The exactness of the above described engine, depends very much on the correctness of the endless screw, which here is required to have some properties, that were not absolutely necessary in the endless screw for the circular engine. In that, as there were but a few threads of the endless screw engaged in the teeth of the wheel, it only required that those threads should have an equal inclina tion to the axis of the screw ; but in this engine, where the whole length of the screw is engaged in the teeth of the moveable plate, it is necessary also that the distance be tween the threads should be the same throughout the whole length of the screw. This is effected by the screw engine hereafter described.
Plate CCLXXXI. Fig. 7. Represents the plan; Fig. 8. The elevation ; and Fig. 9. A section on the line BO. The same letter re fers to the same part in each of the figures. A represents a strong circular plate of brass, having its edge ratchet] by the method given in the description of the circular engine. On its centre is firmly fixed the pulley B by four screws ; a groove is turned on the cylindric part of the pulley, per fectly concentric to the plate A.
Figs. 8, 9. C is a steel axis two feet long, terminating in a point whereon it rests. The upper part of the axis is firmly screwed to the plate A, and turns in the collar D.
Fig. 7. E represents an endless screw, which, being turned on its axis, moves the plate A round its centre. F, a divided circular plate, which may be turned with or without turning the endless screw. On the other end of the screw arbor is a wheel a, having its outer edge cut into teeth. X is a winch whereby the endless screw is turn ed round.
Figs. 7, 8, 9. G represents a triangular bar of steel, which passes over the circular plate A, and is firmly screwed to the frame at H and I.
K, a piece of steel whereon the screw is intended to be cut, having its pivots formed in the manner before describ ed. On one end of this steel is a wheel L, having teeth round its circumference, which take into those on the wheel a, on the arbor of the endless screw.
Figs. 7, 8. M and N represent two strong pieces of brass, in which the steel whereon the screw is to be cut turns. They are firmly fixed to the triangular bar G, by tighten ing the piece I by the screw II.
Figs. 7, 8, 9. 0 is a piece of brass, which slides on the triangular bar G. Its two extremities are made to fit the bar, it slides regularly thereon, and is prevented from ris ing by the two springing pieces c, c. Near one end of the piece 0, is an angular groove q, that holds the tool by which the threads are cut. As it was necessary to cut the screw after the steel was hardened and tempered, there fore the tool was pointed with a diamond. The cock \V
serves to fasten the tool, which may be set to take proper hold on the steel, by turning the finger screw S, and is fix ed there by the screw V.
To make a screw perfect, it is only required to give the point which cuts the threads an uniform motion pa rallel to itself, and to the axis of the intended screw, and that this motion be proportioned to the revolution of the intended screw, as the number of threads may require.
To effect this, a piece of thin tempered steel, exactly of the same thickness th•oughout, is fastened to the slide O at r ; (Fig. 7.) the other end of the spring is fastened to the pulley B, in the groove. Now, while the circle A, with the pulley, is turning round its centre, by turning the endless screw towards the right hand, the spring t draws the slide 0, with the cutter q, along the triangular bar ; at the same time the steel K, whereon the screw is to he cut, is turned round its axis by the communication of the wheel a, on the endless screw with the wheel L.
It hath already been mentioned, that the screw of the engine before described bath 20 threads in an inch ; there fore, if the number of teeth on the wheel a be to the num ber on the wheel L, as the number of teeth on the wheel A is to the number of twentieths of an inch round the circumference of the pulley B, allowing for part of the thickness of the spring t, the spaces between each of the threads of the screw to be cut will be twentieths of an inch.
The size of the pulley was determined in this manner : the endless screw being disengaged from the wheel A, the slide 0 was drawn back till the end of it came nearly to the piece M; the endless screw was again engaged in the wheel A ; then having two very small clots on the slide 0 set off parallel to one side, and exactly five inches distant from each ether ; the slide was moved by turning the end less screw, till one of the dots was bisected by a small silver wire fixed across a hole in a thin piece of brass attached to the piece N ; then the 0 on the divided wheel F was set to its index, without moving the endless screw, and the pulley was reduced, till 600 revolutions of the end less screw brought the other clot to be exactly bisected by the fixed vrire. These bisections were examined by a lens of half an inch focus, set in a small brass tube, which was fixed perpendicularly over the wire." We believe that the above described tight-line engine has been chiefly employed in dividing such scales as sur veyors use for mapping their work, barometer plates, &C.