CUTTING EXTERNAL SCREW THREADS

Cutting external threads is similar in principle to cutting internal ones; however, instead of making the thread on the inside of a drilled hole, you are making it on the outside of a piece of rod or tubing. For this you use a die. held in a stock or holder.

Types of die

Unlike tapping, usually only one die is needed to make a complete thread because the type of die you are most likely to use – the split ring die – is adjustable. As with the tap, the die thread is tapered off towards the start of the thread so the first cut can be made; but because the die works only on a small portion of the new thread at any one time. this tapered section is very small. SPLIT RING die This is most commonly used because it is adjustable for the first and any subsequent cuts needed until the final thread is produced. It is usually used for threads below 13mm in diameter.

Die NUT Depending on the job in hand, it is often not possible to operate the conventional die and a die nut must be used. Unlike the ordinary die this nut is solid, with escape holes for the swarf or broken thread, and can be turned with an ordinary spanner. A ring spanner is preferable, but an open-ended spanner can be used. The nut has no adjustment, restricting the thread to the exact size of the die. It is used in cases where there is no room to operate the die stock and also for reducing an existing thread or renewing a damaged thread of the same diameter.

DIE STOCK

While the tap holder is important for tapping screw threads, the die stock is essential when cutting external threads. It has an expanding screw in the middle, which is tapered at its point, and a locking screw at either side, both of which are substantially flat. When operated, these screws alter the diameter of the cut and therefore the thread.

When the locking screws are loosened and the expanding screw tightened, the split in the die is forced open and the resulting thread diameter is enlarged; as the expanding screw is loosened and the locking screws tightened, the split closes and the thread diameter is reduced.

Using dies

When cutting external threads, lubrication is important because there is much more risk of taking too large a cut and therefore more chance of tearing the metal. The resulting thread will be too rough and often too much material will have been removed for a final finishing cut to be made. For a lubricant, use oil with steel, copper and bronze and paraffin with aluminium; brass and cast iron are worked dry.

Hold the work securely in a vice or cramp and locate the die on it. The rod or tube being worked must have a slight chamfer filed on the leading end to enable a start to be made. This must be carefully tiled since, if the chamfer is not even, there is a danger the die will not sit squarely on the work and the resulting thread will not be parallel to the axis of the work – this is known as a drunken thread.

Unscrew the expanding and locking screws in the die stock and insert the die, making sure the lettering on it is showing on the top side. The split must align with the expanding screw. Tighten the expanding screw so it is embedded in the split and the locking screws so they hold the die firmly. Make sure there is no waste metal around the die which would throw it out of line with the stock. When the die and stock are placed over the work the lettering on the die must face downwards away from you. Check the die is squarely on the work and the stock horizontal.

You can help ensure accuracy by using an electric drill held in a drill stand directly over the work. Put a piece of pipe between the chuck and the die stock, making sure the ends of the pipe are at right-angles to the work. This will help support the work and ensure the die cuts squarely to the work.

Apply downward pressure to make the first couple of cuts, turning the stock in a clockwise direction for a right-hand thread. Turn it in an anti-clockwise direction if you want a left-hand thread, using the appropriate thread die. The die will then pull itself forward down the work as it cuts I he thread and no more pressure will be needed. Keep the die well lubricated once the cutting has started. To clear any swarf from the work move the die forward a little, then back to break the swarf spiral and allow it to fall clear of the die. Experience will tell you when to back off, since the need for extra forward pressure will indicate the swarf is jamming up the cutting. Never apply extra force.

When vou have cut a thread down to the required depth, check with the internal thread to be matched with the work; if the fit is too tight, you must repeat the process after adjusting the split ring die. Repeat the procedure until you achieve a perfect fit.

REPAIRING THREAD

Sometimes a thread will get bruised, even when you take the precaution of holding it between soft jaws in a vice when tackling any work. This may not be serious, but it can be enough to make the thread bind. Slight damage can often be repaired with the help of the edge of a half-round file. You may be able to restore a partially damaged thread by using the correct diameter die nut. Much the best results will. however, be achieved by making a thread restorer. THREAD RESTORER This is made by cutting an identical thread nut diametrically in half through opposite corners of its hexagon. The cut faces must be lightly dressed with a file and the two halves hardened. This hardening is not critical, so the two halves of the nut can simply be heated in a blowlamp flame and quenched in water. This will make them extremely brittle, SO you should temper them. Polish one face of each nut half with emery cloth until bright, place the halves on a metal plate and warm them over the blowlamp flame until the polished face turns a straw colour; remove them from the heat and quench them quickly again in water.

Fit the halves of the nut around a section of the offending thread that is not damaged and grip the split nut in a vice. By adjusting the grip of the vice and running the damaged thread through the split nut several times, the thread can be restored.

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