How to screw in metal

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If you want to fix something to metal, you can use different kinds of bolts and screws, for example: hex-head bolts with full-length or short thread, Allen-head bolts, carriage bolts, metal screws and board screws. The usual sizes in Europe are in millimeters, although in the UK and USA inch sizes (”) are commonly used.

  1. Bolts and screws

    Bolts and screws for use in metal are available with many different head shapes, such as:
    1. hex head
    2. round head
    3. countersunk head
    4. cylindrical head
    5. countersunk Allen head
    6. cylindrical Allen head
    7. countersunk crosshead
    8. knurled head

  2. Nuts

    Nuts are used for different purposes and are available in many different types and sizes. The most common types are:
    1. hex head nut
    2. ring nut
    3. lock nut
    4. cap nut
    5. castle nut
    6. wing nut

  3. Washers

    A washer is fitted under a screw, bolt or nut and spreads the pressure to protect the workpiece while the nut is being tightened or loosened. A washer can also be used to seal the joint against moisture.
    1. plain washer
    2. split washer
    3. star washer
    4. circlip (internal)
    5. circlip (external)
    6. O-ring

  4. Choosing the right wrench

    The sizes of nuts and bolts are often indicated in metric screw thread sizes – for example an M8 bolt. Each nut or bolt size has a corresponding head or wrench size. In Europe these sizes are given in millimeters (mm). The bolt and wrench sizes are shown in the table below:

    Bolt    Wrench
    M3      5.5
    M4      7
    M5      8
    M6      10
    M7      11
    M8      13
    M10    17
    M12    19
    M14    22
    M16    24
    M18    27
    M20    30
    M22    32
    M24    36
    M27    41
    M30    46
    M33    50
    M36    55

  5. Open-end wrench

    Open-end wrenches normally have two head sizes, so you can tighten and loosen all the most commonly used nut and bolt sizes with a small number of wrenches (6-7 mm, 8-9 mm, 10-11 mm, 12-13 mm, 14-15 mm, 16-17 mm and 18-19 mm).

  6. Box-end wrench

    The head of a box-end wrench (or ring spanner) has 12 internal teeth and fits entirely round the head of the nut or bolt. This wrench allows a lot of force to be used (but make sure the wrench fits securely all round the head of the nut or bolt!).

  7. Socket

    A socket wrench with a selection of sockets of different sizes is a useful and versatile tool. The socket wrench has a left/right direction selector so it can be used for both tightening and loosening nuts and bolts.

  8. Spark plug wrench

    A spark plug wrench (or box spanner) is a tube shaped at the end to fit a nut or bolt of a specific size. This type of wrench is useful especially for nuts and bolts which are hard to reach because they are too deeply recessed for an open-end wrench, box-end wrench or socket. At the ends of the tube are two holes in which you can insert a metal bar or screwdriver so you can use more force to turn the wrench.

  9. Adjustable wrench

    This wrench has jaws which can be adjusted by turning a small wheel at the top of the handle so it fits nuts and bolts of different sizes. Note: the head of the nut or bolt can easily be damaged if the jaws of the adjustable wrench don’t fit it securely.

  10. Hex key or Allen key

    A hex key or Allen key has a right-angled handle with a hexagonal head at one or both ends. Hex keys are used to tighten or loosen hex-head screws and bolts.
    Standard sizes are: 1, 2, 3, 4, 5, 5½, 6, 8 and 10 mm.

  11. Hex-head bolts (short thread)

    Hex-head bolts with metric screw thread are available in sizes from M4 to M24 and in lengths from 20 to 200 mm. Hex-head bolts are used for joining components and materials together. They are tightened using an open-end or box-end wrench. You’ll need two wrenches: one to hold the nut and the other to prevent the bolt from turning at the same time. Nuts fit onto bolts with the same size indication.

  12. Hex-head bolts (full-length thread)

    Hex-head bolts with a full-length metric thread are available in sizes from M4 to M16 and in lengths from 6 to 200 mm. These bolts are the same as the hex-head bolts above (with short thread), but in this case they have a full-length thread that extends to just under the hex head.

  13. Allen-head bolts

    Allen-head bolts with metric screw threads are available in sizes from M3 to M12 and in lengths from 6 to 120 mm. These bolts have a head with an internal hexagon fitting, and they are tightened or loosened with a hex key or Allen key. A hex-head or Allen-head bolt has the advantage of a very secure grip with the matching hex key or Allen key. You can also use it in hard-to-reach places.

