CUTTING INTERNAL SCREW THREADS

A screw is probably the most commonly used device for fixing; when working with metal, screw threads can be cut by hand by using taps for making internal threads and dies for external ones. Dies will be covered in Week 46. Cutting accurate threads is within the scope of the home metalworker, but a choice will have to be made on what thread system to adopt, since the cost of buying a full range of screw-cutting equipment would be prohibitive, especially if only a few threads are to be cut at any time. Remember threads are not interchangeable and you must therefore always keep to one system.

THREAD SYSTEMS

There have been a number of thread systems devised since Sir Joseph Whitworth produced a standard one nearly a century and a half ago. The importance of the Whitworth system was that the angle between the sides of the thread was fixed at 55 degrees and the number of threads per inch was related to the diameter of the nut or bolt.

Coarse threads are suitable for work where there is no likelihood of vibration, such as a metal framework for a garage or garden shed. Where vibration is likely, or tight joints are necessary, a fine thread should be used. Take care when using fine threads in soft metals such as aluminium, since the thread in the softer material could be stripped if overtightened.

SPANNERS These are marked according to their use. Those for BSW or BSF are in sizes relating to the diameter of the bolt or screw on which the spanner is to be used. Those used on Unified bolts or screws are marked according to the size of the hexagon on the head of the bolt or screw. The same method of sizing is used for spanners suitable for ISO Metric bolts and screws. BA screws are identified by numbers and the spanners marked accordingly. Spanners will be dealt with in greater detail later in the Course.

THREAD IDENTIFICATION This is not an easy operation. Thread gauges are available, but they are expensive and hardly worthwhile if the volume of work is low; and with the range of threads available, several sets might well be needed. The gauges will enable you to read larger diameter threads, but smaller threads are difficult to check. When you need to identify the thread – where you have broken one or need to replace a missing screw or nut – you should check the gauge with your local tool shop.

DRILL BIT SIZES

For every size of thread there is a precise size of drill bit needed to produce the correct tapping hole. If you are going to carry out a lot of thread cutting, it will be worth your while investing in a fitter’s/ machinist’s data book. This will give you tables indicating the necessary drill bit sizes. If you are able to measure the pitch of the thread you want, you can calculate the tapping drill bit size by subtracting the thread pitch dimensions from the thread diameter. To calculate the clearance drill bit size, multiply the thread diameter by 1.02. Always use the nearest size drill bit over the calculated size.

Metric standards and metric size tools and parts are becoming more common. But you may have to work in Imperial measurements if you are dealing with older equipment or pieces of work.

The correct tapping drill bit is vital for really accurate precision work, but for many normal purposes you will achieve satisfactory results if you use the nearest available drill bit size to the specified one. You should choose one slightly larger since, if a smaller one is used, you may have difficulty in getting the tap to start the thread.

The clearance drill bit is used to clear out A threaded hole; it is often needed where a thread has been spoiled and a new, larger one has to be used. Here the object is to clear out the old thread with a drill bit the same as the tapping size of the new thread, although this is not always practical.

Types of TAP

The actual tapping operation is not just a matter of screwing a tap into a prepared hole; for every size of thread there are, in fact, three types of tap- taper, second cut and plug.

TAPER TAP This type is used most since it is the one which starts the work. The end is tapered down until, at the tip, it is slightly less than the diameter of the tapping drill bit; this enables it to be inserted in the hole. For through holes, you may only need the taper tap; since only about two thirds of the tap is tapered, if the tap is screwed far enough through the hole a finished thread is produced through the whole of the work.

SECOND CUT TAP Also tapered but only for a very short distance at the front, this type takes a second cut and extends the thread. PLUG TAP This tap is not tapered, except for a slight chamfer at the working end; it is threaded full width right up to the end and is used last of the three taps – and always with a blind hole.

Each tap is carefully produced with the threads cut so there is some relief behind each cutting edge of thread. The flutes also provide an exit for the swarf produced when tapping holes.

TAP WRENCH

This tool is almost as important as the tap itself, since it has a simple but effective holding mechanism and equal length arms which help ensure a balanced torque is applied to the tap and there is no sideways strain; otherwise the tap could break. It is not possible to achieve accurate results simply by holding the tap with a spanner or pliers.

Provided you ensure the tap enters the hole at right-angles to the work, you are guaranteed reasonable accuracy; if possible, use an electric drill positioned centrally over the hole as an extra support. You can use a centre punch fitted in the drill chuck; the end of the punch rests in a hole at the top of the tap. This is not so crucial when working with thicker metal, since with a little experience it is quite easy to get the tap to follow the hole; the freehand method is not so easy, however, when working with very thin metal.

Lubricating the work will make tapping easier and reduce the chances of the tap breaking; use paraffin for aluminium and light oil for steel, copper and bronze – brass and cast iron will not need any lubricant.

Punch your marked hole and drill with the correct diameter bit; for blind holes you must drill slightly deeper than required to prevent the taps binding. Place ihe nose of the taper tap in this hole, align the tap with the centre line of the hole and turn it, making sure you keep an even pressure on each side of the tap wrench. For a right-hand thread, turn the tap clockwise; turn it anti-clockwise for a left-hand thread. Bear in mind you will need different taps for left and right-hand threads. You will need to apply some pressure at the start to make the tap cut for the first couple of threads; from then on it will pull itself into the work.

Once the tap has started cutting, you should move it forward a little, then back to break the swarf spiral and allow the swarf to fall clear of the tap. Experience will tell you when to back off, since the need for any forward pressure will indicate the swarf is jamming up the cutting action. You should never exert force on the tap if it is jamming.

Having cut as far as you can with the taper tap, you can then proceed with the second cut and plug taps if needed – unless you are making a through hole, in which case the taper tap may be sufficient.

When cutting the thread you may often find a bump forming at the point of entry into the hole; after you have finished tapping, to ensure a neat, accurate finish to the work, you can countersink the top of the hole slightly.

BROKEN TAP If a tap does break during the cutting operation, it can be removed. Usually there is enough of the tap protruding from the hole to be gripped with an adjustable wrench and unscrewed. If not, you can use a tap extractor; this has four slim metal rods which slide down the broken tap flutes, enabling you to unscrew the tap from the hole. If neither of these methods proves successful, you can soften the tap by applying heat with a blowtorch and then drill it out. This last method cannot be used with metals such as brass which have a lower melting point than the annealing temperature of the tap;

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