The proper shapes of the flutes in a hob are shown by Figs. 5G6 and 567, the width and depth of each flute being indicated by Fig. 567, which denotes the flat extremity of a hob's plug-end. The flute of a hob is much narrower than that of a tap, because a hob is often used to screw dies, and to shape the teeth of screw-tools; it is therefore necessary for the screw to be nearly circular, to provide a good bearing for contact with the work while it is being cut with the hob. These flutes are made with narrow grooving tools, termed circular cutters, and also with ordinary slide-rest tools, having cutting edges of a curved shape. The circular cutters consist of round plates having teeth around their rims partly resembling saw-teeth. A cutter of this class is rotated while tightly fastened on a spindle, and the two together are shown by Fig. 557. The slide-rest grooving tools used for fluting are those having straight thin ends extending from the thicker portions; these thick parts are fixed in the tool-holders of the sbide-rests belonging to shaping-machines, lathes, and planing-machines. It may therefore be inferred that any one of these machines which a maker may have, may be selected for fluting, and this is the fact, no special fluting apparatus being necessary unless a large number are to be fluted with the apparatus when made.

The simplest mode of fluting a hob consists in filing each flute to its proper shape with thin fdes, termed knife-files, such as are indicated by Fig. 555. Small hobs not exceeding half an inch in thickness are thus easily fluted, if conveniently fixed, and such fixing consists in gripping the screw of the hob in a vice which is provided with a pair of thick lead clamps. The fluting of those that are more than half an inch thick is easily commenced with an edge of a rough, three-cornered file, each flute being afterwards smoothly filed to its proper depth and shape with a knife-file, and also with a small round file. Knife-files are both rough and smooth, long and short, some being twelve inches long, and others only two inches. Hobs that are to be fluted by such means, without a lathe or other machine, require a kind of lining to denote the places for the flutes, the lines being made on the flat extremity of the screw part. This portion is smoothly filed, and upon it a circular line is scribed with a divider; after which the line is divided with the same divider into five, seven, nine, or other odd number of equal parts, an odd number being necessary that two flutes may not be opposite each other. The number for a half-inch hob is five or seven; a hob about an inch in diameter has nine, and one which is two inches, thirteen. For such dividing no line of polygons or other Tables are required, a small springy divider being quite sufficient.

Hobs are easily fluted while on the lathe-pivots in the same position as they were when turned and screwed. Such fluting may be done in the lathe that screwed the hob, or in any other lathe provided with a carriage and slide-rest. While the hob remains gripped with the carrier, and is fixed on the pivots tight enough to prevent unintentional movement, a narrow grooving tool, which is fixed in the tool-holder, is moved to and fro along the hob-screw, and the flute is gradually deepened to its proper depth. This depth is attained by working the slide-rest screw to gradually advance the tool further into the hob at each cut. The to-and-fro movement is effected by an operator who rotates the carriage-spindle forwards and backwards with the carriage-handle. If the lathe possesses a dividing-plate and stop, they are brought into use for moving the hob a proper portion of a rotation after each flute is made ; but if the lathe has no dividing apparatus, one of the disk chucks may be divided, and a few dots put in for temporary use, or the flat extremity of the hob may be marked as for fluting in a vice. If thus marked, the hob is rotated by an assistant, who moves the lathe-band a sufficient distance each time a flute is formed, and keeps the hob in its proper position according to the directions of the operator near the hob, who watches the marks on the work. During such fluting the poppet-pivot is supplied with oil, and screwed rather tight, to prevent movement when it is not required. In order to easily move the lathe-band the power gear of the mandril-frame must be connected with the lathe-spindle, as if slow turning were to be effected. A hob in position for fluting in a lathe, by the means just mentioned, is represented in Fig. 564.

Hob-fluting by moving the lathe-carriage to and fro is suitable when only a few hobs are to be fluted; but to avoid the labour of fluting a large number by such means, a circular cutter is made to rotate while in contact with the hob-screw, and the hob is moved along in a direction which is parallel to its length, which produces a flute whose length is in the same direction. Fluting by this method is easily accomplished with a lathe by rotating the cutter and spindle while on the conical pivots, and placing the hob beneath the cutter, so that the length of the hob shall be at right angles to the length of the cutter-spindle. The mode of fixing the hob consists in placing it between two poppets, the screws of which fit in the centre recesses of the hob, so that it can be moved partly round whenever it is necessary to do so. The poppets are fastened to a wrought-iron bracket, which is forged to suit the lathe-carriage, or they may be fastened in a special fluting table which belongs to the lathe. A fluting table of this class, with a hob attached, is shown by Fig. 597, and the entire table, with the work to be fluted, is moved across the lathe by means of the ordinary across traverse screw, or by a special screw which belongs to the table. To raise the hob to the exact height during the fluting, and for making the flutes to the proper depth, a screw is provided at each end of the table, both being rotated at one time by means of two couples of wheels with bevel teeth, which are connected to a spindle having a handle worked by the workman; or the two lifting screws may be actuated by means of two worm-wheels. At each end of the table is also a dovetail guide and slide, so that the affair is a sort of slide-rest. To prevent the hob moving round when not required, one of the poppet-screw points, which is square or triangular, is screwed into the centre recess in the hob's square head; this recess is also square, to fit the screw-point, and when both poppet-screws are tightened, while the work is between, a secure fastening is effected. Those who object to make recesses of this shape, may use the slotted holdfast denoted by Fig. 558 ; this is bolted to the fluting-table, and the angular grips of the instrument are made to grip either the hob's square head, or its stem at any convenient distance from the head. The holdfast is fastened to the hob, after it is put between the poppets, and when it is necessary to move the hob partly round, the grip of the holdfast is merely loosened, and the poppet-screws also loosened, which allows the necessary shifting of the hob without removing any of its fastenings.

Hob-fluting is also easily accomplished with a small shaping-machine; if such is to be used, the hob is supported at a proper height above the table with poppets, that the hob may be rotated when required, similar to the mode for fluting with a cutter in a lathe; but no special apparatus is necessary for a to-and-fro shaper, because the machine-table is easily raised or lowered, for applying the cuts, by means of the usual screw of the machine. To fix the hob tightly, a poppet-screw with a square point is used, or a slotted holdfast, in the mode described for fluting with a circular cutter. The cutting tool employed to flute a hob on a shaping-machine is an ordinary slide-rest tool, having a cutting end of suitable width. This is fastened in the tool-holder and moved to and fro with the usual movement of the machine's shaping-head, while the hob remains stationary, except when moved partly round to operate upon another flute.

Marking each hob to indicate the places for the flutes may be done in a lathe while the hob is on the pivots, whether the hob is to be afterwards fluted in a vice, with a lathe, or with a to-and-fro shaper. The lathe for this marking must have a division-plate, or have a division chuck of some sort, that the required odd number of lines may be made along the hob's screw which is to be marked. After the hob is fastened in a gripper and tightly fixed on the pivots, a grooving tool having a thin cutting end is put into the holder, and advanced to the hob-screw by the slide-rest screw; the tool is then made to cut a sort of broad line along the screw by moving the carriage along the lathe; the hob is next rotated a short distance to put the part for the next flute to the tool, and the tool is again moved along with the carriage handle to make another line; after which the same operations are continued until all the places for the flutes in all the hobs are lined. Lining by this means avoids the use of division plates while the hobs are situated on the fluting-tables.