GlossaryKind of term: Manufacturing termSTEP model
What is a STEP model?
A STEP model is a 3D model in the neutral exchange format of ISO 10303, used to pass a part's shape between different CAD and CAM systems.
The file can hold a part or an assembly and usually carries the extension .step or .stp. The customer exports it from their own CAD system; you import it into your CAM system without needing the same CAD package. The file usually describes the shape as exact faces and edges (a B-rep), not as a triangle mesh like STL, although a STEP file can also carry a mesh. The design history and parameters of the original model are usually lost in the export.
What does a STEP model hold, and what not?
A STEP model gives a single nominal shape, without tolerance. Whether the designer modelled at nominal, at mid-tolerance or at a limit cannot be seen from the model. Threads are usually modelled cosmetically rather than as a helix, so a tapped hole arrives as a tapping-drill hole or as a cylinder at nominal diameter. Tolerances, roughness, material and finish are only in the file when the customer added them as PMI and the export carried them. On import, faces can go missing or edges fail to close; that depends on the modelling kernels at both ends and on the export settings.
What does the standard say about a STEP model?
STEP is ISO 10303, a series of standards. What a file can carry depends on the application protocol (AP) named in its header. AP203 and AP214 carry mainly geometry and product structure; colours, layers and a limited presentation of tolerances depend on the protocol and its edition. AP242, first published in 2014, brings the two together and extends support for PMI: dimensions, tolerances and datums, as presentation on screen or as semantic data that software can read. An AP242 file does not contain them automatically; the protocol only makes room for them.
How does a STEP model work in practice?
From the model you take the geometry for the estimate: outer dimensions for the stock, the volume to remove, the holes, pockets and faces, and the directions from which you reach them. What else sets the price is usually not in the model: a ⌀12 bore is the same cylinder in the model with or without H7 on the drawing. Whether that means reaming or boring and then measuring, you find on the drawing. Which document leads when model and drawing differ belongs in the request or the order; if it is not there, ask.
What is a STEP model often confused with?
A STEP model is not the technical drawing: the model gives the nominal shape, the drawing usually the requirements. Nor is it the original CAD file, which is the native format of the system it was designed in, with history and parameters; STEP is the neutral export of it.
How do you see a STEP model in Blake?
The STEP model goes into Blake together with the PDF drawing. Pairing per part follows the file name; where that is not conclusive, the pairing waits for your choice instead of a model being picked silently. More on requests.
Frequently asked questions
Can I estimate from a STEP model alone, without a drawing?
You can take shape, outer dimensions and stock from it, but usually not tolerances, roughness, finish or material. If you price anyway, state those assumptions in the quote and ask for the drawing.
Are the tolerances inside the STEP model?
Only when the customer put them in the model as PMI and the export carried them; AP242 is the usual protocol for that. Often they are only on the PDF drawing. Agree which of the two applies.
Why are there no threads in the STEP model?
Threads are usually modelled cosmetically, without a helix. The model then shows a tapping-drill hole or a cylinder at nominal size. Size, pitch, tolerance class and depth of the thread come from the drawing.