belts. It would be just as appropriate to have only one feed for all work, as only one speed for all kinds of jobs. While a planer with a bed 12 feet long is able to run at the rate of 15 feet per minute, a 25-foot bed planer is driven at 13 feet per minute, or a little less than the first mentioned tool of this class, because heavier work is done upon it. A shaper, or quick working planing machine for short jobs, has the change of speed we speak of, and at the slowest speed, with a seveninch stroke, also runs at the rate of 12 feet per minute. It will therefore be seen, from the examples mentioned previously in this article, that the average speed at which cutting tools work is, on work of an ordinarily heavy class and of a standard kind, about 13 feet per minute for cast-iron, and 15 feet per minute for wrought iron. Brass we have not considered at all; but with free-working yellow brass, 25 feet per minute is not an unusual speed for diameters of ten inches. The speed of brass work, however, is a very difficult thing to fix, for while a large brass ring may run at a rapid rate and turn economically, a shaft of the same size could not be run at half the speed the ring does and work well on the tool. Besides, this brass is such a "fickle" metal, so to speak, being at one time hard and another soft, now free from "blow-holes" and again honey-combed with them, each and all phases requiring different speeds to suit circumstances; for these reasons we say we

have omitted any discussion of the speeds proper to cut brass. And it must be also borne in mind that the rates mentioned as proper for cutting cast and wrought-iron vary greatly at times. Of course chilled iron cannot be turned on speed that would be proper for a soft loam or a green sand casting neither can a scrap iron shaft be run economically at the same rate as a rolled one of similar size. But as a standard rule for ordinary work, 13 feet per minute for cast-iron turned in lathes, 15 feet for turned wrought-iron, and 15 feet for planers, alike on all metals, is a fair estimate. It is very possible that our readers may have some experience which conflicts with these statements: if so, we would be pleased to hear from them.

The practice of knocking off the centres of turned work is a mischievous one. It is merely doing work that is not only needless, but that at some future day will have to be done over again. When a centre is once properly made in a shaft or any other part, it is unalterable except by chipping or purposely changing its position, and work once turned true on good centres will always be true, provided no damage occurs to it. It is just in this particular that the true centre is useful, for if a shaft is bent or an arm on one thrown out of line, the old centres are available, and the injured piece can be made as good as new in a short time. Suppose, however, that the journal of a shaft is worn oval, or that the collar is bat

tered and jammed up, how is it possible to find the true centre of the shaft? It never can be found; the shaft may be made to run straight, but not by its old centres if they have once been cut off. When shafts are forged too long, in cutting them to the right length great "tits" are left on the ends, which are both ungainly and in the way. This is the blacksmith's fault, and must be remedied by the machinist; cut the shaft to the right length first, knock off the centres if they are too long, and then re-centre the job and finish it according to the drawing. In steam-engine work especially, the centres of shafts are essential to nice adjustment, and they should never be re.noved.

A foolish notion prevails among some nechanics that centres injure the finished appearance of the work, but it seems to us that this is an erroneous view, which ought not to be tolerated. Drill every centre, and drill it deep; countersink it so that it will have a good bearing on the centres of the lathe, and the workman will have the satisfaction of knowing that, all other things being equal, he will have a good job, and one that can at any time be easily repaired if damaged.

CHAPTER V.

CHUCKING WORK IN LATHES.

ONE of the most indispensable adjuncts of a lathe is a chuck for holding work that cannot be turned between the centres, or requires to be bored out. Very great ingenuity has been displayed in constructing chucks so that the piece held, if round, should run perfectly true without any further adjustment. To this class of chucks belong the scroll, the worm aud spiral gear chuck, and others; their utility is very great, and on some work they are indispensable.

Ordinary chucks have four jaws, which slide in grooves in the face plate, and are set up by screws running through them. Such a chuck plate can be altered to take an irregular form, or one that has a hole out of the centre, as an eccentric, but the scroll chuck cannot. The jaws in this move arbitrarily, or toward the centre, and are therefore unchangeable; although we believe there is one variety of scroll chuck in the market that can be shifted so as to take an irregular form. It is surprising to see what clumsy work some men make in chucking a job. To set a simple pulley takes them half an hour, and at the end of that time the face is so covered with chalk marks that it looks

as if it were whitewashed; hammer marks indent the work, and the workman loses his patience and gets out of temper for nothing. It is the simplest thing in the world to set a round job true in a few minutes, and without chalk, sticks, or any other aid. When a pulley is to be bored, the centre should be put in the spindle, and the size measured off to the chucks; one of them should be drawn out a little to let the work in, and when it is in place, setting this slack jaw up will bring the pulley fair. One or two revolutions of the lathe will show in a moment if the outside is true. It is unnecessary to tell the mechanic that no work must be turned from the hole cored out rough. Many unthinking persons have done this to their own and the proprietor's sorrow; the cores not unfrequently get pushed on one side in casting, which makes the work all wrong if they be taken as the centre.

Scroll chucks, in fact chucks of any kind, are costly tools, and not within the reach of every mechanic. To such, a common block of wood is by no means a useless thing. It is astonishing how much can be done in a wooden chuck when properly made. Very large sizes can be employed. and for very small work, it is unequalled as a substitute for the metal chucks. Very frequently cements, such as gum-shellac, etc., are used in connection with the wooden chuck to hold smail flat pieces that have no flange or other point to catch An eccentric may be bored for the shaft and turned