454 



♦ KNOWLEDGE ♦ 



[May 29, 1885. 



customary to make the head of the screw flush with the 

 work. This is done by countersinking. 



Fig. 30t^, illustrates the form of tool mostly used, it is 

 known as the rose countersink. It is made by forming a 

 series of cutting edges on a conical piece of steel of the 

 same angle as the head of the screw, which is to be driven 

 into the hole. 



Fig. 30e illustrates another and almost equally good 

 form, called the shell countersink, which consists of a 

 conical piece of steel with a single cutting edge, produced 

 by boring a hole down the length of the cone, and thereby 

 removing a small section of the cone's face. 



A common form used for rough work is that known as 

 the drill countersink (Fig. 30/'). It is a modification of 

 the simple drill. The disadvantage of this tool is that with 

 soft wood one cannot ensure a perfect countersink, but will 

 more probably obtain only a succession of ridges. 



It may be observed that these countersinking bits, 

 although mentioned with reference to woodwork, are also 

 applicable to metals, oil being used for lubricating purposes 

 in the case of iron, &c. (Ex. XXXVI/). 



Ex. LIX. — As with drills, so with bits, a stock or 

 brace is necessary for the purpose of rotating the tool. 

 Fig. 31 illustrate.? a cheap and, at the same time, quite 

 efficient form. A curved iron bar, B C A, is rotated in a 



tools has been thought up till now in a measure un- 

 necessary. It is essential, however, that a little attention 

 should now be bestowed upon them. As a matter of fact 

 there is no one warning so important or fraught with 

 such dire consequences if neglected as that enforcing the 

 necessity for the strictest possiVile attention to measure- 

 ment and fit. A neglect of this kind will often produce a 

 failure where good workmanship in other directions 

 warranted the anticipation of success. 



Ex. LXI.^For measuring straight lines rules answer 

 best. Tapes are used by surveyors, but they are inaccurate 

 when the measurements are small. There are several forms 

 of rules, but perhaps the best is the steel 12-inch rule, 

 which is marked with English and French measurements. 

 Although some boxwood rules are also so marked, it is 

 noteworthy that the edges of a steel rule are made suffi- 

 ciently straight to permit of its use as a straight-edge. A 

 second purpose is thus answered by the one tool, and no 

 advantage is gained by unnecessarily multiplying the 

 number of tools. 



Ex. LXII. — An instrument is necessary for describing 

 circles. With wood a pair of ordinary compasses may be 

 used, but with metal a form of compass known as the 

 spring divider is most usually employed. It has an adjust- 

 able leg and the points are fine and made o£ hardened 



Fig. 31. 



socket at the extremity of a wooden handle, D, and is pro- 

 vided at A with a holder for the bits. This holder consists 

 of a square piece of iron, with a tapered square hole passing 

 through it, and a thumbscrew passing through one side of 

 the square. It will be observed that the bits (Fig. 30) are 

 each provided with a notch near the upper end. Into this 

 the screw of the holder (A, Fig. 31) is turned, the bit being 

 thereby held securely in the stock. In other and more ex- 

 pensive forms there is a spring instead of a screw for hold- 

 ing the bit in place. The rotation is produced by pressing 

 on D and turning the handle C. 



Ex. LX. — In Fig. 32 is depicted another form of brace, 

 which is, however, relatively expensive. Its primary 

 features are the same as those of the less elaborately 

 finished article (Fig. 31). The points of difieience are in 

 the means provided for holding the bits, and the introduc- 

 tion of a ratchet arrangement. The ratchet is a device for 

 enabling one to bore holes in a corner or in any other 

 similar position in which the rotation of tlie stock is im- 

 practicable. The holder is adjustable to suit the various 

 sizes of bit-shanks, which are secured by screwing down 

 the collar or casing forming the outside of the holder. 



MEASURING IXSTRUMEXT.S. 



Simple measuring instruments, such as the rule, are to 

 be found in most households, whence a reference to what 

 may be regarded as one of the most important class of j 



Fig. 32. 



steel. As a tool it is cheap and should not, therefore, be 

 dispensed with. 



Ex. LXIII. — Means for measuring the internal and 

 external diameters of cylinders must be procured. Such 

 tools are known as callipers. Fig. 33 furnishes a good idea 

 of the form of calliper used for measuring the external 



Fig. 33. 



diameters. It consists of two pieces of steel cut to the 

 shape shown and hinged together so as to allow the 

 distance between the points to be increased or decreased 

 at will. To measure with this tool, the points aie opened 

 until they pass accurately over the cylinder, rod, A-c, that 



