Screwed plates have for many years been made of old flat files. Good plates are made of such materials if the steel were of the proper quality at the time the files were made, some being very brittle and others comparatively tenacious. At the present time a large quantity of brittle Bessemer product is made for files, which is only good because of its capability of hardening, having but little durability through not possessing much tenacity. The files selected for screwed plates and other tools should be the smoothest that are made, which are termed dead smooth, and are of better steel than the rougher sorts, consequently are much more useful when no longer fit for filing. Files are often made into screwed plates by drilling holes, but without altering the outer shapes ; the preferable mode is to reduce each end of the file to form two handles, and to equally hammer all that part which is to contain the screwed holes. The forging of a file will indicate its quality; if it is brittle and comparatively worthless, it will not bear stretching without cracking, and will sparkle, while properly heated at a bright red, as much as a tough good fire would sparkle at a bright yellow.

Rough files that are to be reduced with forging require the teeth to be partly ground off with a grindstone ; if not, the hammering will reduce the file without erasing its teeth, because during much of the stretching the teeth are driven upon each other, instead of being separated, this closing together resulting from the slant positions of the teeth. Another reason for grinding off the teeth is to prevent the file breaking asunder at the bottom of a tooth-groove while being hammered; this sometimes happens with hard steel, although it may be tolerably good.

In order to prevent the two handles of a plate coming into contact with the vice or the piece being screwed, the ends of the handles are placed a short distance above the mid-portion of the tool, either by cranking each handle or by bending it. Such a plate is shown by Fig. 600, and is forged to this shape with cylindrical handles, to avoid the handling of sharp corners.

After each plate is softened by a slow cooling in charcoal they are lathe-turned, if they possess round handles, and the mid portions are next filed to prepare them for being marked to indicate the places for the screwed holes. This filing should be rough, and if the plate is being made of a file no attempt need be made to entirely erase all the original file-teeth, except from the handles; if the mid part is left rough, there will be less risk of breakage in hardening. The thickness of a plate's screwed part should be about three-quarters of the diameter of the holes; such a thickness will allow one end of a hole to be tapered after being drilled; consequently a plate which is to have several holes, whose diameters average three-eighths of an inch, is about five-sixteenths thick.

The plate being properly marked to allow sufficient metal around each hole, becomes ready for drilling, and the first drilling consists in making all the larger holes that are intended to be screwed, the smaller holes to receive the shavings being made afterwards. In order to drill the holes correctly, a correct measurement must be effected by means of callipers, and the taps or hob from which the plate is to be screwed. It is especially necessary for each hole to have a completely formed or full thread, and it is advisable to drill each one a little smaller than the proper diameter rather than to incur the risk of making it a little too large by drilling it to near the proper dimensions. The diameter of the drilled hole should equal the diameter of the hob at the bottom of the thread-groove ; and if the usual Whitworth screw is to be made into the hole, the proper diameter is obtained from Table 5, at page 177. In the sixth column of this Table, the diameter nineteen sixty-fourths of an inch, is given for the bottom of a hob-screw or other screw whose outer diameter is three-eighths of an inch; therefore the hole in the plate to be screwed must not be larger than nineteen sixty-fourths. After drilling, one end of the hole is tapered about a quarter of its length by means of a rosebit, whose cutting part is of suitable angle. The end of the hole thus tapered is that which will be upwards while using the plate for screwing a bolt-stem at the corner near the head; during such screwing the end of the hole not tapered is undermost, and in contact with the corner, because this corner cannot be screwed with the tapered end of the hole.

Forming the screw into the hole should be effected with a long taper tap made according to the directions given. The tap's teeth must be sharp, and while screwing the tap through the plate it is griped in a vice; a slow rotation while tapping and plenty of oil are also necessary. As soon as it has advanced about one-third of the tap-screw's length, it should be screwed out, and the plate put upside down in the vice, to allow the tap to be screwed in from the opposite side. The operator can then observe whether the tap's length is at right angles to the broad sides of the plate, and if not, he presses the tap in the desired direction at the same moment as he rotates it, and he continues so to incline it until it is put square and will remain square while screwing; the hole is then screwed until the tap is about half its length through the plate, when it is again brought out and put in from the opposite side, to advance the tap until its full-thread portion begins to cut, when it is again taken out and the hole examined. At this stage of progress the thread at one end of the hole may be quite fully formed, through the hole being rather too small; if so, it needs a careful filing with a small round file. The tapping is next resumed, and the plate reversed about once more while finishing the screwing. Near the conclusion, it is important to administer plenty of oil, that the work may be properly smoothed.

By means of such gradual tapping, plates may be screwed without making rugged threads,

2 c 2 although the tap may not be so good as it should be ; but a thoroughly good long taper tap will screw a steel plate by reversing it only once or twice during the process, if the hole is not more than half an inch in diameter; the larger the hole the greater the care required.

Another mode of screwing a steel plate consists in using short taps termed hand-taps; these are employed in various ways to screw plates, because such taps are of various shapes and sizes. When a plate is screwed by means of such taps, the screwed hole should be finally screwed with a hob of proper diameter, and having its screw tapered according to the directions given for hob-making.

When a long taper tap is the only instrument used for screwing a hole in a plate, and the thickest part of the tap is of the same diameter as that of the piece to be screwed with the plate, only one hole can be made for each size of screws to be produced, or for each piece of wire or small bolt to be screwed ; it is therefore necessary to provide a thicker tap or hob which shall be screwed through one hole in the plate if two holes are to be used for screwing one bolt. The diameter of this larger one is about half the depth of the thread greater than the diameter of the smaller one. For general work, a screwed plate for screwing quarter-inch wire and smaller sizes needs only one hole for each size, those that are larger than a quarter of an inch requiring two.

The next operation consists in making the gaps or channels that are to receive shavings and admit oil while the plate is screwing a pin or bolt. This gap-making is done by drilling three holes near to each screwed hole, and at equal distances around it; an opening is next made from each small hole to the screwed one, with a thin file, and the edges are carefully smoothed. These gaps are thus principally made by drilling, merely because it is the quicker process; but if thin knife files and round files of suitable sizes can be obtained, they are as efficient as drilling. Screwed plates are comparatively thin, and therefore easily filed, and filing avoids the risk of breaking small drills.

When the hole is completely screwed and the channels made, all the burs made on that end of the hole which was coned previous to tapping, are carefully scraped off", and the surfaces polished with emery cloth. The opposite end of the hole also is trimmed, but not tapered, and the trimming consists in filing off the sharp end of the thread by means of a smooth file ; this part is next scraped and polished. This filing off the thin end of the thread prevents it breaking off while in use.

Hardening the plate is performed by heating all the mid-part at one heating and dipping it edgeways into water. The fire for heating may be a charcoal fire, or an ordinary forge fire of cinders; and when a dull red heat is obtained, both the plate's broad sides may be covered by sprinkling thereon powdered prussiate of potash, which prevents the tool cracking through direct contact with the water, and also imparts a sort of steely surface to the work being hardened, resulting from the heated steel absorbing a portion of the carbon, which is liberated from the cyanogen belonging to the powder applied. After hardening, the plate may be smoothed by grinding it on a grindstone that rotates slowly, to avoid shaking the work and breaking it, and when the two broad sides are thus cleaned, the plate is ready for tempering.

The tempering of a screwed plate is effected by allowing it to remain on a red-hot bar or plate until a light golden tinge appears, being careful to put both sides into contact with the tempering iron, in order that one side may not be harder than the opposite when tempering is completed. The junctions of the handles need not be hardened, and if hardened, should be tempered till quite soft.