Calculate sheet metal bend allowance as the bend rotation in radians multiplied by the neutral-axis radius: BA = θ(R + Kt). R is inside bend radius, t is sheet thickness and K is the neutral-axis offset divided by thickness. The neutral-axis model and allowance relationship are documented by Bricsys; Autodesk Fusion defines the K-factor convention. [1] Bricsys. [2] Autodesk Fusion. A useful flat pattern requires consistent flange references and a rule validated for the intended forming setup.
This guide explains bend allowance, K-factor calculation and bend deduction using a reproducible single-bend example. It then proposes a coupon and drawing-review workflow for stamped sheet-metal parts. It does not select a material, establish bendability or prescribe springback compensation.
Define bend allowance and K-factor before choosing a value
Autodesk Fusion describes the outside of a bend as stretching and the inside as compressing, with the neutral axis between them. Its K-factor is the neutral-axis offset from the internal surface divided by material thickness. [2] Autodesk Fusion. In this guide, let that offset be n, so K = n/t and n = Kt. K has no units; n and t must use the same length unit.
Bricsys expresses the neutral-surface radius as R + Kt and identifies bend allowance as its developed arc length. [1] Bricsys. The practical distinction is between the curved zone and the straight flanges. Bend allowance belongs to the curved zone; it is not a blanket addition to every dimension printed on a folded drawing.
For the proposed engineering review, write down the inside radius, thickness, angle definition and flange references before entering any value into CAD. Ask which dimensions define the finished component and which dimensions are only derived manufacturing aids. A flat drawing with an unexplained K value should be treated as an incomplete manufacturing record until its basis is clarified.
A supplier table is a process reference, not a universal material constant
Protocase’s published radii and minimum-bend tables explicitly apply to its stocked material and tooling combinations, and it notes that geometry can limit the tooling available for a chosen radius. [4] Protocase. Those tables illustrate why a manufacturer’s tooling context matters. They do not establish SSP’s machine capabilities, nor do they supply an automatically transferable K-factor for a stamping die.
When discussing the proposed part with SSP’s tooling and manufacturing team, provide the formed model, drawing revision and intended material specification. Request a review of the unfolding basis and the evidence needed for approval. Avoid choosing a familiar K value merely because another shop used it on a superficially similar bracket.
Fix the angle and dimension conventions
For the examples here, θ means the rotation from a flat strip into the finished bend, not the included opening angle between the finished flanges. A flat strip has θ = 0° under this convention. For the simple convex bends considered here, an included opening of 120° corresponds to a bend rotation of 60°. This is an explicitly declared geometric convention, not a claim that every CAD field uses the same angle definition.
Convert degrees to radians before multiplying radius by angle: θrad = θdeg × π/180. The bend-allowance expression then returns a length in the same units as R and t. If millimeters are used for the radius and thickness, BA is in millimeters. Keep an unrounded calculation internally and round the reported result only at the end.
| Symbol or quantity | Definition here | Check before calculation |
|---|---|---|
| t (mm) | Sheet thickness used by the model | Distinguish a nominal design value from a measured calibration input. |
| R (mm) | Inside bend radius | Do not substitute an outside radius or assume punch radius equals final part radius. |
| θ (degrees or radians) | Bend rotation from flat | Convert the explicitly stated angle convention; use radians in the arc expression. |
| l₁, l₂ (mm) | Straight flange lengths from free edges to bend tangent points | Add BA to these lengths. |
| A, B (mm) | Outside flange dimensions to the virtual sharp intersection | Subtract the corresponding BD from their sum. |
| K (dimensionless) | Neutral-axis offset divided by t | Record its source and the conditions for which the rule is approved. |
The two flange-reference systems produce different arithmetic expressions. With tangent lengths, the idealized flat length is L = l₁ + l₂ + BA. With outside dimensions to the virtual sharp, it is L = A + B − BD. Bricsys documents these relationships through its allowance and tangential-deduction definitions. [1] Bricsys. Do not mix a tangent length from one leg with a virtual-sharp dimension from the other without first resolving the geometry.
