Fifty tons of molten metal may have to be poured into one mould for the casting of a huge propeller for a transatlantic liner. Precision and accuracy are essential in every operation of casting, fettling, boring or milling, and amazing machines have been devised for these purposes
TWO LADLES OF MOLTEN BRONZE pour the metal into the mould of a large propeller. The mould is buried in a pit and the metal flows in through the runner boxes, which act as reservoirs so that the rate of flow can be controlled. In the centre is the riser, an extension of the propeller boss, provided so that any oxide will rise to the top and can be skimmed off by the men.
THE greatest accuracy and care must be used in designing and making large propellers. It calls for the highest skill of the foundry and workshop and still remains an art rather than an exact science. There is no such thing as a stock size in propellers. Each must be specially built for the ship to which it is to be fitted. The design depends upon so many factors of different kinds that it is almost impossible to lay down the exact procedure to be adopted in each particular design. In making a propeller a great deal depends upon past experience.
Large modern propellers are always cast in one piece. That is to say, the blades and the “boss” or hub are in one piece. The propellers for those fine ships, the Umbria and the Etruria, built in 1884, were made with removable blades. These propellers had a diameter of 24 ft. 6 in. and weighed 39 tons, four tons heavier than the individual propellers of the Cunard White Star transatlantic liner R.M.S. Queen Mary.
The four propellers of the Queen Mary are among the largest used in modern marine practice. They are four-bladed, and have a diameter of nearly 20 feet, with a pitch of 18 ft. 2 in. The transport of these 35-tons propellers from the foundries to the shipbuilding yard was a problem of considerable difficulty. Each propeller was taken to Surrey Commercial Docks, London, in a 10-tons eight-wheeled lorry, and loaded into a cargo vessel by a mammoth floating crane. Then they were shipped to the Clyde and fitted to the huge liner. Eight similar propellers were made for the Queen Mary, four for working and four spare ones.
Before the work of making one of these large propellers can be put in hand, the final design must be carefully worked out. For this purpose large numbers of experiments on model propellers are carried out, until the exact pitch and diameter to give the required speed have been determined. A perfectly shaped wax model of the finished vessel without propellers is towed through water by a structure resembling an overhead travelling crane, in a large tank similar to a swimming bath. The forces acting on the model are accurately measured by delicate balances, and are automatically recorded.
Other tests are made also with model propellers, rotated by an electric motor in the wax hull, which takes its current through a flexible cable from the towing structure. Calculations based on the two performances indicate the horse-power required and the suitability or otherwise of the type of propeller proposed.
BUILDING UP A MOULD for a propeller casting. The mould is a large structure built up of bricks on an iron foundation plate. In the preparation of the sand mould the striker board (top right) plays an important part. One end of this board moves up and down a central steel shaft and the level is determined by the sloping timber guide. Notches in the striking board raise marks on the sand for fixing the zinc templets.
Then it is the task of the drawing office at the propeller foundry to make the working drawings. These show the exact shape of the sections at certain fixed points, and the work of casting and finishing the propeller can then be started.
The metal generally used is a tough bronze alloy, the ingredients of which have been found by long and patient research. The tips of the propeller blades of a vessel such as the Queen Mary travel through the water at about sixty miles an hour, so that the metal has to stand up to heavy stresses. Furthermore, as the propeller turns in the water it forms whirls and vortices which are being continually broken up by inrushes of the water flowing past the hull. This means that the faces of the blades are subjected to a persistent hammering, and pressures amounting to hundreds of tons on every square inch are imposed upon them.
When this effect becomes too pronounced, which may be due to the fact that the blades are not large enough, vibration will be set up in the blades and transmitted to the vessel, and serious pitting, or erosion, may occur. This gives some indication of the reason why the correct choice of metal is so vitally important.
