Key takeaways
- A STEP file carries boundary geometry and, in AP242, optional PMI. Material, thickness tolerance, K-factor and flat pattern basis are supplied by the drawing or derived by the supplier.
- Measure wall thickness at five points before anything else: a model built at 2.00 mm against 1.92 mm coil moves the flat length about 0.1 mm per bend, or 0.8 mm over eight bends.
- One solid per part number. A file holding a body, a door and a bracket derives three flat patterns and quotes a part that does not exist.
- Leave the formed geometry at nominal. Bend direction, radius and flange are design intent; the bend deduction is a tool decision and belongs in the supplier's bend table.
A formed bracket opens in any viewer and looks finished. The same file tells a supplier almost nothing about how to make it: no grade, no thickness tolerance, no record of which way each bend runs, and no statement of what the flat pattern was derived from. The shape is the least ambiguous part of the handover.
STEP files for sheet metal are a geometry exchange, not a manufacturing instruction. The format carries boundary representation — faces, edges, vertices and the surfaces between them — and, in AP242, optional semantic PMI. Everything else a toolmaker needs to turn that solid into a routed part has to be encoded deliberately or supplied alongside.
The cost of getting it wrong is measurable. A model built at nominal 2.00 mm against coil that runs 1.92 mm shifts the flat length by roughly 0.1 mm per bend, which is 0.8 mm over an eight-bend enclosure. That is enough to miss a hole pattern that spans two mating panels, and it appears at first-off rather than at quotation.
What does a STEP file actually carry into a quote?
It carries the formed shape, exactly, at whatever scale the exporter used, plus optional annotations. It does not carry the material, the thickness tolerance, the K-factor that produced the flat pattern, or the reason the designer chose a 2 mm inside radius.
ISO 10303-242 defines the application protocol most CAD systems now write as AP242; it extends the older AP214 geometry set with managed model-based engineering, which is where semantic PMI lives. A tolerance typed into a 3D model as a graphic note is not semantic. A tolerance attached to a face as PMI is, and it can be read back by software rather than by a human zooming in.
The practical difference at intake is the number of questions. Geometry-only files are quotable when the drawing is complete. They are not quotable when the drawing says "see model" and the model says nothing.
What STEP files for sheet metal must carry: seven attributes
The table is the intake list, in the order it is checked. Order matters: a scale fault invalidates every measurement after it, so units come first and body count second.
| Attribute | What the supplier needs | Why it changes the part | Ganesh check or limit |
|---|---|---|---|
| Units and scale | Millimetres at 1:1 | A 0.1-scale export quotes a part one tenth full size | Thickness read first; 2.00 mm nominal must measure 1.95–2.05 mm |
| Body count | One solid per part number, each named | A file with three solids derives three flat patterns | Body count logged before any dimension is taken |
| Wall thickness | Constant within ±0.05 mm across each face | A 0.08 mm light wall moves the flat 0.1 mm per bend | Five points: four corners and centre of the largest face |
| Inside radius | At least 1.0 × thickness in mild steel | A 1.4 mm radius in 2 mm CRCA cracks on the outside of the bend | Radius matched to a punch that exists, not to the model default |
| Bend direction | Encoded as PMI or a bend table | Geometry alone leaves up and down ambiguous on symmetric parts | Direction recorded per bend on the setup sheet |
| Flat pattern | Labelled derived, or absent | Two flats 1.5 mm apart cannot both be cut | Formed solid controls unless the customer says otherwise in writing |
| Material and finish | On the drawing | Grey is not a grade; an unlabelled body prices as mild steel | No material, no quote |
Anything in that table that is missing moves the part to a query list rather than a quotation. A query costs a day; a wrong assumption costs a first-off batch of 50 and a drawing revision.
Units, scale and origin: the three faults that survive a visual check
A model at the wrong scale still looks like a bracket. That is why it survives review by eye and fails at the machine.
An inch model exported without conversion reads 25.4 times too large: a 2 mm wall becomes 50.8 mm and the thickness check catches it immediately. The dangerous case is the 0.1 factor, where a 1,240 mm panel reads 124 mm, the wall reads 0.2 mm and every proportion looks plausible. Nothing in the viewer's zoom level reveals it. Only the thickness and the overall envelope do.
Origin placement is the third fault. A solid modelled 400 mm from its own origin quotes fine and nests badly, because the CAM import inherits the offset. Put the origin on a datum that the assembly uses — a mounting face corner, a hole centre, a fold line — and the model, the drawing and the checking fixture then share one reference.
