Loft a duct between two openings

Round at one end, rectangular at the other — a loft skins between outlines that have nothing in common.

A shop-vac hose is round and a tool port is a rectangular slot; a fan is square and the thing it blows into is a circle. A loft is the shape that gets from one to the other — you draw the two openings and it works out every cross-section in between.

Start each outline at the same corner, going the same way round. The skin joins the sections point for point: the first point of one runs to the first point of the next, the second to the second, and so on. Draw the second outline starting from a different corner and the whole run twists like barley sugar between them — a result that is either exactly what you wanted or a defect you will never find by staring at the numbers. When two sections have different point counts the smaller one is filled out with extra points first, and every point you actually drew is kept, so a hand-drawn corner never gets rounded off behind your back.

The sections are the OUTSIDE of the part. Draw them at the size of the openings you want and shell afterwards. Drawing the wall into the outlines instead gives you a duct whose bore is a wall thickness too small at both ends, and nothing on screen shows it — you find out when it will not go on the hose.

Two open faces turn a solid into a duct. It is the same move that turns an extruded outline into a cookie cutter: one open face gives you a funnel with a closed end, both gives you something you can blow through. Shell first and open second, on the finished loft rather than on the sections.

Put the bigger end on the plate. A run that narrows as it rises holds itself up however fast it narrows, and prints as a stack of horizontal rings with nothing to support. Turn it over and every millimetre of the transition is leaning out over thin air — and the shorter the run, the worse that lean gets. If the two ends are the same size this never arises; if they are not, one way up prints cleanly and the other needs supports all the way through the bore, where you cannot reach them.

A loft keeps at least two sections, each one a single closed outline — no holes, and no splitting into two lumps part way up. If you need a hole running through it, cut that afterwards with a channel, the same as on any other solid.

1. Add a Loft from the Sketch group. It drops as two stacked square sections, one at each end of the run.
2. In the Properties panel every section has a z — its height — and a Shape button. Set the two z values to the ends of the run.
3. Press Shape on the first section to redraw its outline, and drag an edge’s arc handle to bow it outward: four points with four bowed edges reads as a round mouth.
4. Press Shape on the second section and draw the other opening. The two ends do not have to be the same shape, which is the entire point.
5. Press Add section for anything that happens on the way — a waist, a step, a flange — and give it a z between the ends.
6. Hollow it to a wall thickness and switch on both Top and Bottom under open sides, which turns the solid into a duct.
7. Run the print check, print it standing on end, then export.