The first step in manufacture is the preparation of the mould, a huge structure built up of bricks on a steel foundation plate. The shape of the propeller face on the sand mould is formed by a board known as a “striking board”, which is constrained to move up and down a central steel shaft and along a curved timber guide. The movement of the striking board gives an exact representation of the final curved surface. The striking board is notched at regular intervals for the purpose of forming raised marks in the moulding sand. It is upon these marks that zinc templets are fixed. Each of these templets is cut to the shape of the cross-section of the propeller at that particular point.
The next stage is to fill the spaces between the templets with sand, the sand being carefully smoothed off to the tops of the templets. This sand will afterwards be brushed away when the top portion of the mould has been made, to form the hollow into which the molten bronze will flow during the casting process. When the sand has been packed between all the templets, another layer of moulding sand will be laid on top of it, and then a thick layer of loam, which is reinforced with stout steel bars. The completed mould is then lifted by a powerful overhead crane and placed in a huge drying oven, where it is baked until it is thoroughly dried out and the moulding sand is quite hard. This drying process must be done carefully because the slightest trace of damp in the mould would cause steam to be formed by the molten metal, with possibly disastrous results. The steam might generate sufficient pressure to burst the mould.
THIS SECTIONAL DIAGRAM shows how a propeller is cast. Metal is poured from the ladle (B) into the runner box (A), equipped with controlling gear comprising a ball (D) whose vertical movement is actuated by a screw and wheel (E). Metal flows through the runners (F) to the casting.
After the baking process has been completed, the two halves of the mould are taken apart and the sand between the templets is removed. All the surfaces against which the molten bronze will be poured are painted with plumbago, or black lead, to give the finished casting a clean smooth surface.
An idea of the size of one of these moulds may be formed from the dimensions of that required for each of the Queen Mary's propellers. In this instance the completed mould weighed over 100 tons and comprised 15,000 bricks, 20 tons of sand and 30 tons of iron and steel reinforcements.
Before the casting process can be put in hand, the two halves of the mould must be bolted together by strong steel bolts. (Although a rare occurrence, it has been known for a mould to burst under the pressure of the molten metal. Should this happen, the liquid bronze will run into the ground as if it were so much water, and the whole job will have to be started again.)
The complete mould is then lifted by the crane and placed in the casting pit, and the drawing on page above shows a section through the centre of the mould ready for casting to begin. In the mould there is a central “core”, made of moulding sand. It forms the central hole which is later machined to fit on the propeller shaft. Another important feature is the “riser”, which is an extension of the boss. This is afterwards cut off and is an insurance that any oxides will rise to the top and not remain in the body of the casting.
Ten Days to Cool
The molten metal is run into the mould through the “runner box” A (see drawing above) from the ladle B. Generally one runner box is sufficient, but for a large propeller two are used. The purpose of the runner box is to act as a reservoir from which the rate of flow of the metal can be accurately controlled. The controlling gear consists of a ball D, whose vertical movement is actuated by the screw handle E. From the runner box the metal flows to the centre of the mould through the “runner” F and thence to the propeller blades.
The process of casting is the most spectacular and interesting stage in propeller manufacture. The men who do this work carry out their duties with almost military precision. Not a word is spoken and every man knows exactly what he has to. do. The metal is melted in a gigantic furnace and run into the huge ladle, which holds about 30 tons of bronze. When ail is ready, the ladle is brought over the runner box and gradually tilted until the metal begins to flow in a golden fiery stream. One man is stationed at each runner box to control the flow, and in the centre stand other men, each of them armed with a long iron rod spade-shaped at the end. With this instrument each man skims the surface of the metal as it flows in from the runner, to make sure that no oxide reaches the blades.
A 35-TONS PROPELLER, one of the set made for the R.M.S. Queen Mary. The propeller has a diameter of nearly 20 feet and a pitch of 18 ft. 2 in. The weight of the casting was 53 tons, and the mould in which it was cast weighed over 100 tons.
The foreman bears a heavy responsibility, for upon his direction depends the ultimate success of the casting. He watches carefully the flow of metal in the centre, making sure that it runs evenly into the mould from either side. The casting operation takes about fifteen minutes, but the foreman still has two hours’ work in front of him. As the metal cools in the mould, it contracts, and to make up for this contraction small quantities of metal must be added to the riser until it shows no further signs of sinking. This operation is known as “topping up”.
