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14.How To Fit Screws

2015-09-20

AS common fixing devices, screws are often taken for granted. Vet on their correct choice and fitting depend the good appearance and soundness of construction of work in which they are used. As is well known, they are of various sorts, shapes and size, those illustrated being the most frequently used. The woodscrew A may be in steel or brass, with a countersunk, round, or cheese head, or with a raised head, partly countersunk, partly round – often used in fixing interior panels and fittings in cars, when the head is chromium plated. The countersink angle is 90 degrees. Large screws with square spanner-heads are screw bolts, or coach screws; dowel screws have a thread each end and no head. The self-tapping screw B is in hardened steel, and can only be shortened by grinding. It may be provided with a countersunk, round, or sun head-partly rounded. This screw is used for fixing sheet metal panels. The outer panel (or part) is drilled clearance for the screw, and the interior panel drilled the core diameter. The screw is then forced through, cuts its own thread and pulls tight. It is often used for minor fittings in motor bodies. In other than sheet metal, it can only be driven a limited distances. 

 

How To Fit Screws 

 

The ordinary metal screw C has a thread like a bolt, and a countersunk, round, or cheese head. It can be in steel or brass. When provided with a hexagon head D, it is a set-screw or sometimes a set-pin. Its uses are widespread. The grub-screw E has a 120-deg. pointed end and a screwdriver slot. It is commonly used for fixing collars, uullevs and small wheels to shafts, the bosses being drilled and tapped to take it. It has the most neat appearance when just flush with the boss.The headless socket screw F has a hexagon socket to take a key or spanner to tighten it. It is used much like a grub-screw, but can be obtained with a variety of ends-pointed, oval, hollow, flat, and with a small plain portion, when it is termed a dog point. It is in hardened steel.

 

Choosing the Screw

The choice of screws is narrowed by the materials being fixed, their thickness, and where the screws will be placed. Woodscrews can only be used on occasion in these materials and in sheet metal. Metal screws and setscrews can be employed in tapped holes, or in clearance holes, and for wood, etc., when nuts and washers are fitted. The length of screw is often governed by the thickness of materials – a woodscrew, for example, should not penetrate a panel for the point to emerge. Similarly, the size is frequently governed by the thickness of the material into which a screw enters edgewise, both from the point of view of splitting the material, and to obviate the head of the screw over: lapping the edge. In the case of grub-screws, they should be large enough to secure the pulley or collar firmly, but not so large as to distort or split the boss.

 

Countersinking

Two countersinking drills (or rose bits) are required, one for metal, one for wood. The one for wood will not cut metal, and the metal one may chatter in wood. Drills with only two flutes may be used with discretion in metal, when the angle has been altered. Before countersinking a clearance hole should be drilled, then the countersink run in so the head of the screw will just lie flush-for which operation care is required to produce neat, uniform work. The continuing hole for a woodscrew may then be made with a bradawl, if the screw is small. If lame, a core drill should be run in. this is important for hardwoods in which screws can be wrung off in fitting-particularly brass. In larger sizes, woodscrews are smeared with grease when fitting to enter and remove easier. When a metal panel is being fixed with countersunk screws, a countersink with a more acute angle can be used on occasion; the screw head will then bite on and hold the panel more firmly. An altered drill can be used for this type of countersinking.

 

Correct Screwdriver 

The width of the blade of the screwdriver should be almost the diameter of the screw head, and its thickness so as just to enter the slot in the screw. The blade should be square and flat, not rounded at the ends, or sharpened like a chisel. A blade which is too narrow will damage the screw head, while one which is too wide will not drive the screw properly, and will score the surrounding surface.

 

Preventing Nuts Unscrewing

The problem of preventing nuts and threads slackening from vibration or other movement has been solved in various ways according to the requirements of assemblies, and, as usual, there are the “ do’s ” and “ don’ts.” Where rotation is in a particular direction, threads can be right or left-hand so there is a tendency for them to tighten. Spindles of cycle pedals, wheel hub nuts of cars and on occasion the actual wheel nuts of lorries are examples of this method and there are numerous other applications. Naturally care must be taken in dismantling to turn in the required direction. Split pins are perhaps the most common locking devices for nuts, whether on studs or bolts, and require the nuts to be either slotted or castle type, A and B. The slotted nut A needs less room where space is lacking, but is not so strong as the castle nut B with its circular portion on top in corporation the slots. Consequently, slotted nuts must not be substituted for castle nuts in important fittings. Split pins should fit the holes reasonably tightly, their heads be tapped into slots, then the legs opened round the ends of the bolts. Where there is continuous vibration (car big-end bearings), loose split pins will eventually wear so the legs break off and the pins fall out.

 

 

The  Lock-nut

Another common device is the lock-nut C on U-bolts of car springs. Usually, the thin nut is on the end or outside, though in this position some engineers declare it takes the load on the bolt, owing to slight slackness in the threads of the thicker standard nut. Plain washers do not prevent unscrewing, but they are used in preference to others where firm pressure is required and it is desired to avoid scoring a metal surface by the underside of nuts. There are two types of tab washer, however, to provide security the single type D and the multiple type fitting on two or more bolts or studs, with a tab at each position to turn up on the nut.

In final tightening of nuts, care is required on multiple tab washers not to twist them, and on single tabs not to turn them into a wrong attitude for locking.

 

Spring Washer Types 

In general use, three types of spring washer provide security: single coil E, double coil (similar) and the serrated type with bore or outside cut. Spring washers should not be used where loads are heavy, or the undersurface can be scored from dismantling. Under pressure, a single coil washer will often break or splay; a double coil, splay, and a serrated type break. Coarse, sharp single coil washers can be much improved by slightly straightening-in the vice, using pliers. Unless assemble is permanent, studs or bolts should not be burred. However, the centre punch dot F is often used on aircraft sub-assemblies. For removal, the displaced portion is chiseled or filed off. The multiple-tab washer G is used with keyed or splined shafts (car hub nuts). For unscrewing, the outside tabs must be fully released or the inner one will be sheared. Nuts fitted with a fibre insert H provide security against unscrewing for two or three removals – after which they are not so secure. They should not be used where heat can dry or burn the fibre. All steel nuts Z have an undersize piece at the end which grips the thread on the bolt or stud. These are a modem alternative for castle nuts on car big-end bolts. They avoid the difficulty of the split pin holes not being in line when the nuts are fully tight to overcome which, castle nuts have to be removed and filed on the bottom. In many fittings, a normal nut can be held by a small screw against a flat J – motorcycle crankpins. Studs can be fitted and removed by means of two nuts locked firmly together K – turning on the top one for fitting, on the bottom one for removing. Immovable nuts may be split L with a sharp chisel down one of the flats, or a small drill down one of the comers M – then a chisel used for splitting. Alternatively, a chisel and support block can be used as N. 

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