The guide is now in position for planing both sides or faces of its slot. When such an article is made with the slot formed by casting, it should be so cast that a clearance space is formed at each end of the slot. This space is a semicircular gap, the width of which is about a quarter or half an inch greater than the width of the slot; consequently the broad surfaces or faces of the slot project into the two clearance gaps, and will form corners or steps, instead of coinciding with the curved junctions of the gaps. These spaces are not noticed in Fig. 902, but can be seen in the comparative large sketch denoted by Fig. 903. Such openings are useful for a convenient planing, because they constitute clearance spaces in which the tool may disengage from the metal, and are also advantageous for preventing the formation of ridges during the future wear of the slot-faces. By making the guide-block of sufficient length to cause its ends to protrude while in action into the half-round spaces, the formation of a ridge is prevented. If it happens that the guide is without such spaces, they require to be made previous to planing, either by a drilling-rod in a drilling-machine, or by chipping and filing. It is also frequently necessary to prepare the clearance spaces, although they are now roughly formed by casting, in which cases a small amount of chiselling and filing is sufficient for the purpose. Those guides which are made without any slot whatever are furnished with clearance gaps by drilling a hole of proper diameter at each end of the place for the slot, the diameter of the hole being necessarily greater than the finished width of the slot.

If the clearance spaces are suitably shaped by some means, the slot-part is ready for planing with grooving-tools, or with right-hand corner-tools and left-hand ones. Springy tools, also, and slotted tools are occasionally used. Supposing that a small gun-metal guide is to be planed which has had a hole drilled at each end of the intended slot, but no portion of the slot formed, the slot-making should commence by using a groover, which is gradually advanced down with the vertical traverse until a narrow slot is formed through the guide, the slot extending from one clearance-hole to the other. This opening is next widened either with another groover having an edge of suitable length, or is widened with corner tools, until the required width of slot is attained. A small guide-slot of this class can be shaped also with a slotted-stock tool (Fig. 716). The cutter of this implement is shown by Fig. 717. Previous to this being used for planing a guide-slot, grooving-tools of proper width are caused to form the slot to nearly its intended width, leaving a small quantity of metal to be removed at each gauge-line indicating the slot's mouth. These two small portions can now be cut out with the slotted tool and cutter at one operation. To do this, the tool must be carefully adjusted to place its edge exactly midway between the two gauge-lines showing the width, after which it can be advanced down with the vertical traverse and caused to cut the two faces of the slot at one time. Such a cutting out will suffice to roughly form a slot, but will not smoothly adjust a slot to a precise width which may be specified. Consequently, a cutter whose width is rather less than the finished width of the slot must be used, which is first allowed to cut two sides at once, but is afterwards made to cut only one side at a time, to finally attain to the exact width intended.

The planing of a large slot of four, six, or seven inches in width, is generally executed entirely with corner tools, whether the metal is gun-metaL iron, or steel, because large slots of guides are always formed by casting, and therefore easily admit the ends of corner tools from the beginning to the end of the slotting. Corner tools for gun-metal are shown by Figs. 431, 432, and 708. The cutting parts of such tools for planing faces of slots are but slightly bent, the amount being only sufficient to cause the cutting part to clear the side of the stalk, and thus allow as much room as possible between the tool and the face of the slot while the tool is therein. Tools of this sort, whether small or large, are available for slots of any width, if wide enough to allow a free movement of the tools employed. When it happens that a large slot is to be planed, and there is not a corner-tool small enough to enter the slot, the slot can be planed with a stalk and cutter similar to Fig. 715, the cutter used being similar to Fig. 714, if the guide is gun-metal; but if iron or steel, the cutter's end is similar to an end of a corner-tool. In the slot of the stock a cutter of any desired shape, left-handed or right-handed, may be keyed, the length of the cutter depending on the amount of room in the slot. The planing of a slot with such a tool somewhat resembles that mentioned in the preceding paragraph for a small slot which has been previously commenced with a grooving-tool.

During all these planing-processes for slot-shaping, the vertical traverse of the slide-rest must be exactly square to the table, and therefore truly vertical, if the machine is properly fixed. This right-angular position is requisite because the broad sides of the standard are parallel with the table, and the direction in which a slot extends through from one broad side to the other, must be square to the broad sides. The rest is therefore adjusted to the proper position by means of a dot or other mark existing for the purpose; but the tool-box, or whatever tool-holder the slide-rest may possess, may be inclined at any angle, if necessary, when a corner tool is in use, that the thick part of the tool's end may be prevented from touching the side of the slot while advancing downwards. Inclining the tool-holder without altering the traverse-slide, has about the same effect as merely shifting the tool, the travel of the slide not being thereby affected; unless the tool-holder happens to be solid with the slide, in which case, only the tool can be inclined.

Near the conclusion of planing a guide-slot, it requires a careful measurement with sheet gauges, or with callipers, the points of which are delicately adjusted that they may only very lightly touch the surfaces in contact, as described in page 208. By such gentle measurement, the operator can ascertain whether the upper mouth or entrance to the slot is of the same width as the lower entrance next the table-face; also whether it is parallel along its length, or wider at one end than at the other. Defects of this character often exist in guide-slots, especially if the metal planed is steel, or a hard iron, either of which may wear the tool-point during the removal of a slice, sufficient to cause one end of the slot to be half a sixteenth wider than the other. It sometimes happens that a slot of several feet in length cannot by any means be made parallel and smooth with a slide-rest tool; and the work is therefore afterwards completed with a considerable filing.

It should be here mentioned that considerable care is required during the fixing of a standard, to avoid distortion, such as described in page 234. The author's method of preventing this injurious bending of a guide, consists in placing each holdfast plate directly over a packing-block, always avoiding, when possible, the fixing of a plate to any portion of the object which is not supported with a block beneath. It is also requisite to use plates having paws of only about three-quarters of an inch or an inch in width; the height or thickness of the paw being sufficient to secure proper strength to compensate for the comparative small width. With this method the tightening of the screw-bolt of a plate causes the paw to bear upon the metal which is exactly over the packing-block, but prevents the paw bearing upon any other part; therefore the fixing of the plate cannot injuriously bend any part of the metal, however tightly the bolt may be fastened, because, although the block is a sort of fulcrum, no leverage exists for causing the distortion, through the plate's paw being situate exactly at the fulcrum, and not at any distance from it, which distance would be necessary to provide the leverage referred to.

It may be also stated that by observing the same rules as these given for fastening guides, during the fixing of all other objects in general, small or large, the distortion referred to will be reduced to the minimum.