1 Dec
2008
1 Dec
'08
7:52 a.m.
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