This section is from the book "Spons' Mechanics' Own Book: A Manual For Handicraftsmen And Amateurs", by Edward Spon. Also available from Amazon: Spons' Mechanics' Own Book.
Reference has already been made (p. 235) to the second iron introduced into the plane for the purpose of curling up and breaking off the shaving produced by the cutter. The arrangement of the 2 irons is shown in Fig. 363, a being the cutter, and b the back or break-iron, the two being united by a screw-nut and bolt c. The united irons are fastened in a hole in the stock of the plane by means of a wooden wedge, and so adjusted that they traverse the stock and project very slightly through a narrow slit in the sole provided for that purpose. The angle ordinarily formed between the sole and the irons is one of 45°, but this is reduced to 35° by the head of the cutter. In adjusting the plane to its work, 2 considerations have to be borne in mind: (1) the degree to which the cutter projects beyond the sole, and (2) the distance between the edges of the cutter a and breaker b. In regulating the position of the double iron, in relation to the sole, it will seldom be necessary to apply a blow of the hammer to either the top or sides of the wedge or irons; by taking the plane in the left hand so that the palm of the hand covers the hole where the shavings come out, a gentle tap with a hammer or mallet can be administered to either end of the stock of the plane: this will effect the purpose.
A blow given in this way even suffices to loosen the double iron enough to permit its complete withdrawal, when it is necessary to sharpen its cutting edge. An occasional side tap may be needed to make the iron set square with the sole. The relations of the edges of the cutter and breaker can be altered by unscrewing the nut c that unites the 2 plates, a long slot being provided in b with that object. The distance between the edges of the 2 irons varies from about 1/8 in. for the coarsest roughing-down work to 1/20 in. for smoothing, the breaker being placed of course that much above the cutter. The higher the breaker, the easier the plane works; the lower it is, the cleaner the cut. It is necessary to caution the operator against wedging up his planes too tightly, as such a procedure will cause the cutting iron to assume a curved form and prevent smooth work being done. Care must be also taken that the projection of the cutting e*dge beyond the sole of the plane be perfectly square with the sole, and level in itself; in fact it is better that the corners be rounded off, to prevent the possibility of their catching.
Many of the planes of modern pattern are made either self-adjusting or so that their adjustment is very easily and accurately performed.