A scale of 1 is spoken of as 'full size,' and a scale of 1⁄2 as 'half size.'
Engineers in this country state dimensions of machines in feet, inches, and fractions of an inch, the latter being the 1⁄2, 1⁄4, 1⁄8, 1⁄16, &c. In making calculations it is generally more convenient to use decimal fractions, and then substitute for the results the equivalent fractions in eighths, sixteenths, &c. The following table will be found useful for this purpose:—
Decimal Equivalents of Fractions of an Inch.
| Fraction | Decimal Equivalent | Fraction | Decimal Equivalent |
|---|---|---|---|
| 1⁄32 | ·03125 | 17⁄32 | ·53125 |
| 1⁄16 | ·0625 | 9⁄16 | ·5625 |
| 3⁄32 | ·09375 | 19⁄32 | ·59375 |
| 1⁄8 | ·125 | 5⁄8 | ·625 |
| 5⁄32 | ·15625 | 21⁄32 | ·65625 |
| 3⁄16 | ·1875 | 11⁄16 | ·6875 |
| 7⁄32 | ·21875 | 23⁄32 | ·71875 |
| 1⁄4 | ·25 | 3⁄4 | ·75 |
| 9⁄32 | ·28125 | 25⁄32 | ·78125 |
| 5⁄16 | ·3125 | 13⁄16 | ·8125 |
| 11⁄32 | ·34375 | 27⁄32 | ·84375 |
| 3⁄8 | ·375 | 7⁄8 | ·875 |
| 13⁄32 | ·40625 | 29⁄32 | ·90625 |
| 7⁄16 | ·4375 | 15⁄16 | ·9375 |
| 15⁄32 | ·46875 | 31⁄32 | ·96875 |
| 1⁄2 | ·5 | 1 | 1·0 |
Engineers use a single accent (´) to denote feet, and a double accent (´´) to denote inches. Thus 2´ 9´´ reads two feet nine inches.
II. RIVETED JOINTS.
Two plates or pieces to be riveted together have holes punched or drilled in them in such a manner that one may be made to overlap the other so that the holes in the one may be opposite the holes in the other. The rivets, which are round bars of iron, or steel, or other metal, are heated to redness and inserted in the holes; the head already formed on the rivet, and called the tail, is then held up, and the point is hammered or pressed so as to form another head. This process of forming the second head on the rivet is known as riveting, and may be done by hand-hammering or by a machine.
Forms of Rivet Heads.—In fig. 1 are shown four different forms of rivet heads: (a) is a snap head, (b) a conical head (c) a pan head, and (d) a countersunk head.
Proportions of Rivet Heads.—The diameter of the snap head is about 1·7 times the diameter of the rivet, and its height about ·6 of the diameter of the rivet. The conical head has a diameter twice and a height three quarters of the rivet diameter. The greatest diameter of the pan head is about 1·6, and its height ·7 of the rivet diameter. The greatest diameter of the countersunk head may be one and a half, and its depth a half of the diameter of the rivet.
In fig. 1 at (a) and (b) are shown geometrical constructions devised by the author for drawing the snap and conical head for any size of rivet, the proportions being nearly the same as those given above.
Geometrical Construction for Proportioning Snap Heads.—With centre A, and radius equal to half diameter of rivet, describe a circle cutting the centre line of the rivet at B and C. With centre B and radius BC describe the arc CD. Make BE equal to AD. With centre E and radius ED describe the arc DFH.
Construction for Conical Head.—With centre K, and radius equal to diameter of rivet, describe the semicircle LMN, cutting the side of the rivet at M. With centre M and radius MN describe the arc NP to cut the centre line of rivet at P. Join PL and PN.
When a number of rivets of the same diameter have to be shown on the same drawing the above constructions need only be performed on one rivet. After the point E has been discovered the distance AE may be measured off on all the other rivets, and the arcs corresponding to DFH drawn with radii equal to ED. In like manner the height KP of the conical head may be marked off on all rivets of the same diameter with conical heads.
Caulking.—In order to make riveted joints steam- or water-tight the edges of the plates and the edges of the heads of the rivets are burred down by a blunt chisel or caulking tool as shown at Q and R.
Exercise 1: Forms of Rivets.—Draw, full size, the rivets and rivet heads shown in fig. 1. The diameter of the rivet in each case to be 11⁄8 inches, and the thickness of the plates 7⁄8 inch.
Exercise 2: Single Riveted Lap Joint.—Draw, full size, the plan and sectional elevation of the single riveted lap joint shown in fig. 2.
Table showing the Proportions of Single Riveted Lap Joints for various Thicknesses of Plates. (Plates and Rivets Wrought Iron.)
| Thickness of plates | 1⁄4 | 5⁄16 | 3⁄8 | 7⁄16 | 1⁄2 | 9⁄16 | 5⁄8 | 11⁄16 | 3⁄4 |
|---|---|---|---|---|---|---|---|---|---|
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Diameter of rivets
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