Screwheads; was 5 floppy?.

[Paul Koning]

> To engage a Torx or Robertson, the screwdriver bit must be fairly
> precisely positioned so it enters the head.  With a Phillips, one can
> still engage the head successfully even if one starts off-center due
> to the property that both the slot and bit are conical in profile--
> the bit will naturally tend to slide to center in the slot.
> 
> The price you pay for this is that it's not easy to preload a driver
> with a screw without some sort of clip or magnet arrangement.  Torx
> and Robertson-headed screws can be preloaded onto the bit which is
> one of the reasons that manufacturers like them--which is also, why,
> I suppose that square-drive drywall screws have largely replaced
> phillips-head ones.

I haven't seen that, though square drives are common for deck screws
which look a lot like drywall screws.

The trouble with Phillips heads is that the bit will "cam out" -- when
you apply torque it pushes the screwdriver out of engagement.  This is
actually by design; the Phillips head was designed back before
torque-limited screwdrivers, and the "cam out" property was intended as
a slipshod way of torque limiting.

Of course that was a century ago, and by now this "feature" is really
just an old crock that no one likes.

	paul