Read this before you rely on the result
Converting a mesh to STEP is not the same thing as converting STEP to a mesh, and any tool that suggests otherwise is being loose with the truth. Going from STEP to STL throws information away, which is easy. Going from STL to STEP has to invent information that is not in the file, which is not.
An STL file contains nothing but triangles. It does not know that a particular ring of triangles used to be a hole, or that a group of them used to be a fillet. So when you convert an STL to STEP, one of two things happens. Either the tool writes each triangle out as a flat face, which is honest but leaves you with a faceted solid, or it tries to recognise shapes and fit real surfaces to them, which is genuinely difficult, needs a clean mesh, and can quietly get things wrong.
This converter does the first one. Every triangle in your mesh becomes a planar face in a proper STEP solid. The file opens in SolidWorks, Fusion, Inventor, FreeCAD and anything else that reads STEP, and the geometry is exactly the geometry you gave it, to the last decimal.
What that is good for
- Getting a scanned or downloaded mesh into a CAD assembly so you can position other parts around it.
- Sending a shape to someone whose software will not take an STL.
- Using a mesh as a reference body while you model the real part properly around it.
- Quoting and planning, where you need the shape and the size rather than editable features.
What it will not do
- It will not give you back a parametric model with a feature tree you can edit.
- It will not turn faceted curves back into smooth cylinders and fillets, so a hole will still be a many-sided polygon.
- It will not be pleasant to machine from directly, because CAM will see thousands of tiny flat faces instead of one pocket.
If you need a real parametric model, the honest answer is that someone has to remodel the part in CAD using the mesh as a reference. That is normal work and it is what most shops do. Text to CAD tools are a separate route, which generates a new solid from a written description instead of repairing a mesh.
Solids and surfaces
Before writing the file, the converter checks whether your mesh is watertight, meaning every edge is shared by exactly two triangles with no gaps. If it is, you get a closed solid, which is what you want and what CAD will treat as a real body. If it is not, you get a surface model instead, and the tool tells you which one you got.
An open result is not a bug in the converter, it means the mesh itself has holes. Scanned meshes and meshes that have been through a lot of editing are often not watertight. Repairing the mesh first, in something like Meshmixer or Blender, and then converting will give you a solid.