Test indicators are a class of tools or in struments used for determining small irregu larities in the accuracy of cylindrical surfaces and small variations from the true rotation of such surfaces. They are also used in determin ing the inaccuracies of a plane surface and in measuring small amounts of end or lateral motion such as the end-motion of a spindle. They are of two types — those which merely indicate the existence of the irregularities and those which give a reading or measurement showing the exact amount of the errors.
Some of the other small tools indispensable to the metal worker may be briefly summarized as follows: The various forms of "hack-saws" used for severing purposes. They are made of hard ened steel to cut metal and mounted in a light frame and may be operated either by hand or by some form of power. Their blades are usually made with 14 teeth to the inch for gen eral work and with 25 teeth to the inch for cutting tubing and thin metal.
The "monkey-wrench" and other forms of wrenches used for screwing and unscrewing the nuts of screw-bolts, etc.
The various forms of "drills" and especially the modern "twist-drill" which is universally used at the present time and which has com pletely supplanted the old flat forged drill which for many years held the first position as a tool for producing circular holes in metal.
The "reamers" and "broaches," which com prise a class of fluted tools used for finishing and truing cored or drilled holes. They are solid when used in a socket or with a wrench and shell or hollow when bored out to fit a mandrel.
The "screw-threads," "taps" and "dies," which, according to their use, may be divided into two classes — those used for fastenings and those used for communicating motion. There are three forms of screw threads used for fastenings—the "V" thread in which the sides make an angle of 60 degrees with each other and in which the top and the root are sharp; the "United States standard" thread which is similar to the "V" thread with the exception that the top is cut off and the bot tom filled in; and the "Whitworth" or "Eng lish standard" thread in which the top is rounded off and the root filleted in and in which the sides form an angle of 55 degrees with each other. There are also three forms
of screw-threads employed for communicating motion — the "square" thread, the °trape thread and "Powell's" thread.
The top is used for producing internal threads and the die is employed for cutting ex ternal threads. They are of two kinds — those operated by hand and those operated by power driven machines. Hand taps are made in sets each of which comprise three taps — the "taper tap," the "plug-tap" and the "bottoming tap.* The taper-tap is parallel on the point for a dis tance equal to one-fourth the diameter of the tap and this point is made the diameter of the roots of the teeth and corresponds to the cor rect size of the hole to be tapped so as to pro duce a full thread. In the plug-tap the first three teeth are tapered off and in the bottoming tap the teeth extend full to the point. The ta per-tap is used for starting a thread, the plug tap for extending it nearly to the bottom and the bottoming-tap for finishing the full thread to the very bottom of the hole.
The dies may be divided into two general classes — those which have to be passed over the work several times in order to produce a finished thread and those by which a finished thread is produced at a single cutting. In the former, the cutting-dies are held in a stock and are capable of being separated so as to permit of their being passed over the work and then closed, by means of a set screw, an amount sufficient to enable them to cut a full thread. In the latter, when they are not of the screw plate type, the "chasers" or "cutters" are held radially in a cast-iron "collet" surrounded by a wrought-iron ring. These dies are capable of being adjusted to compensate for wear. The bevelled outer ends of the chasers fit into cor responding bevelled grooves in the wrought iron ring so that when the ring is forced down the chasers are moved toward the centre. A great variety of self-opening and adjustable dies are also used for machine threading.