Disc Ellipsograph

Disc Ellipsograph

This device is used to draw ellipses with selected values of the major & minor axes. It is similar to the commercially available Haff No. 97 Ellipsograph https://th.bing.com/th/id/OIP.WWccI6yxNY7WoNviI2-XQAHaF8?pid=Api&rs=1. A device using the same principle was invented in 1813 by John Farey Jr., which is seen in the illustration here :

https://upload.wikimedia.org/wikipedia/commons/thumb/c/ca/Elliptograph_in_Encyclop%C3%A6dia_Britannica%2C_1824.jpg/255px-Elliptograph_in_Encyclop%C3%A6dia_Britannica%2C_1824.jpg

The ellipsograph consists of two acrylic discs having slots which allow the discs to be fixed in variable alignments and carrying a pen holder. By rotating the discs within a guide frame, the pen is made to trace out the perimeter of an ellipse.

Circular acrylic discs 150 mm & 110 mm diameter, 3 mm thick.*

Flat aluminium bar 1.2 mm x 20 mm

Aluminium channel 10 mm x 10 mm

Machine screws 3 mm diam.

Pop rivets

Wing Nut & washers 3 mm

Clear plastic ruler ( or photocopy of the scales )

MDF or wood 6 mm x 40 mm x 360 mm

*These can be bought ready-cut, or else cut from acrylic sheet. Laser cutting is preferred, but not essential. The edges must be smooth and accurately circular for best results.

Holes and slots are cut in the acrylic discs as shown in Fig.1

Cut two 10 cm lengths of the plastic ruler scale and attach the scales along the sides of the slots with clear adhesive tape ( see photo )

The pen holder is constructed using the aluminium hardware with pop rivets and machine screws as shown in fig.2 and the photo.

The two discs are assembled with the wing nut, washers & screw and the pen holder as shown in the photos.

This is made from two lengths of MDF or wood joined by a lap joint as a 90 degree L shaped frame. ( see Fig.3 & photo ). The legs of the frame are 150 mm and 210 mm long, with the longer leg on top of the shorter. This leaves a clearance of 5 mm or so under the long leg so that the larger acrylic disc can slip underneath it. A peg is located at the end of the long leg to keep it raised 5 mm above the paper.

A pen is fixed in the pen holder by means of a small self tapping screw. On the drawing sheet two axes are drawn at right angles and the major and minor axis lengths are marked out. The guide frame is placed on the paper and aligned parallel to the axes of the ellipse.

The ellipsograph is placed so that the large disc is in contact with the short arm of the guide frame and the small disc is in contact with the long arm. ( The large disc is slipped underneath the long arm.) The centre of the large disc is aligned over the ellipse centre and the slot in it is aligned above the minor axis and parallel to it. The pen holder is adjusted so that the pen tip is located at the semi-minor axis position and then locked in position.

Next the discs are carefully rotated by 90 degrees so that the slot with the pen holder is aligned above the major axis and parallel to it, keeping the centres aligned and keeping both discs in contact with the guide frame.

The disc clamp wing nut is then loosened slightly and the position of the smaller disc is adjusted so that the pen holder is located at the semi-major axis position. The wing nut of the disc clamp is then tightened, and the pen point lowered onto the paper ready to draw.

The drawing is made by carefully rotating both discs together while maintaining both discs in contact with the guide frame, until the full ellipse is completed.