Establish Material and Geometry Limits Before Selecting Tools

A tonnage chart is useful only after you confirm the material, required geometry, and forming method. Nominal thickness and the term “carbon steel” leave too many factors unresolved: the steel must withstand the required radius, the part must fit and remain supported on the tooling, and the calculated force must remain within every applicable load rating.

Create a setup record before selecting the punch and V-die. Keep drawing requirements distinct from predictions of the radius and springback that the tooling will produce.

Measure Actual Thickness and Confirm Steel Grade, Condition, Tensile Strength, and Material Lot

Measure the sheet thickness at several representative locations using a suitable calibrated instrument. Avoid burrs and damaged edges that could distort the reading, and record the measured range. Thickness is a squared term in the air-bending force calculation, so variation affects both the load estimate and tooling selection.

Confirm the grade and supplied condition using traceable material documentation, and record the tensile strength and its units. Grade alone does not fully define forming behavior: differences in condition can affect strength, ductility, and springback. Tensile strength supports the force calculation, but it does not by itself establish a safe minimum bend radius.

Record the material lot so that trial-bend results and subsequent production changes can be traced to the stock used. If any properties remain unknown, identify the gap explicitly. Any provisional strength assumption requires a defensible basis; a successful trial bend cannot establish the properties of unidentified material or justify an unchecked load.

Record the Required Inside Radius, Finished Angle, Bend Length, Flange Dimensions, and Tolerances

Enter the drawing requirements into the setup record before selecting a die opening:

• Inside radius: the required radius and its tolerance, if specified.
• Finished angle: the angle after unloading, including whether the drawing specifies an included angle or a bend angle.
• Bend length: the length formed in each stroke, which determines the loaded length used to calculate force; for extended bending lengths, ADH Machine Tool’s CNC tandem press brake provides a practical production solution.
• Flange dimensions: their datums and measurement convention, including whether the dimensions refer to inside faces, outside faces, or theoretical intersections.
• Tolerances: the permitted variation in angle, radius, flange length, and other controlled features.

These requirements jointly constrain the tooling. A short flange must satisfy the selected tooling’s minimum flange requirement and remain properly supported during bending. Tool clearance must also accommodate the part’s shape and bending sequence.

Treat the radius shown on the drawing as a requirement to be achieved. In air bending, the resulting inside radius depends on the die opening, material, and punch geometry; it is not necessarily equal to the punch-tip radius. Likewise, the programmed forming angle must account for springback to produce the specified finished angle. For practical implementation, ADH Machine Tool’s CNC-based bending portfolio includes an NC press brake suited to programmed forming workflows.

Identify rolling direction, damaged edges, and holes or notches near the bend

Mark the rolling direction and record the orientation of the bend line relative to it. Where the layout permits, position the bend line perpendicular to the rolling direction. Bending with the line parallel to the rolling direction generally increases the risk of cracking and may require a larger minimum inside radius. Preserve this orientation when preparing trial pieces so that their results apply to production blanks.

Inspect cut edges and nearby holes, notches, and cutouts. Burrs, edge damage, and abrupt changes in section can concentrate strain and initiate cracking. Openings near the bend can also distort as the surrounding material stretches.

Record the locations of these features relative to the bend line. A minimum-radius recommendation for intact sheet does not establish feasibility for a damaged or interrupted bend region. Carry the verified material condition, grain orientation, and required radius into the initial feasibility check: whether this blank can withstand the specified bend.

Use this calculation sequence for air bending; obtain separate force and tooling guidance for bottoming or coining

The following tooling rules and force calculation apply to air bending, in which the punch forms the sheet across the die shoulders without fully seating it against the die faces. Confirm that this is the intended process before using an air-bending chart.

Follow this sequence: verify the material and geometry, check radius feasibility, select a compatible punch and V-opening, calculate the force, and then check the machine’s capacity, permitted load distribution, and tooling ratings. A CNC press brake from ADH Machine Tool provides a practical next step for controlled production bending, supported by CNC-based operation and frame and ram designs verified through finite element analysis. Only after completing these checks should measured trial bends establish springback compensation and confirm finished dimensions.

Bottoming and coining alter the contact conditions and force requirements. If either process is planned, obtain guidance specific to that method; an air-bending estimate does not determine the required capacity or appropriate tooling.

