In order to enable a learner to fully understand and appreciate the making of straight-edges in general, he should entirely make one or two by hand-planing. This will teach him the value of machine-planing of straight-edges, when several are required at one time. He will also observe that much more lining is required for hand-planing than for machine-planing, if he desires to perform the work in an orderly and easy manner.

For making straight-edges in great numbers, smoothly-rolled steel of proper thickness is always used, so that little or nothing is done to the broad sides of the metal, because the entire straight-edge making consists in cutting the steel to proper lengths and widths. But a learner who is commencing the practice of planing is often obliged to use remnants of sheet steel or bar steel having rough broad sides, so that to make one or two straight-edges a large amount of filing, to reduce and smooth the sides, may be requisite. When a straight-edge is to be produced by such extensive reducing, it is necessary to have a proper idea of the form of the tool to be made, and to consider it as a right-angled parallelopiped. This is the form that should be intended by the operator, because, although the tool should be made so that the two edges shall be very accurately planed, and also parallel to each other, the straight-edge would be defective if its broad sides were rugged or irregular near the planed edges. Such a defect would not exist in a straight-edge which is made of the proper smoothly-rolled steel or iron; consequently, no attention need be given to such defects unless the rough filing referred to for reducing the broad sides is to be effected. A projection or a hollow existing at some part of a broad side is of little consequence when the straight-edge is in use for trying flat surfaces, but such a fault is often sufficient to prevent a straight line being scribed with a scriber which is moved along in contact. The small bevelled part which is sometimes made at one edge of a straight-edge is a means of remedying defects in the broad side; but previous to making such a bevel the operator should produce the parallelopiped, after which the bevelling may be easily and accurately done. A parallelopiped for a straight-edge is shown by Fig. 616, the letter C showing the place for the cut, if one tool is being made previous to dividing it into two.

The marking of a rugged piece of plate intended for a straight-edge is denoted by Fig. 622, in which a straight line is shown along the middle ; this line is the primary, and is the one first scribed by means of a standard straight-edge, after which a number of circles are marked along the line, the number depending on the length of the intended tool. Each circle is of the same diameter as the desired width of the straight-edge, and after these are marked, two other straight lines are scribed, so that one line shall touch the circumferences of all the circles at one edge of the rugged plate, and the other line shall touch all the circumferences at the opposite edge, in order to make all the three straight lines parallel to each other. When thus marked, the piece is ready for reduction, either by rough filing, by grinding on a rotating stone, or by chiselling while red hot on an anvil, the mode adopted depending on the quantity to be taken off", and the particular implements accessible to the operator.

If a surface-table of sufficient dimensions is accessible, the scribing is conducted by another mode, which consists in first roughly hand-planing one edge of the plate, and next placing the piece with its planed edge in contact with the table's surface, as seen in Fig. 623 ; while it is held in this position by an assistant, who holds it at right angles by means of an el-square, an operator moves a scriber-block along and marks a line at the desired distance above the table.

By this means one broad side is marked with one boundary of the hidden plane which is to be produced and is to be called the straight-edge. Another straight line is next scribed by similar means, but upon the other broad side of the plate, to indicate another boundary of the small plane to be formed, the el-square being shifted and put into contact with the side first scribed. By this mode of marking, no primary line along the middle nor circles are necessary, because the table's plane surface constitutes the primary base or plane which guides the scriber-block in the desired path.