(wikipedia entry)

 

Close-up of a Robertson screw

A Robertson, also known as a square,^[15] screw drive has a square-shaped

socket in the screw head and a square protrusion on the tool. Both the

tool and the socket have a taper to make inserting the tool easier.

Robertson screws are used mainly in Canada (where they are very common), though they can be found elsewhere.^[16]

Robertson screwdrivers are easy to use one-handed, because the tapered

socket retains the screw, even if it is shaken. They also allow for the

use of angled screw drivers and trim head screws. The socket-headed

Robertson screws are self-centering, reduce cam out, stop a power tool

when set, and can be removed if painted-over or old and rusty. In

industry, they speed up production and reduce product damage.^[17]

Canadian P.L. Robertson invented the Robertson screw and screwdriver in 1908 and received a patent in 1909. An earlier square drive wood screw patent, U.S. Patent 161,390, was issued to Allan Cummings on March 30, 1875.

Robertson had licensed the screw in England, but the party that he was dealing with intentionally drove the company into bankruptcy and purchased the rights from the trustee, thus circumventing Robertson.^{[citation needed]}

He spent a small fortune buying back the rights. Subsequently, he

refused to allow anyone to make the screws under license. When Henry Ford tried out the Robertson screws he found they saved considerable time in Model T

production, but when Robertson refused to license the screws to Ford,

Ford realized that the supply of screws would not be guaranteed and

chose to limit their use in production to Ford's Canadian division.^[18][19][20]

Robertson's refusal to license his screws prevented their widespread

adoption in the United States, where the more widely licensed Phillips

head has gained acceptance. The restriction of licensing of Robertson's

internal-wrenching square may have sped the development of the internal-wrenching hexagon, although documentation of this is limited.

Robertson-head screwdrivers are available in range of tip-sizes.  A given driver's suitability for use with a particular size of screw

and recess size is indicated upon its handle according to the following

color code: