The face of the end so hardened must be cleaned either with a piece of emery - paper or on a grindstone - either will do, as long as the part to be tempered is cleaned so that the colour can be seen when it appears. Having got all the tools that are to be tempered ready, make a piece of bar - iron about 3/6 in. by 1 1/2 in. red - hot, and on it place the tools, with the hardened end extending beyond the heated surface, and in a short time it will be easily noticed that the bright part gradually changes colour - of course being deeper where it is nearest the iron. As soon as the cutting part gets to a straw - colour, it must be again dipped into water, to prevent its going deeper and becoming too soft for its purpose. There are also many different degrees to which cutting - tools for turning various substances should be what is termed "let down to." For example, a tool that is to be appropriated for turning steel, especially if it is not well softened, will require to be harder than one that is to be used for turning brass or gun-metal; therefore, if the latter is to be the material worked, leave the tool on the hot iron a little longer, until it becomes a darker straw - colour, or orange tint. Only practice will perfect any one in this most useful branch of mechanics.

Any workman of experience can tell in a moment what temper will be required for the many different purposes the various tools are used for. Another way, which by men of practice is generally adopted, is to heat the tool, then dip it into water as far as necessary, and let down the temper with the heat left in the other end of the tool. The colour being obtained, the whole tool is cooled in the water, and the necessity for heating the iron is dispensed with. Assuming that a quantity of small articles are made, and ready to be hardened and tempered, great care must be taken not to overheat them; the best way to avoid this will be to build a small hollow fire, and in this place an iron box, about 4 or 5 in. square, and open at the end. Make it red - hot, and on the lower side place the cutters in a row. The fire must be kept going, or the box will cool; they will soon get hot, subjected to the heat in the box. This not only makes a certainty of not burning the tools, but enables the operator to have a dozen at a time getting hot. When the whole lot are so hardened, the bar of hot iron will be again required.

While this is getting hot, the faces of all the tools can be cleaned, and then placed in a row on the hot iron; and as they run down very quickly, being small, it is better to hold in the left hand a vessel of water, and with the right hand push off each tool as it assumes the required colour. By this means a number of articles may be soon effectually hardened and tempered. There are other means for effecting the same purpose. One of these is the blowpipe. Have a large piece of charcoal, in which make a hollow; there place the object to be heated, and blow the flame into the recess; it becomes a miniature oven, and all the heat of the flame is utilized. In making a spiral spring hot, prior to plunging, it will, if heated in an open fire, twist and bend all ways; the best way to avoid this is to get a piece of old gas - pipe, as near the size as convenient, and lay the spring in it, and then make the pipe hot. The spring will soon get sufficiently hot, and a regular heat will be the result, and it will be more likely to remain straight. The next thing will be to plunge it, but not in water. There are various opinions as to the best liquid to be used for this purpose; some prefer oil and water mixed.

Another mode is to have a vessel containing a body of water, with about 1 in. of oil on the surface, so that the heated spring passes through the oil before it reaches the water. I give preference to a simple body of oil, and tor a long, thing like we have under consideration, a cylinder biscuit - tin will be found as good as anything to hold it. The spring being now hardened by the immersion in the oil, it must of course be tempered, and the best way to effect this is to blaze it-that is, after it has been placed in the oil, it should be taken out and held over the fire until the oil upon it catches light, and according to the degree of temper the spring is required to have, so long must the oil be allowed to flare. If a mild action is only wanted, it may be allowed to burn out, and be immediately dipped into water; experience alone can make any one proficient. The process of hardening a plain mandril is a piece of work with which great care should be again taken. Many people are under the impression that the mandril of a first - rate lathe should be all steel. This is an error, and it is only from want of knowledge that any one is impressed with this idea. To make a thoroughly good lathe the mandril must be composed of iron and steel.

I am speaking now of a plain lathe; traversing mandrils are made of steel, but this is special. Taking, then, the plain mandril, the very best fagoted iron must be used, and round the part which is to form the bearing a thick ring of shear - steel must be welded. Cast steel will be of no use for this purpose, as it cannot be welded. The nose which eventually is to receive the chucks must be of iron. The mandril being fitted into its place as it must be, it will be ready for hardening, and the first thing to do will be to protect the nose from getting hot. To effect this, it must be covered with clay, bound on with thin binding - wire. It was at one time considered necessary for the heating of a mandril to have a hollow fire, but this idea has gradually died out. A mandril is a thing especially liable to break when heated and dipped into water; before dipping or plunging the mandril into the water, make a piece of iron hot, and stir the body of water with it. This is, to a certain extent, a safeguard; but nothing will make certain of its not breaking. The mandril must not be simply put into the water and taken out again, but moved about and allowed to remain until cold or nearly so.