JCE Technical Drawing cheat sheet

To bisect a line segment, set compass radius greater than half the segment; draw arcs from both endpoints; the line through the two arc intersections is the perpendicular bisector.

To bisect an angle, draw an arc from the vertex cutting both sides; from those two intersection points draw equal-radius arcs; the line from the vertex through the arc intersection bisects the angle.

To divide a line into n equal parts, draw an auxiliary line from one endpoint at any angle; step off n equal divisions using a compass or scale; connect the last division to the other endpoint; draw lines parallel to that connector through each division.

To draw a perpendicular to a line at a given point, with the point as center draw an arc cutting the line on both sides; from those intersections draw equal arcs; connect the given point to the arc intersection.

To construct a regular hexagon given the circumcircle radius, step off the radius around the circle six times; for a hexagon given the across-flats distance, construct the circle from the across-flats using the 30-60 triangle.

To construct an ellipse by the concentric-circle method, draw major and minor circles; draw radial lines; from each intersection with the major circle drop a vertical, from the minor circle draw a horizontal; their intersection is a point on the ellipse.

To find the center of a circle or arc, draw two non-parallel chords; construct their perpendicular bisectors; the intersection is the center.

To draw a tangent to a circle from an external point, bisect the line joining the point to the center; draw a semicircle on that diameter; the intersections with the circle are the tangent points.

First-angle projection: the object is between the observer and the plane; the view from the front is placed above the plan, and the left-side view is placed on the right.

Third-angle projection: the plane is between the observer and the object; the top view is placed above the front view, and the right-side view is placed on the right.

The front view shows width and height; the top view shows width and depth; the side view shows depth and height. Width aligns vertically between front and top; height aligns horizontally between front and side; depth transfers between top and side by a 45° mitre line or compass.

Hidden edges are drawn as short dashed lines of uniform length; center lines are long-short-long chains; visible outlines are continuous thick lines.

A view is chosen as the front view to show the most characteristic shape and the fewest hidden features.

In first-angle projection the symbol is a truncated cone with the small end toward the observer and the circle on the left; in third-angle the circle is on the left and the cone points right.

Adjacent views must align: features project across at 90° to the folding line between the views.

When a surface is perpendicular to a plane of projection, it appears as an edge (line) in that view and as a foreshortened shape in the other views.

Isometric axes are at 30° to the horizontal; all three axes are equally foreshortened, so true lengths are used along the axes (isometric drawing).

Non-isometric lines are lines not parallel to an isometric axis; they are drawn by locating endpoints on isometric lines and connecting them; their true length is not measured directly.

Isometric ellipses: the major axis is perpendicular to the isometric axis of the circle's normal; the ratio of major to minor axis is about 1.22:1 for true isometric projection; in isometric drawing the ellipse is often approximated with a four-center method.

Cavalier oblique: the receding axis is drawn at 45° and full scale; cabinet oblique uses half scale on the receding axis to reduce distortion.

In oblique projection the front face is drawn true shape and size; the receding axis is typically 30°, 45°, or 60° to the horizontal.

Isometric drawing angles: vertical lines remain vertical; the two horizontal axes are drawn at 30° above the horizontal on each side.

To draw an isometric circle on the top face, construct an isometric square and use the four-center method with centers at the corners and midpoints.

Hidden lines are generally omitted in pictorial drawings unless needed for clarity; center lines are shown for holes and cylinders.

Freehand lines: keep the eye on the endpoint, move the whole arm from the shoulder, and rotate the paper for long lines.

Proportions are estimated by comparing dimensions to a chosen unit; use the pencil as a sighting tool at arm's length.

Block in the overall bounding box first, then subdivide into proportional parts before adding detail.

Freehand circles: mark the center and four quadrant points, then sketch arcs through them; for large circles use a square box as a guide.

Visualization: a missing view is reconstructed by projecting known features and applying the rule that each point must appear in every view at the same height and depth.

Oblique sketching is useful for flat objects; isometric sketching is preferred for objects with features on all three axes.

Use construction lines lightly; darken only final outlines; keep line weights consistent (thick visible, thin construction).

Check symmetry by folding the sketch or measuring from a center line; asymmetric features should be verified against given dimensions.

Dimension lines are thin continuous lines terminated with arrowheads; they are placed outside the view where possible and never coincide with object lines.

Extension lines start with a small gap (about 1.5 mm) from the object and extend about 3 mm beyond the dimension line; they do not touch the object.

Numbers are placed above the dimension line, centered, and read from the bottom or right side of the sheet; for aligned dimensioning they are parallel to the dimension line.

