On 25 Nov 2008 at 18:45, David Griffith wrote:
I don't follow on the automatic centering. Yes, a
slotted screwdriver can
slide out, but torx or robertsons?
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
That's why you're supposed to use a drill with
a slipper clutch.
I did better than that--I used a special-purpose screwgun that
releases as soon as the head is driven flush with the surface. The
problem is that stainless deck screwheads are rather soft. Driving
4" #12 screws into green Doug-fir joists requires a fair amount of
torque, even if one predrlls the first inch or two of the hole.