At ordinary temperatures ebonite is hard and brittle and breaks with a well-marked conchoidal fracture. At the temperature of boiling water the ebonite becomes somewhat softened, so that it is readily bent into any desired shape; on cooling it resumes its original hardness.
This property of softening at the temperature of boiling water is a very valuable one. The ebonite to be bent or flattened is merely boiled for half an hour or so in water, taken out, brought to the required shape as quickly as possible, and left to cool clamped in position.
The sheet ebonite as it comes from the makers is generally far from flat. It is often necessary to flatten a sheet of ebonite, and of course this is the more easily accomplished the smaller the sheet. Consequently a bit of ebonite of about the required size is first cut from the stock sheet by a hack-saw such as is generally used for metals. This piece is then boiled and pressed between two planed iron plates previously warmed to near 100° C.
With pieces of ebonite such as are used for the tops of resistance boxes, measuring, say, 20 X 8 X 11 inches, very little trouble is experienced. The sheets when cold are found to retain the flatness which has been forced upon them perfectly well. It is otherwise with large thin sheets such as are used for Holtz machines. I have succeeded fairly, but only fairly, by pressing them in a "gluing press," consisting of heavy planed iron slabs previously heated to 100° C.
I do not know exactly how best to flatten very thin and large sheets. It is easy to make large tubes out of sheet ebonite by taking advantage of the softening which ebonite undergoes in boiling water. A wooden mandrel is prepared of the proper size and shape. The ebonite is softened and bent round it; this may require two or three operations, for the ebonite gets stiff very quickly after it is taken out of the water. Finally the tube is bound round the mandrel with sufficient force to bring it to the proper shape and boiled in water, mandrel and all. The bath and its contents are allowed to cool together, so that the ebonite cools uniformly.
Tubes made in this way are of course subject to the drawback of having an unwelded seam, but they do well enough to wind wire upon if very great accuracy of form is not required. If very accurate spools are needed the mandrel is better made of iron or slate and the spool is turned up afterwards. The seam may be strapped inside or at the ends by bits of ebonite acting as bridges, and the seam itself may be caulked with melted paraffin or anthracene.
Working Ebonite. —
Ebonite is best worked as if it were brass, with ordinary brass-turning or planing tools. These tools should be as hard as possible, for the edges are apt to suffer severely, and blunt tools leave a very undesirable woolly surface on the ebonite. In turning or shaping ebonite sheets it is as well to begin by taking the skin off one side first, and then reversing the sheet, finishing the second side, and then returning to the first. This is on account of the fact that ebonite sometimes springs a little out of shape when the skin is removed.
Turned work in ebonite, if well done, requires no sand-papering, but may be sufficiently polished by a handful of its own shavings and a little vaseline. The advantage of using a polished ebonite surface is that such a surface deteriorates more slowly under the influence of light and air than a surface left rough from the tool. If very highly polished surfaces are required, the ebonite after tooling is worked with fine pumice dust and water, applied on felt, or where possible by means of a felt buff on the lathe, and finally polished with rouge and water, applied on felt or cloth.
Ebonite works particularly well under a spiral milling cutter, and sufficiently well under an ordinary rounded planing tool, with cutting angle the same as for brass, and hardened to the lightest straw colour.