Convert STL to DXF — a real drawing, not a flat outline

Why an STL is hard to draw from

An STL file is a bag of triangles. It has no faces, no edges, no holes and no units — a cylinder is just a few hundred flat facets pretending to be round. Anything that converts it by tracing outlines inherits that mess: your circles arrive as polygons with two hundred vertices, and nothing is dimensioned.

What Ortho2D does instead

The mesh is welded so shared vertices become one, adjacency is rebuilt, and each candidate edge is ray-tested for visibility rather than flattened into a picture. Tessellated curves are measured and re-fitted, so a facetted hole comes back as a true circle carrying a ⌀ callout instead of a 200-segment polyline.

What happens to a hole, in numbers

Export a plate with a ⌀20 mm hole from any CAD system at the usual 0.04 mm chord tolerance and that hole leaves as roughly thirty-five straight facets. This is not a fault in your export; it is what STL is. A facetted circle measures 20.00 mm corner to corner and about 19.92 mm across the flats, so a converter that simply traces the outline hands you a thirty-five-vertex polyline that is not round, is not 20, and carries no callout.

Ortho2D measures each chain of short segments, tests whether they sit on a common centre within tolerance, and replaces the chain with a true circle entity at the fitted radius. The callout then follows the ASME Y14.5 rule: a full circle, and any arc greater than 180°, takes a ⌀; an arc of 180° or less takes an R. On the plate above you get ⌀20 on a single leader instead of thirty-five line segments.

• A denser export helps. At 0.01 mm chord tolerance the same hole leaves as about seventy facets and fits more tightly.
• A very coarse export does not. Below roughly a dozen facets a circle genuinely is a polygon, and Ortho2D leaves it as one rather than inventing a radius you did not model.
• Slots, counterbores and filleted corners are fitted the same way, working inward from the ends of each arc.

The unit field that STL does not have

A binary STL is an eighty-byte header of arbitrary text, a four-byte triangle count, and then fifty bytes per triangle — twelve floating-point numbers and a two-byte spacer. Nowhere in that structure is there a field for units. The same file is the same size whether the part is eighty millimetres across or eighty inches.

This is why Ortho2D stops and shows you the measured bounding box before it draws anything. Getting it wrong is the most common way an STL drawing goes quietly wrong: a part authored in inches and read as millimetres is out by a factor of 25.4, and every dimension on the sheet will be confidently, uniformly incorrect. There is also a separate scale factor for the case where the model itself was exported at the wrong ratio.

What an STL can never tell you

Some things are simply not in the file, and no converter can recover them. An STL carries no tolerances, no surface finish, no material and no thread data — a tapped M8 hole is stored as a plain cylinder, so it draws as a plain circle. It has no assembly structure either, so a printed bracket and the boss it was printed with arrive as one undifferentiated shell.

Ortho2D gives you the geometry, the views and the sheet. The notes that make a drawing contractual — tolerance class, finish, material, thread callouts — are yours to add in the title block and free-text fields. Any tool that claims to infer them from a mesh is guessing.