Why a DXF opens at the wrong scale, and how to fix it

A DXF records coordinates as plain numbers with no unit attached. The unit is stated, if at all, in one header variable, $INSUNITS: 1 is inches, 4 is millimetres, 0 is "unitless". Many programs ignore it when they open a file, older R12 files cannot contain it, and plenty of exporters leave it at a default that does not match the drawing. When the receiving program guesses differently from the one that wrote the file, the drawing arrives off by the ratio between the two units, which is 25.4 for inches and millimetres. The fix is to find out what the numbers really are, and then either tell the receiving program or correct the header. Both can be done free in a text editor; Foundry, the 2D half of the Engineering Suite, does it with a menu.

Recognise the number

Measure one feature whose real size you know, and divide what the software shows by what it should be.

Read what the file says

An ASCII DXF is a text file of pairs of lines: a group code, then a value. Open a copy in Notepad or TextEdit and search for $INSUNITS. You will find it in the HEADER section, like this:

9
$INSUNITS
70
1

The last line is the answer. The values that matter are 0 (unitless), 1 (inches), 2 (feet), 4 (millimetres), 5 (centimetres) and 6 (metres); the full list runs to 20 and includes microns (13), which is how chip and MEMS layouts are drawn.

If there is no $INSUNITS at all, look for $ACADVER. A value of AC1009 is an R12 DXF, the oldest format still in common use and the one many laser, CNC and CAM programs write because every reader accepts it. $INSUNITS arrived with AutoCAD 2000, so an R12 file has no way to state its units: the receiving program always guesses.

There is also $MEASUREMENT, 0 for imperial and 1 for metric. It chooses which hatch and linetype patterns apply, and some programs fall back on it for the unit; it is a hint, not a statement.

Fix it free

• If the receiving program asks for the unit on import, tell it what the numbers really are. Many laser, CAM and vector programs have a units choice or a scale factor in their DXF import dialogue. This fixes it without touching the file.
• If the header is wrong, change the value after $INSUNITS and 70 in a copy of the file and save it as plain text. A millimetre drawing labelled 1 becomes correct when the 1 becomes 4. This only helps programs that read $INSUNITS, which not all do on opening.
• If there is no header value, the receiving program's own setting decides. Set it to match the drawing, or scale by the factor in the table above after import.

Whichever route you take, measure the known feature again afterwards. That one check is worth more than anything the header says.

What Foundry does with it

Foundry reads $INSUNITS when it opens a DXF, converts the file's units into the drawing's, and says which units it opened in. If the file states none, or states "unitless", it says so rather than guessing quietly: The file does not say its units; it is shown as it stands.

For a file whose header is wrong, File → Drawing settings → Units changes what the numbers mean without moving anything, so a 100-wide plate labelled inches becomes a 100 mm plate. File → Export DXF then writes the file back with $INSUNITS set to match. If the geometry itself was drawn at the wrong size, the Scale command (SC) scales it by a typed factor such as 25.4.

We checked this on a 100 × 50 plate with a hole in it, written as an AutoCAD 2000 DXF whose header said inches. Foundry opened it as 100 × 50 inches, as the header told it to. Changing the drawing's units to millimetres kept every coordinate where it was, and the exported DXF said $INSUNITS 4. Writing this also found that Foundry did not know three of the documented unit codes, angstroms, nanometres and microns, and opened a micron drawing a thousand times too large while reporting that the file stated no units. It knows them now.

One thing to be straight about: Foundry writes R12 DXF, because every program reads it, and adds $INSUNITS to the header. A program that reads the variable gets the unit; one that treats the file strictly as R12 ignores it and guesses, like any other R12 file. Measure after import.

Foundry is part of the Engineering Suite, which is part of OBSCURA Pro; everything above it on this page is free and needs nothing but a text editor.