Elements of machine construction and drawing: or, Machine drawing, with some elements of descriptive and rational cinematics by Warren S. Edward (Samuel Edward) 1831-1909

Author:Warren, S. Edward (Samuel Edward), 1831-1909
Language: eng
Format: epub, pdf
Tags: Machine design, Machinery, Kinematics of
Publisher: New York : J. Wiley & son

EXAMPLE XXXYIIL The Abridged Drawing of Screws.

When either the scale or the pitch, employed in representing a screw, is so small as to make the apparent curvature at points 00' 6,6', etc., PI. III., Fig. 9, so sharp as to be sensibly pointed, the helices may be thus pointed in vertical projection, or, in general terms, in the projection on a plane parallel to the axis of the screw.

The figure illustrates this modification in the drawing of a <m0-threaded screw of a screw, of 8 inches outside diameter, on a scale of one-fourth.

Similar points in the two projections are numbered with the same figure, a very convenient method in many cases.

PI. III., Fig. 10, represents a triangular threaded screw of two threads, to the same scale and dimensions, except pitch, as the last one. Here the thread is generated by the isosceles triangle, a'Q'Q", whose base, ao6", is vertical. In this case, the space between two threads extends from the middle of one thread to the middle of the next, as from 0' to b''. Hence, although the screw is plainly two-threaded, as seen by following an outer helix, 0',6',12'j yet the pitch, O',12', is but twice the height, a'§", of a thread; while in the square-threaded screw, Fig. 9, a pitch, Q'b', of double the height, a'b' ', of a thread gives only a ow<?-threaded screw. To have made Fig. 10 one-threaded, we should have made O'J' the pitch, and made the horizontal lines half as far apart as now, and projected 6 upon the horizontal line through a'. The helix beginning at 0' would then have reappeared at V.

The student should construct a triangular-threaded screw on a large scale, as partly shown on PI. XXIII., Fig. 3 ; also the internal screw for the same. Strictly, the visible contour of a helicoid is curved, being tangent to the successive elements, but is nearly straight for so short a distance as ab / hence it is a sufficient refinement of construction, unless the scale is very large, to make ab straight, but tangent, as at a and b, to the outer and inner helices, instead of running from A to n, as in PL III., Fig. 10.

All the helices shown, but by straight lines. —PI. III., Figs. 11 and 12, illustrate this abridgment. The curvature of a helix,

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