The iron plate of the upright is the chief supporter of the strains imposed upon the structure by the strings. It is necessary, as in the grand, that it should be built so as to have absolute rigidity under these strains, and that it should be always capable of withstanding any others that may by any chance be imposed upon it. Remembering this, what shall we say of the designs that we sometimes see where the iron is cut and shaved away at every possible point in order to save a few pounds of weight? There are pianofortes in existence where the iron framing is so much cut down that the treble can never be depended upon to stay in tune for a reasonable length of time, and where the unevenness of the distribution of metal throughout the body of the frame has caused a warping of the whole instrument out of its proper shape. All this is unnecessary, but it will continue so long as manufacturers are willing to effect small savings at the expense of the future reliability of their product.
It is certainly much better to have the plate of the upright pianoforte so arranged that the total volume of iron, if it is to be decreased, may have this process applied evenly and all through. The most correct design would consider a moderately heavy plate of iron, or better, of steel. This plate would be so arranged that the hitch-pin plate (the portion which carries the hitch-pins) covers the entire surface of the instrument at its bottom end and is bolted into the back with as many and as heavy bolts as are employed at the other end. The upper portion of the proposed plate need not extend over the surface of the wrest-plank, for such a construction can add little to the resisting power of the frame, while it inevitably tends, for reasons that have already been described, to the production of dissonant partials and the consequent emission of a metallic tone. If it is desirable, for any reason, to cut down the weight of a frame, it can better be done at the upper end than at the lower, where diminution of the resisting power of the frame is most severely felt.
We may, then, contemplate a form of iron frame which covers that portion of the back-framing whereon the edges of the sound-board are glued, and that is not continued up and over the wrest-plank. It will be cut away in the centre, as in the grand, and will thus expose a large part of the sound-board to view. It will be provided with a certain number of bracings for the purpose of taking up the strains that are imposed upon the structure at the points of greatest resistance, such as those where the over-stringing begins and ends, and those around the middle registers of the scale. There must not be more of these bracings, however, than is necessary, as it is very important that the scale should be broken up into the least possible number of divisions. The bracings will be so designed that no one of them interferes with the true curve of the belly-bridge. The whole structure will be solidly bolted into the back-framing through the sound-board by a series of long and heavy screws and bolts that will be inserted at frequent intervals along the edges.
We cannot conclude this survey of plate-construction without taking note of a property that is common to all cast-iron structures. We refer to shrinkage. The calculations of the designer, no matter how carefully worked out, must take into consideration the fact that cast-iron shrinks largely in the cooling. It must be noted that the design for the iron plate is the chief portion of the work of scale draughting. As will be explained later on, this work is first done on paper and then transferred to a wooden pattern. From this pattern the casting is made. After the first casting is thus completed it is taken in hand by the designer, who marks and punches it for the boring and pinning and corrects it where necessary. The corrected casting is then used as a model for the future plates that are to be turned out for the same scale. But these also shrink; so that we are compelled to take into account both shrinkages, and allow for each. Fortunately, however, there is no difficulty in arranging this.
The method is as follows: After the string-lengths have been calculated and the size of the iron plate thus determined, a complete drawing is made, showing the exact dimensions, shape, and other features of the proposed plate. This is to be used as the guide for the wooden templet. But this difference exists between the two paper drawings; namely, that the second is multiplied as to every dimension of the plate by a quantity that represents the amount of shrinkage that is known to occur in the two castings.
The average shrinkage of cast-iron in the form of pianoforte plates is estimated at about one per cent. Consequently to account for the double shrinkage it becomes necessary to multiply by the square of 101⁄100; that is approximately 51⁄50.
A great deal of trouble will be avoided if the shrinkage is carefully worked out in this manner. Every dimension of the plate is affected, and the greatest care must be taken to ensure that the corrected calculations are accurate. Only thus will it be possible for the ideas of the designer to be brought out in the completed instrument as he expects to see them.