The traversing mandril is made of the very finest steel to be obtained, and as it has to be bored through, and is not of sufficient substance to admit of welding, the nose is in the solid with it. In hardening one of this description, a somewhat different process will have to be gone through. An experienced turner will detect at once, when turning a mandril, whether it is likely to require to be made to a greater or lesser heat; for instance, if the steel turns short and crumbly, so to speak, it will require less heat than if it will admit of a long shaving being taken, and is, in fact, of a much softer texture. A traversing mandril will be about 14 in. long, and will only require to be hardened where the fittings or bearings take place; that is, about 2 in. from the collar at the front, and 31/2 in. at the small end, leaving 1 in. quite soft at the extreme end, where the guides are to be fitted. The first thing to do will be to protect the nose from being made hard; this can be done, as in the case of the plain mandril, with clay. There are differences of opinion as to whether the nose should have the screw cut upon it before or after it is hardened.

One thing is sure: that if cut first, it will certainly require correcting afterwards; but if it can be kept perfectly soft, it is better to do it when the mandril is hardened. One reason why more care is required with this kind than with a plain mandril is that it is longer, and there are two bearings to harden instead of one, and the nose being of the same steel as the mandril itself, it has more risk of becoming hard. Having protected the nose, proceed with all care to harden. First make a clean fire, free from clinkers, and heat the end that has the collar on it first; when sufficiently hot, which will be a deep blood colour, plunge it into cold water. To keep it as straight as possible, the best plan is the following before dipping, have a pail of clean water ready, and when the work is hot, take a stick and twist it round about in the pail, and so form a kind of diminutive whirlpool, the mandril being placed in the centre. The action ,of the water, in its circuitous course, tends to assist in keeping it in its original form; but take what steps you may, it is impossible to rely upon its being straight after it has been through such a course of treatment.

The one end having turned out well-that is to say, hard - and not very much distorted, the other part may be done in precisely the same way, and when this is finished, it will be consoling to find the mandril is still intact, as it has often happened that, before going so far, it has broken. But here even the anxiety does not cease: it may be perfectly hard, not scaled nor broken, but, when placed between the centres of the lathe, it may be so crooked that it would be out of all character to try and grind it into straight line again. Mandrils of this kind were at one time always tempered so that they could be turned with a diamond point; but since the introduction of emery - wheels, this is avoided, and they are left perfectly hard, and ground true by means of a small emery - wheel running between two centres driven at great speed. But before grinding it true- that is, absolutely so - it must be brought as near as possibly so by other means; and it is here that the anxiety comes in. It may possibly be found when the spindle is placed between the centres, that it is bent in 2 directions; should this be the case, it is not at all an easy matter to correct it.

It must, however, be done with the pane of a hammer, and the mandril itself placed upon a flat surface, and the blow from the hammer take effect upon the opposite side to where the error is; this is where the danger of breaking is, because a sudden or uneven jar is likely to make the mandril suddenly 2 pieces. Sometimes the steel, from various causes, will bear the blow upon the same side. The mandril must be held firmly in the left hand, and with the right hand the hammer is applied gently. Do not blow the fire fiercely, but let the material get gradually hot through, and then, when it is so, do not plunge it.in the water, and withdraw directly to see if it is cold; let it remain there till it is so. I have frequently heard a piece of steel crack some little time after it has been out of the water, but at the same time this may be no fault of the person who is hardening it - it is a thing that will, and unfortunately does, occur. Some authors direct that tho nose of a mandril should be left as hard as possible. This is quite an error: let the nose of all such work be kept as soft as possible.

In the hardening of small centre screws, and for such things as horizontal cutters and the like, many are broken from the fact of being made hard on the screw part; it will be clear that if the screw fits at all tight upon entering, it leaves the chance of the point being twisted off. To avoid this, do not harden it, and the way to prevent it is to have only sufficient water in a flat - bottom saucer, and when hot place the screw point downwards iato it and move it about. Before the small body of water gets hot, remove it quickly into a greater body, and cool it. (J. H. Evans, Eng. Mech.)