Choose a Bend Radius the Material Can Withstand

Accept the required inside radius only if the material’s bendability guidance supports it under the planned bending conditions. If the drawing specifies a radius below that supported limit, increasing tonnage or adjusting the bend angle cannot make it a reliable production choice.

Use grade- and condition-specific minimum bend-radius guidance; tensile strength or elongation alone cannot determine a safe radius

Obtain minimum inside-radius guidance for the specified grade and supplied condition. Verify the applicable thickness range, bend orientation, bend angle, and edge preparation. A recommendation for thinner stock or a more ductile condition does not establish a limit for the material on the brake.

When guidance states the minimum as a multiple of thickness, apply it to the actual stock within the guidance’s specified range. For example, in a hypothetical specification requiring an inside radius of at least twice the thickness, 3 mm stock requires an inside radius of at least 6 mm. This calculation translates an established requirement; it does not prove that twice the thickness is suitable for a different grade or condition.

Tensile strength helps estimate forming force, while elongation describes stretching during a tensile test. Neither property alone accounts for the localized deformation and edge sensitivity of a tight bend. As the inside radius decreases relative to thickness, the outer surface must withstand greater stretching. A general radius rule can therefore guide an initial inquiry, but it cannot replace applicable bendability guidance. If that guidance is unavailable, obtain material-specific advice or an approved qualification procedure before accepting the radius.

For material-specific tooling and bend-radius guidance, contact ADH Machine Tool to discuss the application with its international sales and service network.

Place the bend line perpendicular to the rolling direction where practical; verify bendability when bending parallel to the grain

Use the recorded rolling direction to assess each bend. A bend line perpendicular to the rolling direction generally provides better resistance to cracking than one parallel to it. Apply the radius guidance for the actual orientation; do not transfer an across-grain limit to a parallel bend or assume that a universal multiplier will correct it.

For parts with bends in different directions, evaluate every critical bend before selecting the blank orientation. Rotating the blank may improve one bend while making another more demanding. The most restrictive combination of radius, orientation, and local geometry determines whether the bend plan is acceptable. A larger common radius may resolve the conflict if the drawing permits it.

Where qualification bends are required, preserve the production material condition, thickness, rolling orientation, and edge preparation in the test pieces. Perform those bends only after verifying tooling feasibility and load ratings.

Remove burrs and inspect damaged or hardened cut edges before treating every crack as a tooling problem

Inspect the cut edges where they intersect the bend’s stretched outer surface. Burrs, rough sheared areas, and pre-existing edge cracks concentrate strain and can initiate failure even when the nominal radius appears acceptable. Deburr and prepare these edges according to the applicable process requirements before assessing bendability.

Deburring does not necessarily remove a hardened or heat-affected region left by the cutting process. If that region is suspect, establish an approved edge-finishing method or cutting-process change; merely smoothing the visible burr does not demonstrate that the affected material has been removed.

Use the crack location to guide the investigation. Cracks that begin at cut edges call for checking both edge condition and radius. Cracking concentrated in bends parallel to the rolling direction calls for reviewing orientation-specific limits. These patterns are diagnostic clues, not proof of a single cause.

Resolve conflicts between the drawing radius and material bendability before setting up the brake

Compare the radius range permitted by the drawing with the material’s supported minimum under the planned conditions. If the ranges do not overlap, stop the setup and obtain approval for a change: increase the inside radius, revise the blank orientation where doing so resolves the limitation, or select a material grade or condition with suitable bendability. Recheck affected dimensions and fit whenever the radius or material changes.

Record the accepted radius range and the conditions that support it. This range now constrains V-die selection: the opening must produce an acceptable radius, while the corresponding punch and die geometry must support the flange and permit the overbend required for springback compensation. A material-feasible radius is the first gate; compatible tooling is the next.

Select the V-Die and Punch as a Compatible System

Select candidate punch-and-die combinations that can produce the accepted radius, support the flange, and permit sufficient overbend to compensate for springback. A die opening selected from a tonnage chart must still pass these geometry checks before its load capacity becomes relevant.

For a practical next step, review the CNC bending equipment and related sheet metal solutions in the ADH Machine Tool downloadable materials.

