This section is from the book "Spons' Mechanics' Own Book: A Manual For Handicraftsmen And Amateurs", by Edward Spon. Also available from Amazon: Spons' Mechanics' Own Book.
The following general remarks on polishing metallic surfaces by hand are from a paper by T. F. Hagerty, in the American Machinist: - The practice generally employed by machinists in grinding and polishing either new or old work is to mix the polishing material with oil, usually refuse machinery oil; in most cases this is a great mistake, and has caused the loss of time, patience, and money. Take, for instance, the grinding to a true bearing of a stopcock, a valve seat, or a slide valve. There are few machinists but what have had more or less of that class of work to do, particularly in jobbing shops, and we seldom find one who uses the same method of accomplishing the job that is practised in shops where that class of work is made a speciality. In fitting and grinding the plug into the barrel of a cock, a little judgment and care will save a great deal of hard labour, and in no case should oil be mixed with any of the grinding material, for the following reasons: If fine emery, ground glass, or sand is used with oil. it requires but a few turns of the plug in the barrel to break up the grains of the grinding material into very fine particles; the metallic surfaces also grind off, and the fine particles of metal mixing in with the grinding material and oil, make a thick paste of the mass.
At this stage it is impossible to grind or bring the metallic surfaces to a bearing, as the gluey paste keeps them apart; if more grinding stuff is applied, it will prevent the operator from seeing what part of the barrel and plug bears the hardest. Again, if the grinding material be distributed over the whole surface, the parts that do not bear will grind off as fast as the parts that touch hard, as the particles work freely between the surfaces; should the barrel and plug bear equally all over when fitted it requires more care than if it were a top or bottom bearing, as that part of the barrel and plug across the " waterway " grinds twice as fast as the other parts; therefore it should be kept the driest. Now this objection holds good in the grinding of valve seats or slide valves, to wit: the separation of the surfaces of the metal by a thick, pasty, grinding material. In order to bring the surfaces to a perfect bearing rapidly and with little labour, the following directions will be found worth a trial : - To grind a stopcock of any kind, first see that the plug fits the barrel before it is taken from the lathe.
Run a half-round smooth file up and down the barrel to break any rings that may be in it; a few rubs of a smooth file back and forth over the plug will break any rings or tool marks on it. Wipe both parts clean. Use for grinding material fine moulders' sand sifted through a fine sieve. Mix with water in a cup, and apply a small quantity to the parts that bear the hardest. Turn rapidly, pressing gently every few turns; if the work is large and the lathe is used, run slowly; press and pull back rapidly to prevent sticking and ringing; apply grinding sand and water until a bearing shows on another part, then use no more new sand, but spread the old that has worked out over the whole surface. Turn rapidly, pressing gently while turning: withdraw the plug and wipe part of the dirt off, and rub on the place a little brown soap; moisten with water and press the surfaces together with all the force at hand, turning at the same time. Remove the plug and wipe both parts clean; next try the condition of the bearing by pressing the dry surfaces together with great force. If the parts have been kept closely together while grinding, and the plug has not rubbed against the lower part of the barrel, the surfaces will be found bright all over and a perfect bearing obtained.
If an iron barrel and a brass plug are used, or two kinds of brass, a hard and soft metal, soap should be used freely when finishing up, as the tendency to form rings is greater when two different metals are used. In grinding a slide valve which has been in use until hollow places have worn in the surface, emery mixed with water, or sand and water, will be found better than oil, unless a light body of oil, such as kerosene, is used. If water is used with the grinding material, soap should be rubbed on hollow places, and the grinding stuff should be applied to the high parts in small quantities, keeping the low parts clean and dry until an even surface is obtained all over; then the worn-out stuff should be used for finishing up. In polishing metal, oil that will "gum up" should not be used with the polishing material unless for a dead fine polish. In polishing old brass-work which has been scratched and tarnished by wear, pumice or bathbrick should be used with soap and water for scouring off with, and rottenstone with kerosene oil for the wet finish, and dry for the final polish. The same method should be used for new brasswork.
New work should require, after leaving the lathe and vice tools, but little polishing or grinding, and every good workman should try to avoid using an emery stick or emery cloth, as with proper care in the use of tools a great deal of grinding and polishing can be dispensed with. The polishing of metals varies somewhat according to their character, but the main principle underlying all is the substitution of progressively finer scratches for those left by the material last used, until they become so delicate as to be invisible without the aid of a microscope.