When an object is to be machine-planed with regard to some special dimension, or planed to fit, or very nearly fit, another object with which it is to be in contact, care is necessary to so finish the machine-planing, that when the article is unfastened from the table it shall possess the dimensions intended, because, in many cases, if the piece is too large after being planed, it cannot be again so accurately fixed precisely as it was before on the table to cut off another very small quantity. In order to fit one piece to another by planing, it is also requisite to have means for properly measuring the pieces, to prevent the one which is being planed being made too small. It is therefore needful to mention the implements and processes by which accurate measurements are conducted.

For measuring small objects of only a few inches in length, width, or thickness, outside callipers and inside ones are employed. For example, to plane a block, or a number of blocks, to special dimensions by means of outside callipers, three tightly-riveted callipers are adjusted to the desired distances between their respective points, one calliper being adjusted to the block's length, and the other two being adjusted to the desired width and thickness. The three tools are thus made the means of ascertaining the three dimensions of the block, and the planing is conducted with repeated reference to the callipers until the surfaces of the block or blocks are planed. Callipers thus used are liable to get their legs shifted during use, and previous to finishing the planing it is necessary to apply them to the standard, and again adjust them, if found to be altered.

To avoid the liability to be misled by unexpected alteration of callipers, gap-gauges are much used, especially by makers who supply numbers of machines of the same size and shape. Such gauges are of sheet iron, unless intended to be extensively used for a great number of articles, for which purposes they are of steel. A gap-gauge for three dimensions is denoted by Fig. 722 ; consequently, if such a guage were used for a surface-block, the largest gap would be the length of the block, and the two other gaps for its width and thickness. These gauges, by long usage, and also by improper usage, such as roughly thrusting them upon pieces of work, become worn, and therefore their gaps become too large; in such a condition, the remedy is to heat and hammer the gap-sides to diminish the gaps, and again smoothly file the sides to the proper dimensions.

Both an inside calliper and an outside one are required for measuring, either when a gap is being made by planing to fit a portion which is to be contained in the gap, or when an object or portion of it is being planed to fit an opening already made. To plane an object so that it shall fit the width of a gap, an inside-calliper's legs or points are separated until their distance apart is about a sixteenth greater than the gap's width, and are next adjusted with gentle hammering to the exact width of the gap. While in this condition, an outside calliper's points are adjusted by similar gentle hammering until they just very lightly touch the points of the inside calliper when both callipers are put together. During such placing of the two callipers together, they should lie with all their four points in contact with a tolerably fiat plate or surface of some kind, to prevent the operator being misled by distortion or bending of the legs, which often occurs with large callipers. When an outside calliper is thus properly adjusted to an inside one, the outside one becomes a gauge to which the work is planed, and if the measurements were properly conducted, the object, after planing, will exactly fit the gap's width.

The object may also be planed to accurately fit the gap's width, by means of sheet gauges,

2 G 2 and, if such are used, also without the risk of shifting, which was stated to be involved in the use of callipers. One of the sheet gauges for this purpose is an ordinary gap-gauge, such as that shown in Fig. 721; and the other sheet gauge is a straight flat piece of iron whose length exactly fits the gap's length or distance between the two arms of the gauge. This simply-formed straight piece is termed the male-gauge, and is smoothly filed to exactly fit the width of the gap to which the piece is being planed; after which the gap-gauge is carefully filed to fit the male-gauge, which renders the gap-gauge fit for use to adjust the object while planing, or that portion of it being planed.

A substitute for a sheet-iron male-gauge is a straight piece of wire slightly tapered, or merely curved at each end, although it is not requisite to make sharp points, pointed ones being specially adapted for lathe work. A straight wire gauge is denoted by Fig. 720; and for the convenience of easily lengthening a wire gauge after being worn, or after being filed too short, it may be made with a cranked mid-part, similar to that of the male-gauge shown in Fig. 721. If the wire is thus bent, the gauge is quickly lengthened by bending it slightly to open the gap of the cranked part; and to shorten the distance between its two ends it is bent the opposite way, which closes the gap.

The male-gauge of any couple is analogous to an inside calliper, whether it is a straight wire gauge, a cranked one, or one made of sheet iron; and the gap-gauge of the same couple is analogous to an outside calliper; therefore the same sort of measurement may be effected with sheet gauges as with callipers, with an advantage in favour of sheet gauges, because they are exempt from liability to mislead through shifting. Those who use great numbers of sheet gauges should keep them properly named and arranged in boxes, in convenient situations for reference when not in use.