Dave Sander [ September 2011 ]
The Pozidriv Screw... Not Phillips
The screws have a head that looks like this:
Absent mail this month, I thought I would pass along something interesting I recently learned. Over the years I have seen a lot of screws on antique cars that I thought were early Phillips screws. I always assumed that it was a particular brand of screw. In addition to the slots for a screwdriver, there are a second set of very faint cross slots 45 degrees away. I've no-ticed these screws seem to be on a lot of British cars.
These are not Phillips, but rather something called Pozidriv. Here is some information and history on Pozidriv courtesy of Wikipedia:
The Pozidriv, sometimes misspelled Pozidrive, screw drive is an improved version of the Phillips screw drive. It is jointly patented by the Phillips Screw Company and American Screw Company. The name is thought to be an abbreviation of positive drive. Its advantage over Phillips drives is its decreased likelihood to cam out, which allows greater torque to be applied.
Phillips drivers have an intentional angle on the flanks and rounded corners so they will cam out of the slot before a power tool will twist off the screw head. The Pozidriv screws and drivers have straight sided flanks.
The Pozidriv screwdriver and screws are also visually distinguishable from Phillips by the second set of cross-like features set 45 degrees from the cross. The manufacturing process for Pozidriv screwdrivers is slightly more complex. The Phillips driver has four simple slots cut out of it, whereas in the Pozidriv each slot is the result of two machining processes at right angles. The result of this is that the arms of the cross are parallel-sided with the Pozidriv, and tapered with the Phillips.
This design is intended to decrease the likelihood that the Pozidriv screwdriver will slip out, provide a greater driving surface, and decrease wear. The chief disadvantage of Pozidriv screws is that they are visually quite similar to Phillips, thus many people are unaware of the difference or do not own the correct drivers for them, and use incorrect screwdrivers. This results in difficulty with removing the screw and damage to the slot, rendering any subsequent use of a correct screwdriver unsatisfactory. Phillips screwdrivers will fit in and turn Pozidriv screws, but will cam out if enough torque is applied, potentially damaging the screw head. The marker lines on a Pozidriv screwdriver will not fit a Phillips screw correctly, and are likely to slip or tear out the screw head.
There are special Pozidrive screwdrivers available from tool manufacturers. Snap-On sells an assortment with five defferent sizes. If you are finding a lot of these "odd philips" screws, it may be worth your while to pick up a set of Pozidriv screwdrivers.
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