Part 58

This Fig. 1. Plate 3. is the brass Tongue with its Back-side upwards. The Two outer Lines a b and c d are the Edges of the upper Surface (tho’ turned downwards in this Figure), which are inclined to each other, as afore-mentioned; but the Two inner Lines e f and g h are nearer to each other, whereby this under Surface is narrower than the upper: Both must be plain Surfaces, but the upper and its Two Edges very free from Flaws, and smooth, or polished.

The Reason why the under Surface is narrower than the upper, is to preserve the Bevel of the empty Triangle: For though the Bevel of the Sides of the Mortise would be sufficient for this, if both Sides of the Tongue were sure to keep equally distant from the Spindle; yet as the Tongue never is so tight on its Axis, but that sometimes one Corner of it may be nearer to the Spindle than the other, in this Case, that Side which is nearest to the Spindle would reverse that Bevel, so as to make the small empty Space that is betwixt the Mortise and the Tongue, wider above than underneath.

C C are the Two little Knobs that prevent the Spring from slipping to either Side, and are at the Distance from one another of the Breadth of the Spring.

Fig. 2. shews one Side, and the Thickness of the Tongue the other Side, being the same. a b shews the polished Surface (being a true Plane), whereon the Seed runs down to the Spindle. c d the Back-side, which lies turned uppermost in Fig. 1. b e d shews one End of the hollow Cylinder of the Tongue, thro’ which its Axis passes.

The Length of the Tongue must be such, as will reach lower than just to touch the Bottom of the great Hole as a Tangent: for, if it be not longer than that, it might happen, that when the Mortise is empty of Seed, and the Tongue set up close, a Wheel might, in Turning, or otherwise, go a little backwards, and cause a Notch of the Spindle to take hold of the End of the Tongue, and tear it out of the Mortise: Therefore let the Tongue reach a little below the Spindle, as the pricked Line g h, in Fig. 3. of Plate 2. doth.

As for the Posture in which the Tongue ought to stand in the Mortise, it is shewn by the Three pricked Lines in Fig. 3. Plate 2. where the pricked Line g h makes an Angle of Forty-five Degrees, being the nearest that it can stand to the Spindle; the pricked Line i h makes a somewhat greater Angle, and it is a mean (or middle) Distance from the Spindle; and the pricked Line k h is supposed to be its greatest Distance, where the Tongue makes its greatest Angle with the Top and Bottom of the Mortise. If the Tongue stood so obliquely as to make an Angle much less than Forty-Five, the Tongue would rise too much against the Bevel of the Mortise, and the Spring would have the greater Difficulty in returning it to its Place, when driven back by the Force of the Notches.

And beside, when the Tongue stood wide from the Spindle, there might be so much Room betwixt it and the Sides of the Mortise, that some Seeds might fall thro’ there.

The Steel Spring is D, properly placed upon the Back of the Tongue, in Fig. 1. Plate 3.

At first, I made the Spring double, i. e. with Two Legs, in Imitation of that in the Organ, and fastened into its Tongue, much after the same manner as the Spring of the Organ is into its Tongue or Flap, which prevents the compressed Air from passing out of the Sound-board, except whilst the Key is thrust down by the Finger of the Player; but the Drill-spring requiring to be of a vastly greater Strength than that, I made it of Steel, of the Breadth of half an Inch, instead of Brass Wire: This performed very well, and several Drills are yet extant, that have only this Sort of Springs: Yet I found there was great Difficulty to set the Legs at their due Distance from each other; for their Seasoning would alter them from what they were, whilst the Steel was soft: They also took up too much Room in the upper Part of the Mortise. Then, to remedy these Inconveniencies, I made it single, with only one Leg, which by full Experience is found to be much better than the double one; it does not contain a Fourth Part of the Metal, and is most easily made, requiring none of that Trouble and Nicety that the double Spring doth. I shall therefore give a Description of the single Spring only.

B, the End of the Screw, which holds the Spring to the Tongue, thro’ a Hole near the upper End of the Spring; D, the Middle, against which the End of the Setting-screw bears.

Its Length is almost the whole Length of the Tongue; the End E reaching very near to the lower End of the Tongue, and the End B is as near the upper End of the Tongue; as it can be placed without touching the Cylinder of the Tongue.

The Breadth is usually about half an Inch; the Thickness must be in proportion to its other Dimensions, and according to the Degree of Stiffness required.

The longer it is, the thicker it must be, to have the same Stiffness; but the broader it is, the thinner it must be of the same Length; so that it is hard to determine its Thickness. It is made stiffer or stronger by being cut shorter; it is made weaker, or less stiff, by filing or grinding it either thinner or narrower.

The common Thickness is about that of a Shilling.

Not quite so thick as a milled Shilling, but rather of an old broad stamped Shilling, which is a little thinner.

The Degrees of Stiffness are measured in this manner; viz. Fix Two Boards together, leaving a Chink betwixt them, in one Place of an Inch long; lay the Spring (when seasoned across this Chink) with its Middle exactly over it; then put a String over the Spring, which may pass with both Ends thro’ the Chink, and tie so much Weight to the Ends of the String under the Boards, that will pull down the Middle of the Spring, till it touch the Chink, and is strait with both its Ends; This will shew the Degree of Stiffness. But note, That the Spring must be crooked, and bear only upon its Ends, with the hollow Side upwards.

