I use toggles all the time on my bracelets and have never lost one
yet nor had it come undone. However, that being said, what I do is to
add in a cross bar in the circle receiving the toggle bar. That way
the bar can be inserted and removed but it cannot “drop” out without
manipulation. I think the trick is to have the circle, square,
triangle, whatever shape you choose, to be big enough to allow you to
insert the bar end of the toggle clasp, but be sure that it cannot
slip back out without actually being manipulated. The photo is of a
bracelet I made some 15 years ago and I’ve worn it countless times,
and have never had a problem with it.
That being said, BEFORE I added in a cross bar, sometimes just
resting my arm on a table would be enough to allow the bar to slip
out of the ring. Since I added in the cross bar, I’ve never had one
slip out. You have to physically manipulate the bar into a vertical
position (at right angles almost to the circle) to get it out.
My experience with toggle clasps on bracelets is that they are
secure if you have the proportions and design right. I have not tried
Kay’s idea of a cross bar on the ring. I usually make mine with a
fairly small ring, sufficient to pass the T bar and attached link
without too much trouble but no more then perhaps a third in diameter
than the overall length of the T bar. I also find them much more
secure if the ends of the T bar are either bent over and/or have some
form of knob, decorative or plain, on the ends. I have a bracelet I
wear most days through all sorts of activities consisting of a series
of linked silver castings of shells. In this case I made some smaller
shell castings to use as decorative ends on the T bar. The whole
bracelet weighs around 50 grams and the clasp has never accidently
come undone. This is more than I can say with other bracelets I have
made using lobster or bolt ring clasps or S hooks. The other distinct
advantage of a toggle clasp is that it is by far the easiest to put
on using one hand. This matters a great deal more with bracelets than
with necklaces for instance.