|
A physics laboratory run on mass production lines. The manufacture of receiving valves still remains fundamentally a laboratory process, but the complicated machinery which has enabled the necessary quantities to be produced with the requisite precision and control is a triumph of engineering skill. We are indebted to Messrs. A C Cossor, Ltd., for the facilities granted in preparing this article.
All joints in he construction of the electrode assembly are spot-welded.
The making of most of the components which comprise the modern Wireless receiver is an engineering rather than a scientific job. The physicist, though he understands the principles underlying the design of inductances and transformers, and can follow general circuital arrangements, has very little say in the general design of a wireless receiver, which is the rightful province of the electrical engineer. In the design of loud speakers the physicist's knowledge of acoustics enables him to share the honours. But when it comes to valves he has the field practically to himself.
Glass manipulation and the making of gas-tight seals are common places of the physical research laboratory, when the technique of producing high vacua, the study of gas occlusion in metals, and the emission of electrons and their subsequent behaviour under the influence of the complex forces existing in a modern multi-electrode valve, are essentially problems for the pure physicist.
Yet the production of valves in sufficient quantities to meet the present demand calls for engineering experience of a high order. Thus we find that the modern valve works is a huge laboratory run on engineering lines, with the physicist in supreme control.
The manufacture of scientific glassware was Cossor's business long before the manufacture of valves was contemplated, so that it was only logical, when the time came, that they should turn to the manufacture of a component in which this art plays so important a part. The production side of valve manufacture is largely a matter of the successful design of automatic rotary machinery for glass manipulation. This has been an process of steady development, and minor additions and improvements are constantly being introduced.
Similarly, in the production of the small metal and mica parts associated with the electrode assembly, special machines have had to be developed. One of the most interesting is the machine for winding grids. The parallel vertical supporting wires of the grid are fed into slots on each side of a mandrel, and the wire is wound over these with a regular spacing depending upon the type of valve characteristic required. The wire is preceded by a knife-edged wheel which nicks the supporting wire and forms a trough in which the grid winding can lie. In forming this trough a slight burr is raised, and this is then folded over by a second wheel and firmly grips the wire. The junction formed is amazingly strong and cannot be pulled apart by hand. Indeed, the method is in every way equal to that of spot welding, and is, of course, much simpler in operation. Long lengths of grid are formed in this way and are stored in special boxes awaiting the operation of cutting to size. They are inspected for the correct number of turns and after this are ready for the assembling lines.
Electrically heated furnaces in which the electrodes are cleaned in an atmosphere of hydrogen.
Before assembly all the metal parts have to undergo a cleaning process. This is carried out in a tubular electric furnace through which a stream of hydrogen gas is passed. The furnace temperature is automatically controlled and the process is continuous, baskets of the parts being pushed in at a door at one end and after a regulated time interval passing through to water-cooled chambers at the lower end of the furnace tube. The stream of hydrogen gas is allowed to burn at a pilot hole in the door of the furnace. The gas in no way contributes to the heating of the furnace, and burning is the most convenient way of getting rid of it after it is used.
First Steps in Assembly
A section of the valve assembly department with the exhausting plant in back-ground.
Other important departments contributing to the work of preparing parts for assembly are the bulb inspection and the filament coating departments. The bulbs, which at this stage have a long neck ready for sealing to the pinch, are cleaned by a baking process and are inspected for bubbles and flaws. The preparation of the active coating for the filaments is in the hands of a skilled chemist.
With all the materials prepared the first process in making the valve is the formation of the pinch. This starts as glass tubing which is fed into a vertical carrier above one of the rotary glass-working machines. The tube is first cut to the required length, then heated and flanged, and the top halt flattened ready for sealing in the wires.
Automatic rotary machine for sealing lead wires in the glass pinch. An auxiliary rotary annealing oven on the left deals with the output from the main machine.
