Fig. 105.
The general construction of the machine is shown in the side view, [Fig. 98] ([Plate I.]), and top view, [Fig. 99] ([Plate I.]), details of construction being shown in [Figs. 100], [101] ([Plate I.]), [102], [103], [104], [105], and [106]. a a is the segment of a ring whose outer edge represents a part of the pitch circle. b is a disk representing the rolling or generating circle carried by the frame c, which is attached to a rod pivoted at d. The axis of pivot d represents the axis of the base circle or pitch circle of the wheel, and d is adjustable along the rod to suit the radius of a a, or what is the same thing, to equal the radius of the wheel for whose teeth a template is to be produced.
When the frame c is moved its centre or axis of motion is therefore at d and its path of motion is around the circumference of a a, upon the edge of which it rolls. To prevent b from slipping instead of rolling upon a a, a flexible steel ribbon is fastened at one end upon a a, passes around the edge of a a and thence around the circumference of b, where its other end is fastened; due allowance for the thickness of this ribbon being made in adjusting the radii of a a and of b.
e′ is a tubular pivot or stud fixed on the centre line of pivots e and d, and distant from the edge of a a to the same amount that e is. These two studs e and e′ carry two worm-wheels f and f′ in [Fig. 102], which stand above a and b, so that the axis of the worm g is vertically over the common tangent of the pitch and describing circles.
The relative positions of these and other parts will be most clearly seen by a study of the vertical section, [Fig. 102].[4] The worm g is supported in bearings secured to the carrier c and is driven by another small worm turned by the pulley i, as seen in [Fig. 101] ([Plate I.]); the driving cord, passing through suitable guiding pulleys, is kept at uniform tension by a weight, however c moves; this is shown in [Figs. 98] and [99] ([Plate I.]).
[4] From “The Teeth of Spur Wheels,” by Professor McCord.
Upon the same studs, in a plane still higher than the worm-wheels turn the two disks h, h′, Figs. [103], [104], [105]. The diameters of these are equal, and precisely the same as those of the describing circles which they represent, with due allowance, again, for the thickness of a steel ribbon, by which these also are connected. It will be understood that each of these disks is secured to the worm-wheel below it, and the outer one of these, to the disk b, so that as the worm g turns, h and h′ are rotated in opposite directions, the motion of h being identical with that of b; this last is a rolling one upon the edge of a, the carrier c with all its attached mechanism moving around d at the same time. Ultimately, then, the motions of h, h′, are those of two equal describing circles rolling in external and internal contact with a fixed pitch circle.
In the edge of each disk a semicircular recess is formed, into which is accurately fitted a cylinder j, provided with flanges, between which the disks fit so as to prevent end play. This cylinder is perforated for the passage of the steel ribbon, the sides of the opening, as shown in [Fig. 103], having the same curvature as the rims of the disks. Thus when these recesses are opposite each other, as in [Fig. 104], the cylinder j fills them both, and the tendency of the steel ribbon is to carry it along with h when c moves to one side of this position, as in [Fig. 105], and along with h′ when c moves to the other side, as in [Fig. 103].
This action is made positively certain by means of the hooks k, k′, which catch into recesses formed in the upper flange of j, as seen in [Fig. 104]. The spindles, with which these hooks turn, extend through the hollow studs, and the coiled springs attached to their lower ends, as seen in [Fig. 102], urge the hooks in the directions of their points; their motions being limited by stops o, o′, fixed, not in the disks h, h′, but in projecting collars on the upper ends of the tubular studs. The action will be readily traced by comparing [Fig. 104] with [Fig. 105]; as c goes to the left, the hook k′ is left behind, but the other one, k, cannot escape from its engagement with the flange of j; which, accordingly, is carried along with h by the combined action of the hook and the steel ribbon.
On the top of the upper flange of j, is secured a bracket, carrying the bearing of a vertical spindle l, whose centre line is a prolongation of that of j itself. This spindle is driven by the spur-wheel n, keyed on its upper end, through a flexible train of gearing seen in [Fig. 99]; at its lower end it carries a small milling cutter m, which shapes the edge of the template t, [Fig. 105], firmly clamped to the framing.