Start near V = 8 × thickness for ordinary mild steel, then verify the resulting radius and flange support

For air bending ordinary mild steel, use this common starting estimate:

V ≈ 8t

Here, (V) is the die opening and (t) is the material thickness, both expressed in the same units. This estimate identifies a candidate opening; it does not establish that the tooling is suitable for the part.

For a hypothetical 2 mm mild-steel part, the initial opening is (8 \times 2 = 16) mm. Check the available die closest to that size against the accepted radius range and the supplier’s minimum-flange requirements. If either check fails, consider a different opening and repeat both checks. Changing the opening affects more than the force requirement.

Estimate the air-formed inside radius at approximately 0.16 × V for typical mild steel; verify it against material and tooling data

In typical mild-steel air bending, the inside radius forms approximately in relation to the die opening:

ri≈0.16V

For the hypothetical 16 mm opening, this gives (r_i \approx 2.56) mm, or approximately 2.6 mm. The rounded value reflects the estimate’s limited precision. Compare it with the radius range already accepted for the drawing and material.

The coefficient is an empirical starting rule, not a material-independent relationship. Grade, supplied condition, actual thickness, and tool contact geometry can alter the result. Use applicable material and tooling data to refine the estimate. If the predicted radius is close to an acceptance boundary, do not assume the rule is sufficiently accurate to approve production. After completing the load checks, measure the radius on representative trial bends.

Choose a punch nose that avoids local creasing; a larger nose can alter the assumed air-bending geometry

The punch-nose radius does not automatically become the finished inside radius. In air bending, the sheet spans the die shoulders and forms beneath the punch; the opening and material response influence the bend profile.

A nose that is too sharp for the material and thickness concentrates contact pressure and can cause local creasing or marking. Check the tooling supplier’s guidance for the proposed thickness, opening, and material instead of selecting the smallest available nose to force a tighter radius.

A larger nose also requires scrutiny. Once it significantly changes how the sheet contacts and wraps around the punch, the simple (0.16V) estimate may no longer describe the bend. Use guidance applicable to that nose-and-die combination, and include the resulting radius and part clearance in the trial inspection.

Check the minimum flange length against the die supplier’s geometry and maintain shoulder support throughout the bend

Compare the shortest flange with the die supplier’s minimum-flange requirement for the actual opening, shoulder geometry, and bend angle. Before comparing it with the drawing, confirm how the supplier defines flange length: a dimension to a theoretical sharp corner may differ from a dimension to a bend tangent.

The sheet must remain supported on both die shoulders as the bend develops. A short flange can lose contact and slip into the opening, causing angle variation, twist, or an inconsistent bend location. Initial contact with a shoulder does not demonstrate that support will remain adequate at full overbend.

Also check cutouts, notches, and angled edges near the bend. They can eliminate local support even when the nominal flange length appears sufficient. If support is inadequate, consider a narrower opening or suitable alternative tooling, then recheck the radius, punch compatibility, and force.

Confirm that punch and die angles permit the required overbend without interference or unintended bottoming

Springback opens the bend after unloading, so the angle under load must be more acute than the required finished angle. The geometry of both the punch and die must permit that loaded angle while maintaining the intended air-bending contact conditions.

For a nominal 90-degree bend, 88-degree tooling may provide some allowance for overbend, but the designation alone does not establish that the allowance is sufficient. Determine the expected compensation using applicable setup data, then check the complete tool profile and planned stroke. After load capacity has been verified, trial bends will establish the actual compensation.

Throughout the stroke, check clearance between the workpiece, punch body, die, holders, and any previously formed flanges. If the sheet seats against the die faces or encounters interference before reaching the required loaded angle, additional ram travel cannot be considered ordinary air bending. It changes the contact conditions and may sharply increase force. Select a compatible profile or revise the bend sequence before proceeding.

Widening the die reduces force but increases radius, flange requirements, and potential springback

For otherwise comparable air bends, a wider opening provides the forming force with a longer moment arm and reduces the required force. It also produces a larger estimated radius, requires greater flange support, and can increase springback and the overbend required to compensate for it.

In the hypothetical example, widening the opening from 16 mm to 20 mm increases the estimated inside radius from about 2.6 mm to 3.2 mm. This option is useful only if the larger radius remains acceptable, the flange continues to meet the supplier’s support requirement, and the punch-and-die combination provides sufficient overbend clearance.

