Preparing your STEP file: designing a welded part that is easy to make

1. The STEP export: the complete assembly, as solids

The STEP format (.stp or .step) is read by every CAD package and keeps the exact geometry. Export the complete assembly, not a single part or a PDF drawing: it is the relative position of the components that lets us find the welds.

• Version: AP214 or AP242, both work. Units in millimetres.
• One body per component: each plate, tube or section is a separate solid. A weldment drawn as a single block is accepted (our software splits it into elements), but the check takes longer.
• Real thicknesses: no surfaces or zero-thickness plates. The modelled thickness is the one that will be cut.
• No hardware: remove screws, nuts and bought-in parts, or put them in a separate sub-assembly.

2. Gaps and bevels: draw the part as it will be welded

For a fillet weld (the most common case: stiffener on plate, tube on plate), the parts must be in contact. A large gap has to be filled with filler metal, lengthens the cycle and increases distortion.

For a butt weld on thicker material, the joint preparation (bevel, root face, root gap) determines penetration. EN ISO 9692-1 gives the recommended preparations by thickness and process: model them if you need them, or flag the joint and we will propose the preparation.

3. Tabs and slots: the part positions itself

Jigs are expensive for small batches. Laser cutting lets you do without them: tabs on one component that fit into slots in the other fix the position before tacking. That is what makes an assembly quick, whether it is placed by a robot or by a welder.

• Allow a cutting clearance in the slots. It depends on the thickness and the machine: ask your cutter for the value, often in the order of a tenth of a millimetre per side.
• Keep the tab no longer than the thickness of the plate it goes into, so it does not stick out where a weld runs.
• Make the assembly foolproof: an offset tab or one of a different width prevents a part being fitted the wrong way round.
• Do not put a tab under a continuous weld: it creates an irregularity in the joint.

We go into these rules in the article designing a welded part for laser cutting.

4. Torch access: every weld must be reachable

A welding torch has a nozzle about two centimetres across and must reach the joint at an angle close to 45° for a fillet weld. A weld at the bottom of a closed corner, under a shelf or between two close walls is difficult for a welder, and sometimes impossible for a robot.

• Leave clearance around each weld, at least the width of the nozzle plus a margin.
• Avoid closed boxes welded from the inside.
• If a weld cannot be reached once the part is assembled, the assembly order may make it possible: that is what our software plans.

During the check, our software tests torch access along each weld. An inaccessible section is flagged: either the model is changed or that section is welded by hand.

5. Limiting distortion at the design stage

Metal heated by the arc shrinks as it cools. A few design choices reduce this effect:

• Do not oversize welds: a larger throat means more heat and more shrinkage.
• Spread welds symmetrically about the part's axis.
• Consider intermittent welds where strength allows.
• Tacking, the small welds that hold the part together before welding, plays a big role: see why tack welding determines quality.

6. What to tell us with the file

The 3D model contains the geometry. A few pieces of information remain, in the app form or in a note:

• the material (S235, S355);
• the weld throat, if it is set by a calculation;
• functional dimensions and their tolerances;
• the expected finish (raw, shot-blasted, painted, galvanised: in the last case, add vent holes to closed tubes);
• the quantity and ordering rhythm.