I’ve noticed, lately, that slotted screw heads are all but gone on new equipment. The only thing that I find remarkable about that is that it took so long. While it is true that slotted heads have been around for ages, better systems are both common and have been around for at least a century.
The reason slotted heads — technically known as the drive — are so common is probably because they are very easy to make. A hacksaw is sufficient for the job and there are other ways to get there, too. The only advantages I know of for the user is that you can easily clean a slotted drive and — possibly — use field expedient items like butter knives and quarters to turn the screw. I’ve heard people claim that it also is a feature that the screwdriver can pry things like paint can lids, but that’s a feature of the tool, not the screw drive.
The disadvantages, though, are significant. It is very hard to apply lots of torque to a slotted screw drive without camming it out or snapping the head off the screw. The screwdriver isn’t self-centering either, so applying force off-axis is common and contributes to the problem.
A Bit of History
Threaded wood for things like olive presses started showing up about 2,000 years ago or so. By the 15th century, metal screws appeared, but being hand-made they were not very regular and were only popular in limited applications. Job and William Wyatt patented a screw machine in 1760, but it would be ten more years before a workable model would appear.
In 1770, Englishman Jesse Ramsden invented the first workable screw-cutting lathe which he used to create micrometers and other scientific instruments. He was also a mathematician and you may know him from his contributions to optics. Around 1800, Henry Maudslay invented a large screw-cutting lathe and American machinist David Wilkinson created a similar lathe. The lathe allowed for the production of interchangeable parts which were key to the industrial revolution where people like Eli Whitney were able to move to mass production of items like guns which had previously been one-of-a-kind.
Prior to Maudslay’s inventions, lathes were generally operated with a foot treadle and the operator simply held the tool to the work piece. This was not accurate enough to cut threads, so screws were made by the freehand use of chisels and files. Maudslay’s lathe had changeable gears that allowed for different pitch threads. Wilkerson’s was a larger machine, but was not easily configurable. Even so, the government used around 200 lathes like Wilkerson’s and awarded him $10,000 for the invention — That would be about a quarter of a million dollars today.
All of this time, the heads were nearly always a simple slot. In 1744, you could get a flat blade that fit a carpenter’s brace sort of like today’s drills will take a drive bit. By 1800, though, handheld screwdrivers were common.
Cutting threads with a lathe is only one way to form threads. While it is precise, it is also expensive and weakens the metal. Cold rolling is economical and doesn’t remove any metal, but the first attempts by William Keane in 1836 failed because cast iron dies were not up to the task. In 1867 Harvey J. Harwood also attempted cold rolling, but practical thread formation using this method had to wait for Hayward Harvey and Charles Rogers in the early 1880s. Some special bolts are still cut, though, and the video below shows a modern operation, and the second video shows the more usual modern process.
Ironically, while assembly lines became possible because of standard screw threads, they also were responsible for dissatisfaction with the slotted drive. Inventors wanted to do better. In 1908, Canadian Peter Robertson invented a square-head screw drive where the driver and the hole had a slight taper. There had been other attempts at producing similar drives, but they were all difficult to manufacture and did not catch on. Robertson’s invention was conducive to inexpensive manufacturing using a die.
This has several advantages. First, the drive is self-centering. Second, the driver won’t slip out even at high levels of torque. The Canadian Model T Ford used over 700 Roberson screws.
Ford and Phillips
Ford found that the Robertson drive saved a few bucks on the production cost of each car, but was wary of having a single supplier. Robertson, however, had a bad experience licensing in England and was reticent to enter into another license deal.
In the 1930s, Henry Philips building on work by John Thompson, had introduced the Phillips drive for screws. This also self-centered and was more amenable to automated methods compared to a slotted drive. Too much torque could cam out the head, though, as you’ve probably experienced. The company claimed this as a feature to prevent over tightening. GM wound up using the Phillips drive in the 1936 Cadillac, and its popularity was assured.
Robertson drive screws account for almost all screws sold in Canada and are virtually unused everywhere else. Even though most people consider the Robertson drive superior, a business decision made it a small player.
There are a staggering number of drive types out there, many of which don’t have the problems the Phillips head has. William Allen’s hex drive is used in many applications. Pozidriv is uncommon outside of Japan. Yet the slotted drive survives and Phillips remains nearly ubiquitous in much of the world.
It makes us wonder. The old saying is that if you build a better mousetrap, the world will beat a path to your door. But that doesn’t seem to be true. It is more like if you build a better mousetrap, market it successfully, build a great distribution network, and defend your intellectual property… well, you get the idea.
I focused on the driver technology, but the screw threads themselves are a fascinating design that our own Dan Maloney dove into a few years back. If you want to know more about other types of bolts and their appeal, the amusing video below has a lot of good information. It still doesn’t answer the question of why we don’t see more use of “better” drives. What do you think?