That remark was made in the shop, and was pretty near correct. Outside of the shop no one ever thinks of calling anything a screw driver except the instrument used to turn slotted screws. I had almost said wood screws, but that would have discarded the screw driver most in sight, that little convenience that comes, and goes, with every sewing machine, to tempt the operators to get them out of repair, and which, it is needless to say, isn’t, sometimes, worth the powder it would take to blow it to Chic—, but after all, it serves its purpose—what more would you want? And it is used by more people than any other mechanical instrument.
In fact few domestic screw drivers are just what they should be; and carpenters screw drivers are not much better. Wood screws are of various lengths and sizes. You can get little bits of wood screws, an inch long, or you can get them, just the same length, as big as a lead pencil.
Here are half a dozen estrays now on my desk while I write; how, whence, or when they came I don’t know. They will do for samples, and they vary from an eighth to more than a quarter of an inch in size. Now it don’t stand to reason that the little screw—not much bigger than a knitting needle, requires the same size and kind of a screw driver that the big one does.
There are different kinds of heads too, some are flat underneath, and some are coned, for a tapered countersink, So it can be shown that to be correct and precise, every kind or size of a screw should have its appropriate, well fitted screw driver. But this would involve too much expense, and most families can get along quite comfortably with a nicely selected set of six assorted sizes. Some families can get along with a broken case knife—a corset spring—or a piece of hoop iron.
The point of a screw driver intended to engage with the head of a common wood screw should not be tapering flatwise, but should be of even thickness, equal to the width of the slot, for by a wise provision of the screw maker, the slot is as wide at the bottom as it is at the top. But the screw driver should be tapering in width to correspond with the taper of the head of the screw, which is about 80 degrees. It should not be any wider at the end than the length of the bottom of the slot; if it is, it will tear the countersink, if the screw is let into wood; or it will not go to the bottom of the slot if the countersink is in metal. It should be as wide at the top of the slot as the diameter of the head, so as to get the full benefit of the leverage, and the body of metal to hold it.
What is a screw but a combination of the inclined plane (made spiral) with the wheel and axle? The body of the screw is the axle, and the head is the wheel, and what is the function of the screw driver but to enlarge the wheel and gain more leverage? The slot, and the point of the screw driver—combined—form the joint connecting the wheel with the greater leverage, and any imperfection in this coupling detracts from the advantage to be gained. The nature of the case demands, whenever the full power of the screw is required, that the joint shall be made as strong as possible, and that the strength of the screw driver shall exceed the strength of any one screw head, because one screw driver has to be jointed to many screw heads.
When it comes to the test, a good many screw drivers fail, or only stand because they are made so blunt and unfitted to the slots as to spoil the screws. The object which should be aimed at is to drive the screws to their full power without injury to them or to the screw driver. So much depends upon screws in mechanical construction, outside of the construction of machines, that the subject has been very thoroughly considered, although its importance is lost to the sight in those agricultural implement manufactories where they drive all the screws with a hammer. The foreman of one of the assembling rooms once complained to me about a certain workman who was not putting in his screws to suit him. “He drives them clear in with his hammer!” said the foreman “and I always told him to give them half a turn with the screw driver to make them set good!”
When screws are well put in they are good mechanical construction, and to put them in good, the screw driver must be well made. Above where it enters the slot it may be made as wide as may be desirable, as wide as a reaper knife if necessary. The side taper of the point may be continued, as there would be no gain in strength by more abrupt enlargement in this direction. But the thickness may be abruptly enlarged to three times the width of the slot. More than this is of no advantage. If the slot is a sixteenth of an inch wide, the bit which enters it should be a sixteenth of an inch thick, the length equal to the depth of the slot. A sixteenth of an inch above, it may be three sixteenths thick, but there should be no sharp corners. The offset should be with a fillet of an eighth of an inch radius. This is the strongest possible coupling that can be made with the slotted head of a screw.
Ordinarily the screw driver may be forged as thick at the point as the width of the slot, and a little wider than the diameter of the head, and then tempered a spring temper. Forget not that the intrinsic value of every steel implement is its temper.
Now to finish up the screw driver, the side tapers and the flatting of the point can be carefully done on a true running grindstone, or emery wheel, or with a file. To get the right side taper, take off nearly as much bevel in width as in height—if the height is an eighth of an inch take off a tenth of an inch bevel, so if the top of the bit part is half an inch, or five tenths across, the end will be three tenths of an inch across. The taper of a wood screw head comes so near being a right angle that if the taper of the screw driver is fitted to the inside of the square, it will be a good deal nearer right that it generally is.
Draughtsmen may figure the taper of the bit of the screw driver by multiplying the height of the tapered bit by 1.71. The product is the difference between the width of the top and bottom of the bit. But the result can be reached graphically with greater expedition.
If the tempered screw driver is shaped with a file it may be remembered that to file hard steel, a firm pressure, slow stroke, and steady hand are the main requisites—a good file has something to do with it, also. Get the teeth of the file engaged, and shove straight and firmly, so they will not lose their grip.
To favor the strength of the screw driver, finish it with the file marks lengthwise, because any little cross mark will sometimes start a crack in steel. If, on the other hand, the prime object is to get the firmest connection with the screw head with the least tendency to slip back, the finish should be with the file or emery marks crosswise.
A screw driver or any other steel instrument is stronger if the last thing done to it is to temper it. A screwdriver bit a sixteenth of an inch thick is twenty per cent better, well tempered the last thing, than one which is ground after tempering. The surface skin left by tempering has extraordinary value. The very outside is the best. Instances which vary from this rule are due to poor quality of steel, poor fuel, dense ignorance, gross stupidity, or treatment of the steel so utterly detestable that it seems incredible that they should ever be mentioned.
There is one point open to dispute about screw drivers which I never will attempt to settle. Common belief and science stand over it with drawn swords. The mooted questions are whether a screw can be set up tighter with a long springy screw driver, or with one that is short and rigid; and which kind will stand the most hard usage. The subject has been discussed from time immemorial with such acrimony, asperity and rancor, that nothing but a bravery, sired by temerity and dammed by rashness, prompts me to suggest that the decision must depend upon the circumstances of each particular case. But I have always noticed when I have offered this salvo to heated disputants, that both parties turned from me with averted looks, and went off grumbling—hunting for daggers.
The Age of Steel – May 7, 1898
– Jeff Burks