Timber in the Telephone Industry
In spite of the increasing use of plastics and other substitutes for wood, the wood-cabinet-making department of a telephone works is still a very important part of the production organization.
The main reasons for the continued use of wood are its economic and technical advantages; for example, from an economic point of view, it lends itself readily to the manufacture of single units or small quantities of equipment at relatively
low cost, particularly in designs of intricate shape, because ordinary hand tools can be employed instead of the costly machines and special tools which are indispensable with some materials, irrespective of the number of parts being made. On the other hand, there is probably less difficulty in arranging for mass production with timber than with any other basic material, because of the versatility of the machines normally used in the cabinet shop. It is not generally necessary to provide special tools or devices.
The physical beauty of well-finished timber is universally appreciated. It is pleasing to the senses of sight and touch and can be made to enhance the most delicate furnishings, therefore it is not surprising that its popularity for telephone switchboards is undiminished.
The world's supply of some raw materials is by no means inexhaustible and timber is one of the few examples of a replaceable substance. Well-planned schemes of
re-afforestation are ensuring continuity of future supplies.
PRESENT DAY USES
The use of hardwoods in the construction of manual positions for main telephone exchanges has continued since the earliest
days of the industry and although challenged by metal and other materials, timber has generally maintained its position in this field. High grade Honduras or similar types of mahogany have proved to be extremely stable and have given excellent service over long periods. In some instances other timbers which lend themselves more readily to light colour schemes have been used and there is little doubt that improved techniques in seasoning and stabilizing processes will result in reliable long-term service from beech and similar woods. An excellent feature in the design of main exchange switchboards is the employment of internal steel frameworks, enabling relatively short lengths and widths of timber to be used for the individual sections, thus ensuring maximum dimensional stability.
Undoubtedly one of the principal reasons for the continued use of timber as the main constructive material for private switchboards is the wide varieties of size and form required to meet individual needs. Conditions of service may be relatively arduous and protection against extreme variations in climatic conditions must be given, but, should the surface deteriorate after long use, woodwork can be refinished locally at fairly low cost. It will be seen later, however, that interesting developments in wood laminates offer new advantages for this class of work.
There is a general tendency for small items of apparatus previously made in timber to be replaced by plastic or other non-conducting materials. The higher standards demanded for insulation have resulted in the gradual elimination of timber for terminal blocks, connection
strips etc. Certain woods have reasonable insulating properties but very careful selection and testing are essential when this point is of importance.
Battery boxes and ladders of various kinds are examples of miscellaneous equipment where the preference for timber has so far been maintained. In the case of ladders, lightness and strength, combined with reasonable electrical insulation, are features which have resisted tendencies toward the replacement of wood by metallic materials and, in addition, wooden surfaces are not cold to the touch and have good frictional properties which reduce the danger of accidents.
Timber is, of course, used in connection with all classes of telephone equipment in the ultimate packing stage. The construction of packing cases for some of the more delicate equipment might almost be termed an art, as adequate strength and resistance to shock and moisture penetration must be achieved at reasonable cost. For the main cases, soft woods and timber laminates are used almost exclusively, as most of the alternative materials do not possess the necessary mechanical properties, while processed and dried wood wool is widely employed as a shock-resistant inner packing.
The extent to which timber is used in the Company's works may be judged from the following quantities for the year 1949:-
Hard woods for cabinet work - 36,500 cu. ft.
Soft wood for packing - 50,000 cu. ft.
Plywood laminates for packing 420,000 sq. ft.
SELECTION OF TIMBER
For many years during which timber of various types was in plentiful supply, the selection of suitable varieties was a relatively simple matter. In the telephone industry
a measure of standardization was achieved, so that good quality Honduras mahogany was generally employed for the construction of switchboards for use in temperate climates. For tropical work, selected teak held an unchallenged position and gave unrivalled service in hot damp climates owing to its resistance to moisture, fungus and insect attack.
Under present conditions, it is not possible to obtain regular supplies of these desirable woods, but fortunately authorities in the timber trade, and the Forest Products Research Laboratory, have studied the problem of finding suitable alternatives and, while some of these differ in appearance from the mahogany and teak which they replace, they nevertheless possess the requisite technical features. In the following descriptions of the more important of the alternative timbers, indication is given of their behaviour under seasoning and machining processes but it is stressed that great care is always taken in selecting timbers for specific purposes.
AGBA. (Gossweilerodendron balsamiferum)
This timber has proved to be a very satisfactory alternative both for mahogany and teak. Whilst it is rather lighter in density, it possesses very good mechanical properties, being at the most only 15% less in compressive and shear strength. It is highly resistant to fungus and termite attack. Further points of importance are that it responds very well to kiln seasoning processes, is available in substantial lengths and widths and will accept a good polish by normal finishing methods.
| Fig. 1
Examples of Laminates
This is a species of Afzelia which grows generally throughout tropical West Africa and a shipment would probably consist of two or three species classified under the general name of Apa. These varieties are similar in character, but the
most noticeable difference is that of weight, which ranges from 40 to 55 lb. per cu. ft. The lighter woods have proved to be the most suitable for switchboard manufacture.
