Sliding bevels are normally set once for a specific angle, the splay of a leg for example, and then left set for the duration of the project. In that context the screwdriver lock sliding bevel has some key advantages:
It is easier to get a really firm setting with a screwdriver, you don't need to give it the white knuckle death grip, but something just a little stronger than finger tight.
Because the screw is flush with the surface you can use it any way around without having to think about it.
Because you need to get a screwdriver out to adjust it, there is less likelihood of inadvertently using it for another job and changing the setting; a bit of masking tape with "workshop stool leg splay angle" written on it is also a good idea.
Sliding bevels are an indispensable tool for recording, storing and transferring angles from drawings to workpieces and from one component to another. Used as a T, Y or an L shape sliding bevels are versatile and adaptable to the situation at hand.
Best rosewood with solid brass fittings and a 4½" blued steel blade.
Made from scratch in Sheffield.