Convert STEP to STL

Your STEP file contains no triangles

This is the thing worth understanding before anything else. When a CAD program stores a cylinder in a STEP file, it stores a radius, an axis and a height. The surface is exactly round, at any magnification, forever. Nothing has been rounded off and nothing has been approximated.

An STL cannot hold that. STL has exactly one thing in it: flat triangles. So converting means covering every curved surface with a mesh of triangles fine enough that the flat bits do not show. That is called tessellation, and the only question it asks is how close is close enough.

Every converter has to answer that question. Most free ones answer it silently, with one setting they never tell you about, and you find out it was the wrong one when the print comes off the bed faceted.

The detail setting, and what the numbers mean

The control above has three presets and a custom option. What they set is the largest gap allowed between a flat triangle and the true curved surface underneath it. Smaller gap, more triangles, bigger file, smoother part.

Draft is coarse on purpose: use it to check that a part is the one you meant and that it fits on the bed. Normal keeps every triangle within about a tenth of a millimetre of the real surface on a part the size of your fist, which is finer than an FDM printer can place plastic — this is the right choice for almost all printing. Fine is five times closer again, and is worth the file size for resin printing, for a curved surface that will be sanded and painted, or for anything going to a machinist.

The presets are proportions of the model, not fixed lengths, so Normal means the same thing on a 5 mm screw and on a 2 m frame. Custom is the opposite: you type the tolerance in the units the file states, which is what to use if a drawing or a print profile has already given you a number.

Watch the triangle count beside the control while you change it. A small bracket with a couple of round holes might move from two thousand triangles to nine thousand across the three presets. A part covered in fillets can move by a factor of fifty. There is no rule of thumb that survives both; that is why the number is on the screen.

Units come out of the file, not out of a guess

A STEP file states which unit its numbers are in, near the top of the file, and it is one of the few 3D formats that does. This tool reads that declaration and sets the input unit box to match, so the measurements shown under the 3D view are the real size of the part.

Most STEP files from European and Asian CAD are in millimetres and need nothing done. Files from American shops are frequently in inches, and this is where parts get ruined: a bracket that is 4 inches wide arrives in a slicer as a 4 mm bracket if nothing converts it. Set the output unit to millimetres and the arithmetic is done for you.

If the file says something you know is wrong — and old files sometimes do — change the input unit box. Everything on the page follows immediately, and you can read the width, depth and height before you commit to anything.

Assemblies, and picking one part out of one

STEP is the format people send whole assemblies in. A gearbox arrives as a housing, a shaft, two bearings and fourteen bolts, each a named part in a tree. STL has no idea what a part is: everything in an STL is one undivided lump of triangles.

So the parts are listed above, with the name the CAD program gave each one and its triangle count, and you choose. Leave them all on and they are merged into one STL, which is what you want when the assembly is the thing you are printing. Use the isolate button on a row and you get that part alone, which is what you want when you need to print one bracket out of a machine.

Parts that share a name — and Pro/ENGINEER calls every solid body SOLID — are numbered so that no two rows read the same. Pick one and the 3D view shows you immediately whether it is the one you meant.

Why the conversion only goes one way

STEP to STL throws information away permanently. The cylinder becomes sixty flat strips and the fact that it was ever a cylinder is gone from the file. No tool can get it back, here or anywhere, because the answer is not in the STL to be found.

Keep the STEP file. If you need a different tessellation later — finer for a resin print, coarser for a quick view — convert again from the STEP rather than trying to repair the STL. It takes seconds and the result is correct.