Build a fan that spins

A rotor and the frame it turns in, generated as a matched pair — and one number carried between them.

A printed fan is two parts that have to agree about one dimension, and getting them from two different generators is exactly where that agreement usually breaks. This build is the answer to that: the frame reports the largest rotor it will take, and you hand that figure to the rotor. Nobody does arithmetic on a clearance they had to guess.

The frame size is a standard, and it carries four numbers. Choose 80 mm and you get the 80 mm square, the Ø76.3 opening, the 71.5 mm hole pitch and the Ø4.3 screws together, because they come from one published table — the same one the panel cutouts use. So a frame you generate bolts where a bought fan of that size would, and a hole you cut for one accepts the other. Only the bore is a convention rather than a specification, which is why it is the one you are allowed to edit.

75.3 is not 76.3. The opening is Ø76.3 and the rotor may be 75.3, because a blade tip that touches the frame is a fan that grinds itself to a halt on the first cold morning. Half a millimetre a side is what the Takes a rotor up to row already subtracts for you. Type the number it gives rather than the bore you can see.

Measure the shaft, do not name it. The rotor's bore is cut to the shaft *plus* whatever the Fit adds — so the field wants the shaft, not the hole. A "5 mm" motor shaft is a nominal size; the one in your hand may be 4.9 or 5.05, and a hub is a part that has to grip rather than turn, so it is the one place in the catalogue where the clearance runs tighter than everywhere else. If your shaft has a flat on it, set Shaft flat to how deep the flat cuts in and the bore takes a D-shape that cannot slip.

The blade underside is shallow, and that is geometry, not a defect. Pitch is how far the fan would advance in one turn if it screwed through a solid, and the blade angle falls away from hub to tip on its own — about 29° at the hub and 11° at the tip on the default rotor. Eleven degrees of underside is far past what any slicer bridges, so a rotor printed flat needs support beneath the blades. The row warns rather than refuses, because the fix is yours to choose: supports, or a pitch so steep it stops being a fan.

Watch the roots, not the tips. Blades run out of room where they leave the hub, not at the outside — that circle is the shortest they cross and the blade stands steepest there. The Gap at the blade roots row measures it perpendicular to the chord, which is the only direction two blades can actually meet. Push the blade count up far enough and the gap closes, the blades fuse into a solid disc, and the part still looks completely convincing in the viewport.

Nothing here claims an airflow figure, and that is deliberate. Every row on both parts measures the solid or says what a printer will do with it. What moves air is the motor you bolt it to, and we have never seen it.

1. Open the Generate panel and pick Fan frame (under Fans & airflow). It drops in as an 80 mm frame: Ø76.3 opening, four Ø4.3 holes on 71.5 mm centres, 25 mm deep — the pattern a bought 80 mm fan bolts to.
2. Read the Takes a rotor up to row: 75.3 mm, at half a millimetre of tip clearance a side. Write that number down — it is the whole joint between the two parts.
3. It arrives already guarded — four struts and one ring — and the Largest opening row says what still gets through: 16.1 mm at one ring, which a finger fits into. Take Rings to three and it closes to 6.5 mm. The rings are what shut a guard; the struts barely move that number.
4. Now pick Axial fan from the same group and set Tip diameter to the 75.3 mm the frame just told you. Set Shaft diameter to your motor shaft, measured with calipers — not its nominal size.
5. Read Blade underside at the tip. On any sane fan pitch it is far shallower than 45°, and the row says so: a rotor printed flat wants support under the blades. That is normal for this part, not a mistake in it.
6. Check Gap at the blade roots is still positive before you print. It closes as you add blades or lengthen the hub chord; the row names the four ways back — fewer blades, a bigger hub, more pitch, or thinner edges. At zero they fuse into a disc that still looks like a fan.
7. Lay both parts flat, run the print check and export. Print the rotor with supports on and the frame without.