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CORE LINKS · DESIGN GUIDES

Plan sheet-metal dimensions

Distinguish folded dimensions from the blank length. Start by agreeing exactly where each dimension is measured.

A 90 degree sheet bend and its flat pattern. Straight legs A and B end at tangency; bend allowance BA is added.ABRt90°SectionABABL
Section example with one 90° bend. A and B are straight lengths ending at the bend tangent points. Add the neutral-axis bend allowance BA for the flat length.

What you will understand

  • Tangent versus outside dimensions
  • Bend allowance versus flat length
  • How hole pitch and tolerances affect assembly

Blue: defined conditions · Red: calculated quantities. Diagrams are schematic.

  • t: thickness / R: inside bend radius
  • A, B: tangent-to-edge straight lengths
  • BA: bend allowance / L: flat length

The examples below are illustrative. Calculator links open the normal calculator; enter the listed conditions yourself. Values are not transferred automatically.

Choose the dimension convention

The calculator supports tangent straight lengths or outside dimensions to the virtual apex. The apex is the intersection of extended outer faces; it differs from an inside dimension or a tangent length.

  • Define thickness t, inside radius R and bend angle θ
  • θ is rotation from flat; the inside included angle is 180°−θ
  • For outside dimensions, subtract setback SB from each leg

SB = (R + t) tan(θ/2)

Add bend allowance to obtain flat length

K locates the neutral axis as a fraction of thickness. It is not fixed by material name alone; establish it for matching material, thickness, tooling and forming conditions.

BA = θrad (R + Kt) / L = A + B + BA

Example: calculate from tangent straight lengths

Inputs

  • Straight lengths A = 40 mm, B = 30 mm
  • Thickness t = 2 mm, inside radius R = 2 mm
  • Bend angle θ = 90°; K = 0.40 (assumed)

Results

  • Bend allowance BA ≈ 4.398 mm
  • Flat length L ≈ 74.398 mm

The same shape has outside dimensions Aout = 44 mm and Bout = 34 mm. Entering 40 and 30 as outside dimensions describes a different part. K = 0.40 is illustrative, not a forming guarantee.

Check dimensions and hole assembly

Assembly dimensions can depend on bend angles and hole positions as well as lengths. Distinguish a signed linear dimension chain from a two-hole clearance check.

Example: both bolts float in both parts

Inputs

  • Part A pitch: 100 ± 0.1 mm
  • Part B pitch: 100.2 ± 0.1 mm
  • Minimum holes in both parts: 6.6 mm; maximum bolt diameter: 6.0 mm

Results

  • Worst pitch mismatch ΔP = 0.4 mm
  • Required offset per hole pair = 0.2 mm
  • Available offset = 0.6 mm → margin 0.4 mm

This is a one-dimensional model with optimal translation along the row. It excludes tapped holes, fixed pins, three or more holes, transverse error and positional tolerances. Margin does not certify the full drawing or real assembly.

Verify forming and finished dimensions

A flat length does not establish crack-free bending, tool access or hole shape. Understand one bend first; for multiple bends, also check interference and dimension references in sequence.

  • Material, temper, bend direction, minimum radius and springback
  • Tool compatibility, minimum flange, bend sequence and interference
  • Distance from bends to holes, cutouts and reliefs
  • Calibrate K/flat length from trials, then inspect the formed part

Keep a review checklist

This checklist is a personal reminder, not a safety score or an automatic approval. It is not sent or saved.

Continue with another guide

Reference information and illustrative calculations. This content does not guarantee suitability or safety of a design. Confirm the actual materials, component specifications and operating conditions.