Depth Gauges.—Depth gauges are used to measure the depths of grooves in milled and planed work, also in lathe face plate work, when one surface has to be turned to a definite distance from another surface. The gauge can be set to the depth required, and secured by the setscrew. Fig. 36 shows a gauge in which the wire is held by a friction spring inside the nut. Fig. 37 is a similar gauge, except that a marked scale is substituted for the wire rod.

The gauge shown in Fig. 38 admits of a more accurate setting; it has a micrometer screw reading to in. Standard collars are passed on the spindle when the depth of the recess to be measured is considerable. A lock nut at the top of the spindle prevents any changing after the gauge has been set.

CHAPTER II.

"MARKING" OR "LINING-OUT"

TABLE AND TOOLS.

WHEN castings and forgings are delivered into the machine shop, they are first placed on a marking-off table, and "set out" for machining. It is the practice for templates to be made to dimensions on drawing for each type of engine or machine, a practice which greatly facilitates the marking, and whereby the templates serve as guides, and sometimes as gauges, for the machine operatives. The above system can be generally applied to duplicate work. When, however, it is not practised, drawings fully dimensioned are supplied.

Whichever system is adopted, a careful and reliable workman is sought for the post of marking off. He is obviously required to thoroughly understand all kinds of drawing details, and be able to trace them out, frequently from complicated elevations, end views, or plans, and is responsible for each part being correctly tooled and fitted to the lines scribed.

"Marking tables' vary in dimensions from 4 ft. to 12 ft. long. The upper surface is truly planed, as are also the edges which are truly at right angles to the face and to each other. The under side of the table is strengthened with ribs, similar to a surface plate, to keep it true under varying conditions. The table should be set by the aid of a spirit-level to lie even in all directions, and supported in such a manner as to remain firm under any load. Such a table is shown in Fig. 39, provided with a complete set of tools, as follows: Nos. 1 and 2, large and small squares; Nos. 3, 4, and 5, scribing blocks; No. 6, parallel blocks; Nos. 7, and 8, vee blocks; No. 9, compasses; No. 12, large straight-edge.

Tools not numbered.-Accurate steel rule, marked in terms of British and decimal measures. A plumb-bob and spirit-level, pair of inside and outside calipers, also odd-leg calipers, trammels and scriber, centre and prick punch.

EXAMPLE. The engine connecting-rod, No. 10, is mounted on parallel and vee blocks after machining, while the centre and other dotted lines are tested with the scribing block (a temporary pin being inserted in the fork end). The example, No. 11, represents a rough, forged fork end, marked off for machining; the centre line shown would be marked similarly on the opposite side of fork. The inner circle

shows the diameter of the hole. The outer circle shows the amount to be removed by shaping, slotting, or profiling machine, as the case may require.

Small and important mechanisms are frequently assembled on a surface plate, which has been scraped up perfectly true. This is found

FIG. 39.-Example of work and tools-marking-out table.

advantageous, especially where the marking has to be done at different stages that is to say, after a piece of work has been lined out and "tooled," it is sometimes necessary to fit it to some other part or parts before the final marking and tooling can be done. When important lines have to be made at a definite distance apart on some forging or

casting of irregular form, an angle plate is used, to which the work is secured. Castings which are provided with cores are bridged over with thin metal, or plugged up at their extremities with wood; even in the latter case it is frequently necessary to insert a tin strip, so that the centre when struck out will be contained by the strip.

Wood is good enough for one or two tooling operations, but where the centres have to be retained, often for a considerable period, wood is unsuitable.

EXAMPLE 1. Suppose the bracket in Fig. 40 has been faced on the foot, and the hole shown cored has to be bored accurately to the dotted circle. There would be no reason why a wooden plug should not be inserted, just to remain until the dotted circle has been described.

EXAMPLE 2. Let us take an engine cylinder where the valve spindle bearings are required to be at a given distance apart, and at a given distance from the centre of cylinder bore.

Here we have quite a different case. Some of these centres will be in use for a considerable time, therefore wooden plugs will be unsuitable. Again, when erecting engines, etc., with holes bridged with thin steel or brass strips, very fine lines and dots may be made, which will serve until the " setting" is finished.

Marking templates are growing in numbers daily, as the interchangeable system, now extensively adopted, becomes universal. When

FIG. 41.-Templet for "marking out" holes in shaping machine bed.

a shaper" body is being marked, a further example is seen in locating all the bearings at once from one templet (see Fig. 41). After the upper surface and vees of the machine body have been planed, a counterpart of that surface and vees is fitted thereon at one end. This counterpart or templet has an apron with a series of holes made in it, each hole being bushed to receive a fitting cylindrical gauge-marker.

The marker consists of a finely pointed scriber, inserted in the end of the cylindrical rod in such a position that by rotating the marker a finely traced circle is made on the boss of the machine to be bored.

By this arrangement the exact centres for each longitudinal bearing is obtained precisely, and the gear-wheels will subsequently be correctly meshed when placed upon their respective shafts.

Besides this, the marking is done without any measurements whatsoever being necessary, and also in considerably less time than is the case where drawings are used.

It may here be pointed out that holes to be bored in huge castings are generally previously "cored out." There are some points of advantage in this, viz.—

1. A "boring-bar" can be passed through the cored holes.

2. There being less metal to remove, the time in boring is reduced. 3. The metal is generally more dense, or close grained.

On the other hand, it is most desirable that the "core" is in its right place when the casting is finished. If it occurs that a core is out of position, the consequence may be serious. Any error in this direction is doubled when it comes to measurement from the boring bar.

Setting out Machine Bed.-EXAMPLE.-Let us suppose a casting for a machine bed or body is made having a long bearing, the outer diameter of which is 4 in., but whose hole or core is 21 in. diameter, and has dropped out of position

in.

Now, if the core were central with the outer diameter, and the casting had to be bored 2 in., when finished there would then be in. thickness of metal each side of the bored hole. But the core in this case is eccentric with the boss, or, to put it another way, it is in. out of alignment with some other hole or surface. If it needs "tooling," the boss will be in. thick only on one side, while on the opposite side it will be I in. thick. But the cutting has been all one-sided, and although the finished size of 2 in. is reached, all the black or scale has not been removed.

It therefore follows that, since it is absolutely necessary to have a truly bored and smoothly finished hole, more metal will have to be removed, leaving the hole in. too large, and reducing the thin side of bearing to

in.

This clearly shows us that a bent or eccentrically made core in any casting which has to be machined may be sufficient to condemn the work altogether. It would be well for the student to note the following important points respecting cores in iron or other castings:

1. The amount to be removed by tooling any cored casting should always be sufficient to "clean up" without using a reamer, i.e., it should be smooth and true.

2. Before "lining out," long holes should be thoroughly examined and tested for alignment.

3. Castings in halves should be well jointed, and, where practicable, small articles may be sweated together, or otherwise made secure with screws or cotters, which pass into lugs, or ears.

This refers to bearings, pulleys, and similar split work, which may be disturbed at the joints if left insecure at any point.

4. After correctly "setting," cored work is best faced before boring, as measurements across uneven surfaces are unreliable, and especially so to a beginner.

Cored Work. There is no economy in "coring" some kinds of work below, say, in. diameter; the time spent in machining is worth. more than the weight of metal removed from a solid casting. Especially is this the case in long holes of small diameter-say, for instance, a -in. core to be bored to 1 in. diameter 2 ft. through an iron