By a due consideration of the processes just mentioned, it may be noticed that bad tools are the means whereby good ones are produced, and this is the secret of all good tool-makers' operations. In the hands of a careless man, a defective straight-edge becomes worse ; but if in the possession of a tool-maker, its defects are discovered and removed. All surface-plates and straight-edges were thus gradually improved forty years ago, and similar tools are now adjusted in the same way, because we have no planing-machine which will plane a surface-plate so that it does not need any further adjustment. This adjustment is always effected with scraping, grinding, and by referring the surface which is in progress to another surface that is being made at the same time, or to another surface made by similar means on some previous occasion.

Those who may be obliged to make straight-edges and surface-plates without any primary implements as standards may proceed in the order thus indicated. First provide two straight-edges, each about three feet long, two inches wide, and a quarter thick, and file one edge of each tool to a thread straight-edge, shown by Fig. 480. The two opposite unfiled edges are to be next filed until parallel to the two first adjusted by the thread. This parallelism is attained by adjusting a tightly riveted calliper to the desired width of the straight-edges, and then gradually filing the entire length of the tools until the calliper will fit any width along their lengths. After this, the four edges are further straightened by applying them to the thread, and their parallelism is also improved by again using the calliper. The next step is to grind all the four edges together ; this would be done on a plane surface, if one were accessible; and instead thereof, the two tools are put on the ground, on the floor, or on to some board nearly flat, with the broad sides of the two tools in contact with the board, and two of the edges in contact with each other. In this situation the four edges are ground with emery by rubbing to and fro, putting the tools upside down, and placing all the edges successively together several times during the process. This treatment is continued until all the four edges appear to coincide with each other, at which time all the emery is cleaned off, and tange is applied to discover which of the four edges are the two hollow ones; these two may be discovered also by putting the edges together and observing the light between. The two hollow edges are next ground until they coincide, at which time they are straight enough to serve as standards while scraping or smooth-filing the two inferior edges; after which, the two concave edges are again detected, if possible, and ground together as in the previous grinding; but if no concave edges can be found, the tools are near enough to be used for making surface-plates. These plates are made in pairs as previously described, and adjusted to the straight-edges, and also with grinding the two concave surfaces together. At this time another adjustment can be given to the two straightedges, which could not be given before; this consists in grinding the straight-edges together on the surface-plates. This grinding includes two processes; one of which is the grinding while the broad sides are in contact with the plate's surface, and the other process is the grinding to make both the straight-edges of one width. By these successive grindings of the implements with each other, good plates and straight-edges are made with very ordinary tools; and if the straight-edges are each a quarter thick as directed, they will serve as twist-finders.

In addition to the trial processes and grinding processes now described for adjusting surface-plates, a few other means should be adopted while making plates or tables that are specially intended to be standards of reference. A couple of tables of this class are finished by placing the two surfaces together and rubbing them as in other processes, but in a vertical position, instead of horizontal. The two plates or tables are stood together edgeways on a block or table which is convex, to cause a rocking motion while moving the work to and fro for grinding, and the plates are stood on all their edges successively, to give a sort of equality to the operation. Makers of plates, who have any fear of a surface being injured with its own weight, should use the pillar-table that was mentioned; the surface of a table of this class is about the same in any position, whether vertical or horizontal, so that while a surface-plate is in progress, or may require a trial, t should be rubbed to the pillar-surface while in a vertical position, and also while in a horizontal position, and if the tange presents the same appearance in both cases, it may be inferred that the plate is strong enough to sustain its own weight without apparent distortion.

It is also necessary to consider the small sides or edges. Every good surface-plate should have the edges at right angles to the principal surface ; and the nearer it is to precision in this particular, the greater will be the future usefulness of the implement. The boundaries of the principal surface should also be as nearly as possible rectangular.