Patent GB-750895-A (GB)
Title: Improvements in or relating to drawing appliances
IPC: B43L13/14
Filed: 1952-09-19
Abstract: 750,895. Perspective drawing-instruments. HAUGHEY, K. Dec. 21, 1953 [Sept. 19, 1952], No. 23618/52. Class 97 (3). A drawing appliance by which perspective views can be drawn from plan or aerial views comprises at least three members, namely a slide member adapted to extend parallel to two parallel side edges of a rectangular support and slidable in that attitude, a first link pivotally connected to the slide member and pivoted at one end to an adjustable location near or on one of the said two side edges forming the rectangle, and the second link connected by separate pivotal connections to the slide member and the first link and pivoted at one end to an adjustable location on the same side edge as the first link, the pivotal connections between the links and slide member having relative sliding engagement, and two of the said pivotal connections, e.g. that between the slide member and the first link and that between the two links, being adapted to mount tracing and marking elements respectively. As shown in Fig. 3, the drawing board 4 carries a sheet of paper F bearing a plan view of a rectangle Ao, Bo, Co, Do. A perspective view of this rectangle, as viewed from a given viewing point, is to be drawn on a second sheet of paper H also affixed to the drawing board 4. A point ISO 1  at the side of the drawing board 4 represents the isocentre, that is, a point on a line in the ground plane containing the plan view which is parallel to the principal line of sight at a distance from the projection of the viewing point in the ground plane which is equal to the distance of the viewing point above the ground plane. The point VP 1  is a similar projection of the vanishing point. It can be shown that, for any point on the plan, for example one of the corners Ao, Bo, Co, Do, if a perpendicular is dropped from that point to a horizontal line passing through the point ISO 1 , then the intersection of a line joining the said point to the point ISO 1  and a line joining the foot of the perpendicular to the point VP 1  represents the said point on the perspective view a, b, c, d. In the apparatus shown in Fig. 1, the drawing board 4 is surrounded by a rectangular framework. A double-headed T-square 5 is guided along opposite edges 4a, 4b of this framework, and links 7, 8 having longitudinal slots 9, 10 therein are pivoted to blocks 17, 18 which are adjustably clamped to the side 4c of the rectangular frame. A pin 21 carrying a tracing point is slidable in a longitudinal slot 6 in the T-square 5, and also in the slot 9 in the link 7, while a pin 22 carrying a drawing point is slidable in the slots 9, 10 in the links 7, 8. A pin 12 is adjustably clamped to the T-square 5, but slides freely in the slot 10 of the link 8. In use, the blocks 17, 18 are adjusted so that the pivot points thereon correspond to the points VP 1  and ISO 1  respectively, and the pin 12 is clamped to the T-square 5 at a convenient distance from the edge 4b. When the tracing point 21 is caused to follow the outline of the rectangle Ao, Bo, Co, Do the drawing point 13 will trace a perspective view on the paper H. If a perspective view of a rectangular block is required, the paper H and the blocks 17, 18 are all moved upwardly from the side member 4b by a distance equal to the height of the block. The tracing point 21 is again caused to follow the outline of the rectangle, and the corners of the two perspective views are joined. The type of perspective view required may be adjusted by selecting the viewing point and the vanishing point, to which the points ISO 1 , VP 1  correspond. These points may, if desired, be obtained by calculation. In the above consideration, it is assumed that the vertical plane passing through the principal line of sight passes to one side of the object viewed. If this plane passes through the object viewed, the apparatus as modified in Fig. 6 is used. A T-square 30, slidable at right angles to the T- square 5, slidably engages a pin 13 which replaces the drawing point 22. A slotted arm 31 is pivoted at the point 35 at a distance from the edge 4b of the frame equal to the distance of the point ISO 1  therefrom. The arm 31 engages the tracing member 21, and a drawing point is mounted on a pin 34 engaging the slotted link 31 and the T-square 30.