How to turn a STEP file into an SVG line drawing

Search for "STEP to SVG" and most of what comes back are format-converter pages. They tend to turn the model into a mesh of triangles and trace that, which gives you thousands of short straight segments where there should be a circle, and edges where the triangles meet rather than where the part has edges. That is not what anyone wants in a manual.

What you want is a projection: the exact geometry in the STEP file viewed from one direction, with the edges you can't see either removed or drawn dashed. CAD people call this hidden-line removal (HLR). Every route below does that from the real surfaces, so arcs stay arcs.

What good output looks like

• Circles and arcs are true arcs in the SVG (A commands or curves), not polylines.
• Hidden edges are either gone or on their own layer or class, so you can dash or delete them in one go.
• The outline of curved faces (the silhouette of a shaft or a fillet) is drawn, even though it isn't an edge in the model.
• The drawing is in real units on a sheet, so a line weight in millimetres means something when printed.
• Each part is its own group, so you can recolour or remove one part without picking through loose paths.

Check any tool against that list on a part with holes and fillets before you commit to it for a whole manual.

Option 1: FreeCAD TechDraw (free, desktop)

FreeCAD is open source and reads STEP. Its TechDraw workbench makes projected views from the exact geometry and can export a page as SVG.

1. 01

   Import the STEP file

   Use File → Import… and pick the .step or .stp file. Large assemblies take a while; let it finish before rotating the model.

2. 02

   Add a page

   Switch to the TechDraw workbench and insert a page from a template (A4 or A3 landscape suits most illustrations).

3. 03

   Insert a view

   Select the parts in the tree (or the top-level part that contains them) and use Insert View. Set the direction: (0,-1,0) is a front view and (1,-1,1) is an isometric view, per FreeCAD's own documentation.

4. 04

   Choose the lines

   In the view's properties, Hard Hidden turns hidden lines on or off and Smooth Visible controls tangent edges such as the edges of fillets. Leave Coarse View off: it approximates every curve with short segments.

5. 05

   Export

   TechDraw → Page → Export Page as SVG. Open the result in a vector editor and check the arcs are arcs.

The catch is speed and setup. A big assembly can take minutes per view, and the exported page includes the template frame, which you may need to delete.

Option 2: a short build123d script (free, needs Python)

If you are comfortable with Python, build123d (built on the same OpenCASCADE kernel as FreeCAD) projects a shape and writes visible and hidden edges to separate SVG layers. This is adapted from the example in its documentation, with a STEP import and a viewpoint added; we ran it with build123d 0.13.0 on a NIST test part. Set view_port_origin to where the camera should be, at a sensible distance for your model's size.

from build123d import *

part = import_step("assembly.step")
view_port_origin = (-100, -50, 30)
visible, hidden = part.project_to_viewport(view_port_origin)
max_dimension = max(*Compound(children=visible + hidden).bounding_box().size)
exporter = ExportSVG(scale=100 / max_dimension)
exporter.add_layer("Visible")
exporter.add_layer("Hidden", line_color=0x636363, line_type=LineType.ISO_DOT)
exporter.add_shape(visible, layer="Visible")
exporter.add_shape(hidden, layer="Hidden")
exporter.write("part_projection.svg")

It is repeatable, which matters when the model changes and you need the same view again. Line weights and grouping per part are up to you.

Option 3: your CAD system's drawing module

If you own the CAD that made the model, its drawing environment already does hidden-line views. Most can save a drawing as PDF or DXF, which a vector editor can open and save as SVG. Expect to clean up: drawing exports often split curves, add the title block, and lose any grouping by part.

Tidying the SVG

• Set line weights by role, not one weight for everything: a heavier outline, lighter interior edges. See line weights for technical illustration.
• Decide on hidden lines per drawing rather than by habit; hidden lines: when to show them covers when they help.
• Delete fasteners and internal parts nobody needs to see before you style, not after.
• Keep the source view settings (direction, parts hidden) with the file, so the next revision of the CAD gets the same drawing.