Carry only geometrically feasible combinations forward into the force calculation. The next check must determine whether the machine and every loaded tooling component can withstand each candidate setup under the applicable load ratings.

Calculate Air-Bending Force and Verify Every Load Limit

Calculate the estimated air-bending force for each geometrically feasible setup, then verify that the press and every loaded tooling component can withstand it under the actual loading conditions. A bend can remain below the press’s total tonnage rating while still exceeding the limit of a punch, die, holder, adapter, clamp, or permitted concentrated load. Total capacity is only the first check.

Use F (kN) ≈ 1.42 × tensile strength (MPa) × bend length (mm) × thickness² (mm²) ÷ [1,000 × V (mm)] as an estimate

For conventional air bending, use:

F ≈ (1.42 × σu × L × t²) ÷ (1,000 × V)

Here, F is the estimated force in kN, σu is the tensile strength in MPa, L is the bend length engaged during the stroke in mm, t is the sheet thickness in mm, and V is the die opening in mm. Use the length actually being formed, not the installed tool length.

Tensile strength is the maximum engineering tensile stress reached during a tensile test. It is the material-strength input for this estimate. Keep it distinct from yield strength—the stress associated with the onset of permanent deformation—which is relevant to the later discussion of springback. Substituting yield strength changes the formula’s intended basis.

The units explain the divisor: MPa is equivalent to N/mm², so σuLt²/V yields newtons; dividing by 1,000 converts the result to kilonewtons. Because the coefficient 1.42 is empirical, dimensional consistency alone does not establish accuracy.

Confirm the formula’s coefficient and application using the tooling supplier’s chart

Check the supplier’s chart for its material-strength basis, units, bend angle, and applicable thickness and tooling ranges. If a chart is based on a stated reference strength, use the supplier’s correction method when the workpiece strength differs.

Some chart-based equations use a coefficient of 1.54 instead of 1.42. Do not average the coefficients or choose whichever produces the lower force. First determine whether the equations use the same units and apply to the same conditions, then follow the supplier’s applicable calculation method. Tooling guidance establishes the estimate and tool limits; the press manufacturer establishes the machine limits.

This relationship applies to conventional air bending. If the intended stroke brings the sheet into bottoming or coining contact, the loading mechanism changes, and this estimate no longer describes the full operation.

Convert explicitly: divide kN by 9.807 for metric tonnes-force or by 8.896 for US short tons-force

Use explicit force units when comparing calculations with ratings:

Metric tonnes-force = F in kN / 9.807

US short tons-force = F in kN / 8.896

For a hypothetical estimated load of 100 kN, these conversions yield approximately 10.20 metric tonnes-force or 11.24 US short tons-force. Both values describe the same force. A rating labeled only as “tons” must be clarified before comparison.

Recalculate for actual thickness and strength: force increases with the square of thickness and decreases as the V opening increases

With all other inputs held constant, force varies directly with tensile strength and engaged length, with the square of thickness, and inversely with die opening. A useful comparison is:

F₂ / F₁ = (σᵤ₂ / σᵤ₁)(L₂ / L₁)(t₂ / t₁)²(V₁ / V₂)

Thickness has a squared effect because increasing it raises the section’s resistance to bending. A wider opening increases the moment arm of the forming force, reducing the force required under otherwise comparable conditions.

For example, a hypothetical thickness increase from 3.0 to 3.1 mm raises the estimate by (3.1/3.0)², or about 6.8%, with strength, length, and opening unchanged. Use measured thickness and documented tensile strength. When production material spans an allowed range, evaluate credible upper bounds for thickness and strength rather than relying only on nominal values.

Check total press capacity, usable force at the working position, and manufacturer limits for concentrated or off-center loading

Check the calculated requirement against every applicable machine constraint:

• Total press capacity: the force the machine is rated to deliver under the specified conditions.
• Usable force at the working position: the force available at the required ram position or stroke condition.
• Concentrated loading: the force permitted over the actual loaded length and at the bed location.
• Off-center loading: the permitted force and load imbalance when the bend is offset from the machine center.

