https://en.wikipedia.org/wiki/List_of_screw_drives
@AES: I'm not sure who told me about Roberstson screw heads being used in aircraft - if I am perpetuating an urban (workshop) myth, I apologise. I've looked at the Wikipedia pages and there are references to time saved on production lines by using Robertson screws (Henry Ford liked them, apparently), but nothing about aircraft:
https://en.wikipedia.org/wiki/List_of_s ... #Robertson
I have to say though, that I have never, ever had one cam out - quite the opposite: they seem to work better than any of the alternatives, except possibly Allen heads. I have, however seen some cheap "square drive" driver bits that are _not_ the Robertson form. So those won't mate properly with Robertson screws and may be the reason for the issue. It's the same with cheap Pozidriv and Phillips drivers (and screws): loose tolerances always cause problems.
As you can see above, the shape of the Robertson screw head and driver is a double-tapered 4-sided pyramid with a blunt tip. You can use them with the driver slightly off-axis, but on-axis I find the shape acts like a Morse taper and the screw fits snugly on the end of the driver. Phillips screws do this too, but nowhere nearly as well. This taper is really handy for driving pocket hole screws upwards, or into awkward spots - sometimes you can put the screws in first, but sometimes you can't (because the heads stick out). there are now a couple of variants used in engineering, the double-square and triple-square, with eight and twelve corners respectively. Tellingly, both keep the Robertson taper (if I correctly understand the Wikipedia entry), and can be used with Robertson drivers (although this may not be wise as they are intended for high-torque applications).
Other candidates for best system would be Allen heads - they get my #2 slot - and Torx.
The trouble with Torx is the geometry for countersunk screws: the "blades" of the screwdriver form stick out. In cross-section you are making a rectangular hole in a triangle. You can't make that hole too deep, or the deep corners will break out of the sides of the countersink. Even if they don't actually break through they could weaken the head of the screw considerably. So either you have a shallow hole, or your driver needs to be narrower to fit in the available metal of the screwhead. Torx screws for retail sale usually have a driver size that's too big (T25), and too shallow (to stop the screw head snapping). So they don't stay on the driver...
... I find this a right PITA, as they also cause the driver bits to wear too fast right at the end, so the fit becomes looser and the bit eventually snaps. I used to be able to get Torx screws with a T20 head - these were far better, both in fit on the driver and wear. I can only assume the manufacturers think decking fitters will be annoyed by having to use several sizes of bit, and thus "standardized" on T25. It does mean they can sell lots of bits though. Of course, if it's domed or cheesehead these issues don't apply (as much).
Back markers in this field have to be the cross-head variants, Phillips and Pozidriv. I still have a genuine, original Pozidriv #2 screwdriver somewhere from the British consumer launch way back inthe 1970s (Pozidriv is a British invention, BTW). Like everybody, I use them all the time, but I don't
like them. Far too many variants, far too many badly stamped screws and drivers, far too many metal splinters...
... I know: far too much detail, too.
E.
