Convert DXF to STL

The part most DXF converters get wrong

A DXF almost never contains outlines. It contains loose pieces. A rectangle with rounded corners saved out of AutoCAD is four separate LINE entities and four separate ARC entities, stored in no particular order, none of them knowing about the others. On screen it looks like one closed shape. In the file there is no shape at all.

Nothing can be given a thickness until those pieces are joined back up. This converter does that: it walks the ends of every line and arc, finds the ones that meet, and rebuilds the loops. The message under the 3D view tells you how many outlines had to be rebuilt that way, which on a normal CAD drawing is all of them.

This is why so many free DXF converters hand back an empty model. They read the entities, find nothing that is already a closed path, and give up. If another tool has told you your drawing is empty, it almost certainly is not.

Units, and the mistake that costs a sheet of material

A DXF can state what one drawing unit means, in a header field called $INSUNITS. Many do. Many do not, and a drawing from an American shop is very often in inches whether it says so or not.

When the file states its unit, this page reads it and says so under the 3D view: "the drawing states its unit as inches", and the millimetre sizes beside the model are that statement converted. When the file says nothing, the numbers are read as millimetres and you are told that too.

So there is one thing to check before you download anything: the width and depth figures next to the 3D view. If your 300 mm panel is showing as 11.8 mm, the drawing was in inches and said nothing about it. Set the scale box to 2540 and it is right. If it shows as 7620 mm, the opposite happened; use 3.94.

Arcs, bulges and splines all survive

A rounded corner in a polyline is stored as a single number called a bulge, attached to the vertex before it. It is easy to ignore, and ignoring it turns every rounded corner in the drawing into a straight chamfer — a part that looks almost right and does not fit whatever it was drawn to fit. Bulges are read here and turned back into arcs.

Splines are read properly too, using the knot vector the file provides rather than the shortcut of joining up the control points. Anything traced, anything drawn freehand, and everything Illustrator writes arrives as a spline, and the shortcut visibly cuts the corners off all of it.

Curves are flattened into straight segments finely enough that the edge is smooth to a laser cutter and to the eye, and coarsely enough that a traced logo does not become a model with a hundred thousand triangles in it.

Blocks: the reason a drawing looks like it has no holes

A drawing with twenty identical mounting holes is usually not twenty circles. It is one block, defined once, placed twenty times with an INSERT entity. A reader that walks only the main entity list sees a plate with no holes in it.

Blocks are expanded here, with their position, rotation and scale, and nested blocks — a block that places another block — are followed too. So are the row and column grids that an INSERT can carry, which is how a perforated sheet is usually stored.

What is left out, and why

Text is left out. A TEXT entity is a request to draw letters in a particular font, not an outline, and the font is not in the file. If you want the lettering as part of the part, explode the text to lines in your CAD program first, or use the text to STL tool on this site.

Dimensions, leaders and hatching are left out. A dimension describes the part rather than being part of it, and a hatch is a fill pattern drawn inside a boundary — the boundary is what becomes the shape.

Any outline whose ends do not meet is left out, and you are told how many. A gap of a thousandth of a millimetre still leaves a shape open, and an open shape encloses no area to give a thickness to. In LibreCAD or QCAD, Modify → Trim closes them; in AutoCAD, the PEDIT command with the Join option.