Under bending and compression tests, Doussie has been found to possess strength about 20% above that of oak. The timber is mahogany coloured and has a characteristic grain structure of considerable beauty, although it is unlike that of mahogany or teak, either of which it can replace.
IROKO (Chiorophora excelsa)
This timber is an excellent substitute for teak, being similar in appearance and having durability with equivalent strength. It has the additional advantage that the shrinkage ratio is very small, and it is not liable to warp, shake or check during, or subsequent to, kilning. From the machinist's point of view, a possible disadvantage is that sometimes a hard calcareous deposit is found in the interior of the log and this may cause difficulty in working.
TRUE AFRICAN MAHOGANIES
These comprise various species of Khaya, whilst other species with very similar characteristics and appearance are normally distributed under their own names and are typified by Gedu Nohor, Sapele, Makore, Guarea and Andiroba (Crabwood). The African mahoganies are normally identified by the districts and seaports of their origin, i.e. Lagos mahogany, Benin mahogany, Sekondi mahogany and Gold Coast mahogany, and this serves as a guide to their general properties.
Thus, broadly speaking, Lagos is consistently of fine texture with good machining and finishing qualities, and Benin is similar, but a harder and stronger wood. Sekondi and Grand Basam (another one of the group) are usually softer timbers of more open texture, whilst Gold Coast mahogany has been known to vary over very wide limits and is generally more uncertain from many points of view. In comparison with American mahoganies, the African substitutes are equivalent in many respects, but are more liable to variation.
SAPELE (Entandrophragma cqlindricum) and Assie (Entandrophragma Utile) are West African timbers with striking and attractive figuring and are in demand for cutting into veneers. They are prone to warping and twisting during seasoning unless radially sawn, but nevertheless constitute valuable additions to the mahogany range.
MAKORE (Mimusops Heckelii), ANDIROBA and AFRICAN WALNUT (Lavoa Klaineana) are lesser known woods which are becoming more popular for cabinet work but are
liable to 'move' during machining unless seasoning is carefully controlled.
IDICBO (Terminalia Ivorensis)
This is a sound timber of general utility and is very suitable for the interior fittings of switchboards, etc. It is steady in
kilning and machines well.
FREIJO (Cordia Goeldiaria)
This South American timber has proved to be excellent for switchboard construction. When quarter cut, the boards exhibit a distinctive grain figuring which requires careful matching in the suite, but in other respects Freijo is a consistent wood and will be widely used when supplies are more readily available.
BLACK BEAN (Castanospermum Australe)
This fine cabinet wood, native to Australia, possesses an attractive grain structure and is a good substitute
for teak. It seasons rather slowly and care must be exercised during glueing operations, but it ranks as a first-class wood for switchboards and similar work. Its electrical insulating properties are better than those of most timbers.
MANSONIA (Mansonia Altissima)
Mansofia closely resembles American black walnut in appearance, but is reputed to be more resistant to attack by fungus and termite. Its kilning properties are excellent and it machines well, with little tendency to tear.
Apparatus for Testing Bursting Strength.
| Fig. 3
Wood Laminate Undergoing Impact Test
Test Pieces for Assessing Strength of Cemented
The earlier types of wood laminates were referred to under the general term 'plywood'
and were built up from varying numbers of thin sheets of wood, each successive sheet being arranged so that the grain direction was opposed to that of the adjoining layers. The sheets were secured by bone glues or casein cements and, whilst they possessed considerable mechanical strength, the resistance to moisture was notoriously bad, so much so
that the product gained a poor reputation. In recent years however, superior types of plywood have been evolved and extensively used in the construction of certain types of aircraft. Tremendous improvement in strength and resistance to moisture resulted from the development and application of new types of synthetic adhesives. The new laminates, examples of which are shown in Fig. 1 are very little affected by immersion in hot water and can be exposed almost indefinitely to outside weather conditions, thus excellent dimensional stability is maintained even under excessively dry or moist heat.
The new adhesives which are facilitating the rapid development of composite soft and hard wood laminates are based on cold-setting urea-formaldehyde resins and phenol-resorcinol resins of various types. Extensive research by firms prominent in this section of the plastics industry has enabled an adequate range of these adhesives
to be placed on the market, and a considerable amount of specialised technical information is issued to the user.
It is now possible to bond satisfactorily and permanently thin sheets of wood to most of the other materials commonly used in the engineering industry, and which include metals, plastics, fibre and rubber. Our
process research and development departments have closely studied the application of these new laminates to switchboard design, with the result that important customers overseas have now accepted boards with caseworks of synthetic
resin bonded wood laminates having extremely hardwearing and attractive plastic facings. A stringent testing procedure has been developed so that these laminates can be assessed for strength, stability, resistance to wear and fastness to light.
The illustrations in Figs. 2, 3 and 4 are examples of some of the testing devices employed for the measurement of bursting strength, impact resistance and the strength of cemented joints. Test pieces are of course subjected for considerable periods to moist or dry heat, in addition to tests under normal conditions.
Reference should be made to other manufactured forms of sheet in which wood forms the main basis. In principle these are comprised of disintegrated wood fibres bonded together with modern adhesives under considerable pressure.
These products are very stable and highly resistant to moisture, although generally lower in mechanical properties when compared with solid timber or sheet laminates.