JCE Technical Drawing key terms

The set of compass-and-straightedge procedures used to build accurate shapes such as bisectors, perpendiculars, tangents, and polygons from given data before any drawing is inked.

A line drawn at right angles through the midpoint of a segment, found by striking equal arcs from both endpoints; it locates centres for arcs and divides lines into equal halves.

Dividing an angle into two equal parts by striking an arc across both sides and intersecting arcs from those points; the resulting line also locates the incentre of a triangle.

The single point where a straight line or arc touches a curve without crossing it; it is found by dropping a perpendicular from the arc centre to the line or by aligning centres.

Joining two lines, arcs, or a line and an arc with a smooth curve so no corner shows, achieved by locating a centre at the correct offset distance from each element.

Methods for drawing equal-sided figures inside or around a circle, such as stepping the radius six times for a hexagon or using the division of a diameter for pentagons and octagons.

A system that shows a three-dimensional object as separate two-dimensional views, with projectors perpendicular to the picture planes so true shape and size are preserved on each view.

A convention where the object sits between the observer and the plane, so the view seen is placed on the far side; the plan appears below the front view and the left view goes on the right.

A convention where the plane sits between the observer and the object, so each view is placed on the same side as the observer; the plan appears above the front view and the right view on the right.

The small truncated-cone graphic placed near the title block that declares whether first- or third-angle layout is used, showing the frustum and its end view in the standard arrangement.

Any of the six standard views obtained by looking perpendicularly at the front, top, bottom, left, right, or rear faces of an object; the front view is chosen to show the most shape.

A view projected onto a plane inclined to the principal planes, used to show the true shape of a slanted face that appears foreshortened in the standard views.

A pictorial view built on three axes at 30 degrees to the horizontal, with all lines drawn true length along the axes, so the object appears equally tilted to the viewer.

The four-centre approximation of a circle on an isometric face; it is drawn as an ellipse whose major axis lies along the long diagonal of the enclosing rhombus.

Any edge in an isometric view that does not lie along one of the three axes; it cannot be measured directly and must be located from coordinates or enclosing box points.

A pictorial projection in which the front face is drawn true shape and parallel to the picture plane while receding edges run at an angle, usually 30, 45, or 60 degrees.

An oblique projection that draws receding edges at full scale, giving a bold depth effect but severe distortion on deep objects; often used for simple blocks and furniture.

An oblique projection that halves the scale of receding edges to reduce distortion, producing a more natural appearance than cavalier while keeping the front face true.

Producing a proportional drawing by eye and hand without instruments, using light construction lines, boxed proportions, and steady strokes to communicate a design idea quickly.

The relative size relationship between parts of an object and the whole; in freehand work it is judged by comparing widths and heights against a chosen unit or the pencil held at arm's length.

A light rectangular box drawn to the overall width, height, and depth of a subject, inside which the sketch is developed so that proportions stay consistent.

A thin line, terminated by arrowheads or oblique strokes, drawn between extension lines to carry the numerical size of a feature and placed outside the object outline where possible.

A thin line that extends from the feature being measured to the dimension line, starting with a small gap from the outline and continuing a little beyond the arrowhead.

A thin line, usually with a short shoulder and an arrowhead, that points from a note, symbol, or dimension to the feature it describes, commonly used for radii and hole callouts.

A convention in which all figures are written horizontally so they read from the bottom of the sheet, regardless of the direction of the dimension line.

A convention in which figures are written parallel to their dimension lines, so they read from the bottom or from the right-hand side of the sheet.

Placing dimensions end to end in a series so each feature is measured from its neighbour; simple to read but it accumulates tolerance errors across the part.

Measuring every feature from one common reference face, edge, or centre line, which avoids the cumulative build-up of error found in chain dimensioning.

A scale written as a ratio of drawing length to real length, such as 1:50, independent of the units used; it is the standard way of stating a drawing's scale.

A graduated graphic line drawn on a map or drawing that allows distances to be measured directly and remains valid if the sheet is photocopied or resized.

A scale that uses similar triangles to subdivide a small unit into tenths or hundredths, allowing distances finer than the smallest graduation to be read accurately.

A heavy line, often chain-dashed with thick ends and arrows, that marks where an object is imagined to be cut and indicates the direction in which the section is viewed.

A view that shows the cut surface of an object plus visible edges behind it, used to clarify internal features such as holes and bores that would be hidden lines otherwise.

Thin, evenly spaced parallel lines drawn across a cut surface at about 45 degrees to show solid material; adjacent parts in an assembly are hatched in opposite directions.

A section in which one quarter of the object is removed, showing half the view in section and half in outside view; suited to symmetrical parts such as shafts and pins.

The flat pattern obtained by unfolding the surfaces of a three-dimensional object onto a plane, used to mark out sheet metal before folding or rolling.

A straight line on a development whose length equals the circumference or perimeter of the round or curved part, divided into equal parts to locate fold or seam points.

The line along which two surfaces meet, such as a branch pipe entering a header; it must be found before a development can be laid out accurately.

The agreed meaning of each line type: continuous thick for visible edges, dashed for hidden edges, chain thin for centre lines, and continuous thin for dimension and construction work.

A short-dashed line showing an edge that is concealed in a given view; dashes should meet at corners and the line is omitted where a visible line occupies the same place.

A chain-thin line of long and short dashes marking axes of symmetry, centres of circles, and pitch circles; it extends a little beyond the feature it defines.

A very light line used to block in proportions, locate centres, or build geometry; it is either erased or left faint so that the finished outline dominates.

The ruled panel, usually at the bottom right, carrying the drawing number, title, scale, projection convention, material, tolerances, and the names and dates of those who drew and checked it.

A tabulated list, often above the title block, giving each part's item number, description, material, quantity, and reference number so the drawing can be read as a complete assembly.

The permissible variation in a dimension, stated as plus and minus limits or as upper and lower values, defining how much a manufactured feature may differ from the nominal size.

A theoretically exact point, line, or surface from which dimensions or geometric tolerances are measured, chosen because it relates to how the part is located or functions.