The cheapest straight-edge is that made of a piece of hoop iron which is flattened with a tin hammer, and next straightened along its edges with filing; or, if the tool is several feet in length, it is straightened with planing. Those who may be obliged to make a long straight-edge without another for reference, and also without a planing-machine, can use the thread straight-edge shown by Fig. 480. The thin thread of such an instrument is a simple and efficacious standard of reference, and the apparatus may be made of any length to suit the particular straight-edge which is being formed. An iron or steel straight-edge is not liable to rust, if the thin scaly coating on its broadsides is allowed to remain, which presents a better appearance than a rough grinding and filing. In order not to disfigure the scaly sides, it is gripped with leather or wood clamps during the filing of the edges.

Straight-edges that are made with a planing-machine may be produced by forming two at one time by using a strip or plate of steel which is more than twice the width of one of the intended tools. Such a piece is bolted to a few blocks which rest on the table, or on to a couple of long packing-strips that are as long as the steel, the broad side of the steel being upwards. The packing-strips are put near the edges, and thus a clear space is allowed between the table and the straight-edges and along their entire length. When the steel is bolted in this situation, a roughing tool having a vee-point is used to make a cut along the length and in the midst of the steel, and two or three cuts will divide the work into two if it is only about a tenth of an inch thick ; but if thicker, a narrow parter is employed to effect the division. When thus cut into two, both pieces are still held with the holding-plates, and in this condition the two contiguous edges are straightened with a sharp springy tool or with a pointed corner tool. During the last cuts, water or soapsuds are applied to the work to produce smooth surfaces. After the two edges are thus prepared, the two tools are released from their plates, and both tools are put close to each other with their broadsides in contact, while the two planed edges are made to rest on the planing-table. The tools are also fixed so that their broadsides shall be at right angles to the table, and this is effected by fastening them to a holder or chuck which has a surface at right angles to the table; to this surface the straight-edges are fixed, and the mode of fixing is by applying poppet screws along the broadsides, and by fastening each end with a holding-plate. While properly fixed, a few light cuts are given to the upper rough edges, which are then made straight, and both tools are made to one width. The next step is to cut the tools into shorter lengths, if short ones are required, and to again divide them along their mid-lengths, if small straight-edges are to be made. After all the planing is done, they are next smoothly filed, and also scraped to make each edge coincide with a surface table of suitable length. During the trial of an edge on a table, a thin layer of oil and soot or lampblack is put on the places which are to serve as standards for the edges being adjusted, and this oily mixture is strained and kept free from filings, dust, scrapings, and other substances moving about. The final adjustment of straight-edges should consist of grinding, and this may be effected by placing two tools together, with their edges in contact with each other and with their broadsides in contact with a table; while thus situated, they are gently pushed together and a few rubs given by moving the tools while in contact. During this rubbing a mixture of the finest emery and oil is applied to the surfaces.