These checks address different mechanisms. A short bend concentrates force within a limited region of the bed and ram, while an off-center bend creates unequal loading across the machine. Neither condition is addressed merely by remaining below the total rated force. Consult the manufacturer’s applicable load diagrams or instructions before approving the setup.

Compare force per engaged length with punch, die, holder, adapter, and clamp ratings

When a component rating is expressed as force per unit length, calculate the average line load:

q = F/Lengaged

Using the hypothetical load of 100 kN over 500 mm gives (q = 0.20) kN/mm, or 200 kN/m. Compare this value with ratings expressed in matching units.

Check the exact punch and die profiles, holders, adapters, and clamps that carry the load, including any restrictions on segment length and mounting arrangement. A rating for a different profile or engagement condition does not establish capacity for this setup.

Average line load is a screening calculation. Interrupted engagement, uneven contact, or local features can cause uneven force distribution, so dividing total force by length does not replace the manufacturer’s guidance on local loading.

Use manufacturer capacity guidance rather than a universal safety factor or bottoming multiplier

Adding an arbitrary percentage to the estimated force cannot establish whether a particular tool profile, short segment, or off-center arrangement is permitted. Apply any reserve or derating specified by the relevant manufacturer under its stated conditions. If an applicable rating is unavailable, obtain it before trial bending.

A generic bottoming multiplier likewise cannot validate an air-bending setup or justify unintended bottoming. Changes in contact conditions require a method and capacity assessment appropriate to that operation.

Record the estimated requirement separately from the applicable machine and component limits. Then apply this calculation and the same capacity checks to two candidate die openings for a single 3 mm part. The lower-force option remains a candidate only if it also meets the established geometry requirements.

Compare Two Die Openings in a Worked 3 mm Carbon-Steel Setup

For this hypothetical setup, increasing the V opening from 24 to 30 mm reduces the estimated force by 20 percent, but increases the estimated inside radius from 3.8 to 4.8 mm. The wider die is a useful option only if the larger radius, flange-support requirement, and available overbend clearance are suitable for the part.

Assume conventional air bending of 3 mm carbon steel with a tensile strength of 450 MPa and 1,000 mm of continuously engaged bend length. These inputs provide a basis for comparison; they do not establish material bendability or machine and tooling capacity.

With 450 MPa tensile strength, a 1,000 mm bend, and a 24 mm V, estimate 240 kN and a 3.8 mm inside radius

The 24 mm opening is eight times the material thickness, making it a common initial candidate. Using the 1.42 coefficient from the preceding calculation method:

F = (1.42 R_m L t²) / (1000 V) = (1.42 × 450 × 1000 × 3²) / (1000 × 24) = 239.625 kN

Here, (R_m) is tensile strength in MPa; (L), (t), and (V) are the engaged length, thickness, and die opening in millimetres. The result is approximately 240 kN, equivalent to 24.4 metric tonnes-force or 26.9 US short tons-force, based on the unrounded force. Over the 1 m engagement, the average line load is approximately 240 kN/m.

This example assumes that the 1.42 method applies. If the tooling supplier specifies a different coefficient or calculation method, recalculate both candidates before comparing them with the applicable ratings.

Applying the earlier rough radius estimate, (r_i ≈ 0.16V):

r_i ≈ 0.16 × 24 = 3.84 mm ≈ 3.8 mm

This is an expected air-formed radius, not a guaranteed finished value or simply the punch-tip radius. The material condition and actual tooling combination still require confirmation.

With a 30 mm V-opening, the estimated force is 192 kN and the inside radius is 4.8 mm: 20 percent less force with a larger bend radius

With thickness, tensile strength, and engagement unchanged:

F = (1.42 × 450 × 1000 × 3²) / (1000 × 30) = 191.7 kN

This is approximately 192 kN, 19.5 metric tonnes-force, or 21.5 US short tons-force, with an average line load of 192 kN/m.

Because force varies inversely with the V-opening under this model:

Force reduction = 1 − (24 / 30) = 0.20 = 20%

Applying the same radius estimate gives:

rᵢ ≈ 0.16 × 30 = 4.8 mm

The wider opening provides a longer forming moment arm, reducing the required force while producing a larger estimated bend radius. Whether the force saving is usable depends on that change in geometry.

