Model-based definition for sheet metal: PMI over a 2D drawing

A bracket with 40 features can carry 60 or more annotations: dimensions, datum references, position tolerances, bend notes, a material callout and a finish. On a drawing they are printed once and read by everyone. In a 3D model they exist as text floating next to faces, and if nobody attached them to the geometry as data, a programmer will re-enter all 60 by hand — which is the same as saying the model did not carry a definition at all.

Model-based definition (MBD) is the practice of putting the definition in the model: geometry, dimensions, tolerances, notes, material and finish, attached to the features they describe, so that the model is the authority and any 2D view is derived from it. For sheet metal the practice runs into three specifics that general MBD advice skips: bends are features with a radius, an angle and a K-factor; thickness is a dimension as much as a geometry; and the flat pattern is a derived representation that the press actually runs.

This guide covers what semantic PMI has to contain for a formed part, what STEP AP242 carries, how to define bends so the blank comes out right, and how the tolerance arithmetic changes when a bilateral note becomes a profile callout. The rest of the design rules sit in the design and CAD hub.

What does model-based definition change for a formed part?

The change is one of authority: the model becomes the controlled document, and the drawing becomes an output. Everything else follows from that. A dimension on a drawing is read by a person and typed into a machine; a tolerance in a model can be read by software and used directly.

For a sheet metal part the change is smaller than it sounds, because the manufacturing file was never the drawing. The laser and the punch run a flat pattern DXF, the press brake runs a bend program, and the welder works from a fixture and a weld drawing. MBD formalises what already happens: the model and its derived files are the manufacturing definition, and the drawing was always a communication layer on top.

What MBD genuinely improves is the inspection hand-off and the change hand-off. A characteristic read from semantic PMI carries its datum reference and its material condition, so the CMM program does not have to reinterpret a frame that was retyped. And a revision that moves a face moves the tolerance with it, which a 2D view regenerated from the model also does — but only if the view was regenerated, which is the failure mode MBD removes.

What the model has to carry

Semantic, not graphic

A tolerance that is only drawn is a picture of a tolerance. Semantic PMI stores the value, the zone shape, the datum references and the material condition as structured data, so a downstream system can query it. The difference shows up the first time a CMM programmer tries to use the model: semantic data populates the inspection plan, graphic data has to be read off the screen and typed.

The trap is that both look identical in a viewer. A model can be fully annotated, pass a visual review, and contain nothing a machine can read. Ask the supplier or the customer to confirm which one they are sending, and confirm it in the format rather than in the viewer.

Bends, thickness and the flat pattern

A bend in a model needs to be a bend feature, not a folded piece of geometry. The feature carries the radius, the angle, the direction and, in a sheet metal environment, the bend allowance rule that generates the flat pattern. Without that, the flat pattern is whatever the modelling kernel guessed, and the blank size will disagree with the supplier's calculation.

The K-factor is the number that makes the two agree. It depends on the material, the radius-to-thickness ratio and the forming method, and it is not a constant: 0.33 is a common starting value for air bending, 0.44 is often used for a nominal calculation, and the real value for a given tool and material is established by a bend trial. The bend allowance calculation sets out the arithmetic, and the important point for MBD is that the K-factor has to travel with the model in a bend table, not sit in a designer's head.

The flat pattern in the model is then a derived representation. It is useful for reviewing bend relief and hole-to-bend clearance, and it is not the file the laser runs. Issue the manufacturing flat pattern as a DXF with its own revision, and treat any disagreement between the model's flat and the DXF as a revision error rather than a tolerance.

The worked example: a ±0.5 mm note is a 1.0 mm zone

Leaving the drawing changes the arithmetic of tolerance zones, and this is where a converted drawing most often goes wrong.

A bilateral tolerance of ±0.5 mm on a surface allows the surface anywhere within 1.0 mm total. A profile-of-a-surface callout of 0.5 mm allows it within 0.5 mm total. Converting the first into the second by writing "profile 0.5" halves the allowed zone, and a process that held ±0.5 mm will now produce rejects. The equivalent conversion is profile of a surface 1.0 mm, and it should be written as such if the intent was the bilateral note.