How bend direction is encoded, and what happens when it is not
A bend in a STEP file is a pair of tangent faces. There is no flag that says "up". The direction exists only in the feature tree that was exported away, or in PMI that was attached deliberately.
Symmetric parts hide the problem: a channel formed with both flanges up and the same channel formed with one up and one down are different parts, and the flat patterns differ by two bend deductions. A supplier working from geometry alone will pick the reading that looks conventional, which is usually the one where both flanges point the same way.
The inexpensive fix is a 2D flat pattern with bend lines annotated up or down, sent as a DXF alongside the STEP. The better fix is AP242 PMI: a bend annotation attached to the tangent faces survives the exchange and can be read by the receiving system. Whichever route you take, one bend direction must be stated per bend, not implied by a note that says "form as shown".
Radius follows the same logic. A model drawn with a 1.4 mm inside radius in 2 mm CRCA is asking for a radius below the material's forming limit, and no standard punch produces it. Mild steel and CRCA hold about 1.0 × thickness across the rolling direction; 304 stainless needs about 1.0 × thickness with the bend line across the grain and more if the bend runs along it. Model the radius the tool will make, or accept that the supplier will change it and the flat pattern with it.
Why the flat pattern is a derivation, not a file
The flat pattern is arithmetic, and the arithmetic depends on the tool. For a 90° bend the allowance is:
BA = π × (R + K × t) × θ / 180
With R = 2 mm, t = 2 mm, K = 0.44 and θ = 90°, the allowance is π × (2 + 0.88) / 2 = 4.52 mm. The setback is (R + t) × tan(45°) = 4.00 mm, so the deduction is 2 × 4.00 − 4.52 = 3.48 mm.
Change only the K-factor to 0.50, which many modellers use as a default, and the allowance becomes 4.71 mm with a deduction of 3.29 mm. The difference is 0.19 mm per bend, or 1.5 mm across eight bends. Change the die opening and the inside radius moves, which changes both the allowance and the deduction again. A 16 mm V die on 2 mm sheet gives an inside radius near 2.5 mm, not 2.0 mm, and the flat grows with it.
That is why the formed model should control and the flat pattern should be marked as derived. The formed geometry is what the assembly needs and what the customer measured. The flat is a manufacturing output that changes with the die, the grade and the coil.
Two further numbers decide how far the derivation can drift. The coil is one: the tolerance that IS 513 Part 1 allows on 2 mm cold reduced sheet is wide enough for the flat to move about 0.1 mm per bend between a heavy and a light coil, and the model cannot know which coil will be loaded. Batch size is the other. A 25-piece prototype can be cut from the derived flat and corrected on the second article; a 5,000-piece release has to be right in the file, because the nest and the tool plan are frozen with it.
The practical rule differs by volume. For prototype and low-volume work, send the formed model, mark the flat as reference and let the first article settle the last 0.3 mm. For a production release, agree the bend table before the drawing is issued so the flat in the customer's PLM and the flat in the supplier's CAM are the same geometry.
What stays on the drawing rather than in the model
A STEP file is a poor place for a material call-out, and a worse place for a tolerance class. Keep both on the drawing, where a human signs them.
- Grade and thickness, with the standard that governs the thickness tolerance. IS 513 Part 1 covers cold reduced carbon steel sheet and strip and sets the thickness and width tolerances that a nominal model ignores.
- Finish and colour, including whether the part is bare, phosphated, powder coated or wet painted, and which faces are cosmetic.
- General tolerance class. ISO 2768-1 covers linear and angular dimensions that carry no individual tolerance, so a drawing that invokes it does not need ±0.5 mm repeated on forty dimensions.
- The dimensions that cannot move: hole positions that mate with another part, flange heights that set a gap, and the overall envelope.
- Datum scheme, where the part is inspected in a fixture rather than on a table.
- Weld symbols and sequence notes for anything fabricated. ISO 2553 governs the symbolic representation of welds on drawings, and a weldment without symbols is a weldment designed by the welder.
The pattern is consistent: the model carries shape, the drawing carries intent and acceptance.
How Ganesh checks an incoming model
Files are accepted as PDF, DXF, DWG and STEP. A STEP intake runs the table above in order, because a scale fault invalidates everything downstream. The solid is opened at 1:1, the body count is confirmed, wall thickness is measured at five points, and each bend is recorded with its radius, flange length, direction and the tool planned to form it.
