Surface-tables or plattens consist of cast iron or Bessemer steel blocks resembling Fig. 295, and a few others adjoining. Such tools are made of various sizes, from an inch across to several feet, and each platten has at least one plane surface. One principal use of a surface-plate consists in applying a piece of work to the plane for the purpose of ascertaining how near the surface of the work resembles the plane of the table; consequently, plattens are standards of reference during the formation of plane surfaces, and are used whenever a plane is being produced upon any piece of work that may be in progress. Surface-plates are also planes from which to exscribe straight lines upon pieces of work by the aid of scriber-blocks. The smallest sort of platten is held in one hand, and applied to the work, instead of the work being moved to the platten ; and small surface-plates of this class may be only a few ounces in weight. A good platten is that which is made of hard iron or steel smoothly finished to a plane, and which will maintain its plane character while being used; therefore any surface-plate, small or large, must contain a proper amount of metal which is so distributed as to prevent the platten bending through its own weight. For some purposes a surface-table is fixed in one place, instead of being portable; when thus placed, with its plane boundary upwards, and in a horizontal position, the metal should be as thick as the space allotted to the table will permit, and the base of the table should rest on as unyielding a body or cement as can be obtained. The number of bearing points or touching points of contact of the base with its support should include all the points in the base, so that no portion of the surface-table's base shall be without a prop which is directly underneath ; and after the table is permanently fixed the plane surface is finally smoothed while in its position for use, if the fixing process has rendered such final finishing necessary. Some plattens are suspended above a piece of work which is being filed or scraped, so that the platten can be let down to try the work when it is necessary to do so, and also easily raised up to allow the surface to be further smoothed or scraped after being tried with the platten. Tables for such purposes are provided with lifting studs; these are screwed into holes that are formed into various sides of the surface-plates; implements of this character are represented by Figs. 296 and 298. Fig. 297 indicates a wood cover which is put over a plate's surface when not in use, or, in some cases, the plate is put into the cover; if so, its bottom is covered with soft clean cloth. Such covers may be made also of sheet iron.

The surface-table shown in Fig. 299 is supporting four additional surface-tables or blocks; the two blocks on the right-hand end of the table are similar to each other in shape and dimensions, and are termed a pair; and the other two blocks on the left end of the table are named a couple of odd blocks, through being of different sizes. The corners of all these movable blocks require to be curved, that they may be put to or moved about the surface-table without injuring it. All the four blocks are useful for trying pieces of work while being adjusted to right angles, or being made parallel; such blocks are useful also for trying el-squares, tee-squares, rulers, and other gauges. A table thus employed with blocks thereon requires to be fixed in a quiet place which has a good supply of equally diffused reflected light, and but little dust; and the surface of the table ought to be high enough to enable an observer to look across and see the light between the blocks without an inconvenient stooping; and to facilitate the observation, white paper may be placed, either near the blocks, or at a distance behind the table.

Radius Gauges

The radius gauge shown by Fig. 300 consists of a steel bar on which slides two scriber-holders ; the small wires, which are the scribers, being fastened in their places with little screws having teeth around the rims of the heads. One of the scriber-holders may be moved along to any desired place on the bar, but the other holder can be moved only about an inch. This one is shown at the right-hand end of the Figure, and is connected with a short screw, denoted by A, having a head with teeth on its rim, for the purpose of easily rotating it with a finger and thumb, or, if necessary, with the palm of the hand. The use of the gauge consists in scribing arcs and circumferences of long radii which are too long for compasses; and when the instrument is required for use, the scriber-holder at the left end of the bar is slid along until the distance between the two scriber-points is about a sixteenth greater or less than the distance when adjusted; after this, the adjusting screw A is rotated slowly, and moves the holder which is nearest to it until the two scriber-points are at the exact distance from each other which is required.

Callipers

The simplest sort of calliper is made of a piece of straight wire, which is first bent to produce a short arm at each end, and afterwards bent in the middle, to make the tool resemble an arch, similar to Fig. 301; in this form the tool is an outside calliper for measuring rods, axles, and other work; and in the form shown by Fig. 302 it is an inside calliper for measuring holes. Fig. 303 denotes an outside calliper of two legs, which are connected with two friction plates and one rivet, the two plates being termed washers. Fig. 304 indicates a springy calliper, whose action is similar to that of a springy divider. Springy callipers are very useful for small work that does not require the calliper's legs to be separated more than seven or eight inches ; but for large work callipers resembling Fig 303 are more frequently used, and are made large enough to measure three or four feet. Next to the springy calliper is shown an inside calliper, whose feet may be, if necessary, only an eighth of an inch in length, by which means the tool is capable of measuring a hole which is only a quarter of an inch in diameter. Fig. 306 represents an inside calliper which is shaped to resemble legs, merely for ornament. It may here be mentioned that all inside callipers may be, if necessary, used as outside callipers, and this is effected in cases of emergency by sliding one calliper leg across the other, so that the toes of the feet are put nearest to each other, instead of the two heels being nearest each other, which is the usual position. By adopting similar means, outside callipers also are made to serve as inside ones. Fig. 307 represents an awkward, inconvenient class of callipers, which are never used by engineers; the tool has four legs, two for measuring holes, and two for rods and other work that needs outside measurement. When such a tool is newly adjusted, the distance between the points of the inside legs is the same as the distance between the points of the outside legs, but after a short time the wear of the points makes the distance between the outside legs too long, and the distance between the inside legs too short; it is therefore necessary to hammer the ends and re-file them, so that the tool is only fit for those who do not understand the use of proper gauges.