For unknown reasons, Mary’s Pixel 3a phone sometimes does not react to her fingertip, so she now has a stylus for such occasions. The cap covers the delicate fine-tip end (with the weird clear disk), leaving the rounded mesh end exposed to dock the cap.
I made a pair of covers for the mesh end, mostly because the styli came in a two-pack and I carry mine in a pocket pouch that will likely abrade the mesh:
They’re made from 3/8 inch = 9.52 mm acetal / Delrin rod, turned down to match the 9.4 mm stylus OD. The thread resembles a standard M8×1.25 with very rounded crests:
While it’s possible to tap such a thinwalled cylinder with some exterior reinforcement, the (standard / normal / regulation / crispy) thread form of the hitherto entirely unused M8×1.25 tap cowering in the back of the drawer seemed a poor fit and, not being a bottoming tap, it wouldn’t cut full threads where they’re needed.
Besides, what’s the fun in that?
Lacking a threading tool small enough to fit inside the 7.4 mm bore, I gnawed one from a snippet of spring steel wire harvested from a dead box spring. The first pass was much too wide, but gave me the opportunity to make a few mistakes while shaping the tip:
The discoloration on the shank betrays the torching required to knock the hardness down to something file-able. A little more Dremel cutoff wheel / grinder / file action produced a tiny tooth matching the rounded thread form on the stylus:
A side / bottom view shows the crude grinding and excessive angles:
A real machinist would harden and temper it, but I didn’t bother for a tool cutting two non-critical threads in plastic.
Somewhat to my surprise, the mini-lathe can cut a 1.25 mm thread without any fancy metric change gears: a simple 35-40-45-50 train did the trick. Running dead slow gave me enough time to poke the power button and let it coast down as it approached the carriage stop marking the end of the thread: cutting plastic is much less exciting than, say, Real Steel.
When all the cutting was done, I beveled the cap for my stylus to fit better into the bottom of the pouch, but that’s definitely in the nature of fine tuning:
The second one went much faster and I should have made a third while I was hot.
A doodle giving the key dimensions:
Another day of Quality Shop Time™!
The Smell of Molten Projects in the Morning 