Derive setback for the stated geometry
For the simple constant-thickness bend in this guide, the outside setback is the distance from the virtual sharp to the outer bend tangent along a straight flange. Right-triangle geometry gives OSSB = (R + t) tan(θ/2). Since each outside flange dimension includes one setback, l₁ = A − OSSB and l₂ = B − OSSB. Substitution into the tangent-length expression gives BD = 2 × OSSB − BA. These are geometric derivations for the declared model, not empirical process limits.
Mark the virtual-sharp intersection explicitly on the review sketch or CAD inspection view. If the actual drawing dimensions to a different surface, edge or datum, re-establish the straight lengths from that geometry. The formula cannot correct a misidentified dimension reference.
Calculate a single-bend flat pattern
Assume a hypothetical bracket with t = 1.00 mm, R = 2.00 mm, K = 0.40 and θ = 90°. Let outside dimensions to the virtual sharp be A = 30.00 mm and B = 20.00 mm. These assumed inputs define the arithmetic example; they do not establish a suitable alloy, tooling radius, minimum flange or allowable tolerance.
The neutral-axis offset is n = 0.40 × 1.00 = 0.40 mm, and its radius is 2.00 + 0.40 = 2.40 mm. The bend rotation is π/2 radians. Therefore BA = (π/2) × 2.40 = 3.769911 mm, before display rounding. This applies the sourced neutral-axis relationship to invented dimensions, rather than reporting a measured allowance.
| Step | Calculation | Result (mm) |
|---|---|---|
| Neutral-axis offset | K × t = 0.40 × 1.00 | 0.400 |
| Neutral-axis radius | R + Kt = 2.00 + 0.40 | 2.400 |
| Bend allowance BA | (π/2) × 2.40 | 3.770 |
| Outside setback OSSB | (2.00 + 1.00) × tan(45°) | 3.000 |
| Bend deduction BD | 2 × 3.000 − 3.769911 | 2.230 |
| Flat length L | 30.00 + 20.00 − 2.230089 | 47.770 |
Check the same result through the tangent lengths. Here l₁ = 30.00 − 3.00 = 27.00 mm and l₂ = 20.00 − 3.00 = 17.00 mm. Their sum plus the allowance is 27.00 + 17.00 + 3.769911 = 47.769911 mm. Agreement between the two routes is a useful calculation check; it does not validate the chosen K value against a real forming process.
A common reference error is to add BA directly to the 30 mm and 20 mm outside dimensions. That would give 53.769911 mm, which is 6.000 mm longer than the correct result for this declared geometry. The discrepancy equals the two outside setbacks that were counted twice. The example illustrates a bookkeeping error, not a measured manufacturing deviation.
Compare bend allowance K-factor sensitivity
Keep R, t, θ, A and B unchanged, and vary only the assumed K value. Because BA = θ(R + Kt), the change in allowance is ΔBA = θtΔK. For this one-bend model, the outside setback does not depend on K, so the calculated flat-length change equals ΔBA when the outside dimensions remain fixed. This follows algebraically from the equations above.
| Assumed K | BA (mm) | BD (mm) | Flat length L (mm) |
|---|---|---|---|
| 0.30 | 3.613 | 2.387 | 47.613 |
| 0.40 | 3.770 | 2.230 | 47.770 |
| 0.50 | 3.927 | 2.073 | 47.927 |
Changing K from 0.30 to 0.50 changes the calculated flat length by (π/2) × 1.00 × 0.20 = 0.314159 mm, or 0.314 mm rounded. This is a sensitivity result under fixed assumptions. It is not the expected error on an SSP component or evidence that a particular value is correct.
Use the comparison to decide which assumptions need verification before blank release. If the allowable dimensional variation is small relative to a plausible change in the unfolding inputs, ask for calibration evidence rather than treating the CAD default as an approved manufacturing rule. The actual tolerance and decision rule belong to the part specification; this guide does not invent an acceptance band.
Calibrate with traceable coupons and a controlled bend table
Autodesk Inventor supports linear K-factor, bend-table and custom-equation unfolding methods. Its documentation describes bend tables by material thickness, radius and angle, and calls for measured sample pieces to establish useful values. [3] Autodesk Inventor. A table therefore needs an identified process basis, not merely a filename copied from a different machine.
