A supplier wants a STEP file and all you have is an IGES — or a legacy machine shop wants IGES and your model is a STEP solid. Converting between the two looks like a one-click Save As, and sometimes it is. Done carelessly, though, an IGES-to-STEP or STEP-to-IGES conversion can quietly cost you the solid body, the assembly structure and the tolerances, and the problem only shows up when CAM or the recipient’s importer rejects the file.

This guide is about the conversion itself: the steps we follow in each direction, how to check the result before it leaves your desk, and the tools that do the job. If you are still deciding which format to send in the first place, start with our IGES vs STEP comparison and come back here when you need to convert.

What actually changes when you convert

Three things decide whether a conversion is clean. Body type: IGES carries surfaces and wireframe entities, so a part can arrive as loose surfaces rather than one closed solid; STEP (ISO 10303) is built to carry a true boundary-representation solid. Structure: STEP keeps assembly hierarchy; IGES flattens an assembly into geometry. Manufacturing data: STEP AP242 can carry tolerances and annotations (PMI); IGES cannot, so PMI never survives a trip through IGES. Every step below exists to protect one of those three. (The .igs and .iges extensions are the same format, as are .stp and .step.)

How to convert IGES to STEP

Import with surface knitting turned on. In SolidWorks, Inventor and most mid-range packages the IGES import options include an automatic knit or “try forming solids” setting. Turn it on.
Check the body type before anything else. Confirm the import produced a single solid body, not a surface body or a stack of surfaces. A model that looks complete on screen can still be open.
Heal the gaps. If it came in as surfaces, run the package’s import diagnostics or knit tool. Adjacent surfaces that fail to meet within the stitching tolerance leave gaps; adjusting the knit tolerance is often the difference between a clean solid and a failed heal. Gaps that no automatic heal can close have to be rebuilt locally.
Confirm units and scale. Measure one known dimension. A part that imported in inches instead of millimetres is easy to miss until a drawing looks wrong.
Export to the STEP protocol the recipient expects. AP214 is the most common request from Australian shops; use AP242 when tolerances need to travel with the model.
Re-open the STEP you just wrote. Import it fresh and confirm it is still one solid with the right volume. This is the only check that proves the file you are sending is the file you think it is.

How to convert STEP to IGES

Ask what the recipient’s system wants inside the IGES. Some legacy CAM and surfacing tools want trimmed surfaces only; others accept IGES solid entities. The export option you pick depends on that answer, not on your own software’s default.
Expect to lose structure and PMI. An assembly becomes flat geometry and tolerances are dropped. If either matters, send a PDF drawing alongside, or keep the STEP as the master.
Export as IGES 5.3 unless the recipient names an older flavour, and keep the units explicit.
Re-import the IGES and look for missing faces. Small fillets and tangent edges are where trimmed surfaces most often drop out.

A real conversion that went wrong — and what it taught us

On a recent mechanical assembly job for a Brisbane manufacturer, we received the parts as IGES files. The client’s own SolidWorks installation did not have a working IGES import path that produced usable solids, so they needed the parts as STEP before they could do anything with them. On the surface a trivial conversion — open the IGES, export STEP. In reality, several of the parts came across as disconnected surface bodies rather than solids, because the original IGES export had never stitched the surfaces into a closed volume.

The fix was not to re-export and hope. We ran each part through a knit/heal step: importing the IGES surfaces, identifying the gaps where adjacent surfaces failed to meet within tolerance, knitting them into a closed boundary, and only then exporting to STEP as a true solid. Two parts had genuine geometry gaps that no automatic heal could close, and those had to be rebuilt locally before the solid would form. The lesson we took from it: always verify that an IGES import produces a single solid body before converting downstream. The failure only surfaces when CAM or a Boolean operation rejects the part.

Common conversion pitfalls

Surfaces instead of solids. The single biggest issue in IGES-to-STEP work. Always check the body type after import.
Collapsed assemblies. Converting an assembly to IGES flattens it. If structure matters, stay in STEP.
Knit tolerance set wrong. Too tight leaves gaps; too loose can merge faces that should stay separate.
Units drift. Confirm the unit system on import, every time.
PMI loss. Tolerances and annotations do not survive IGES. If you need them, convert within STEP to AP242.

Tools that do the conversion

SolidWorks / Inventor. Open the source file and Save As to the target format. Both expose import options for knitting surfaces into solids — turn that on and verify the result.
AutoCAD. Imports and exports both formats; best suited to simpler or 2D-derived geometry rather than complex surfacing.
FreeCAD. A no-cost option that reads and writes IGES and STEP through its OpenCASCADE core. Useful for one-off conversions when no commercial seat is free.
Online converters. Fine for a quick, non-confidential single part. Avoid them for anything under NDA, or for assemblies where structure must be preserved.

Before you send: a five-point check

The converted file re-opens as the body type the recipient expects (solid for STEP, the agreed surface or solid entities for IGES).
One known dimension measures correctly, so units and scale are right.
Part count matches the source assembly, or the recipient knows it has been flattened.
Any tolerances that must travel are in STEP AP242 or on an accompanying drawing.
A one-line note goes with the file: units, source CAD system, and solid or surfaces. A surprising number of “format problems” are really a part that imported at the wrong scale, or a recipient who did not know what to expect.

What Australian shops tend to expect

Across Australian manufacturing and fabrication, STEP has become the default request: most CNC shops, laser cutters and fabricators ask for STEP first because their CAM software ingests it cleanly as a solid. AP214 is the version you will be asked for most often; AP242 is appearing more where tolerancing needs to travel with the model, particularly in defence and aerospace-adjacent work. Requests to convert back to IGES now come mainly from older equipment and tooling suppliers on legacy systems, and from industrial-design workflows where freeform surface data is the point.

If you would rather hand the whole problem to a team that converts these formats every week, that is exactly the kind of work we do. See our CAD conversion services and 3D CAD conversion services, or our guide to converting PDF drawings to DWG. Send us a sample file and we will tell you exactly what is in it and what it will take to get it into the format you need.

How to Convert IGES to STEP (and STEP to IGES) Without Losing Data