Bend review and flat pattern derivation are covered separately in STEP files for sheet metal bend review, which deals with what happens after the model is accepted rather than what it must contain. The cut file side is handled by the DXF preparation checklist.
Cutting runs on a 12 kW fibre laser with a 40 ft × 16 ft bed, supported by two 6 kW and two 3 kW machines, on mild steel, CRCA, stainless, aluminium, brass and copper. Forming runs on a 200 T, 5-axis CNC press brake with a 4,450 mm bed and a Delem control, working to 10 mm. Welded assemblies go to three CLOOS robotic MIG cells or to one of more than 30 manual MIG, MAG, spot and SPM stations, with fixtures built in-house.
Custom parts run from 0.8 mm to 40 mm, with ±0.5 mm as the typical sheet metal tolerance and tighter features agreed against the drawing. First article inspection is standard and PPAP documentation is available where the programme requires it. Manufacturing has run since 1992 across five units in Mumbai (Vasai) and Nagpur, under ISO 9001:2015 and ISO 14001:2015.
What to send with the model
- The STEP file at 1:1 in millimetres, AP242 if your CAD exports it, named with the part number and revision.
- The drawing revision that the model matches. If the two disagree, say which one is current.
- Grade, thickness and finish, even when they also appear on the drawing.
- Bend-critical dimensions marked, so the review knows which flange cannot move.
- The assembly or mating part number, so tool access can be checked in context.
- Any proven bend table for the grade, when the flat length matters to your build.
Send those six and the quote is built on the same numbers the shop will use. Send a model alone and the quote rests on assumptions that are written down nowhere.
When a model-only handover is the wrong route
A STEP file cannot define a part that is controlled by tolerance rather than shape. A welded frame with a 0.5 mm flatness callout on a machined pad, a bracket inspected in a customer's checking fixture, or a panel whose datum scheme does not follow the model origin all need the drawing and often the fixture definition.
It is also the wrong route for a surface model, for a file exported through a viewer rather than native CAD, and for a revision whose file name does not match the drawing. Ganesh does not reverse-engineer a part from a scan and does not release a flat pattern that contradicts the formed model without written approval. Those are limits, and they are cheaper to state now than to discover at first-off.
If the model is right, the route is quick: send it to the design and CAD hub reading list first if you are still choosing between processes, then put the file through the laser cutting capability review and request a quote with the drawing attached.
Frequently asked questions
Can a sheet metal part be quoted from a STEP file alone?
Yes for cutting and bending, provided the model is one solid at 1:1 in millimetres and the drawing states grade, thickness, finish and the critical dimensions. Anything welded, coated to a colour standard or inspected against a datum scheme needs the drawing as well.
Is AP242 better than AP214 for sheet metal?
AP242 can carry semantic PMI, so bend direction, datums and tolerances survive inside the file. AP214 carries geometry only. Both are accepted here, but AP242 removes a round of questions when the part has more than four bends.
Why does the same STEP file show a different number of bends in two CAD systems?
Because a bend exists in the file only as a pair of tangent faces. Where two faces meet at a very shallow angle, one system reads a fold and another reads a flat with a crease. A part with 14 real bends can read as 13 on import, and the flat pattern then loses one deduction.
Should the flat pattern be in the STEP file?
Only as a labelled body, marked derived. The formed solid is what the assembly needs; the flat pattern changes with the die opening and the material batch. Send both and say which one controls, or send the formed model and let the supplier derive it.
What file size is too large for a quote?
Size is a symptom, not a limit. A 300-face enclosure normally arrives under 2 MB as exact geometry; a 40 MB bracket is usually a mesh export that no bend review can measure. Strip assemblies and send the single part.
Sources and further reading
- ISO 10303-242 — Industrial automation systems and integration: managed model-based 3D engineering (AP242, the protocol that can carry semantic PMI) · reviewed 3 Nov 2026
- IS 513 Part 1 — Cold reduced carbon steel sheets and strips, which sets the thickness tolerance a nominal model ignores · reviewed 3 Nov 2026
- ISO 2768-1 — General tolerances for linear and angular dimensions where the drawing gives no individual tolerance · reviewed 3 Nov 2026
Send the model before the drawing is frozen
Attach the STEP file, the drawing revision and the bend-critical dimensions. We check units, body count and thickness, then return the flat pattern and a list of features that cannot be formed as modelled.
Send a STEP file