Compare both options against the minimum bend radius, drawing limits, flange support, and tool load ratings

Apply the following checks to each complete punch-and-die combination:

Check	24 mm V candidate	30 mm V candidate
Material minimum radius	Confirm that approximately 3.8 mm satisfies the documented bendability requirements for the material condition and bend direction relative to the grain.	Apply the same check at approximately 4.8 mm; the larger radius alone does not establish acceptability.
Drawing limits	Confirm that the allowable finished-radius range includes the expected result.	Reject this candidate if the drawing cannot accommodate the larger radius.
Flange support	Check the actual flange against the supplier’s minimum requirement for the selected profile and bend angle.	Explicitly recheck support: the wider opening generally requires a longer flange.
Punch compatibility and overbend clearance	Confirm suitable punch radius, tool angles, and clearance throughout the required stroke.	Confirm that the selected combination can achieve the necessary overbend without interference or unintended bottoming.
Machine capacity	Check approximately 240 kN against total capacity, the force available at the working position, and concentrated or off-center loading limits.	Apply the same limits to approximately 192 kN.
Loaded tooling components	Check approximately 240 kN/m against the applicable ratings for the punch, die, holder, adapter, and clamp.	Check approximately 192 kN/m against the ratings for this exact combination.
These line loads assume continuous, reasonably uniform engagement. Segment restrictions, interrupted contact, and localized loading still require the applicable manufacturer guidance. Also assess credible upper production limits for thickness and strength; nominal calculations do not automatically cover stronger or thicker material.

Neither candidate can be approved based on the supplied numbers alone: the material’s minimum radius, drawing tolerances, flange dimensions, clearance, and applicable ratings are still required inputs.

Select a feasible tooling combination before calculating the flat pattern or programming angle compensation

Select the 24 mm V if its tighter estimated radius is suitable for the part and the complete setup can support the higher load. Select the 30 mm V if the larger radius and flange requirements are acceptable and its tooling, clearance, and capacity checks pass. If neither meets all conditions, revise the tooling or part requirements before trial bending.

After selecting a feasible combination, calculate the provisional flat pattern using its expected formed geometry and an applicable bend allowance. Angle compensation cannot correct an incompatible radius or insufficient flange support.

Treat the selected setup as a trial candidate. Measure the unloaded angle, inside radius, and finished dimensions using representative material, then use the results to establish springback compensation and refine the bend allowance as needed. Release the production settings only after a confirming bend meets the drawing.

Set Springback Compensation and Validate the First Bend

Use the released part to establish springback compensation, then use its finished geometry to refine the flat pattern. The selected tooling remains a trial setup until the unloaded angle, radius, flange dimensions, and surface condition meet the drawing across the production bend length. Programmed overbend and finished-part bend allowance serve different purposes; keep them separate.

Expect springback to vary with yield strength, elastic modulus, radius-to-thickness ratio, and material condition

Springback is the elastic recovery that opens the bend after unloading. It generally increases with higher yield strength, lower elastic modulus, and a larger ratio of formed inside radius to thickness. Material condition also affects the response: prior cold work and lot-to-lot differences can change the required compensation even when the nominal grade and thickness are the same.

A tonnage calculation cannot determine the correction angle. It estimates forming force under stated assumptions but does not establish how much elastic recovery remains in the part. Treat any standard correction as an initial setting that must be verified by measurement.

Start with proven material data or the controller’s bend model; verify overbend using a representative coupon

Begin with a verified shop setting for comparable material, thickness, tooling, radius, and bend direction. If no such record is available, use the controller’s bend model with the actual material and tooling inputs. Confirm whether the control expects a finished angle, an angle correction, or a direct depth adjustment; entering compensation twice can cause excessive penetration.

Make the first coupon bend with this provisional setting. Keep the blank dimensions and setup traceable so the released result can support both angle correction and subsequent bend-allowance calibration. A stored setting becomes transferable evidence only when its underlying conditions are known.

Measure the released angle and adjust ram penetration in controlled increments within tooling and load limits

Measure after unloading with an angle instrument appropriate for the flange length and required tolerance. Under the included-angle convention, a released angle greater than the target is too open; a smaller angle is too closed.

• Too open: increase penetration by a small, controlled increment, provided the tools maintain clearance and the setup remains within its verified limits.
• Too closed: reduce penetration.
• At target: repeat the bend on fresh, representative material to confirm the setting.