If ten or a dozen Pounds Weight pull it down to the Board, it is a good Degree of Stiffness, for a large Box: We are not confined to be very nice or exact in the Degree of Stiffness; for by our Fingers pressing it, we that are practised in it, know well enough, whether a Spring be of a sufficient Degree of Stiffness, without weighing it; but for such who are unacquainted with them, it is best not to trust to Guess, but Weights; and to adjust the Stiffness to that of a Spring, that has been known to perform well.

The Spring must bear against the Back of the Tongue at each End, and lie hollow in the Middle: But the Degree of Hollowness of the Spring is very material; for thereon depends the Distance of the Tongue’s Motion towards the Spindle by Force of the Spring, and back again quite to the Setting-screw, by the Seed that is pressed against it by Force of the Notches, when they are moved by the Wheels; because the more the Spring is curved, the farther will it thrust the Tongue from its Middle, if its Strength be superior to the Force that resists it, as it ought to be when a Notch is passed and before the next: This Motion of the Tongue is called its Play.

In order to measure the Distance (or Quantity) of this Motion, we must consider, that the Tongue, moving on its Axis above, describes with its lower End the Arch of a Circle, the Chord of which Arch is the Measure required.

To measure this by the Angle the Tongue makes at its Centre, would be no Rule for making Boxes; because some Tongues are longer, some shorter, in proportion to the different Diameters of the Spindles they move against; and yet the Play of the shortest must be as much as that of the longest, that is, it must describe as great an Arch at the Place of Pressure (described in Fig. 3. Plate 2.); and therefore the shortest Tongue would make the greatest Angle.

A short and easy Way, then, for a Mechanic to measure, is thus: Screw in the Setting-screw until the Tongue come within a quarter of an Inch of touching the Spindle; then take out the Spindle, and from the Centre of the Hole draw a Line on the Side of the Mortise, perpendicular to the Tongue, and at the Tongue’s Edge make a Mark with the Compasses, or a Pen; then force back the Tongue against the Setting-screw as far as it will go (that is, until the Spring touch the whole Back of the Tongue); produce the said Line to the same Edge of the Tongue, or set the End of the Rule thereon, and draw another Line, by the Rule, from the Mark to the Edge of the Tongue, when farthest back, and there make the second Mark. The Ruler used this Way will shew both the Perpendicular, and the Measure.

But yet a quicker Way is, to set the Tongue, by the Setting-screw up to the Edge of the Hole; and, when it is forced back, measure from the Tongue to the nearest Part of the Hole, which will ever be a perpendicular Line drawn from the Centre of the Hole to the Place of Pressure above-mentioned, and make another Mark there: Now the Distance between these Two Marks is the Measure (near enough) of the Tongue’s Play at the Place of Pressure. Tho’ this Line drawn on the Side of the Mortise be not exactly perpendicular to the Surface of the Tongue, but only to its Edge; yet the Difference is next to nothing, and not to be regarded.

If its Measure be a quarter of an Inch, it is what Experience shews to be of a good Size for all Corn and Peas; a little less is no Harm, but greater is the most fatal Error, into which most of the Pretenders to the making of this Machine have fallen; they give the Tongue half an Inch, sometimes Three quarters of an Inch Play. The Mischief of this Error is yet farther increased, if the Spring be weak, if the Mortise have too great a Bevel, or if the Angle made by the Tongue at the upper Edge of the Mortise be too acute.

When the Tongue has too great Play, the Seed is apt to be turned out too fast, or else too slow, in spite of the Driller. For when the Tongue is set at its due Distance from the Spindle, and is thrust quite back by the Seed pressed against it by the Turning of the Notches; but the Spring being unable to return the Tongue to its former Place at such a Distance, at the time of passing the Intervals which are betwixt the Notches; then the Space between the Spindle and the Tongue being too open, the Seed is sent down too fast.

To prevent that, they set up the Tongue to the Spindle; and then, as often as the Spring happens to overcome the Force of the Seed’s Pressure (as sometimes it will), it is sent out too slowly.

The Inequality of the Running of the Seed makes such Boxes useless, which the Expence of Two-pence (for another Spring, or new Seasoning of that) at most would rectify, if the Maker understood how to mend his own Work. If time did permit, more should be said on this Point, because I find it is the Pons Asini of a Workman. Sometimes it may be prevented, when the Spring is too hollow, and gives too much Play. Screw the Screw, that holds it on the Tongue, down closer, so that the lower Part of the Screw’s Head press against the Spring, and thereby force its Middle nearer to the Tongue, until you find its Play lessened to its just Distance.

The Spring, remaining in this compressed State, has lost the weakest, and retains only the strongest, Part of its elastic Force. Therefore, if you find it then too stiff, make it weaker by Filing or Grinding, or else put another into its Place, which is honestly worth no more than Two-pence.

This Holding-screw has a pretty broad Head, and is screwed in by a Notch, like the Screw-pin of a Gun-lock.