This article will focus on screws, simply because they are used very often in the practice of a home craftsman. Mainly because they are mostly screwed into wood. There are a lot of items made of wood that sometimes need to be repaired. In addition, wooden parts are easier to process than metal ones, so you often have to work with wood when building something from scratch..
Everyone has seen the screws, but few know how to use them correctly. It is better to buy imported screws, which are often called self-tapping screws. (To be honest, I do not quite understand how a screw differs from a self-tapping screw.) Imported screws are stronger and of much better quality, although they are somewhat more expensive. And if we take into account that the thread on them is rolled smooth, and the domestic ones are shaggy (and therefore it is difficult to wrap them), then it becomes clear that everything has been done in domestic screws to make it convenient to rip off their slot.
First, how the screw works. In general, it has a head, an unthreaded rod and its continuation with a thread. The screw is always used to hold at least two parts together. So the thread must all be located in one part, that is, this part must have a thickness of at least L (in the figure). And the rest of the parts should not be thicker than l (again, according to the picture). If you attach, say, a thin metal plate to a wooden board, then the length l can be very small, that is, there will be a thread on the entire screw (except for the head, of course). If you screw the screw with the threaded part into two different parts, then a gap will necessarily form between them, and it will not be possible to pull it off. One of the parts must necessarily fall on the smooth part of the screw.
Now about different hats. The picture shows different types of screw heads, the blue line shows where the surface of the material into which the screw is screwed should go. The slots are not shown conventionally. The first in the figure is a countersunk screw, the most common. Used where the surface needs to be smooth. The hat is buried in the material, in a pre-made countersink. The second type is with a lenticular head protruding above the surface of the material, but looking better with a countersunk head. The third type – with a head like a bolt, completely ugly, but powerful. There are no slots on such a head at all; the screw is tightened with a wrench. The last, fourth type is a semicircular hat. Used when attaching thin parts, when there is nowhere to make a countersink.
Countersink is done with a special drill that makes a tapered recess. Camber angle of the cone – 90o, therefore, a conventional large diameter drill will not work, it has a corresponding angle of only 70o. Since the countersink does not drill deep holes, it blunts a little, and there are no strict requirements for the quality of sharpening. Therefore, you can re-grind a drill of a suitable diameter to the countersink. The slots on the cap are very different. The simplest is a straight slot, just a slot, the screw is tightened with a flat-blade screwdriver. The most common is a cruciform, a screwdriver looks like a cone with four slots, and the slot on the head is in the shape of a cross.
There is also a reinforced cruciform recess, it looks like a cross with a four-pointed star superimposed on it, rotated 45o relative to the cross. Some screws have an internal hex slot that looks nice, but requires a very accurate and therefore expensive screwdriver. It is most convenient to work with Phillips or reinforced Phillips slots, because the screwdriver does not slide sideways and you can screw the screw even blindly without seeing it. But if the screw or screwdriver is not very high quality, with a Phillips head you will only suffer, in this case it is better to take screws with straight slots. Sometimes a decorative plastic plug is included with the screw (and sometimes sold separately). It is inserted into the slot of the screw wrapped in place, hiding it. Less often, the plugs are metal, with a threaded leg, and the counter thread is cut at the end of the screw. But in any case, screws with caps are more expensive.
Now let’s talk about how to properly screw the screw. How to choose the length of the screw and its threaded part is described above. You also need to choose the diameter of the hole that is drilled for the screw. More precisely, two holes are drilled with two drills of different thickness plus (possibly) a countersink. One, large, diameter (D) is needed such that the threaded part of the screw and its smooth part under the head just enter without effort. And the second diameter should be equal to the diameter of the thin screw shank (d). As already mentioned, the screw can only be screwed into one part; the screw must pass through the other part freely. If your material is soft, or the screw is very short, use a thin drill bit slightly smaller than d in diameter (or even prick with an awl instead). In the case of hard material and / or a long screw, a thin drill can be taken a little thicker.
There are special drills for the whole screw at once, they are stepped. You fold the two parts to be screwed together, and at one time drill a hole of the desired shape, and even with a countersink. Minus – you need to have such a drill for each individual size of screws.
Well, now for some tips