After each trial, record the penetration or controller correction and the released angle. Use consistent measurement locations and methods so any apparent improvement reflects the bend rather than a change in inspection technique.

There is no universal depth increment. Follow the machine’s adjustment procedure and monitor actual force as penetration changes. Stop if further travel would exceed any applicable machine, tooling, or material limit, cause interference, or shift the setup into unintended bottoming. Repeatedly re-striking one coupon can aid exploration, but confirm the final setting on a fresh coupon using the intended production stroke sequence.

Use a coupon from the same material and grain orientation, then verify angle consistency across the full bend length

Cut the coupon from the production material lot, maintaining the same bend-line direction relative to the rolling grain and representative thickness, surface condition, and flange geometry. If these conditions differ, the coupon provides only provisional evidence.

A short coupon can establish an initial local angle correction. It does not reproduce full-length machine and tooling deflection or validate crowning—the compensation used to counter deflection along the bend. After confirming capacity for full engagement, make a representative production-length bend and measure the released angle near both ends and at intermediate positions, including the center.

A consistent error along the bend supports adjusting the overall angle setting. A varying error requires checking crowning, tool seating and alignment, and material variation before adding more global penetration. Record the measurement positions so subsequent trials can show whether the correction improved the entire bend.

Calculate BA as bend rotation in radians × (inside radius + K × thickness), using a K-factor calibrated to the formed geometry

Bend allowance, BA, is the developed length allocated to the curved portion of the finished bend:

BA = θ(R + KT)

Here, (θ) is the finished bend rotation in radians, (R) is the measured formed inside radius, and (T) is the material thickness. The dimensionless K-factor locates the neutral axis—the layer treated as having no longitudinal change in length—at a distance (KT) from the inside surface. When (R) and (T) are measured in millimetres, BA is also in millimetres.

K is not a fixed material constant. Use a value calibrated for comparable tooling and formed geometry. For a simple single-bend calibration coupon with a known blank length, determine the straight lengths from its edges to the bend tangency points. Subtract those straight lengths from the blank length to obtain an effective measured BA, then calculate:

K = BA/θ − RT

Do not directly subtract outside flange dimensions, because they include geometry beyond the tangency points. If inspection uses outside dimensions, convert them consistently using the applicable setback and bend-deduction geometry. Confirm the resulting allowance on another part before adopting it for production flats.

Use bend rotation instead of the included angle, and keep programmed overbend separate from finished-part bend allowance

For a finished included angle (α), with a flat sheet defined as (180°):

θ = (180° − α)π/180°

Consider a hypothetical finished bend with a (120°) included angle, a measured inside radius of (4.0) mm, a thickness of (3.0) mm, and a trial-calibrated value of (K = 0.40). Its rotation is (60° = π/3) radians:

BA = π/3 (4.0 + 0.40 × 3.0) ≈ 5.45 mm

Using the included angle of 120° directly would double the allowance in this example. Regardless of the temporary overbend required under load, the finished flat-pattern calculation uses the released 120° geometry. If the measured finished radius changes during setup refinement, recalculate the allowance using that radius and an applicable calibrated K-factor.

Verify the inside radius, flange dimensions, cracking, and marking before releasing the setup

Inspect a confirmation part made using the final settings and intended production sequence. Release the setup only when:

• Released angles meet tolerance at the recorded positions along the bend.
• The actual inside radius meets the drawing requirements and applicable material bendability requirements.
• Flange dimensions meet the specified datums and tolerances.
• The bend and edges pass the required crack inspection.
• Tool marks and surface damage remain within the specified acceptance criteria.

Use failures to guide the next change. A correct angle with incorrect flange dimensions points to the flat allowance, backgauge reference, or blank accuracy. Cracking requires reassessing the radius, material condition, grain direction, and edge quality; deeper penetration may make it worse. Excessive marking requires checking the contact surfaces and the suitability of the tooling or surface protection.

Record the material identification, tooling, angle correction or penetration, crowning setting, measured radius, calibrated allowance, flange measurements, and inspection results. This record provides evidence for troubleshooting later deviations and documents the conditions under which the setup was approved.

