developed; also called Half-portal).-A form of portal, or gantry crane, the horizontal and vertical framings of which, making a right angle, permit of the passage of trucks beneath (see Fig. 94, Plate VI.). The vertical framing carries the ground wheels, while another set of wheels at the opposite end of the horizontal members runs on an elevated track, usually supported by the wall of a warehouse. The advantage is that one vertical framing is saved, with the space that it would occupy. See Gantry Crane, Portal Crane.

Angle Rolls, or Angle Iron Rolls.-The general section of these rolls for equal-sided

is considerable difference in the diameters of the rolls. When this happens, the larger radius moving at a higher velocity than the smaller, slides with much friction on the parts of the metal which it operates against, stressing the metal so that it twists and curls badly when it leaves the rolls. Further, the rolls are so shaped that the diameters are about equal at the edges of the rolls, which again is done to avoid risk of curling of the bar. Another thing is that the formation of fin is prevented by the close manner in which the rolls fit one another. Sometimes the angles are turned over for the finishing passes, in order to allow the scale

angles is shown in Fig. 95, the points to note being the following:

The angles lie so that both the flanges make equal slopes from the horizontal. In unequalsided angles both flanges approximate to the same condition. This lessens the stress imposed on the metal by rolling, and favours free delivery. Angles are therefore more favourably shaped for rolling than some other sections, as channels, joists, and rails, in which some portions must stand perpendicularly, and be provided with draught, or bevel on inner faces, and in which also in some cases one section is considerably thicker than others, which increases the time occupied in rolling, and tends to produce curved and twisted bars. The design shown avoids the evil effects which result when there

to fall away, instead of being squeezed into the bars. Differences in the thicknesses of angles having the same width of legs are provided for by the open shoulders of the rolls, which permit of bringing the axes farther apart from each other.

Angles. See Angle, Angle Iron, Fillets, Protractor.

Angles of Cutting Tools.-The wide subject of the various shapes of cutting tools will be found treated under that head. The present article has reference only to the particular angles which experience has demonstrated to yield the most efficient results.

The theories which have been formulated relating to the angles of cutting tools do not always harmonise with the practice of the shops.

The fact remains that good practice presents many points of divergence which it is difficult to reconcile on the supposition that tool angles are governed by hard-and-fast conditions.

There are four essentials in the formation of any true cutting tool-penetrative capacity; adequate clearance, strength, and permanence. These vary with the nature of the material operated on, and the degree of hardness of the tool. The removal also of two obstructions to the good operation of such tools must be provided against, and are partly included in these

One may learn a very great deal about the action of cutting tools from the wood-worker's chisel. It is a keen-edged instrument which cannot be sharpened much too keenly for soft wood, for which a cutting angle of from 15 to 25 degrees is suitable. Clearance between the face of the chisel and the wood is practically nothing. If, however, this thin chisel is used on a piece of lignum vitæ, or greenheart, or of soft copper, its edge will turn over, and be destroyed, for the simple reason that it lacks the necessary strength, and the

conditions. One is the character of the chips or shavings cut, the other that of the lessening, and removal of the heat generated by the cutting. In the abstract these would appear easy of accomplishment. In practice they are subject to many variations in the hands of those who have to use the tools.

A cutting tool is a wedge. But a wedge is not always used as a cutting tool. Thus the axe, Fig. 96, A, is operating by splitting only when it cleaves the block. It cuts when it removes chips or shavings, as shown at the side B.

tool angle must therefore be increased. Taking two such chisels, let one be ground as represented at c, or say to an angle of about 15 degrees, and the other as shown at D, or to an angle of about 30 degrees, and let them both be sharpened similarly upon a hone. Now try them each in succession on a piece of soft endgrain timber, and the result will be that c will cut sweetly, and D only do so with difficulty; c will remove a nicely curled and clean unbroken shaving or chip, while D will only remove small fragments, by a process which is more akin to tearing than cutting. Moreover, a very much

greater expenditure of muscular effort will be necessary to actuate the last named than the second.

Following the chisel into other forms, as a simple method of grasping principles :-Imagine a chisel end supported on a rest, and power operated, as at E, in a position for removing material from the flat piece of wood (or metal) indicated by the dotted line, or from a circular piece, as in the lathe (full line). The objection to the position of E, when cutting metal, is that too much friction is set up between the chisel face and the face of the work. In C and D, being hand-operated chisels, this is of no importance; in fact, the control which the face of the work affords to the chisel is a necessary aid to true cutting. But that reason no longer exists in machine-operated tools, hence the tilting of the imaginary chisel as at F, gives clearance at a, which avoids undue friction, with its resulting waste of power. Even in turning wood by hand the workman instinctively tilts the side chisel a little, G, to lessen its frictional contact with the work.

Going a stage farther, take the graver, a hand tool of triangular ection used for turning metal. It is in principle a chisel point, as a comparison with the shaded areas in C, D, E, F, will show. Presented as at H it does not act as a true cutting tool. At J its action is almost identical with that of the chisel E, or F, but at к it resembles more that of a tool for turning cast iron. In each case the angle of the tool itself remains unaltered, but the methods of presenta tion differ, so that the clearances vary, and so does the slope of the top face, or the "top rake." We are now able to trace the subject a stage farther. Evidently there are three elements involved, the tool angles themselves, the angle of front rake, or relief, or clearance, and that of top rake, or slope, each of which is subject to considerable variations.

To cut the hard metals and alloys the tool angle must be much greater than that of the soft wood chisel, the keenness of the wedge being sacrificed to strength; penetrative power being therefore less, and permanence greater. Front clearance too, the amount of which is a matter of little or no moment in wood cutting, is a highly important factor, essential in cutting

metal, being necessary in order to diminish the heat generated by the friction of metallic surfaces. There are thus a number of opposing and contradictory conditions to be observed in the formation of any cutting tool, and because of this no rigid set of rules can be possible.

Since the mode of presentation of the tool to the work is of as much importance as the cutting angle itself, there are thus two conditions which are of a fundamental character. The aim should be to obtain as close an approximation to these as possible, consistently with the conditions imposed by the character of the material being operated upon. These are the following:That the top, or cutting face of the tool shall make the smallest angle practicable with the face of the material being cut, or in other words shall approach the tangential line. Also that the largest amount of material possible shall be massed immediately behind the cutting edge in the direction of the line of thrust of the work. As the fulfilment of these leaves little angle between the front face and the face of the material being cut, the conclusion arrived at is that the minimum angle of relief should be settled first of all, and the others be afterwards fixed. These are of the nature of axioms, to which there are no exceptions. Whatever the types of cutting tools, we find that these principles have their application therein.

The graver, and the old heel tool, once common in engineers' shops, depended for their best results upon the way in which they were manipulated by the turner. They were ground to constant cutting angles-about 60 degreesbut were made to cut all metals and alloys, and both roughed and finished, simply by varying the method of presentation. This variation being impracticable in the slide lathe, the settlement of suitable angles for the different tools which are fixed is an important problem.

L, M, and N illustrate three common forms of lathe (or planer) tools, in which areas are shaded in order to illustrate the relation of such tools to the graver and the chisel. The only difference lies in the shape of the tools as forged, and not necessarily in angle, since each of the three forms has its angles varied for different materials.

The standard tools which have been evolved