What this viewer does
A STEP file holds the real geometry of a part, which is why it is the format shops send each other when the part has to be manufactured. The trouble is that you need CAD installed to open one, and that is a problem when a customer emails you a file and you just want to see what you are being asked to make.
This viewer solves that narrow problem. You drop the file in, it opens, and you can spin it around and read the overall size. It uses the OpenCascade geometry kernel compiled to WebAssembly, which is the same open source kernel behind several commercial CAD packages, so what you see is the actual geometry rather than a rough preview.
What you get
- The part in 3D, which you can rotate by dragging and zoom with the scroll wheel.
- The overall bounding box in millimetres, which is usually the first thing you want when you are checking whether a part fits your machine.
- The number of separate solids in the file, which tells you quickly whether you have been sent one part or a whole assembly.
- A button to save what you are looking at as an STL, if you want to print it or open it somewhere else.
When the file holds a whole assembly, the drawing that usually travels with it is a general arrangement drawing. It shows the overall size of the product and where it connects to its surroundings.
Why nothing is uploaded
Shops are careful with customer drawings, and rightly so. Most free online viewers work by uploading your file to a server you know nothing about. This one does not have a server to upload to. The geometry kernel runs inside your browser tab, so the file is read on your own computer and never crosses the network. You can disconnect from the internet after the page loads and it will still work.
About the STEP format
STEP is defined by ISO 10303, and the two versions you will run into are AP203 and AP214. AP214 carries colour and a little more assembly information, and both open here. The file extension is usually .step or .stp, and either is fine.
What makes STEP useful is that it stores real surfaces and solids rather than a pile of triangles. A hole in a STEP file is a cylinder that knows its own diameter. That is why CAM software can recognise features in it, and why STEP is the right thing to send when someone is going to machine your part. It is also a fair test for AI CAD tools: whether they give you a solid like this, or only a mesh of triangles.