This huge mass of metal, which may amount to as much as 50 tons, will take about ten days to cool off before the mould is stripped from it. When the casting is lifted from the mould it weighs far more than the finished propeller, the riser alone weighing about 10 tons. The finished weight of each of the Queen Mary's propellers was 35 tons, whereas the rough casting weighed 53 tons.
The next operation is known as “fettling” and consists of removing the surplus metal from the rough casting. This is done by a number of highly-skilled men armed with pneumatic chisels. The “skin” of the casting is taken off in strips, the depth of cut being carefully measured. This work makes a deafening noise, for the huge bronze casting reverberates under the rapid blows of the chisels in the manner of a deep toned bell.
The machining of such an enormous piece of metal cannot be carried out in an ordinary turning lathe, because of the tremendous size and unwieldy shape. At the Charlton (Kent) foundry of J. Stone and Co., Ltd., there is a special boring machine for doing this work. It is the largest of its type in Great Britain and can accommodate work up to 25 feet in diameter. The machine itself weighs more than 165 tons and is built upon a massive concrete foundation weighing 200 tons. The turning operations must be finished to such a high degree of accuracy that it is essential to have a perfectly rigid foundation.
The first operation to be done on this monster machine is the removal of the riser. For this purpose a huge steel saw is mounted in the spindle of the machine and it cuts through the tough bronze as a knife cuts through cheese.
The Archimedean Spiral
Arrangements are made to support the weight of the riser as the saw cuts through it; thus is avoided any possibility of the saw being nipped during the cutting process. The gigantic saw has a diameter of 6 feet and is fitted with special hardened steel teeth, which are renewable.
PLANING MACHINE for machining the surface of propeller blades. This machine at Charlton, Kent, is the only machine of its type in the world. By an ingenious arrangement of gears the cutting tool is moved backwards and forwards across the blades at a predetermined rate, while the table on which the propeller rests is slowly rotated. The rate of rotation is calculated exactly to give the required curve on the blade. The speed of the cutting tool is about 80 feet a minute, and the maximum depth of cut about ¼-in. The propeller is not rotated continuously, but in a series of steps of exceedingly small dimensions. Thus a remarkably high degree of finish is obtainable on this machine. The vertical feed on the cutting toll is from 1/16 in to 3/32 in for each stroke of the tool. When the propeller has an accurately machined working face the blades are finished by hand in the fettling shop. In the final stages the whole surface of the propeller is ground with abrasive disks rotated by small electric motors.
THE BORING MILL for propellers at the Charlton foundry of J Stone and Company Limited, its the largest boring mill of its kind in Great Britain. The machine weighs more than 165 tons, and is built on massive concrete foundations weighing 200 tons. Before boring is done the riser of the newly cast propeller is cut off by a steel saw with a diameter of 6 feet. Arrangements are made to support the riser as the saw cuts through it, to prevent the saw from being nipped during the cutting process. A tapered bore is then accurately machined, and a groove is made in the propeller to take the key which locks it onto the propeller shaft. This boring mill is capable of accommodating propellers which have a diameter of as much as 25 feet, yet all the operations are carried out with the highest degree of precision. An ordinary type of machine lathe would be unsuitable for the work because of the unwieldy shape of the propellers.
The riser removed, the face of the boss must be machined accurately and the hole through the centre bored out to its exact size. It has long been the general practice to make this hole tapered, so that the propeller may be easily removed for repair or renewal when the ship is in dry dock. The propeller is held in place on the propeller shaft by means of a key. This key fits into a groove cut in the propeller itself and in the propeller shaft. The keyway to take this key is also machined on this huge borer, which can thus do four separate operations with the propeller in one position.
The next operation, that of machining the working face of the propeller, is carried out on a machine which has been specially designed for this one operation. It is the only machine of its type.
