Instructions Of Screw And Screwdrivers
While 90 per cent of all carpentry can be completed by the use of common nails, the remaining 10 per cent requires some special fastening device and, in most cases, it will be wood screws which are called upon. Screws are specifically recommended in the following instances: When the work may have to be taken apart at some future date.
When greater holding power is required of the joint.
When it is important that the finished job be unmarred.
Screws should always be used when the pull of the load is to be directed along the length of the fastening devices. A nail in such a position will pull out but a screw will hold tightly.
Since there are literally hundreds of types of screws from which to choose, selection of the right one is important. Screws are not, like nails, identified by length alone. A screw has two dimensions: the diameter of the shank just below the head is the number of the screw, and the length is indicated in inches or fractions of an inch.
In choosing a screw for a specific purpose, first select the proper head style for the job. Round heads will remain exposed and can actually become a part of the decorative scheme; otherwise they must be used where they will remain unobtrusive. Flat heads will either be countersunk flush with the surface or buried deeper and concealed by wood putty or filler.
When selecting a screw by its number, remember that the number indicates thickness only and that the length must also be specified. Use a thin screw for hardwood, a screw with a thicker shank for soft –wood. A fiat-head screw is measured by its over-all length; a round-head screw is measured from point to the base of the slot in the screw head. Select a maximum length that will not pass entirely through the wood. At the same time, remember that, as with nails, the maximum area of wood fiber gripping the screw provides the maximum holding power, in determining the safety limit to be placed on a single screw, consult the table. It provides a guide for selection when 1" of screw thread is embedded in wood across the grain. This, of course, means 1" in the holding piece of wood. For 1/2" of embedded screw, use one-half the load shown. For 2" of embedded screw thread, double the load value. When the screw is set with the grain, use 60 per cent of the indicated load figure.
In softwood, it is sometimes necessary to make only a short indentation as a starting hole for the screw, and your screwdriver plus wrist action will sink it into place. Larger screws require pilot holes. For softwoods such as pine and spruce, drill a hole only half as deep as the threaded part of the screw. For hardwoods such as oak, maple and birch, drill the hole as deep as the screw. When using exceptionally large screws in hardwoods, first drill a pilot hole that is slightly smaller than the threaded part of the screw; then use a second drill (of the same diameter as the unthreaded portion of the screw) to enlarge the hole at the top.
To speed up the driving action on setting screws, it has long been customary to soap the threaded end of the screw. The idea was that this made driving the screw easier, and this is correct. However, soap eventually turns to a form of glue and cements the screw into the hole. It also produces rust on steel screws, corrosion on brass screws. As a result, withdrawing the screws later is almost impossible. It is far better to use candle wax or graphite. Don't use oil - it will stain and penetrate the wood grain for some distance around the screw head.
Since a screw is often used with the idea that it may later be removed and replaced, it is well to bear in mind that, when withdrawn, it will leave a much larger hole than will a nail. Before the screw can be replaced, the hole must be filled to provide a firm gripping area. You can fill it with wood putty and reset the screw in this new material, or use plastic plugs, driven into the old screw holes. Both provide an excellent base. Also, you can use a longer screw for replacement.
To remove a tight screw, be sure to use a screwdriver blade with perfectly parallel sides and one which fits the screw head exactly. If turning the blade counterclockwise does not loosen the screw, reverse the direction (which will drive the screw in a bit further) and then again turn counterclockwise. By working the screw both ways, you will often find that it will back up a bit more each time until it is entirely out.
Screws are used instead of nails when greater holding power is needed. The "power" referred to is in an outward direction, opposite that in which the screw point is aimed. In other words, outward pull against the threads is holding power.
The quality of the wood into which the screw is driven has a definite bearing on its ability to hold. Screws hold best in hardwoods. A glance at the table shows the differences.
The table also indicates safe loads for each Y of screw thread inserted into the "holding" piece of wood. For a screw set with threads 1/2" into the holding piece, reduce the safe load figure by 50 per cent. Remember that the threads of a wood screw represent 2/3 the total length of the screw. Setting into end grain reduces holding power 40 per cent.
The screwdriver will be one of the most essential tools in your tool kit and, used correctly, it will contribute much to keeping your home in tiptop shape.
A quality screwdriver has a tip of high-grade steel which will not become burred easily, and its sides are almost parallel. Most screwdrivers may look somewhat alike, but they vary considerably in the jobs for which they were made. The use of the right size for a particular screw will cut down considerably on time and effort involved on any job. A screwdriver's size is designated by its blade - a 7" screwdriver has a 7" Blade, and so on. Naturally, those with heavier tips are specifically designed for heavy jobs, those with narrow-tipped blades are made for more delicate work. There is no such thing as an all-purpose screwdriver - the screwdriver must be suited to the job, and you always use the longest screwdriver convenient for the job. There are combinations which utilize one handle and several different blades, and these are fully practicable for a variety of jobs. The four types of screwdrivers most generally used are:
The common screwdriver - available in a wide variety of sizes, each for specific screws. Remember that the tip of the screwdriver must fit the screw slot. If too wide, the blade tip may ruin the wood around the head of the screw. If the tip is too narrow, there is a good chance that it will slip and chew up the head of the screw.
Spiral ratchet and ratchet screw drivers - may be used with blades of various sizes. Use these to drive in screws or to extract them; they work semi-automatically, much faster than the ordinary screwdriver. The handle of the ratchet type turns back and forth but the ratchet allows the blade to turn only in the direction you set.
Phillips screwdrivers - used only with Phillips screws. The head of each Phillips screw has two slots which cross at the center, guiding the blade of the screwdriver to the right location.
Offset screwdrivers - used where there isn't sufficient space to use the common screwdriver. One offset type has two blades at opposite ends, made at right angles to the handle.
There are also heavy-duty types of screwdrivers with square shanks. You can use a wrench on the shank and apply enough force to turn and loosen large, rusted screws.
In use, the handle of the screwdriver should be held firmly in your right hand, the head of the handle should be against your palm, and your thumb and fingers should grasp the handle near the ferrule. To drive the screw in, turn the handle clockwise. To remove the screw, turn it in the opposite direction.
A screwdriver does not have to be sharpened, but occasionally it must be ground on an emery wheel or the sides must be filed with a flat file. Never bevel the tip of a screwdriver. When filing the blade, use a good vise, and make the tip straight across the end. It must be at right angles to the sides and shank. Make certain that the faces of the blade near the tip are parallel to each other. This is a vital point; if it is neglected, the blade will slip often and jump out of the screws slot.
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