The making of a phonograph horn in segments, as illustrated by Fig. 278, is particularly suitable for amateur's work, as it can be readily constructed with few tools and at little cost of material. It may be made out of tinplate, zinc, brass, aluminium, or hard rolled copper. The horn, as shown, is made up in twelve strips jointed together and fitting into a thimble.
To obtain the pattern for a strip or segment, the profile or section of one strip is set out, as shown in Fig. 279. A joint line is drawn, making an angle of-
360 / twice number of strips = 360 / 24 = 15° with the base line. The section curve is divided into any number of parts, four being chosen in this case. The length of this curve is carefully set out to form the girth line of the pattern for a segment. This is done by making the lengths 0 to 1, 1 to 2, etc., on the pattern equal in length to the parts of the curved line 0 to 1, 1 to 2, etc., on the section. Lines are drawn across through each point on the girth line, and these cut off on each side equal in length to the corresponding line between the base and joint lines. Thus line 4 A on the pattern is the same length as the line 4 A on the section, and so with the other lines through points 3, 2, 1, and 0, all measured between base and joint lines. A piece of hoop iron or a lath should now be bent to pass through each point, and the pattern curve drawn in. If it is desired that the bell-mouth of the horn should come an exact circle when the strips are joined together, then the compasses must be set to a radius equal in length to the joint line, and the curve at the bottom of the strip pattern marked along as shown. The allowance for wiring around the mouth must be added on as seen in the pattern by the dotted curve.
The strips can be jointed together either by soldering or grooving. If soldered, the allowance for the lap will be as the dotted line on the right-hand side of the pattern. If grooved, an allowance on both sides will have to be made, the lap on one side being twice the width of that on the other. The double lap is shown on the left-hand side of the pattern.
To make up the horn, the strips will first be bent so that the girth line will have the same shape as the section or profile. If to be soldered, the small lap will then be slightly bent over with a mallet, so as to lie on the adjoining strip. The strips should all be tacked together before any joint is completely soldered down. The laps and soldering should be on the outside of the horn, the joint being made as neatly and cleanly as possible. The wire edge on the bell-mouth should now be turned over, the ring of wire inserted, and the edge hammered down with the mallet, and carefully tucked in with the pane end of hammer.
If the joints of the horn are to be grooved, then the single edge must be edged up and half of the double edge turned down, this, of course, taking place after the strips are shaped. The strips can now be hooked together and grooved
(see sketch of outside groove, Fig. 279) by placing on a square bar or bick-iron and hammering the groover gently along the joint. The grooving of the narrow part of the horn will present some little difficulty; but this can be overcome by fixing on the small end of bick-iron, or by the amateur on a piece of round bar-iron held fast in a vice, or by other means.
If it is desired to have the outside of the horn plain, and consequently the groove formed on the inside, this can be accomplished by placing the joint on the edge of bick-iron or bar, hammering down with mallet to form groove, and then flattening the groove with the mallet or hammer in the usual way. A sketch of this method of forming an inside groove is shown on Fig. 279.