The latest type of rotary machine used for this purpose has twenty-five heads, i.e., twenty-five successive processes are carried out on each pinch before it makes one continuous circuit of the machine. The loading of the partly formed pinches in the clamping jaws of each head and the insertion of the leading-in wires is done by hand, After this the machine is entirely automatic, and even the thin tube by which the complete valve is attached to the vacuum pumps is picked up and sealed in at the appropriate point mechanically, If by any chance any part is omitted in hand loading the 'automatic hand' fails to pick up an exhausting tube for that pinch, so that at the output end of the machine any faulty specimens are easily recognised. Nevertheless, all the pinches which are complete are individually inspected in front of a special lamp to detect any bubbles and flaws near the seal between the wires and the glass.
The pinch is now passed to the assembly line where the first operation is to cut the wires to length. A single press is used for this operation, which, in addition, flattens the filament wires and turns them over to form a hook in which the filament itself is located. This is then welded and the next process is the welding in position of the grid. In the case of a battery valve the filament is then hooked on to side supports and the top is supported by a fine spiral spring and suspended from the mica bridge piece at the top. Next, the anode assembly in which the suppressor grid, in the case of a pentode, has already been located is slipped over and welded. The getter support is then attached.
One of the assembly lines with Its own separate bulb sealing machine at the output end.
The electrode assembly is now ready tor sealing in the bulb, and this is done in machines which are situated at the end of each of the assembly benches. Through out the process of assembly none of the parts is touched by hand, and all the operatives wear special cotton gloves.
The exhausting plant; On the left the getter is being fired with an HF heater coil.
The valves are next taken to one of the exhausting machines, which are continuously operating, and accommodate sixty four valves in a complete cycle. The vacuum pumps rotate with the machine and two valves are allocated to each pump. The thin glass stem of the valve is inserted in a rubber joint, and all the wires leading out of the base are tucked away, with the exception of the filament leads which are used later to heat the filament. After passing under a height gauge to ensure that the valves will not foul any part of the apparatus they pass through a long annealing oven in which there is a progressive rise of temperature with the glass and to some extent the heaters warmed up, they next undergo a process of eddy current heating under high-frequency coils, which cover six valves at a time and travel with them for a short distance. At this point, with the electrodes at a bright red heat, the filaments are heated, and it is at this stage that the active oxide coating is finally formed. Finally, the getter is fired by a high-frequency coil which is placed over the valve by hand and causes local heating at a higher temperature than the general eddy current heating which the valve has previously received. As soon as the getter which removes the last trace of gas is fired, the stem is fused with a sharply pointed blow-pipe flame and the valve removed.
The valves, as they come off the pump, are placed on a conveyor for capping. The wires are straightened by hand and pushed through the hollow valve pins simultaneously, another operation calling for considerable practice. The caps, which have previously been lined. with paste, are pushed home, and the wires soldered at the ends of the pins, after which they are cut off and the paste is set in a rotary baking oven.
Ageing
Ageing is carried out on a slowly moving continuous belt.
The next process is ageing, and in the Cossor works a novel continuously operating, belt system accommodating nearly a thousand valves at a time is used. The process is continuous, and the speed of the belt is timed to give just the right period for the ageing process. First of all, the filament is flashed under overload conditions, and the remainder of the time the valve emits under more normal conditions in order that the emission may thoroughly settle down. Ordinary metal filament lamps associated with each valve are fitted in sockets at the side of the belt, and serve the dual purpose of load resistances and indicators of short circuits. When these occur the lamps burn brightly, and the associated valve is removed from the belt.
One of the testing panels in which meters are provided for measuring every electrical characteristic of the valve.
After leaving the ageing-belt the valve goes to the testing department, where it undergoes a searching examination for any defect in the vacuum or general performance. From the photograph of one of these test units it will be obvious from the number of meters in use that the test is something more than the perfunctory, check of filament continuity; and emission. As a matter of fact for some special types of valves a cathode-ray type of test gear is employed in which the whole length of the characteristic can be viewed on the screen.
So the quantity production, of valves in the current list proceeds but meanwhile research into new types and designs for the future is being carried on in the laboratory. Here a complete miniature valve manufacturing works is at the disposal of the staff, so that experimental valves can be made up at a moment's notice without interfering in any way with the flow of the main production. Here we are back to the prototype laboratory in which the new industry of valve production in vast quantities had its origin.
Removing the final trace of gas from Ferranti valves by means of induced high-frequency currents.
| 