Troubleshoot by Symptom and Record the Proven Settings

Use the defect pattern to determine the next check. Change only one setup variable at a time and compare the results at the same inspection positions. If force or contact behavior indicates an overload, stop trial bending until the revised setup passes the applicable capacity checks.

Outside-radius or edge cracks: check material condition, bend radius, grain direction, edge quality, and punch-induced strain

Cracks on the outside radius indicate that tensile strain has exceeded the material’s local ductility. Compare the measured inside radius with the material’s bendability requirements, confirm the grade and condition, and check for prior work hardening. A bend line parallel to the rolling grain can increase the risk of cracking; where the part layout allows, consider bending across the grain.

For cracks that start at an edge, inspect for burrs, notches, and cut-edge damage. Determine whether a small punch tip is concentrating strain along the bend line. Correct the edge preparation or select a larger compatible radius as supported by the evidence; consider a wider V-opening only if the resulting radius, flange support, and clearances remain acceptable.

Uniformly open angle: verify the released-angle measurement, penetration, and available overbend clearance

Confirm that the error exists in the unloaded part and that the gauge follows the drawing’s angle convention. Then check the actual punch penetration, tool data, and stored angle correction. If the error is consistent along the bend, adjust the penetration or angle correction in a controlled trial, using the measured released-angle error as feedback. First confirm that the tools allow the additional overbend without interference or unintended bottoming. Retain the correction only after a confirming bend meets tolerance.

Angle variation along the bend: check crowning, deflection, alignment, tool wear, and thickness variation

Compare the center with both ends. A more open center suggests uncompensated ram or bed deflection; check the crowning against the actual load and bend length. An end-to-end difference points to tool seating, alignment, wear, or thickness variation. These patterns guide inspection but do not establish a single cause. Verify the suspected condition before making an adjustment, then repeat the measurements at the recorded positions. Increasing overall penetration can overbend acceptable areas without resolving the variation.

Excessive force or deep marking: check material inputs, die opening, local tool loading, and unintended bottoming

Recheck the measured thickness, material strength, engaged bend length, units, and installed V-opening against the force estimate. Inspect the punch and die contact surfaces, and confirm that the stroke still produces air bending. Unintended bottoming changes the loading conditions and invalidates an air-bending force estimate.

Compare the total force and force per engaged length with the applicable ratings. Short tool engagement or a specific step in the bend sequence can concentrate the load even when the total force is below machine capacity. Deep marking alone does not prove overload, but it requires checking the contact condition and local loading before increasing penetration.

Correct angle but incorrect flange dimensions: check the actual radius, bend deduction, and backgauge references

Measure the formed inside radius and compare it with the radius used for the flat layout. If they differ, review the calibrated bend deduction—the amount subtracted from the applicable outside flange dimensions to determine the flat length. Also check the blank dimensions, backgauge contact, and reference face used after each bend. Inspection must follow the drawing’s datums. Retain the accepted angle setting while determining whether the dimensional error originates from the developed length or the bend location.

Overload risk: reassess die width, approved bend sequence, tooling, or machine capacity before proceeding

Before making another stroke, evaluate a wider V-opening, a different compatible tool set, an approved bend sequence, or a machine with adequate capacity. Any sequence change requires a new load assessment; reducing the engaged length may still result in excessive local loading.

Recalculate the requirement and verify all applicable ratings for the machine, punch, die, holder, and adapter, including working-position and concentrated-load restrictions. After any tooling change, repeat the checks for radius, flange support, overbend clearance, and interference. Resume trials only after confirming that both geometry and capacity are acceptable.

Record the material lot, thickness, grain orientation, tooling, angle correction, crowning, bend deduction, and inspection results

Maintain a reusable setup record containing:

• Material: grade and condition, lot identification, measured thickness, grain direction relative to the bend, and edge preparation.
• Tools and sequence: punch and die identifiers, punch radius, V-opening, tool position and engaged length, and the approved bend sequence.
• Settings: penetration or angle correction, observed springback, crowning, backgauge references, and calibrated bend deduction.
• Verification: released angles and measurement locations, measured inside radius, flange dimensions, crack and marking inspection results, and applicable load checks.

Treat any material or tooling change as a reason for revalidation. Saved controller settings qualify as a proven setup only when the recorded combination of material and tooling produces verified angle, radius, and flange dimensions.