Before describing its method of operation, it is necessary to explain exactly what happens when a propeller does its work in the water. The expression “screw propeller” is no idle term, for the propeller cuts its own screw thread through the water. In relation to the propeller, the water is similar to a solid body and the action of driving the ship forward corresponds to that of a screw being driven through a piece of wood or metal. More than two-thirds of the driving force in the water is derived from the front face of the propeller, as distinct from the back or driving face. For this reason the shape of the front of the propeller is extremely important and has a direct bearing on its efficiency.
POURING MOLTEN BRONZE into the mould of one of the Queen Mary’s propellers. The huge ladle is tilted so that the metal runs into the casting through the runner box. Men continuously skim the oxides from the riser round the central core of the mould.
The driving face of a propeller is formed to a definite mathematical curve, known as an Archimedean spiral, so that there is no insuperable difficulty in designing a machine in which the tool follows this particular curve. The best example of an Archimedean spiral is a spiral staircase, and it is upon that principle that this marvellous machine operates. The tool cuts the metal from the working face of the propeller and finishes it to the particular curve that is necessary.
By means of an ingenious arrangement of gears, the tool is fed on to the work automatically at a predetermined rate. At the same time as the tool is being moved backwards and forwards across the face of the propeller, the table of the machine is also rotated slowly, the rate of rotation being exactly calculated to give the required curve on the blade.
This rotating movement of the machine is not continuous, but is arranged so that the tool always has a fresh length of metal to cut on the inward stroke. In effect, the machining operation is done in a series of small steps. These steps are an exceedingly small distance apart and a remarkably high degree of finish is obtained on this machine. The usual cutting speed is 80 feet a minute and the depth of cut ¼ in. The vertical feed of the tool is from 1/16 in. to 3/32-in. for each stroke of the tool.
The propeller now has an accurately machined working face and it remains to finish the blades to their exact thicknesses and shapes. The front surface of the blade varies tremendously with the width, the reason being that the cross-section of a propeller blade is exactly similar to that of an aeroplane wing - in other words, it is an “aerofoil” shape. Because of this variation in thickness and of the complicated curves which are formed on the surface, it is difficult to design a relatively simple machine for finishing the front of a propeller.
Although a machine is being developed for this purpose, it is still in the experimental stage. The present practice is to finish the blades by hand. This work is also done in the fettling shop, pneumatic chisels being used for the purpose.
Special equipment has been designed for this work, and the process is known as “lining out”. The equipment consists of a huge steel table, accurately levelled and perfectly fiat, upon which the propeller is set with the machined surfaces of the blades uppermost. In the centre of the propeller is placed a vertical column on which pivots a long graduated ruler.
Turned Round with a Finger
From this ruler is suspended a plumb line and the plumb bob is used to mark a series of definite points on the surface of the blade, where the thickness is carefully measured to check whether or not it conforms to the design. All high spots are marked with chalk and the propeller is taken to the fettling shop for removal of the surplus metal. Particular care is taken to shape the edges of the blade to the correct curves, for these also have an important bearing on the efficiency of the propeller. So that the points at which measurements are taken may be clearly defined, the blade surface is covered with a special marking paste, on which any marks can be clearly seen.
Such work demands skill and care of a high order, for it is upon accurate lining out that the perfect balance of the propeller ultimately depends. Even the largest propellers - such as those of the Queen Mary - must be so carefully balanced that they can be turned round with one finger.
The final process consists of grinding the whole surface of the propeller with abrasive disks rotated by small electric motors. This brings out the beautiful rich colour of the bronze, which is even more strikingly emphasized later, when the emery cloth has been applied by hand.
The finished work is a thing of beauty, in a class all by itself, a wonderful tribute to the skill and patience of modern craftsmanship. Here is one of the many instances in engineering where human skill is essential to final perfection. Those who have seen a propeller made will realize what this skill means.
A COMPLETED PROPELLER for the quadruple-screw Canadian Pacific liner Empress of Britain, a vessel of 42,348 tons gross. The propeller has a pitch of 19 feet and a diameter of 19 ft. 3 in. It weighs 25 tons.