| absolute coordinates | (n) Coordinates associated with an origin that never changes location and thus gives a stable method of locating geometry in space. The absolute coordinate system is also called the world or global coordinate system.
(See 141)
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| Bezier curve | (n) A special case of the B-spline curve. Unlike a standard B-spline curve, the Bezier does not provide for local control, meaning that changing one control point affects the entire curve.
(See 160)
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| B-spline curve | (n) A parametrically defined freeform curve that approximates a curve to a set of control points and provides for local control. Multiple 2-D curves are often combined to create 3-D surface patches.
(See 160)
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| circle | (n) A closed, planar curve that, at all points, is an equal distance (the radius) from a point designated as the center. A circular arc is an open, planar curve equidistant from a center. The arc will subtend an angle of less than 360 degrees. A circle is sometimes described as a 360-degree arc.
(See 149)
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| continuity | (adj.) A term used to describe the transition between two elements. Elements are continuous if there is no gap or break between them and there is a single mathematical function used to describe the two combined elements. Continuity is often used to describe the connection of two curved lines or surfaces.
(See 175)
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| convolute | (n) A single-curved surface generated by a straight line moving such that it is always tangent to a double-curved line.
(See 168)
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| curved line | (n) A line which does not follow a straight path. Curved lines are often classified by their underlying mathematical functions. Examples are circular and elliptical curves.
(See 146)
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| cycloid | (n) A curve generated by the motion of a point on the circumference of a circle that is rolled in a plane along a straight line.
(See 157)
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| cylinder | (n) A planar geometric solid described by a straight line (the generatrix) that traces a closed, curved path and always stays parallel to itself. The most common cylinder is a right circular cylinder for which the curved path is a circle and the generatrix is perpendicular to the path.
(See 168)
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| cylindrical coordinates | (n) A system for locating points in space with one angle and two lengths. Cylindrical coordinates describe a point as a distance from the origin, its angle in the X–Y plane, and its Z value. Cylindrical coordinates are useful when designing circular shapes and geographic applications.
(See 140)
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| double-curved surface/line | (n) A surface or line that curves in two orthogonal dimensions at the same time. A sphere is an example of a double-curved surface.
(See 164)
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| ellipse | (n) A single-curved line primitive. An ellipse is a conic section produced when a plane is passed through a right circular cone oblique to the axis and at a greater angle with the axis than the elements. An ellipse also describes a circle or circular arc viewed at any angle other than normal (perpendicular).
(See 152)
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| focus point | (n) A location where reflecting rays from a (parabolic or hyperbolic) surface converge. Focus point describes both the physical phenomenon of light rays reflecting from a mirrored surface and the abstract geometric calculations of line paths.
(See 151)
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| hyperbola | (n) A single-curved surface primitive, created when a plane intersects a right circular cone at an angle with the axis that is smaller than that made by the elements.
(See 151)
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| intersecting lines | (n) Lines that share one or more common points in space. Lines that share all their points in common, or lines for which one could be considered a subset of the other, are called coincident.
(See 145)
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| involute | (n) A curve defined as the spiral trace of a point on a flexible line unwinding from around a line, circle, or polygon. The contacting surfaces between gear teeth are designed as involutes.
(See 158)
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| line | (n) A geometric element that connects two points in space. Although a line itself is 2-D in nature, it may connect points in 3-D space. Lines are typically classified as either straight (linear) or curved. Lines are the most prominent element in technical drawings, defining edges of objects, indicating symmetry, relating text elements to geometric elements, creating borders, etc.
(See 145)
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| local coordinate system | (n) A transient coordinate system that can be positioned anywhere in space. The local coordinate system is used to assist in the construction of geometry, and the origin is usually defined relative to the feature of current interest.
(See 142)
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| major diameter | (n) A threading term referring to the largest diameter on an internal or external thread.
(See 152)
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| minor diameter | (n) A threading term referring to the smallest diameter on an internal or external thread.
(See 152)
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| parabola | (n) A single-curved surface primitive, defined as the curve of intersection created when a plane intersects a right circular cone parallel to one of the cone’s elements.
(See 151)
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| parallel lines | (n) Two lines in a plane that stay equidistant from each other along their entire logical length. The lines can be straight or curved. Circular curved parallel lines share the same center point and are referred to as concentric.
(See 145)
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| plane | (n) A region of space defined by a minimum of three noncoincident points in space. For the simplest type of plane surface, all points can be described by two coordinate axes; that is, the plane has no curvature.
