ERICSSON BULLETIN
No. 5 PAGE No. 4


From Tree to Telephone
July 1934

The supremacy of timber as one of the main materials used in the manufacture of telephone exchange equipment and instruments has been seriously challenged during the last few years. Automatic apparatus, housed in steel framework, and moulded synthetic resin products have become the greatest rivals to the familiar switchboards and wooden instruments.

Nevertheless, as a visit to our Cabinet Factory would show, wood still plays a very important part. It has many desirable advantages which retain it in high favour, apart from the general appeal of well finished cabinet work. There are practically no limits to size and shape as in the case of mouldings. Ability to withstand severe usage, ease of repair and the wide varieties of instruments which may be obtained at low cost to please individual taste and requirements are points which in themselves
will no doubt ensure the continuation of woodwork in the telephone industry. Even in the case of apparatus of uniform design, monotony is relieved by ever differing markings of the grain and the depths of colour.

Timber arriving at the works
Drying sheds which also contain the seasoning kilns are in the background

The cabinet maker has many types and sources of timber from which to choose. A short description of the most important in telephone manufacture will doubtless be of interest.

Mahogany
This is by far the most largely used wood for exchange and subscribers’ apparatus, and it is specified extensively by the British
Post Office. It possesses uniformity of grain and texture, may readily be machined and has good hardness and strength. Supplies are obtained from several parts of the British Empire. The West Coast of Africa exports large quantities, Lagos and Benin being sources of the most favoured varieties. The trees grow to very large dimensions, many of the logs received in this country being four feet square and twenty feet long.

These logs are hewn square by natives who are experts with the axe. Although shaped by hand, large logs will not vary more than, say one half inch in width over the whole length.

Cutting into boards is usually left to be carried out in this country as the Coast arrangements are somewhat primitive and large modern machines as used here are not generally available.

Mahogany of very fine quality has been imported from British Honduras for over two hundred years. Close grained, stable and uniform in colour, this wood is used for large exchange and other switchboards. The industry is now becoming highly developed and supplies are accurately graded. On an average, over 10 million feet per annum are exported by British Honduras to various parts of the world.

Mahogany is used also for the manufacture of telephones, bell cases, mounting boards, and desks. A fine example of workmanship in this wood may be seen in the Ericsson loudspeaking intercommunication master set, which is excellent in design and finish. (Bulletin No. 3, page 14).

Teak
This wood is dark in colour when seasoned, is heavy, strong and has an oily appearance. Being immune from insect attack, it is a very important material for the manufacture of equipment for use in tropical countries. Portable sets and mining switchboards exposed to damp or outside conditions give excellent service when constructed from teak.

Again many sources of supply are available, but the best known and most used for telephone work is British Burma from the Port of Rangoon. The timber is shipped both in the log form and as sawn boards and is usually partly seasoned. There are extensive forests of teak in Burma and the work is carried out by natives under expert supervision. For haulage or transport, elephant power is used and these animals are specially trained for the work. Naturally they are greatly valued by the owners and may cost between £600 and £700 each.

Teak is rather more difficult to machine than the average wood. Joints are not readily secured by glue owing to the oily surface, so that additional strength is obtained by the use of non-ferrous screws. Finishing also presents certain difficulties although if precautions are observed, the usual types of varnishes may be applied. The most serviceable method is finally to wax polish the surface, and this process is specified by many Colonial Authorities, particularly where equipment must withstand severe use and be maintained by partly skilled native operators.

Oak
A comparatively limited amount of this timber is used for telephone work, although it is one of the few hardwoods of which supplies grown in this Country may be obtained. Canada, Africa and India also export various types of oak.

The timber, when quarter sawn, usually shows a very attractive figuring and is much prized on account of this, apart from its hardness and strength. This class of timber is chosen by many railway companies because it will withstand very hard service and unlike mahogany it is difficult to mark.

Hard Whitewoods
In this class we find beech, maple and ash as the woods used for the miscellaneous parts where fineness of grain and strength are of prime importance. Ladders for exchanges, jack spacers, fanning strips, terminal boards and blocks of many types are suited ideally by these timbers. They are capable of being machined to a smooth finish and when drilled give clean and sharp edges.

Mention might also be made of pearwood which is noted for its fine turning properties and its high electrical insulation. This wood holds a small amount of moisture and is suitable for terminal mountings exposed to damp conditions.

Soft Woods
Parts such as battery boxes, and rear doors of switchboards which are not seen when in service are naturally made from the less expensive woods. Columbian pine and other Canadian products of similar type are used extensively for these purposes.

Much is being done and remains to be done in the development of Empire timbers. The British Post Office has given every encouragement to the use of lesser known types. Names such as chuglam, pyinma, haldu and Tasmanian myrtle are unfamiliar though attractive woods of the mahogany type and are worthy of more extensive use. Australian and New Zealand woods of various kinds are now establishing themselves in our markets and there is no doubt that as methods of production, grading and distribution are developed, their use will be extended considerably. With present-day manufacturing conditions it is, of course, essential that supplies are available in this country at short notice.

Seasoning of Timber
This is the first and most important treatment which the wood receives before being machined and assembled. Seasoning does not, of course, mean simply the drying of the material. The actual object of the process is to adjust accurately the moisture content of the wood to the amount which is retained under normal atmospheric conditions. At the same time the operations must be carried out under conditions that will not strain or otherwise damage the structure of the fibres. The efficiency with which the seasoning is carried out determines to a large extent the stability of the finished cabinet work and its freedom from warping, shrinkage or expansion. Certain grades of timber are more sensitive than others, and even the direction of sawing the original log has a marked effect on the tendency to warp.

