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 you take into account that the thread on them is rolled smooth, and that of domestic ones is shaggy (and therefore it is difficult to wrap them), it becomes clear that everything has been done in domestic screws to make it convenient to rip off their slot.
First, about 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 the 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. The angle of "camber" of the taper is 90 o, so a conventional large diameter drill will not work, it has a corresponding angle of only 70 o. 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, turned 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 recess 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 commonly, 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, a screw can only be screwed into one part; the screw must pass freely through another part. 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, the thinner drill can be taken slightly thicker.
There are special drills for the whole screw at once, they are stepped. You fold the two parts to be screwed together and drill a hole of the desired shape at one time, and even with a countersink. Minus - you need to have such a drill for each individual standard size of screws.
Well, now for some tips
If the connection is not too important, you can take a screw with a thread along the entire length, and consider the part of the screw under the head smooth and thick. Of course, this is not very correct, but not fatal. Naturally, you still need to drill a hole of diameter D.
It is very useful to secure the screw with glue. When tightening the screw, the glue will act as a lubricant, and then it will reliably keep the screw from unscrewing under variable loads.
It is necessary to tighten the screws with a screwdriver exactly suitable for the slot, because otherwise you will either spoil the screw without turning it all the way, or spoil the screwdriver. It is easy to check the correspondence of the screwdriver to the slot. Press a piece of plasticine into the slot of the screw, and then insert the tip of the screwdriver. If all the plasticine is squeezed out, the screwdriver is suitable. Usually, in the practice of a home craftsman, three straight-blade screwdrivers and three cross-head are enough.
A short screwdriver is always worse than a long one. The human hand is designed in such a way that when you try to turn a screwdriver around its axis, you also get an inclination of this axis. The longer the screwdriver, the less this slope, and the more reliable the contact between the screwdriver and the screw. Each screwdriver break from the slot pulls out a piece of metal either from the screw or from the screwdriver, and you either will not be able to tighten the screw, or you will throw the screwdriver through a few dozen wrapped screws.
It is always better to screw in several small screws than one large one. The whole structure will be more reliable, if only because the load is distributed evenly. You can draw an analogy with a broom and a thick stick - if there is a knot or a crack in the stick, it will not be difficult to break it, and a few cracked twigs in the broom will hardly reduce its overall strength.
Pulling out a screw along its axis is easier than sliding it sideways. This must be taken into account when planning the connections on the screws.