(See 162)
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| polar coordinates | (n) A 2-D coordinate system used to locate a point in a plane by specifying a distance and an angle from the coordinate origin. When another distance normal to the coordinate origin is added, cylindrical coordinates can be specified.
(See 140)
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| polygon | (n) A plane figure bounded by straight lines. If the sides are of equal length and form equal angles with each other, the polygon is considered a regular polygon (e.g., a square or hexagon).
(See 165)
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| polygonal prism | (n) A geometric solid consisting of two equivalent polygonal bases parallel to each other. Each equivalent edge of the bases is connected to form a series of parallelograms, bounding the sides of the solid.
(See 170)
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| polyhedron | (n) A geometric solid bounded by polygons. If the polygons are equal, regular polygons, the solid is called a regular polyhedron.
(See 168)
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| pyramid | (n) A geometric solid consisting of a polygonal base and a series of triangular lateral faces. The triangular faces each share one side with the polygonal base and the other two sides with the neighboring triangular faces. The triangular faces all meet at a common point called the vertex.
(See 170)
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| quadrilaterals | (n) Four-sided polygons of any shape. The sum of the angles inside a quadrilateral always equals 360 degrees. Quadrilaterals are classified by the characteristics of their sides. If opposite sides of the quadrilateral are parallel, the shape is a parallelogram.
(See 164)
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| regular curve | (n) A bent line composed of constant-radius arcs generated around a single centerpoint. With traditional tools, regular curves are drawn using a compass or circle template; with CAD, they are constructed with the CIRCLE and ARC commands.
(See 147)
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| relative coordinates | (n) Coordinate locations specified in reference to a previously defined location other than the origin. Relative coordinates are sometimes referred to as delta coordinates, meaning changed coordinates.
(See 141)
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| ruled surface | (n) A surface produced by the movement of a straight-line generatrix controlled by a directrix to form a plane, a single-curved surface, or a warped surface.
(See 166)
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| shape | (n) The internal spatial relationship of vertices and edges that make up a face or the arrangement of faces on an object. Examples of characteristics used to describe a face are the number of edges (sides), the angle between edges, and the ordering of edges around the perimeter. Shape is independent of overall scale but not of viewpoint.
(See 135)
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| single-curved surface | (n) A surface that curves in only one dimension. A cylinder is an example of a single-curved surface. Single-curved surfaces can be developed without distorting or altering the topology of any of the faces.
(See 164)
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| spherical coordinates | (n) Coordinates used to locate points on a spherical surface. Spherical coordinates are described by specifying a distance and an angle from the origin measured in the X–Y plane and then an angle from the X–Y plane.
(See 140)
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| spiral | (n) A curved line that begins at an origin point, moves further away from the origin, and decreases in curvature as it travels around the origin. A spiral is sometimes referred to as a spiral of Archimedes.
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| straight line | (n) A line generated by a point moving in a constant direction. Straight lines can be either infinite or finite in length. A finite straight line is an entity of specific length but no depth or breadth. An infinite straight line is an entity of unspecified length but no depth or breadth.
(See 145)
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| surface | (n) A finite portion of a plane, or the outer face of an object, bounded by an identifiable perimeter. A surface represents the path of a moving straight or curved line, called a generatrix. The path that the generatrix travels is the directrix. In a 3-D model, the topological equivalent of a surface is a face.
(See 162)
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| tangent | (n) A condition in which a straight line is in contact with a curve at only one point. Tangents describe the smooth transition from a linear/planar element to a curved one. Geometric construction techniques are used to define tangent curves in an engineering drawing.
(See 147)
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| undevelopable surface | (n) A surface of an object that cannot be unfolded or unrolled onto a plane without distortion. Double-curved surfaces, such as spheres, are undevelopable.
(See 164)
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| warped surface | (n) A single- or double-curved transitional surface (e.g., cylindroids, conoids, helicoids, hyperbolic paraboloids). Warped surfaces are often approximated by triangulated surface sections, and may join other surfaces or entities together.
(See 164)
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| world coordinate system | (n) A fixed coordinate system, also referred to as a global coordinate system, used in CAD to define the geometric properties of elements stored in the database. The world coordinate system typically uses either a 2-D (X,Y) or 3-D (X,Y,Z) Cartesian coordinate system.
(See 142)
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2002 McGraw-Hill Higher Education