The Throwing House



Reproduced from the Post Office Electrical Engineers' Journal of April 1934. The illustrations are by courtesy of Messrs. Bullers, Ltd., of Milton, Stoke-on-Trent, Staffs.

It is exceedingly doubtful whether the users of porcelain insulators realize in full the variety and number of processes which the finished article will, and must have, gone through before being pronounced satisfactory for delivery from the factory. Many items .of telephone use, such as instruments, automatic equipment, including dials, and other apparatus, have obviously, from their multiplicity of small parts and intricate character, called for the application of much ingenuity in their assembly. There are more than a dozen different operations involved in the manufacture of one of the porcelain insulators used by the Post Office, whilst a steel insulator spindle, to take another example from external construction work, when made under modern mass production methods, has some sixteen different stages to go through.

Electrical porcelain is made from the four ingredients, ball clay, china clay (kaolin), silica and felspar (chiefly aluminous silicate). The first material is obtained from Dorset, the second from Cornwall, whilst the silica and felspar normally come from Norway. The steps taken to convert these into a homogeneous vitreous material of the required shape will be described. Telephone and wireless relay insulators are made by a "throwing" process, whilst larger articles, including H.T. insulators, as used by the Grid system, are made in the "jolly" shop.

What are colloquially termed the "slops" are the primary substances each thoroughly puddled with water until a definite weight per pint of fluid is secured. Naturally the silica and felspar, being rock-like in character, have to be ground down very finely, before this is done. A certain number of gallons of each slop, according to the type of porcelain, are run into large revolving cylinders. Heavy balls inside the cylinders grind and mix these up together. The "slip" is then passed to a rotating cage with 120-mesh sides to retain any particles of appreciable size; any iron particles are extracted from the slip as it runs down over a series of magnets on its way to the filter presses. These contain flat canvas bags about 2ft. square with an opening in the centre of each for the slip to enter. The presses are about 20ft. long and pressure from the ends forces out the surplus moisture so that a flat pinkish-buff pancake of moist clay is taken from each bag.

To complete the conditioning of the material, it passes in succession through two large sausage machines, from the second of which it is extruded practically free from air bubbles and with the moisture evenly distributed.

In the manufacture of the telephone insulator, the throwing house is the scene of the next stage, and here one of the oldest mechanical aids to industry, the potter's wheel, is used - the only difference being that the wheels are now driven by electrical machinery in place of the treadle arrangement. In this shop, lumps of clay are cut off and weighed roughly on scales in order to check their size and are given to the "thrower" who turns each piece on his wheel to a cylindrical form approximating to the insulator being made. The clay at this stage is still quite soft and a short period of drying off at a low heat has to be given to the cylinders of clay before the next operation can be carried out. A view of the throwing house is shown in the Frontispiece.

In the turning shop (Figure 1) each cylinder receives its final form, although all dimensions are somewhat greater to allow for shrinkage. First of all, the inner sheds are formed by means of correctly shaped cutters; then at the next lathe, the spindle thread is formed as the insulator is placed on the chuck and, guided by the profile edge of a jig, the cutting tool removes the surplus clay, trimming the outer surface as required. If leading-in insulators are being turned out, these have several extra operations including the cutting of the cavity and the threads for the cap and the drilling of the holes which will take the cable leads and wires in service.
The stages which follow lead up to the firing of the material, and for this it must be as thoroughly free from moisture as possible. The insulators or other articles are transported on boards some 6ft. long to the drying room, where they remain  exposed to the dry hot air for several days, until only combined water is left. These long boards are used throughout the factory for the conveyance of the insulators and the rather top heaviness of the No. 1 insulators when stood on the extending inner shed has led to the familiar name of "tumbledown" being given to this by the workers.

Figure 1. The Turning Shop

Figure 2. The Jolly Shop

High tension insulators with sheds 10 or 12 inches in diameter are made in the jolly shop (Figure 2). Each shed is worked up separately by roughly shaping the lump of clay and dropping it into a mould conforming to the outer surface. It is then well pummelled into contact with the sides of the mould and when this has been done the whole is rotated after the manner of the potter's wheel whilst the shape of the inner surface is produced. These insulators, too, must be dried out completely, but where a pin-type insulator has two sheds, each part is "jollyed" separately and carefully welded together with some of the same material, which later fuses as one complete mass.

In yet another shop, die pressing machines turn out small articles in repetition for uses not essentially electrical. These may be resistor blocks for fires, door knobs, accumulator stoppers, electric light switches or the small egg insulators sold by wireless retailers for aerials. The wet clay mixture used for electrical porcelain is not suitable for this work, although much the same ingredients are of course required in the form of a powder with a little lubricant for die working. This powder is granular in appearance and brick red or grey in colour, but the finished product generally is an entirely different colour. The interesting feature of these machines is the ingenuity which has been used to obtain holes at right angles to the movement of the dies and to form screw threads by pressure. The tool maker's shop where the intricate little steel dies are being prepared is of no less interest than the machines themselves.