Diameter is prefixed with Ø; radius with R; spherical radius with SR; spherical diameter with SØ; countersink with a symbol and angle; counterbore with a symbol and depth.

A scale is the ratio of drawing length to actual length. Scale 1:2 means the drawing is half size; 2:1 means double size.

To construct a plain scale, divide a line into equal parts representing units, subdivide the first part for fractions, and mark the representative fraction (RF = drawing length / actual length).

A diagonal scale gives three successive units by using diagonal lines to subdivide a small vertical distance into ten parts.

Tolerances are written as limits (e.g., 25.00/24.95) or as a basic size with plus/minus values; general tolerances may be noted in the title block.

A cutting plane line is a thick chain line with arrows showing the direction of sight; it is labeled with capital letters (e.g., A-A).

Section lines (hatching) are thin continuous lines at 45° to the horizontal, equally spaced; adjacent parts in an assembly are hatched in opposite directions or at different spacings.

Ribs, webs, shafts, bolts, nuts, and similar solid cylindrical parts are not hatched when cut longitudinally; they are shown in outside view.

A half section removes one quarter of the object and shows half in section and half in outside view; a broken-out section uses a freehand break line to expose a local area.

A revolved section is drawn on the view itself by rotating the cross-section 90°; a removed section is drawn elsewhere and labeled.

Hidden lines are omitted in sectioned views unless required for clarity.

The cutting plane may be offset (stepped) to pass through features not in one plane; the section is drawn as if all cutting planes were rotated into one plane.

Section lining spacing is reduced for small areas; very thin parts may be shown solid black instead of hatched.

Development is the true shape of a surface unfolded onto a plane; parallel-line development is used for prisms and cylinders; radial-line development for pyramids and cones; triangulation for oblique surfaces.

For a right cylinder, the development is a rectangle of width equal to the circumference (πD) and height equal to the cylinder height.

For a right cone, the development is a sector of radius equal to the slant height; the sector angle θ = 360° × (base radius / slant height).

To develop a truncated cylinder, divide the base circle into equal parts, project heights to the development, and plot the curve through the points.

The intersection of two cylinders is found by projecting common points from auxiliary views; the curve is plotted point by point.

For a pyramid, find true lengths of lateral edges by revolution or auxiliary view; lay out the base and triangulate the faces.

A hole in a development appears as an opening; its shape is found by projecting the intersection curve onto the developed surface.

Allowances for seams, bends, and thickness are added to the developed pattern; the neutral axis is used for bend allowance.

Pencils: H or 2H for construction and guidelines; HB or B for visible outlines and lettering; sharpen to a long taper for accuracy.

A T-square is used against the drawing board edge for horizontal lines; set squares (30-60 and 45) ride on the T-square for vertical and angled lines.

A compass is used for circles; adjust the needle point slightly longer than the lead; rotate the compass by the knurled knob, not by the legs.

Line weights: visible outlines thick (about 0.5-0.7 mm); hidden and center lines thin (about 0.25 mm); construction lines very thin and light.

Alphabet of lines: continuous thick (visible), continuous thin (dimension, extension, leader, hatching), dashed (hidden), chain (center, cutting plane), and phantom (alternate position).

Lettering: use single-stroke Gothic capitals; guidelines for height and slope; vertical or inclined at 75°; minimum height about 3 mm for readability.

Standard sheet sizes: A0 841×1189 mm, A1 594×841, A2 420×594, A3 297×420, A4 210×297; the title block is usually at the bottom right.

A drawing border is a continuous thick line with a margin (e.g., 10 mm) and a filing margin of about 20 mm on the left edge.

A working drawing includes orthographic views, dimensions, tolerances, notes, and a title block sufficient for manufacture without further instructions.

A bill of materials (BOM) lists part numbers, descriptions, quantities, and materials; it is placed above the title block or on a separate sheet.

A title block contains drawing number, title, scale, date, drafter, checker, and projection symbol.

A flowchart uses standard symbols: oval (start/end), rectangle (process), diamond (decision), parallelogram (input/output), and arrows for flow.

A wiring diagram shows physical connections; a schematic uses standard symbols and does not show physical layout.

A site plan or layout drawing shows the arrangement of buildings or equipment to scale, with north arrow, scale bar, and key.

A graph is plotted with independent variable on the x-axis and dependent on the y-axis; label axes with quantity and unit; include a title and legend.

A pictogram or infographic uses simplified symbols and consistent scale; avoid misleading area scaling by using length or bar charts rather than area for quantitative comparison.