Most woods in service in factories, offices or homes are found to take up a definite amount of moisture. This is usually 9% to 11% by weight and the figure varies but little from season to season. Decided changes may occur in the volume of timber, dimensions increasing or decreasing with corresponding variations in the water percentage. For example, wood containing 15% moisture would shrink when put into service, while if the figure were 6% when manufactured the finished parts would expand when the moisture increased to its normal figure.

Modern requirements rarely permit the natural seasoning of timber. The wood in this case is simply stacked so that air has access to the whole of the surfaces and the moisture gradually adjusts itself to normal conditions. The much more rapid kiln drying is generally employed for factories where time is limited. Contrary to certain beliefs, excellent results are obtained, equal to or even better than those given by the natural process.

Each stage of the seasoning is carried out under careful control at our Beeston factory. As soon as the timber is received, a test is made to determine the moisture content which may be as high as 50% of the total weight - usually, however, the amount is about 25%. These figures give an indication of the time which will be needed before the material is ready for use.

The latest type of drying kiln is used by the Ericsson Company. The timber for seasoning is first loaded on to steel trucks, suitable spacing strips being inserted between the boards to permit free circulation of air. A number of representative boards are selected and from each of the ends test pieces are cut and accurately measured for moisture content. From this result is calculated the weight that the boards will possess when containing the desired 9% of moisture. By regularly weighing these pieces and by reference to a graph, the progress of the timber while being seasoned may be constantly observed.
After loading the trucks are hauled into the kiln which is then sealed.

The actual adjustment of the moisture content is carried out by an efficient apparatus for circulating a uniform mixture of air and steam. The proportions may be varied to give the desired temperatures and humidity. On the sides of the kiln, travelling baffles are fitted and these direct the stream of vapour through the individual spaces between the boards. When the bottom of the stack is reached, the flow is reversed, so that the air and steam passes in the opposite direction. This procedure gives very uniform results, each sequence occupying about one minute.

No attempt is made to commence drying the timber immediately as this would tend to seal the surface, resulting in a dry exterior with a wet centre. The kiln is first saturated with steam, and air is not introduced until the temperature is raised to the desired degree. As air is gradually introduced, the percentage of humidity and the temperature are consequently lowered. Drying commences gradually at this stage and the test pieces are weighed daily until the wood contains the desired moisture content. A final short period of steaming to relieve strains in the boards completes the process. After cooling, check tests are made on boards selected at random, and the timber is then ready for machining.

The whole time occupied by the seasoning is from ten to twenty days, depending upon the original moisture content, the thickness and the type of timber. The tests already described are supplemented by practical cutting trials which ensure that the boards are free from strain.

The high temperatures attained during kiln seasoning effectively kill any grubs or larva which may be hidden away. This advantage is of considerable importance as naturally seasoned timber may give trouble in this respect.

Cabinet-making
A wide range of machines is necessary for the manufacture of telephone equipment from wood. Our Beeston factory is equipped with modern and efficient tools which produce a wide variety of work. Large quantities of parts are also provided for the wireless and other electrical industries, a high standard of workmanship and finish being maintained.

The boards are first cut to the required lengths and then issued to the cabinet shop for the various processes of manufacture. As a typical example of the numerous operations which the wood undergoes, we may follow the progress of a switchboard.

The sanding and fitting up section of the Cabinet making Department

Firstly, the material is carefully examined, being graded so that the colour and type of grain on the completed work will match to give a natural appearance. This needs judgment coupled with experience and has no small bearing on the final effect obtained.

The boards then pass through a series of machines, and as it is rarely possible to obtain sound timber of adequate width they are joined together by an ingenious jointing machine. This is a caterpillar-like device which automatically carries out the whole process. On being fed into the machine simultaneously at both ends, the boards are propelled towards the centre of the bed plate where they are joined in pairs. Before they reach each other, dove-tail shaped grooves are cut into the edges along their whole length. These faces are then coated with a thin layer of glue and eventually are fitted together by sliding, board to board. As a matter of fact, the machining is so accurate that the pieces of wood are secured firmly even without the joint being made permanent by glue. This process is found to be practically the only one which gives trouble-free results on equipment used in tropical countries.

After careful inspection, the boards pass through the flattening and planing machines which are fitted with high speed rotary steel cutters. These produce a smooth level surface and the material now has an attractive appearance as the true character of the wood is revealed. Next comes the process which adjusts the width to within the required limits, followed by a sandpapering operation which is carried out by passing the boards between revolving steel cylinders fitted with the abrasive material.

Not until these stages are passed does the switchboard commence to assume a really discernable form. Bandsaws, aided by guides in the form of templates, produce uniform curves for the sides. Moulding machines with shaped rotary cutters prepare the neatly designed ornamental rails. Joints for fitting the parts together are prepared on mortice and tenon machines, while complicated recesses are produced by an interesting machine where the cutting is performed by means of an eccentric drill revolving at a very high speed.

Preparations are made on drilling machines for the fitting and housing of the actual telephone apparatus, and the parts are then ready for fitting up. Accurate machining facilitates this process and before long the switchboards are ready for hand sand-papering and final inspection.

Lastly, the products are placed in the hands of the finishing department where high grade cellulose finishes, applied by spray, soon transform the appearance of the woodwork. The methods used in this department have been described in a previous issue of the “Ericsson Bulletin.” The wood cases are then ready to be fitted up with apparatus and wired as complete switchboards.

It will be seen that experience and equipment enable the Ericsson Company to produce a wide range of cabinet work of superior quality, and the highest satisfaction in service is assured by the care taken at every stage.

BACK Home page BT/GPO Telephones Ericsson Telephones Ericsson Home Page Ericsson PABX All Telephone Systems

Last revised: February 18, 2003