Guide 03 / 06 · 5 min read

Bend deduction in aluminium composite: why “deduction = thickness” is only a default

ACM 3D Studio · Published 21 September 2026

01What the deduction removes from the developed size

The real developed length of a folded panel is shorter than the sum of its faces. At the fold, the material does not follow the sharp corner of the drawing: it turns around the residual skin and “consumes” less than the two outer dimensions. The difference, per fold, is the deduction. On the default flat cassette — 600 × 400 mm, returns 40 mm, tabs 20 mm — each direction crosses four folds (two returns, two tabs):

developed = Σ lengths − Σ deductions
          = 600 + 2 · (40 + 20) − 4 · 4 = 704 mm   (width)
          = 400 + 2 · (40 + 20) − 4 · 4 = 504 mm   (height)

The engine splits each deduction in two halves: each face loses r / 2 along the fold edge, then the whole sub-tree of the child face is translated by −r towards the parent so the two trimmed edges meet on one junction line. The groove axis ends up r / 2 on the parent side, where the two edges touch. This is why a drilling margin must cover half the deduction: the fold axis moves into the face.

02Why “deduction = thickness” is only a default

The deduction depends on the machine, the material and the groove depth — hence on the cutter angle, the residual skin and the sheet batch. The engine does not hard-code it: it reads it from the material profile. When the table is empty it falls back to deduction = thickness (4 mm for ACM 4 mm) and writes it on every part: DEDUCTION_UNCALIBRATED, “run the calibration wizard before a batch”. The cut sheet carries the note “deduction NOT calibrated”.

That default is an order of magnitude, not a measurement. On a cassette with four folds per direction, a 0.5 mm error per fold makes 2 mm on the finished dimension: enough for a 10 mm façade joint to become 8 or 12.

03The test-piece method

The calibration coupon is a U-shaped test piece: a 60 mm wide strip, 2 folds at the same apex angle θ, three faces of nominal length L = 80 mm. It is cut with the assumed deduction r0 of the current profile, frozen at download time; a U<angle> mark is engraved on the REPERE layer to recognise it once folded.

1. Pick the cutter in the studio wizard — cutters used by the open family and not yet calibrated are flagged — and download the test piece.
2. Machine the piece and fold both folds into a U.
3. Measure with a caliper only the middle face, on the outside, from one fold to the other.
4. Type the measurement: the wizard states the gap plainly (“folds 0.4 mm longer than planned”) and records it as a calibration point of the material profile, replacing the point at the same angle.

middle face after folding = L − r0 + r
r = r0 + (measured − L)

example: L = 80 mm, r0 = 4 mm, measured = 79.3 mm → r = 4 + (79.3 − 80) = 3.3 mm

general method, k folds, sum of the k + 1 faces:
r = r0 + (measured − (k + 1) · L) / k

Each fold gives its deduction back equally to the two faces it joins: the V-groove is symmetric, and the engine trims r0/2 on each side when cutting. The middle face of a U touches two folds, so it carries a whole deduction: one measurement is enough, with no addition or division. One measurement, one unknown, exact in one step — which is why there is still one test piece per cutter angle.

04How the table is read

Each calibration point carries an apex angle, a deduction and a date. Between two points the engine interpolates linearly. Above the largest calibrated angle it clamps to the last point. Below the smallest, it goes down towards (0°, 0 mm): a fold of zero angle consumes nothing, and clamping to the smallest point would give a 10° fold the deduction of a 45° fold.

05The two cases where the deduction is not applied

• Cyclic pattern (continuous sheet, origami, closing folds): no sub-tree can be shifted without breaking the cycle. The pattern is machined at design size and DEDUCTION_SKIPPED_CYCLIC states how much the finished part will shrink per fold.
• Apex folds of a relief cassette: the fan of facets around a vertex would each shift in its own direction. These folds are a few degrees, their deduction is under a millimetre: DEDUCTION_SKIPPED_APEX. Returns and tabs keep their full deduction.

06What the deduction changes elsewhere

The corner relief radius is max(tool radius, r / 2 + clearance): max(3; 2 + 0.5) = 3 mm with the Ø 6 mm contour tool, the default deduction and the 0.5 mm clearance. A return must exceed groove width + r / 2 = 7.2 + 2 = 9.2 mm on 4 mm, otherwise GROOVE_TOO_DEEP. A drilling margin must cover r / 2 + half groove + 1 mm, otherwise DRILL_IN_GROOVE. See returns, tabs and corner reliefs and V-groove.

Read next: the coupon calibration, step by step.