SLDPRT, STEP, Parasolid or STL: what to send, and to whom

Someone has asked you for "the CAD file" and you need to know which one.

Most exchange problems are one decision made badly at the start. The question is never "which format is best" — it is "what is the other side going to do with it". Answer that and the format follows in one step.

Short answer. Send STEP unless you know otherwise: it is exact solid geometry that every CAD and CAM system reads. Send Parasolid (.x_t) to someone on SOLIDWORKS, Solid Edge or NX who asks for it, 3MF (or STL if they insist) for 3D printing, and OBJ or GLB for visuals. Avoid sending the native SLDPRT: the other side may not have the software, or a recent enough version of it.

Updated 2026-10-01 · first published 2026-09-08

The short answer

They are going to…SendWhy
Machine it, quote it, inspect itSTEPExact solid geometry every CAM and inspection system reads. The default for twenty years.
Keep modelling in another CAD systemSTEPThey get a solid body they can cut, mate and measure. No system inherits your feature tree, whatever you send.
Model in SOLIDWORKS, Solid Edge or NX specificallyParasolid (.x_t)Those systems share the Parasolid kernel, so nothing is translated on the way in or out.
3D print it3MF, or STL if they insist3MF records units, colour and separate objects. STL records none of those and causes scale accidents.
Render, visualise or put it in a game engineOBJ or GLBThey want a mesh with materials and groups, not a solid they would have to tessellate anyway.
Look at it and commentSTEP or a PDF/screenshotDo not make someone install CAD to give you an opinion.
Open it in RhinoSTEPRhino reads STEP surfaces directly and keeps them as NURBS.

When in genuine doubt, send STEP. It is readable by more systems than anything else on this list, it carries exact geometry, and nobody has ever been annoyed to receive one.

Why not just send the native file?

Sending a SLDPRT or SLDASM is the commonest mistake and it feels like the most generous option — you are sending the real thing, after all. It fails for three reasons.

They may not have the software
A machine shop runs CAM, not necessarily SOLIDWORKS. A client runs email. A native file is only useful to someone with that exact product, and often that exact version or newer.
Version compatibility runs one way
A newer file will not open in an older seat. "Please save it down to 2021" is a real message people send, and it costs a day each time.
You may be sending more than you mean to
A native part can carry your feature history, sketch dimensions, custom properties and configurations. That is your method, and it is not always what you intended to hand a supplier.

The last point cuts both ways and is worth thinking about deliberately. If you want a supplier to understand your design intent, native or Parasolid is generous. If you want them to quote a shape, STEP tells them exactly what they need and nothing else.

Exact, tessellated and recovered — the only distinction that matters

Every format on this page falls into one of two families, and every conversion between them is one of three kinds. Once this is clear, most exchange surprises stop being surprises.

Exact (B-rep): STEP, IGES, Parasolid, native CAD, Rhino NURBS
Geometry is described mathematically. A hole is a cylindrical face with a radius. Converting between two of these preserves the shape precisely — a STEP to IGES round trip keeps true planes and cylinders.
Mesh: STL, OBJ, 3MF, PLY, GLB
Geometry is a list of triangles. A cylinder is a fan of flat strips, and how many strips was decided when the file was written.
Exact → mesh (tessellation)
Always possible, always one-way. Fine when the destination needs triangles anyway. Keep the original: the triangles cannot become surfaces again.
Mesh → exact (surface recovery)
Produces a watertight solid with true surfaces: planes, cylinders, cones, spheres and tori are recognised in the triangles, and organic regions become smooth B-spline patches. Each surface is only as accurate as the mesh, and there is no feature tree — that was never in the mesh.

Ask what a "STL to STEP" tool actually returns. Some give one flat face per triangle, so a hole is hundreds of slivers. MeshWeave returns true surfaces — a hole is one cylinder you can measure. Neither is a rebuilt CAD model with a feature tree, and a service that implies otherwise is setting you up for a bad afternoon.

The five-second version

  1. Manufacturing or another CAD system: STEP.
  2. A Parasolid-kernel shop that asked for it: .x_t.
  3. Printing: 3MF, falling back to STL.
  4. Pictures: OBJ or GLB.
  5. Anything else, or you are unsure: STEP.
No account needed
Drop a CAD fileSLDPRT · SLDASM · STEP · IGES · Parasolid · 3DM · STL · OBJ · 3MF

On our servers. No account: deleted within 24 hours. Signed in: kept until you delete them. Never shared or used for training.

Questions5 answered
Is STEP or IGES better?
STEP, unless someone has specifically asked for IGES. Both carry exact geometry, but STEP is the current ISO standard, is better supported, and avoids IGES's long history of surface-only files that do not close into a solid. IGES remains correct when a CAM system or toolmaker names it.
What format should I send a machine shop?
STEP. It is what their CAM system reads and what they will ask for if you send anything else. Include a drawing or PDF for tolerances and finish — the solid carries the shape, not the requirements.
Should I send STL or 3MF for 3D printing?
3MF where the printer or bureau accepts it: it records units explicitly, keeps colour and separate objects, and is compressed. STL records no units at all, which is the classic cause of a part arriving at twenty-five times its intended size. Send STL when the other side specifically wants it.
Will STEP keep my feature tree?
No, and neither will any other exchange format. STEP transfers exact geometry, not modelling history. The receiver gets a solid body they can measure, mate and cut, but the sketches and features that built it stay in your system.
When is Parasolid the right answer?
When the other side works in SOLIDWORKS, Solid Edge or NX. Those systems share the Parasolid modelling kernel, so a transmit file goes in without any translation step, which occasionally survives better than a neutral format. Outside that group, send STEP.
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