Cam out (also cam-out or camming out) is a process by which a screwdriver slips out of the head of a screw being driven once the torque required to turn the screw exceeds a certain amount. Repeatedly camming out damages the screw, and possibly also the screwdriver, and should normally be avoided.
The Phillips screwdriver design may have a tendency to cam out at lower torque levels than other, newer Pozidriv-type designs. There has long been a popular belief that this was actually a deliberate feature of the design, for the purpose of assembling aluminum aircraft without overtightening the fasteners. The feature is not mentioned in the original patents. However, a subsequent refinement to the original US Patent #2,474,994 does describe it.
In recent years, automated manufacturing insertion tools can now precisely sense fastener torque. Consequently, it is typical for computer parts, automobiles, and other highly engineered products to be assembled with Torx or Pozidriv head screws, which have been specifically designed not to cam out.
Robertson head screws
Robertson screws are commonplace in Canada, and significantly reduce cam-out when compared to Phillips screws.
- Alexander III, Adler, W. (1998). "2. Literature Review" (PDF). Testing and Understanding Screwdriver Bit Wear. Virginia Tech Digital Library and Archives (M.Sc.). Virginia Tech. Retrieved 2015-01-20.
- "What are the differences between the two types of drive – Phillips and Posidriv?" (PDF).
- US 2474994, Tomalis, Joseph & American Screw Company, "Screw Socket", published December 30, 1942, issued July 5, 1949
- "US Patent #2,474,994 Claims, Page 7".
- Pozidriv at Phillips-Screw
- When a Phillips is not a Phillips! at Instructables.com