This "coupler" is designed for standard PVC ducting to be joined to flexible Polyurethane/PVC hose. The hose join is on the outside of the hose which compared to couplers made by placing the hose over ducting means air flow is less restricted.

The STL files are designed to suit 4" hose and 4" PVC ducting.

The .sin.STL file produces a sinusoidal threaded section that is the most forgiving as you don't need to be too specific about getting the radius of the flex side of the coupler just right, but it will also be the more restrictive, although no more so than a shallow bend in the flex.

The .cir.STL file produces a semicircular threaded section that less restrictive than the sinusoidal threaded coupler but is not as forgiving in terms of fit so you may have to play around with the sizes in your slicer. near in mind that if you adjust the thread you will end up adjusting the unthreaded section. If you can use the SCAD file you can adjust these independently.

The SCAD files uses the Revolve2 OpenSCAD library module developed by Dario Pellegrini (https://www.thingiverse.com/thing:3215997) and for that I am truly grateful.

The SCAD file allows for considerable flexibility. The couple is divided into 3 sections. Threaded, Unthreaded and a small lip in between. The radii, length and wall section of these sections can be adjusted to suit your needs.

For the unthreaded side of the coupler the ID is lightly tapered to get a high fit with PVC ducting - you may have to play around with the internal radius and taper variable to get a super tight fit for your PVC.

Three types of thread are available (see image of graph showing thread profiles)
Sinusoidal, Semicircular (as described above) and a Manual threading section that can be used to generate any type of thread profile. The one shown in the graph is square but also , triangular or trapezoidal or . .... can be produced.

The pitch and magnitudes of the threads can be adjusted within SCAD but be aware if you make the magnitude too large it may cut through the coupler.

photographic images showings examples of the sinusoidal and semicircular threads are provided

The SCAD file also allows for the threaded and unthreaded sections of the coupler to be printed independently ie just the threaded or unthreaded sections. This is useful to get the sizes right to suit your specific ducting and flex, ie one you have one side right there's no need to print both sides.

Creality

CR10S PRO

Doesn't Matter

Doesn't Matter

0.28

The sinusoidal thread prints easily withouts support but the semicircular thread will produce strings from the very outermost edge of the top part of the semicircular thread. I figure its easier to remove these than remove any support.

There will be quite a few strings to remove if printed unsupported but scraping with a razor will fix most o these.