This section is from the book "Spons' Mechanics' Own Book: A Manual For Handicraftsmen And Amateurs", by Edward Spon. Also available from Amazon: Spons' Mechanics' Own Book.
The use of these instruments is for marking out work at right angles. The most usual forms are illustrated below. Fig. 244 is a common brass-mounted square; Fig. 245 a mitre square. It consists generally of a wooden stock or back with a steel blade fitted into it at right angles, and secured by 3 screws or rivets; the sizes vary from 3 to 30 in., and the prices from 1s. to 10s. They are also made of plain or nickel-plated steel, with scales engraved on the edges. In use, the stock portion of the square is placed tight against the edge which forms the base of the line to be marked, so that the blade indicates where the new line is to be drawn. The making and application of squares have been well described by Lewis F. Lyne in the American Machinist. He remarks that the 2 sides of a square should form an angle of 90°, or the 1/4 of a circle; but hundreds of tools resembling squares in appearance, and so named, when the test is applied to them, are found entirely inaccurate : the angle is in some instances more, and in others less, than a right angle. The way these tools are generally made is by taking a piece of steel for the stock, planing it up to the right size, and squaring up the ends, after which a slot is cut in one end to receive the blade.
The blade is neatly fitted and held securely by 2 or 3 rivets passing through the end of the stock and blade. It is a very difficult undertaking, with ordinary appliances, to cut this slot precisely at right angles to the sides and ends of the stock; and, when the blade is finally secured, it will be found that it leans to one side or the other, as shown in Fig. 246, where a represents the stock, and b the blade; c is an end view, the dotted lines showing the position of blade, as described.
The best way to produce a square without special tools is to make a complete flat square of the size desired out of thin sheet steel, the thickness depending upon the size of square desired. In almost every instance where squares are made by amateurs at tool-making, the blades are left too thick. After the square has been trued up and finished upon the sides, 2 pieces of flat steel should be made exactly alike as to size, to be riveted upon the sides of the short arm of the square to form the stock. To properly locate these pieces, the square should be placed upon a surface plate, and the parts clamped in position, care being taken to get them all to bear equally upon the surface plate, after which, holes may be drilled and countersunk, and the rivets inserted. The angle formed by the cutting edges of the drills for countersinking the holes should be about 60°, so that when the rivets are driven, and the sides of the back finished, there will be no trace left of the rivets, which should always be of steel.
Close examination may reveal the fact that the blade is winding, or is slightly inclined to one side. If inclined, as shown at e, in Fig. 240, the end of the blade only will touch a square piece of work when the tool is held in a proper position, as shown in Fig. 247, where i represents the piece of work, and f the square. It is a custom among machinists to tip the stock, as shown at h and I, to enable the workman to see light under the blade. This only aggravates any imperfection in the squareness of the blade, for when the stock is tipped, as shown at k, it will touch the work at g, occupying the position indicated by the dotted lines g,g; whereas, if the stock be tipped, as shown at I, the blade will assume the position indicated by the dotted lines h, h. These conditions will exist when the blade of the square is inclined, as shown at e, in Fig. 246. If the blade is inclined to the left, a precisely similar condition will exist, except in the reverse order. It is next to an impossibility to perform accurate work, or test the same with a square having a thick edge, because of the reason already stated that the light cannot be seen between the edge of the blade and the work.
The most ingenious tool for overcoming the foregoing difficulties is a sort of self-proving square, made by a machinist in New York. This is shown in Fig. 248, and consists of a steel beam j, shown in bottom view at k. In the end of this beam is a hole for the reception of a screw, with a common bevelled head. A square piece of steel, l, m, forms the blade of this square, n representing the end of the blade. The blade is first planed, then tapped and hardened, after which it is ground to bring the sides exactly parallel and of equal size, which makes the bar perfectly square. The stock is of a rectangular section, and, with this exception, is hardened and ground in the same manner as the blade. The end for the screw is then carefully ground at right angles to the sides, after which the parts are put together and the screw is tightened. If the blade is not precisely at right angles to the stock, it will occupy a position indicated by the dotted line o; then, if the screw be loosened and the blade turned half a revolution, the edge will stand as shown by the dotted line at p.
The end must be so ground that the blade will occupy precisely the same relation to the beam when turned in all positions. When this is accomplished, the square is a very close approximation to perfection. The accuracy of work is tested with one of the corners; when it becomes worn, another may be turned into position; and when all are worn, the blade is removed and trued up by grinding, as at first. In testing the accuracy of the ordinary square, it is usually placed upon a flat surface having a straight edge, as shown in Fig. 249, where s represents the surface with the square upon it. The stock is pressed firmly against the edge of the surface, and with a scriber a fine line is drawn along the edge of the blade. The square is then turned to the position t, indicated by the dotted lines, and a second line is drawn along the edge of the blade. If the tool is less than a right angle, the line with the square in the former position will incline towards q, while in the latter position it will appear as shown at r; whereas, if the square be correct, the two lines will exactly coincide with each other.
This is not a reliable test for the accuracy of a square, but it answers very well in case of emergency.
It is difficult to draw the lines to exactly represent the edge of the blade, owing to the fact that the slightest inclination of the hand holding the scriber to either side will make a crooked line. The form of square shown in Fig. 248 always presents a fine edge to work to, and may always be relied upon for accuracy when properly fitted up. This square would seem to be quite as easily made as the common one, but the construction of an accurate square with ordinary appliances is a job that tests the skill of a good workman.