This section is from the book "The Boy Mechanic Vol. 2 1000 Things for Boys to Do", by Popular Mechanics Co.. Also available from Amazon: The Boy Mechanic, Vol2: 1000 Things for Boys to Do.
By CHELSEA CURTIS FRASER
An efficient ellipsograph is the only-device that will make true ellipses of various sizes quickly, and such a machine is in demand on some classes ot work in a drafting room. Its cost, however, is prohibitive where only few ellipses are to be drawn, but a person handy with tools can make an apparatus that will do the work as well as the most expensive instrument.
The completed ellipsograph, herein described, will appear as shown in Fig. 1. It consists of two main parts, the base and the arm. The former is a hardwood piece, A, Fig. 2, to the side center of which is attached another hardwood piece, B. This latter piece may be dovetailed into A, but an ordinary butt joint will suffice, as a one-piece base, C, also of hard wood, is glued to the bottom surfaces of the pieces A and B, thus securely holding them together. Before gluing, however, care must be taken to see that the piece B is exactly at right angles with the piece A. This can be done with a try-square. Dimensions are given in Fig 2. The same letters are used throughout for the same parts.
Ill: Fig.1 Practically Any Size or Proportion of Ellipse, from 1 1/2to 16 In. Maximum Diameter, can be Drawn with an Instrument of This Size, and Larger Instruments will Work Equally Well
A 1/16-in. groove, H, is cut out at the juncture of A and B, to admit the flange of the minor-axis swivel head. Another groove, J, is made along the longitudinal center of the piece B, to receive the spine of the major-axis swivel head. Sheet brass plates, D and E, are cut from 1/16-in. stock and attached to the top of the piece A with screws. The plates F and G are of the same material and fastened in a like manner to the upper surface of the piece B. All plates project slightly where they touch the groove borders, as shown. This is to provide a bearing for the projections on the sliding parts against metal instead of wood. A piece of thin celluloid or brass plate, K, is inserted in the bottom side of the base C so that the point will be exactly in line with the center groove J, and extend outward 1/8in. This point indicates the center of all ellipses to be drawn. In the bottom of the base C, at each end, drive an ordinary pin and cut it off so that a part of it will project beyond the surface of the wood. These extending ends are pointed with a file, and serve to keep the instrument from slipping when in use.
The arm L is shown in Fig. 3. It is made of either maple or birch. Follow the dimensions given, cutting a slot through it longitudinally, as shown at M, narrower on the upper side than on the lower, the upper being 1/4in. wide. An octagon-shaped piece of wood, N, 1/2 in. thick, is glued to the end, to give a good seating for the sleeve O, which is a piece of 1/2 -in. solid-brass rod, 2 in. long, drilled to receive closely the pencil sheath. The latter, P, can be made from a section of brass tubing such as is used in a bicycle-pump valve. The upper end is notched to receive a rubber band, and an ordinary pencil can be cut down to fit closely into the other end. The piece O fits tightly in a hole bored through N and L.
The detail of the minor-axis swivel head is shown in Fig. 4. This swivel head consists of two pieces of brass, one, marked Q, being % in. square by 3/4 in- long and the other, R, 7/8 in. square by 1/2in. long, with a notch cut out as shown. These parts, as well as the somewhat similar ones for the major-axis swivel head, can be cast cheaply, or block brass may be cut with a hacksaw and filed to the right shape. A shortened dry-cell screw, S, with washer, to fit a tapped hole drilled in the piece Q, serves to bind the head where desired on the arm. The flange T is a piece of 1/16-in. brass driven into a slot cut in the piece R. The piece R is pivoted to the piece Q, as shown, by means of a piece of wire nail which engages Q, by friction, the lower end being fitted with a washer and riveted loosely so that the parts will turn freely.
The major-axis swivel-head detail is shown in Fig. 5. The piece U is the same size as Q, Fig. 4, with its screw set a little farther forward to make room for the pivot V, which loosely joins U and W together. The pivot is made of a wire nail, riveted on both ends. The piece W is of brass, 1/2in. thick, 3/4 in. wide and 7/8 in. long. The spine X is made just thick enough to pass freely in the groove J, Fig. 2. A screw taken from a discarded dry-battery cell is used to bind the head to the arm.
To operate the ellipsograph, draw a line, Y, Fig. 1, on the paper, which is to mark the major axis of the ellipse. About midway of its length make a point to represent the center. On the latter set the point K, Fig. 2, and adjust the bottom forward edge of the base C parallel with the line Y. Set the minor-axis swivel head at such a point on the arm L that, when the latter is directly on top of the piece B, the pencil will touch the paper at a distance from the center, marked by K, equal to half the minor axis of the proposed ellipse. To secure the major axis swing the arm until it is parallel with the piece A, leaving the major-axis swivel head unset, and set it when the pencil point has been adjusted to the proper major radius.
Grasp the arm with the right hand between the swivel heads and bearing down, swing the pencil end from right to left. The rubber band will keep it constantly on the paper with even pressure. 'When half of the ellipse is completed detach the rubber band and reverse the instrument to the opposite side of the longitudinal line Y and draw the other half in the same manner.