-The extruding machines turn out rods, tubes, long wedge-shaped sections for coil mounts and so on, and here we cannot resist the simile of a sausage machine ! The plastic ebonite is fed in lumps through a hopper, and is forced out through dies, which determine the resulting shape. As the rods, tubes, or whatever shape is required, emerge in a continuous length from the nozzles of the machines, they are cut off into pieces about 3ft. long, and dusted with French chalk to prevent them sticking. They are now ready for the vulcanising process, for which purpose they are enclosed in iron boxes, which are in turn placed inside the huge vulcanising ovens. Here they are kept under super-heated steam for long periods, varying from 5 to over 20 hours, depending on their size and thickness.

Although the extruded material in its various forms requires only vulcanising and finishing to fit it for delivery, the calendered sheet goes through severa processes before, in its turn, it reaches the ovens.

As has been mentioned, sheets of any desired thickness are built up from the initial sheets, each " ply " being rolled by hand on top of the previous one. The finished sheets are then placed between tin foil and passed to the vulcanising ovens. Tin foil is used during vulcanising, the whole purpose of the foil is to protect the surface during the vulcanising process, otherwise there is a possibility of pitting or roughening of the surface occurring. All panels after vulcanising are either sand-blasted, or rubbed down by hand, and they owe their highly polished finish solely to buffing on silk buffers.

So much for panels, but there are many other purposes for which the calendered sheet is required, one of the biggest being accumulator cases. These are made up on steel formers, the sheet being rolled on with hand rollers, the joints closed, and the edges finished and squared off. The lids for the cases are moulded in hot moulding presses, and vulcanising hardens and completes both cases and lids alike.

In the grinding machines mentioned above, the object of steam heating one roller, and water cooling the other, is to cause the mixture to adhere to the hot roller, and so be continuously ground between the two rollers.

About 15% of sulphur and a temperature of 275° F. for 4 hours will produce an elastic rubber, while 30% of sulphur and at temperature of 315° F. will make a hard vulcanite.

Machining Or Working The Material

Owing to the very large variations to be found in different makes of vulcanite and allied substances some trial is always advisable before carrying out extensive work. Where possible, work should be executed without any cooling fluid, but a mixture such as " Automatics " will be found suitable for ebonite and vulcanite, and will also ensure the best possible results.

Red and black fibre are particularly liable to be softened by cooling fluids, and the danger of breakage is considerable in the case of these substances.

Also, to ensure the most uniform results, the workshop temperature should be kept as constant as possible.

Tools

The angles of rake in lathe tools for this class of work are more obtuse than those for ordinary metal work, no top rake is permissible and the side and bottom clearance should not exceed 10 per cent. For form tools, which are usually circular, this clearance is exceeded, but the points of the tools are kept at a suitable angle by giving them negative top rake.

For milling work, the cutters should be very much accelerated and a coarse feed used. Usually it is better to run the cutter dry. Similarly, for drilling work, the same principles apply.

It is where repetition work of accuracy has to be carried out in vulcanite and similar substances that special methods of machinery become necessary. If a few components are to be made on the lathe, a steel tool will be found quite satisfactory; although the final parts should be checked for size.

The peculiar tendency of vulcanite, ebonite, bakelite and similar substances to wear away the sharp tip of a tool is fairly well known, and steel tools that will last weeks when cutting hard steel will only stand up for a few days. The necessity of meeting this difficulty has led to much experiment, and finally to the adoption of the diamond turning tool.

Diamond Tools

As this type of tool will run for months under normal conditions, the extra cost of a diamond tool (which is from £3 to £5) will soon be justified by the saving in replacements, cost of supervision, and any expense due to an added finishing operation which is often necessary with the steel tool.

There are many products made with the ordinary tool that require filing, polishing or burnishing, while even a component of, say, bakelite, having a brass or copper inlay, can be finished right off with the diamond tool.

Apart from the wireless industry, such articles as fountain pens, the fine sliding tubes of telescopes, etc., are regularly turned out ready for immediate assembly.

It will be found that the use of a diamond tool will be often preferable to grinding, as any residue of abrasive grit is eliminated, while the finish obtained on metals is comparable to that of a lapped or burnished surface.

To obtain these results, a high surface speed should be given to the work, say for turning 220 ft. per minute drilling about half this, and threading 80 ft. per minute.

Heavy cuts cannot be taken and care should be exercised to avoid jars, overheating or sudden cooling, such as might be caused by switching on the cooling fluid after the tool has been cutting for a period.