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Technical Drawing - Coggle Diagram

- - - - Nice to look at, impossible to measure: in an isometric
        drawing the edges are foreshortened, so the same edge is drawn
        shorter than it really is.
    - - The orthographic front view of the same block: that
        edge is parallel to the paper, so it is drawn at its true length and
        the drawing can be measured.
  - - - Orthographic. The rays are parallel and perpendicular to
        the projection plane, so the drawing keeps the true size of the
        object.
    - - What a perspective does (like a torch and a shadow): the
        rays diverge, so the image grows with distance. You cannot
        measure it.
- - - - Why the top view goes below. The object is between you and the planes, so the top view is painted on the plane on the far side.
        When the box is opened flat, that plane swings down: the top view ends up under the front view. The fold is the ground line
        (línea de tierra).
  - - - First angle (European system). An L-shaped plate, 60 mm wide, 40 mm high and 20 mm deep. Because the views are projections
        of one object, width, height and depth have to match across them — that is what the thin blue lines check.
      - THE THREE CORRESPONDENCES
        
        Width — the front view and the top view are exactly
        the same width, one above the other.
        
        Height — the front view and the side view are exactly
        the same height, side by side.
        
        Depth — the top view and the side view carry the
        same depth. The 45° line (línea de 45° o de mitra)
        takes the depth from one to the other: down, across,
        and up.
      - TWO DETAILS THAT WIN MARKS
        
        The step in the middle of the plate is a solid line in
        the top view (you can see that edge from above) but a
        dashed line in the side view (the tall part hides it from
        the left). Same edge, different line, because you look
        from a different place.
        
        Choose the front view first: the position that shows
        the shape best (here, the L). Everything else is placed
        from it.
- - - - Same views, different position. The symbol tells you which system is used: the circles go on the same side as the view of the left
        face — right in the first angle, left in the third angle. The symbol keeps the proportion 20d / 10d (d = line thickness) fixed by UNE-EN
        ISO 5456-2.
    - - The object is between you and the plane.
        Top view below the front view.
        The view from the left goes on the right.
        Used in Spain and the rest of Europe. This is what you
        draw in the exam.
    - - The plane is between you and the object.
        Top view above the front view.
        The view from the right goes on the right.
        Used in the USA, Canada and Australia. You will see it
        in imported drawings.
- - - - The number never changes. The three drawings show the same plate at three scales, and all three are dimensioned 60, because
        60 mm is the real size. Only the size of the drawing changes — so you can always measure the real object on the drawing, multiplying
        by the scale.
    - - Reduction (the object is big): 1:2 · 1:5 · 1:10 · 1:20 ·
        1:50 · 1:100.
        Full size: 1:1 — the object fits on the sheet.
        Enlargement (the object is small): 2:1 · 5:1 · 10:1.
        Always use a standard scale, never 1:3 or 1:7.
        The scale is written in the title block
        (cuadro de rotulación), never next to each
        measurement.
    - - Real size → paper: divide by the second number.
        60 mm at 1:5 → 60 ÷ 5 = 12 mm on paper
        Paper → real size: multiply by the second number.
        12 mm on paper at 1:5 → 12 × 5 = 60 mm real
        A classroom plan at 1: 100: 8 m of wall = 8000 mm ÷
        100 = 80 mm on the sheet.
    - - "The plate is 60 mm, so at 1:2 I write 30." No. You write 60: dimensions are always the real size. The scale changes
        the drawing, not the number.
- - - - One object, four line types. A plate with a blind hole. The hole cannot be seen from the front, so its walls are drawn dashed; the
        hole's axis is a centre line, which starts a dash before the outline and finishes one after it.
    - - The outline and every edge you can see from that
        position. The thickest line on the drawing
    - - An edge that exists but is covered by material. It must
        start and finish with a dash, and never touch the outline.
    - - The axis of a hole, a cylinder or a symmetrical part. It
        goes 2–3 mm past the outline.
    - - All the lines that measure the part. They are never drawn
        thick, because they are not part of the object.
    - - If you draw a hidden edge solid, the reader sees material where there is a hole, and the part is machined wrongly.
        Line type is information.
- - - - The same plate, done twice. On the right, three faults that appear in real work: the dimension is written inside the part, the
        extension lines touch the object, the numbers sit on the line and one dimension is repeated with the unit written.
    - - Dimension each measurement once, on the view
        where it appears at true size.
        Extension lines: they start 1 mm away from the
        outline and go 2–3 mm past the dimension line.
        Dimension lines: thin, parallel to the edge, with an
        arrow at each end, drawn outside the shape, 8–10 mm
        from the outline.
        The number goes above the line (or to the left of a
        vertical line), never on it, and never inside the part.
        Millimetres, no unit written. Angles in degrees,
        diameters with Ø, radii with R.
        Group dimensions in rows (parallel to a common
        base) instead of chaining them all over the drawing.
    - - Using the outline as a dimension line, with the
        number written inside the part. It is confusing: the
        reader cannot tell which line is the object and which
        is the dimension.
        
        Extension lines touching the object (they need a 1
        mm gap) and the number written on the line instead
        of above it.
        
        Writing the unit ("55 mm") and repeating the width,
        which was already given. Each dimension is written
        once and millimetres are never written.
- - - - The difference. In 2D CAD you draw the lines; in 3D CAD you build the solid once and the drawing is generated from it. That is
        why the 3D model can also feed a 3D printer (as an STL file) and why its views can never contradict each other.
    - - You do: Draw lines, circles and arcs on a flat plane, with
        exact coordinates.
        Sketch a flat profile and then extrude or revolve it
        to get a solid.
        You get: Plans, electrical and electronic circuits, layouts,
        templates.
        A solid model: you can measure its volume, check
        that parts fit, render it, print it.
        Views: You draw all three views yourself, one by one. The program generates the three views from the
        model in seconds.
        Advantage: Fast for diagrams and 2D plans, and lighter to
        learn.
        One change in the model updates every view. No
        contradictions between views.
        In class: Schematics, circuit diagrams, the plan of a room. Onshape for the parts and the assembly (Tasks 2.2
        and 2.3), then STL to the 3D printer.
    - - A 3D model is not a prettier 2D drawing: it is a model of the object. From it you can extract the views, the
        measurements, the volume and the file for the printer.