The proposed calibration record includes material grade and condition, sheet lot, thickness basis, rolling-direction orientation, approved tooling identification, forming method, radius and angle measurement methods, blank size, operator/setup revision and inspection stage. Define how coupons will be labeled and how the corresponding CAD rule will be versioned. These are recommended records for review, not a claim that any one coupon proves production capability.
Reverse a hypothetical coupon into a K-factor calculation
Assume, only for this exercise, a coupon with flat length L = 47.80 mm, outside dimensions A = 30.00 mm and B = 20.00 mm, inside radius R = 2.00 mm, t = 1.00 mm and θ = 90°. All are invented measurement-like inputs, not actual test results. BD = A + B − L = 2.20 mm. With OSSB = 3.00 mm, the inferred BA is 6.00 − 2.20 = 3.80 mm.
Rearranging the allowance relationship gives K = (BA/θ − R)/t. The hypothetical inputs give K = (3.80/(π/2) − 2.00)/1.00 = 0.419155, approximately 0.419. This inferred value is valid only as arithmetic for the assumptions. It cannot establish a universal K-factor for the material.
Before fitting real data, check the final angle, inside radius and dimension references rather than adjusting K to hide every discrepancy. Record measurement uncertainty, repeatability and the inspection team’s decision rule. If repeated coupons disagree, retain that spread and investigate it; do not publish a selective “best” value without its conditions.
Autodesk distinguishes bend-table data for different material types and thicknesses and explains interpolation between available values. [3] Autodesk Inventor. The proposed release review should identify the validated domain and how requests outside it are handled. Software interpolation is a calculation behavior, not proof that an untested tool/material combination is qualified.
Review the drawing, CAD rule and tool enquiry together
Use the following proposed sequence before issuing a blank: confirm the formed geometry and critical dimensions; check thickness and radius conventions; select the approved unfolding method; compare the flat pattern with its documented basis; and preserve the drawing/model/rule revisions together. Confirm who is authorized to change the blank after tryout and how the change will be communicated.
For multiple bends, apply the allowance only to the curved zones represented by the model and retain the straight lengths between the appropriate tangent points. This guide’s single-bend example does not validate hems, complex drawn transitions or other geometry outside its cylindrical-bend assumptions. Review those features with the responsible tooling engineer rather than forcing them into the example.
When evaluating the physical tooling, consult SSP’s stamping-die product configurations to frame the tool enquiry. For the drawing-review, build and acceptance discussion, use the stamping-die manufacturing service. Send the formed model, critical dimensions and unfolding basis so the proposed scope and trial evidence can be assessed.
Springback compensation remains a separate question. Keep the finished-part angle definition clear while the process team establishes the required tooling and setup. Do not alter K simply to relabel an angle problem as a length correction. The objective is a traceable relationship between the specified formed part, the flat pattern and the approved forming evidence.
Frequently asked questions
Is bend allowance the same as bend deduction?
No. BA represents the developed curved-zone length. BD is subtracted from the sum of the declared outside virtual-sharp dimensions. For the hypothetical bracket, BA is 3.770 mm and BD is 2.230 mm, rounded.
Can K-factor always be set to 0.40?
No universal value is established here. K = 0.40 is a teaching assumption. The approved rule should be tied to relevant material, thickness, tooling and validation records.
Should the formula use the included angle?
This guide uses rotation from flat. Convert the drawing’s angle convention before calculating: the stated simple 120° opening corresponds to 60° rotation. Check the software field rather than assuming identical terminology.
How is a K-factor obtained from a coupon?
Establish its allowance from traceable flat and formed dimensions, then use K = (BA/θ − R)/t under the stated model. The worked coupon gives approximately 0.419 from hypothetical inputs. Measurement quality and process scope still require review.
Does a bend table replace part inspection?
No. A table supplies unfolding data within its documented scope. The proposed release workflow still checks the actual component against the drawing and retains the relevant inspection evidence.
Technical references
Reviewed on 2 November 2026. Documentation supports the definitions and CAD behaviors cited nearby. The V16 Bricsys page is used for geometric definitions, not as current interface guidance. All worked inputs are illustrative; supplier tables are not SSP capability evidence.