  14. Carriage bolts

    Carriage bolts with metric screw thread are available in sizes from M5 to M12 and in lengths from 16 to 180 mm. Directly under the smooth head is a square-section neck. The bolt is usually driven in by striking the head so the square-section part enters the material. The bolt is then firmly fixed and does not turn when the nut is tightened. The rounded head not only gives this bolt an attractive finish, but it also increases security – the bolt cannot be unscrewed from the outside, which makes it ideal for fixing gates or garage doors.

  15. Metal screws

    Metal screws with a cylindrical head and metric screw thread are available in sizes from M2 to M4 and in lengths from 4 to 30 mm.

  16. Board screws / self-tapping screws

    Board screws or self-tapping screws with cylindrical head, PH or PZ fitting and self tapping thread are available in sizes from 2.2 to 3.5 mm and in lengths from 6.5 to 38 mm.
    The shaft of the screw has a sharply pointed tip and a coarse thread that extends right up to the head of the screw. This means the screw easily engages in the material, and even forms its own screw thread as it is tightened. You should always predrill the hole using a drill with a smaller diameter than the thickness of the screw shaft.

  17. Self-drilling screws

    To fix two sheets of metal together it’s handy to use self-drilling screws. With these screws there is no need for predrilling.

    Self-drilling screws are suitable for fixing thin sheets to thicker ones, or brackets to metal posts. In combination with a locking plate they can be used to fix parts together firmly. Assembly is much quicker than with pop rivets or nuts and bolts, also because there’s no need for predrilling. These screws are available in lengths from 6 to 27 mm.

  18. Cutting an internal thread I

    If you want to cut an internal screw thread in metal, for example so you can insert a bolt, you need to use a tap. Before you can use the tap you first have to predrill the hole with a drill diameter that is slightly larger than the core of the tap but smaller than the outer thread of the tap.
    To find the right size of the hole to drill, use the following rule:
    The diameter of the hole to be drilled is 80% of the size indication on the tap.
    Example: to cut an M6 thread you need to drill a hole of 6 x 0.8 = 4.8 mm, or rounded-off 5 mm (see the table).

  19. Cutting an internal thread II

    You then use a 3-piece standard tap set to cut the thread in the predrilled hole. You do this in 3 stages using the 3 taps together with a handle.
    • 1st cut: using a taper tap to pre-cut the thread
    • 2nd cut: using an intermediate tap with a deeper thread
    • 3rd tap: using a plug tap to cut the thread to the final depth
    If you’re cutting the thread by hand you need to make sure the tap is at right-angles to the workpiece. It’s very important to break off and remove the metal shavings constantly: after every 2 turns in the cutting direction (clockwise), turn the tap back half a turn. This helps to prevent the tap from breaking. Using cutting oil makes the process smoother. You can also tap the thread directly with an electric drill, using special machine taps which cut the required thread in one action. These machine taps are only suitable for small diameters.

  20. Cutting an external thread I

    You can cut an external thread on material with a round cross-section such as threaded rod, shaft or pipe.
    This is done using a die and a holder. Dies have ground cutting teeth to cut the thread on the outside of the metal workpiece. The thread is cut to the full depth in a single action. A different die is needed for each rod diameter. The dies can also be used to repair damaged bolt threads.

  21. Cutting an external thread II

    When you cut the thread you need to exert a lot of force while turning the die to the right (clockwise). After each complete turn of the die, turn it back a quarter of a turn to remove the metal shavings. Check constantly that the die is a right-angles to the workpiece and use cutting oil, as otherwise you won’t get an even, good-quality thread and the cutting will be very heavy.

  22. Using pop rivets I

    Pop rivets or blind rivets are used to fix thin materials or sheets together. A pop rivet is a metal pin with a short length of aluminium tube and a thicker section at one end. Drill a hole through the two materials to be fixed together and insert a pop rivet in the hole. The head of the pop rivet must protrude completely from the other side of the material. The pop rivet is then secured using riveting tongs.
    Pop rivets are available in different diameters. When you choose a pop rivet don’t just look at the size but also at the length of the fastening section of the rivet.

  23. Using pop rivets II

    The process of making a pop riveted joint is as follows:
    1 When the riveting tongs are first closed the pin of the rivet is clamped and pulled forward in the material
    2 The aluminium tube is compressed at the back of the material
    3 When the tongs are closed further the aluminium tube expands
    4 The pin breaks off when enough force is exerted on the tongs
    A pop rivet is very stable and can’t easily be removed.
    The big advantage of using pop rivets is that you can make a strong joint even in places where you can’t reach the rear of the material. This isn’t possible using nuts and bolts.

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