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Q: What’s the advantage to square-drive screws over Phillips-head screws?
A: The primary advantage to square-drive screws is they are much less prone to “cam-out.” Cam-out refers to the slipping of the bit in the screw head as the screw is driven.
The square-head screw was invented by P. I. Robertson, a Canadian, in 1908. It offered a big advantage over the slotted screw head because it was self-centering and not prone to slipping when driven. But, an early attempt to market these screws in the United States failed. This left the Robert-son screw confined to the Canadian market.
The Phillips-head screw was initially developed in the 1930s for industrial use. It offered the same self-centering advantage as the square drive but was designed to cam out. Cam-out was considered an advantage in the industrial assembly line, preventing screws from being overtightened and giving a little cushion to the furious power drivers of the day. But the advantage for industry was the bane of woodworkers. Cam-out for them meant marred finishes and scarred wood.
Eventually, the Robertson-head or square-drive screw migrated across the border and, by the 1970s, was making strong inroads in the furniture and woodworking trades.
A new hybrid screw-head design is a square-drive screw that, in a pinch, allows you to use a Phillips driver.
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