How to Choose Screws and Bolts

These work by pulling tightly together the pieces of metal being joined.

Machine screws pass through a hole in one piece of metal and usually screw into a matching thread already cut in the other piece. They usually have slotted heads.

Bolts have square or hexagonal heads and are usually threaded along only part of their length. A stud is threaded at both ends.

Nuts and bolts are very easy to use and provide a good strong joint. A hole slightly larger than the bolt diameter should be drilled through the parts to be joined, the bolt passed through the hole and a nut screwed on to the protruding thread by hand. The bolt should then be held and the nut tightened using a suit­able spanner. A washer placed between the nut and the metals being joined spreads the load and avoids damage.

Screws and Bolts

Screw threads and sizes

Screws, bolts and nuts come in a wide variety of shapes and sizes, in most cases specified by their diameter and thread type. The diameter of a bolt determines what size hole it will go through; the thread type determines what nut will it, and the head size determines what spanner is needed. There are sometimes two versions of a thread available fine and coarse. A fine thread screws in less quickly than a coarse thread but it is able to exert a greater clamping force for a given tightening torque than its coarse equivalent.

Most nuts and bolts used nowadays have one of the four threads recognised by the International Standards Organisa­tion (ISO). All four use the same shape of thread, but three have inch dimen­sions, called Unified (cither UNC or UNF) or ISO Inch, and the fourth is metric [SO Metric. All have both coarse and fine versions. The coarse metric thread is usually given the name Metric as it is the most common.

A number of other screw threads, now officially obsolete, are still found British Standard Whitworth (BSW), British Stan­dard Fine (BSF) and British Association (BA). There is also British Standard Pipe (BSP), used for gas and water pipe fittings.

All these types of nut and bolt are dif­ferent, though one or two can be interchanged.

BA screws are found on radio and TV equipment, particularly if it is old. The sizes are numbered from 0 BA (largest) to 10 BA. Odd numbered sizes are least common and sizes are 6mm or less in diameter; 2 BA is   in diameter.

BS (BSW and BSF) bolts are bigger -from {in diameter upwards and are known by their diameters stated in frac­tions of an inch. They are generally found on old equipment.

Unified nuts and bolts sometimes have identifying marks. The hexagonal head of a bolt may have a circular recess on the top, or a line of little circles indented on one of the six flat sides. The nut may have a similar row of circles, or a groove running round one of its two faces. Sizes smaller than {in are numbered so that the larger bolts carry the higher number; over {in the bolts are called by their actual size. Unified bolts can be found on many British cars and domestic appliances.

Metric bolts cover a whole range of sizes and are marked with an M and the diameter in millimetres M10 for example. Bolts larger than M5 may have M or ISOM marked on the head.

Both ISO bolts Unified and Metric -have hexagonal heads that increase in fractions of an inch or whole numbers of millimetres from one of the flat sides to the one directly opposite. This is often known as the width across the flats (A/F), and the spanners that fit these bolts are often labelled with this dimension. Do not confuse the bolt diameter with the head size a M8 bolt (8mm thread) with a 13mm head takes a 13mm spanner.

Spanners for BA and BS bolts are labelled with the same name as the screw thread.

Self-tapping screws

A self-tapping screw is by virtue of its shape able to cut or form its own thread in metal as it is driven in. Self-tapping screws are most commonly used for join­ing sheet metal.

There are two basic types: thread-form­ing and thread-cutting. Both types need a pilot hole the diameter of this hole depends on the gauge of the screw and, to a lesser extent, on the metal and its thickness.

As the thread-forming type is driven into the pilot hole, the hardened thread deforms the sides of the hole and forms a matching thread. This type of screw should not be used 011 brittle materials but is suitable for most metals. Two types of thread-forming screw are avail­able, known as AB and B. The pointed end of AB means that it can be used with a slightly smaller pilot hole and in thinner materials.

Screws and Bolts

When the thread-cutting type is driven into the pilot hole, the hardened thread cuts its own matching thread. This type can be used on brittle materials. The two main types of thread-cutting screws are known as BT and Y. The culling edge of type BT is a nicked-out groove in the end of its shank. Type Y has a broken thread which allows it to cut along more of the length of the shank.

The most common type of self-lapping screw available is the thread-forming type AB. Many different head shapes can be found, but the slotted pan head is the most common. The most common sizes and gauges are 10mm to 25mm and 4 to 10 gauge though self-tapping screws can be found in lengths up to 50mm and in gauges from 2 to 14. Plated, rustproof screws are also available mostly nickel or zinc.

Spring steel fasteners

It is quite common for manufacturers, particularly in the motor industry, to use spring steel fasteners for joining sheet metal components. These are used in preference to self-tapping screws because they allow the holes in the components being joined to be slightly out of line. They are also known as spire or speed nuts. They are useful for coping with over-large pilot holes and for joining thin sheet metal. The fasteners are often used with sheet metal screws which look sim­ilar to self-tapping screws but do not have a cutting edge.

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Filed Under: Home & Maintenance


About the Author: Jason Prickett loves to write about home maintenance and stuff you can do yourself instead of hiring any professional. His step by step guides will assist you in completing your home maintenance tasks.

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