I have a new tuner design that is easier to make. Basically, you take a slot headed screw and cut straight down with a saw until the head is cut through. It is much easier to cut as well, because I make a jig that is simply a piece of wood with a 3/16" hole drilled in one end. I clamp the jig onto a bench and lock the screw into the jig using 2 nuts. It works very well.
There may be some concern that the strength of the screw is being compromised and that the head will split apart. I have found that the tensions for the current Sq'Ukuleles are low enough that there is adequate strength left.
I've been experimenting with not pounding the nut, but just using superglue. It seems to be working, But because most of the Sq'Ukulele designs have the inside nut burried in the body, I wanted to ensure that they were not going to be removable. If the superglue deteriorates and loosens, your only option is to cut the body open. I'll keep pounding the inside nut.
Designing tuners was probably the most difficult task in this project. I looked at several designs even buying some from ebay.
Commercial tuners are designed to fit on a head stock and are designed so that where you hold the tuner (the knob) and where the string is attached (hte post) are in different places. Because the Sq'Uke is headless, I wanted the knob and the post to be close.
I first tried eye-bolts, a design that I gleaned from a CGB community, I was concerned about the holding power and unscrewing the nuts. I was going to use nyloc nuts, but my experience with them is that they lost their holding power after a couple of turns. Besides, I couldn't control how tight they locked. The breakthrough came when I made my own locknuts by hammering them oval rather than round.
I used the eye-bolt idea on a couple of Sq'Ukes, but they took up so much space. The next idea was to use normal machine screws and drill a hole. Several drill bits later, I came upon the idea of cutting a slot.
And that is the design to date.
Tuners are used to change the pitch of the string by increasing or decreasing tension.