Whatever process the article has been through it must be absolutely dry and free of moisture. At the end of the drying period, each article is examined and should there be the slightest hair line crack, the trained eye of the examiner will observe this, remove the piece and return it with other waste material to the starting point. For, once fired, a defective article is only fit for the scrap heap, but waste from the lathes or drying shops, being unfired, is valuable and easily worked up again.

The glazing fluid is similar in composition to the material of the body, but the proportions of fusible elements are much higher so that a smooth surface of the character of glass will be obtained. The colour again is no guide to that of the fired article, for whilst white insulators are dipped into a cream or buff solution, those which are to be brown are dipped into a mauve-coloured fluid.

The large champagne-bottle-shaped brick ovens, some 30 or 40 feet high, are a familiar sight in the pottery district. The internal space, 12 to 15 feet in diameter, is packed with the fire clay containers which carry the articles to be fired. When the oven is full, the fires around the circumference are lit and the heat very gradually raised to the correct temperature and then almost as carefully cooled down. It is interesting to note that actual temperatures are not recorded, but in order to judge the state of the oven a number of clay rings are put in initially at certain points and withdrawn one by one as firing progresses. The shrinkage of the ring, measured on the diameter by a special form of caliper, indicates to those in charge whether the required firing point has been reached. The same shrinkage, too, takes place in the insulators which are being fired and the dimensions change by approximately one-seventh in height and one-tenth in diameter during the process. This naturally has to be allowed for in forming the clay shapes.

In addition to three or four such ovens, there is a truck oven over 100 yards long. Along the length of this the trucks carrying the fire clay containers pass at slow speed, taking four days to arrive at the far end. The greatest heat is at the centre and is maintained constant day in and day out by gas burners fed from a producer gas plant. As the trucks travel up towards the furnace, their loads are being gradually warmed up by the heated air and waste gases coming down the tunnel from the centre. Similarly, after the trucks have passed through the furnace, the air coming in at the exit cools down the fired truck loads and at the same time becomes preheated for the burners; seemingly, a very convenient and ingenious arrangement. Figure 3 shows a truck load being withdrawn from the oven.

Once the material has been fired, no further work can be done on it and the remainder of the passage through the factory is concerned with the examination and testing prior to despatch. The Post Office requires that every single insulator shall be electrically tested to ensure that it is sound, for in service each will represent a separate parallel path to earth and the overall insulation resistance will be largely affected by the inclusion of any inherently bad insulator points. For the electrical tests - carried out in the majority of contracts by the manufacturer with occasional check tests by the Post Office Engineering Department's inspectors - the insulators are inverted in a large shallow trough and water run in until the surface is inch below the lips of the outer sheds. The interior of each cup, including the space between the two sheds, is then filled with water to half an inch from the lip and the whole left to soak for 12 hours. The tests are made between the trough and the water in the spindle hole and the water in the space between the two sheds and with a 500 volt d.c. battery and galvanometer the deflection must indicate more than 10,000 megohms to be satisfactory. Generally, if the insulator is good, it is well above this figure and, if poor, well below; but rejections are surprisingly few. Figure 4. shows insulators under test.

The insulation test is thus mainly through the material from the spindle hole, which is unglazed, to the head of the insulator, which is also unglazed. This is also the case with leading-in insulators but, in view of the connecting holes between, the insulator is only immersed so that the water is half an inch below the top of these holes, the spindle hole being filled to half an inch from the top.
The glaze is not put on to render the insulator waterproof as is shown by porosity tests which are occasionally made.

For this test, the insulator is broken to give a fresh surface and the pieces immersed in a 0.5 per cent, alcoholic solution of fuchsin for 24 hours under a pressure of 2,000 lbs. per sq. inch. When taken out and wiped with a cloth there must be absolutely no signs of impregnation by the alcohol.

The chief function of the glaze is to give a surface from which rain will readily clean off any soot and dirt. As the surface leakage may exceed by several times the leakage through the material from the spindle to the groove, this aspect is naturally an important one. It also speaks well for porcelain insulators that when they are recovered after many years and the surface well cleaned with a solution of caustic soda and "Quebracho" extract, they are found to give results practically equal to new. Experiments have been made, however, with unglazed insulators and extended field trials, which are being carried out in a variety of atmospheric conditions, show a somewhat unexpectedly good performance.

To ensure interchangeability of insulators and good fit on the spindles - on which they are to be screwed with the interposition of a 5/32in. rubber washer - the cordeaux thread of each spindle hole is tested by screw gauges. These are a "go" screw gauge to check that the insulator will screw on any spindle, a "not go" screw gauge and a "not go" plug gauge to see that it will not be too loose on the spindle, and a "tell-tale" plunger gauge, which measures both the depth of the screwed cavity and the length of the shed. Certain limits must be observed on these gauges; and the insulators which pass are ready to be packed and despatched to the depot of the Post Office Stores Department, from which they will in due course be issued for the erection of circuits throughout the country.

Figure 3. Exit End of the Truck Oven

Figure 4. Electrical Testing of Insulators


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