The second conversion is position. A ⌀6.0 +0.2/−0 hole with a position tolerance of ⌀0.3 at maximum material condition gets a bonus as the hole grows: at ⌀6.2 the bonus is 0.2 mm, so the total allowed deviation from true position is ⌀0.5 mm. A hole whose axis sits 0.2 mm off in X and 0.15 mm off in Y has a radial deviation of √(0.2² + 0.15²) = 0.25 mm, which is a diametral position of 0.5 mm — exactly at the limit, passing only because of the bonus. Programmed against the nominal ⌀6.0 with no bonus, the same hole fails.

Neither conversion is exotic. Both are invisible on a drawing that says ±0.5 and ⌀0.3 M and both become explicit when the model carries semantics, which is the argument for MBD rather than against it.

Free state, restraint and the flattest part in the pack

Sheet metal brings one tolerance question that a machined part does not: a thin panel is not flat when nothing holds it. A 1.0 mm panel 1,000 mm long, laser cut and stress-relieved only by the cut itself, will sit with a wave of 1–3 mm across its face, and every dimension taken from that face depends on where it is supported.

A model that carries a flatness or profile callout without saying how the part is held has not defined the requirement. The callout has to state the restraint: free state, or clamped at named points with a stated force. Free state is the simpler contract because it can be checked on a surface table without fixtures, but it pushes the requirement onto the process: a 0.8 mm panel will not meet a 0.5 mm flatness callout in free state without a straightening operation or a stiffening rib.

The practical answer for most parts is to tolerance the features that matter — hole positions and the sealing face — and leave the free-state flatness to a general note at a value the process can hold, typically 1 mm per metre of panel for 1.0–1.5 mm sheet. A model can carry that as an annotation on the face, and an inspector can measure it on a table.

What Ganesh runs with a model-based definition

Ganesh accepts PDF, DXF, DWG and STEP. The STEP is read for geometry, the DXF for the flat pattern, and the PDF for anything the model does not carry — which today is most notes, weld symbols and the coating specification. That is the honest description of the route: a model-based hand-off works when the model carries the definition, and most customer models still arrive with geometry only.

Cutting runs on the 12 kW fibre laser with its 40 ft × 16 ft bed or on the 6 kW and 3 kW support machines, forming on the 200 T, 5-axis brake with its 4,450 mm bed and Delem control, welding on three CLOOS robotic MIG cells or at the 30+ manual stations. Press tools, welding fixtures and checking fixtures are built in-house, and first article inspection reports and PPAP documentation are issued against the revision received.

What we do not do is program a CMM directly from semantic PMI. Inspection is planned from the drawing characteristics and the checking fixture, and the FAI report lists what was measured and against which revision. A customer who needs an inspection programme generated automatically from the model should say so at quotation, because it changes the hand-off rather than the part.

What to put in the model

• The material grade and standard as a model attribute, plus the thickness with its tolerance.
• Every bend as a bend feature, with radius, angle, direction and the bend table that holds the K-factor.
• Datum features on the faces that locate the part in assembly, referenced by the tolerances that follow.
• Semantic tolerance frames, not text, for every characteristic that will be inspected.
• Notes attached to faces or to a named annotation view, readable from one direction.
• A revision property on the model, matching the revision on the DXF and the inspection report.
• The flat pattern marked as derived, with the manufacturing DXF issued separately and revision-stamped.
• The coating system, the dry film thickness and the masking list, because a model rarely carries finish data well.

The STEP and DXF hand-off is covered in STEP files for sheet metal bend review, and the note side in the ten drawing notes that prevent rework. If the model is the deliverable, send it through the quote form with the DXF and the finish specification so the review covers all three files.

When a 2D drawing is still the right document

A drawing is still the right controlled document in four situations. Where the supplier or the inspector cannot consume PMI, a model without a PDF is a definition that stops at the gate. Where the part is welded, the weld symbols and the sequence are easier to read on a sheet than in an annotation plane. Where the contract is the drawing, the drawing is the legal object, and a model referenced only by a filename is weaker evidence. And where the part is simple enough that the drawing is one page, the conversion to MBD costs more than it saves.

The practical arrangement for most sheet metal work is both: a model that carries the geometry and the tolerances, a PDF generated from that model for reading and for the contract, and a DXF for the flat pattern. The model is authoritative, the PDF is a view, and the DXF is the manufacturing file. Any of the three that drifts out of step with the others is a revision error, and the revision property on all three is what keeps them together.