MATSEC SEC Graphical Communication flashcards

32 cards. Read the term and the definition here. The deck at the top of this page is optional.

First-angle projection

The object lies between the observer and the projection plane; the plan is drawn below the front elevation and a left side view is placed on the right.

Third-angle projection

The projection plane lies between the observer and the object; the plan appears above the front elevation and the right side view is placed to its right.

Hidden detail line

A thin dashed line showing edges not visible in a view; dashes are evenly spaced, about 3–4 mm long with 1 mm gaps.

Centre line

A thin chain line of long and short dashes marking axes of symmetry, centres of circles and pitch circles, extending slightly beyond the outline.

Mitre line

A 45° construction line used to transfer depths from a plan across to an end elevation so corresponding features stay aligned in the layout.

Line precedence rule

Where lines coincide, visible outlines are drawn first, then hidden detail, then centre lines; construction and projection lines never overpower object outlines.

Dimension units convention

Dimensions are written in millimetres without the unit symbol; if any other unit is used, the unit must be stated after the number.

Isometric axes

Three axes drawn 120° apart: one vertical and two at 30° above the horizontal, with equal lengths stepped off along each axis without foreshortening.

Isometric circle ellipse

A circle drawn on an isometric face appears as an ellipse whose major axis is about 1.22 times the diameter and minor axis about 0.71 times.

Isometric scale

True isometric projection foreshortens every axis by 0.816; isometric drawing avoids this by stepping off full lengths, giving a slightly larger image.

Cavalier oblique

An oblique projection with the front face parallel to the picture plane drawn true shape, and the receding axis usually at 45° to full scale.

Cabinet oblique

Oblique projection in which the receding axis is drawn at half scale, usually at 30° or 45°, to reduce the apparent distortion of depth.

Non-isometric line

A line in an isometric view not parallel to any of the three axes; its length cannot be measured directly, so its endpoints are located by coordinates.

Oblique front face

The face parallel to the picture plane is drawn true size and shape, so circles lying on it remain true circles rather than becoming ellipses.

Perpendicular bisector

The locus of points equidistant from two given points; it is constructed with equal arcs from both ends and a line through the two arc intersections.

Angle bisector locus

The locus of points equidistant from two intersecting lines consists of the two angle bisectors, found from arcs struck from the vertex and from points on the arms.

Dividing a line equally

Draw an auxiliary line at any angle, step off the required number of equal divisions, join the last division to the endpoint, then draw parallels through the other divisions.

Tangent from external point

Draw a circle on the midpoint of the line joining the external point to the circle centre; where it crosses the given circle gives the tangent points.

Ellipse property

For any point on an ellipse, the sum of its distances to the two foci is constant and equal to the length of the major axis.

Cycloid locus

The path traced by a point on the circumference of a circle rolling without slipping along a straight line; one complete revolution produces one arch.

Parabola focus-directrix

A parabola is the locus of points whose distance from the focus is always equal to their perpendicular distance from the directrix line.

Cone development

A right cone opens into a sector whose radius equals the slant height; the sector angle equals 360° multiplied by base radius divided by slant height.

Cylinder development

The lateral surface of a right cylinder develops into a rectangle of width π times the diameter and height equal to the height of the cylinder.

True length by rotation

A sloping edge appears true length when rotated in an auxiliary view until parallel to the projection plane, then projected onto the development.

Section true shape

The true shape of a section is found in an auxiliary view projected perpendicular to the cutting plane and looking along that plane.

Non-developable surface

A sphere or other doubly curved surface cannot be flattened without stretching; approximations use lunes, gore strips or the zone method.

Prism lateral surface

A right prism develops into a rectangle whose width equals the perimeter of the base and whose height equals the height of the prism.

Freehand straight line

Draw from the shoulder with a relaxed grip, mark the endpoints, make light overlapping strokes, then darken the final straight line.

A4 sheet size

210 mm by 297 mm; each A-series size is half the area of the previous one, with A0 measuring 841 mm by 1189 mm.

Cutting plane line

A thick chain line with arrowheads at its ends showing the direction of sight for the section view; it may be bent to pass through features.

Hatching convention

Section areas are hatched with thin continuous lines usually at 45° and evenly spaced; adjacent components are hatched in opposite directions or at different spacings.

Scale notation

Written as a ratio such as 1:2 or 2:1 in the title block; 1:2 means the drawing is half the size of the real object.