This was my quick make of a dumbbell that mimics common brands of adjustable dumbbells.

NOTICE: please print horizontal for part strength. Yes you need supports but it will be infinitely stronger that way. Also, be aware of the integrity of plastic. I'm confident my girlfriend (whom I designed this for) won't put crazy weight on it to risk it breaking.

Be smart... This isn't for maxing out your skull crushers or dumbell bench press. This is for lightweight use.

Handle length: 8.75in
Length thread region: 3in each
OD handle: 1 in
OD thread: 1 inch (whereas the plates I was designing for where slightly bigger).

The sizes I designed for were so my girlfriend could have lesser weights on some dumbbells while I had the heavier ones on the original metal ones. To fit my print bed, the length of the thread region was shortened as well as a slight adjustment to the handle region. The handle would still work for me though and isn't too small.

I printed horizontally for strength, and at around 30-42% infill for weight. I left supports to Superslicer. Sanding was needed on the very top and bottom for threads and knurling, but everything with very minor sanding ended up smooth.

Tolerance for threads in the STL ended up feeling similar to the original, although the original had cornered threads which weren't offered by OnShape.

I have included STEP files if that helps anyone, but it is good to print as is.

Unless you're going crazy with your lift speed the single nut should be sufficient to lock the weight down, but if needed an additional nut could be printed as a lock nut on each side. I don't find it necessary.