[Milsurplus] Screwing

[Brooke Clarke]

Hi John:

Phillips (crosshead, +) screws are designed so that when a certain 
amount of torque is present the driver rides up out of the screw (cams 
out).  The big benefit of the Phillips head is that the driver does not 
slip off the head like the sloted (-) head screw.  They are intended for 
hand held drivers.
http://en.wikipedia.org/wiki/Henry_F._Phillips
The first patent was in 1933
http://www.phillips-screw.com/history_about_phillips.htm

Torx heads are designed to avoid cam out and so must be used with torque 
limited power drivers (or by hand).  Note that conventional slotted (-) 
screwdrivers have sloping sides on the blade and will cam out but 
gunsmiths slotted screwdrivers are hollow ground to avoid cam out.

-- 
Have Fun,

Brooke Clarke
http://www.PRC68.com