accounting for bend deduction:

• For R/T < 1: linear interpolation between 0 and 0.27324.
• For 1 < R/T < 4: linear interpolation between 0.27324 and 0.5.
• For R/T > 4: constant at 0.5 (material stretch gets neglected once the bend radius is at least four times the thickness).

The only value you actually control in the Mechanical Browser is the one at R/T = 1; setting it to 0.6, for example, reshapes the whole interpolation curve accordingly. A distinguishing feature of this curve: varying R within (0, T] keeps the total unfolded length (L1 + L2 + BA) constant, in other words, a constant bend deduction, reflecting that radii smaller than the material thickness exist for design convenience rather than physical accuracy, since an actual bending tool’s radius is usually at least equal to the material thickness.

Worked Example

With T = 2, R = 2, B1 = 100, B2 = 100:

• K = 0: L1 = L2 = 96, Unfolded Length = 195.14, so BA = 3.14 (matching a straight angle in radians, roughly 1.57, or π/2).
• K = 0.27324 (default): Unfolded Length = 196, so BA = 4, whether computed as 196 – (2 × 96), or directly from the formula: BA = 1.57 × (2 + 2 × 0.27324) = 4.
• K maximized: Unfolded Length = 198.28, the maximum achievable in this example, since the neutral surface shifts all the way to the bend’s external side.

The Bend Deduction for k = 0.27324: BD = 100 + 100 – 196 = 4.

Frequently Asked Questions

What’s the default K-Factor value BricsCAD uses, and for what bend radius? 0.27324, defined for a bend radius equal to the material thickness (R = T).

Why does the K-Factor stay constant at 0.5 once the bend radius exceeds four times the material thickness? Because at that point, material stretch effectively becomes negligible, so BricsCAD stops adjusting the value further.

Can I set a different K-Factor for radii smaller than the material thickness without changing the unfolded length? Yes, that’s exactly the interpolation behaviour within (0, T]: varying the radius there keeps the total unfolded length, and therefore the bend deduction, constant.

When should I use a bend table instead of just overriding the K-Factor? When you need maximum precision, a bend table supplying tangential bend deductions directly is more accurate than the simplified K-Factor calculation.