Most things were 'eyeballed', with just a couple of careful measurements. I'm a real novice on a lathe and this is not a precision job (the method could be precise if necessary). I used my lathe with a vertical milling slide and an end mill.

The square shank fits beautifully in the slots and does not protrude above the surface. The finished bolt is strong and has an 8mm thread diameter. The beauty of using these Carriage/Coach bolts is the square shank near the circular head which makes it an almost trivial task to measure, align and cut them. These Carriage/Coach bolts seemed to be an almost perfect size for modification to fit my tee-slots. They were also a whole lot cheaper than 'official' tee-slot bolts. The tee-slot bolts described here have a much bigger flat support area, than the ones I bought from a supplier (this is one of the criticisms made of modified hexagonal bolts). The finished result seems to be fine for my purposes. Despite reading some negative comments about modifying the hexagonal heads of set screws and bolts, I set about producing my tee-slot bolts from some 'Carriage Bolts' (aka 'Coach Bolts') bought at my local 'pound' shop. It is surely relevant to any lathe, although the specifics given here are for my Myfold ML10. I needed some tee-slot bolts to fit my lathe.

Make a t slot bolt