Detail Drafting in CAD

§1One model at full size, two routes to paper

Before 1970 a change to a drawing meant erasing and redrawing, and a change that propagated meant a draftsperson remembering which other sheets it touched. CAD removed that burden and introduced a different one: the geometry is now stored at true size, and the sheet has to be reconciled to it.

In the model, an eighteen-metre house is 18 000 units long and a spectacle-frame screw is 2 units long. Nothing is ever drawn to scale. That single rule keeps the geometry honest — measurements interrogated off the model are real measurements, and a part copied between drawings arrives at the correct size — but it means the sheet border, the text, the dimension arrows, the welding symbols and the dash pattern of a hidden line all have to be reconciled with the model at some scale factor N.

There are two ways to do the reconciling, and a drawing office should pick one and standardise on it.

Route B scales better in every sense, and the more complex the project the stronger the case for it: commit the whole job to one model and take snapshots of it, rather than carving the work up so each piece fits a sheet. Route A survives because it is quicker on a single-view part and because it is what most people were taught first.

§2Coordinate entry — absolute, relative, polar

Geometry picked freehand off the screen is geometry nobody can trust downstream. Three input methods put points where the calculation says they go, and each suits a different kind of leg.

All three describe a point as a displacement from a datum along the X and Y axes, written with a comma between them. Absolute coordinates measure from the drawing origin at the intersection of the axes — 125,63.30 is 125 to the right and 63.30 above 0,0. Relative coordinates move the datum to the last point entered, for that entry only, using the @ prefix: @25,43.30 steps 25 right and 43.30 up from wherever the last click or keystroke left the cursor. Polar coordinates give a distance and a bearing separated by <, measured anticlockwise from the positive X axis, so @50<60 travels 50 at 60°. A negative angle runs clockwise, which is why @150<200 and @150<-160 arrive at the same place.

Orthogonal constraint (ORTHO, toggled on F8) covers the common case without any typing at all: drag in the direction wanted, type the length, and the line is exactly horizontal or vertical at that distance. It is the fastest correct method for rectilinear work and should be the default state of the cursor.

Object snap is the other half of accuracy. It constrains a pick to an exact geometric feature — endpoint, intersection, midpoint, centre, node, quadrant, tangent, perpendicular — so a line genuinely starts on the end of another line rather than a pixel away from it. Temporary snaps invoked per pick are useful while learning; production work runs on a considered set of running snaps, kept small enough that the wrong feature is not offered.

§3The template file carries the standard

No published standard formalises how a CAD drawing environment should be set up — AS 1100 governs what appears on the sheet, not how the file that produced it is configured. The template is where a practice writes that second standard for itself, and it is the highest-leverage artefact in a drawing office.

A template is a master drawing holding settings and, usually, a border. It is opened only to amend it — anything drawn on the master appears on every drawing made from it afterwards, which is the trap that catches people who open a template to check something and save on the way out. A serviceable template carries:

Style names are for other people. Tolerance Standard and Tolerance Leader tell the next operator what they are for; Dim2 and Dim3 tell them nothing and guarantee the wrong one gets picked. The same discipline applies to layer names.

§4Layers, colours and lineweights

Layers are the overlays of manual drafting made addressable. Their real value is not tidiness — it is that visibility and editability become properties you can switch, per drawing and, in a layout, per viewport.

Two layers are reserved rather than chosen. Layer 0 should carry no drawing content: entities created on it and made into a block inherit the properties of whatever layer the block is later inserted onto, which is exactly what a symbol library wants and exactly what a detail does not. Defpoints is created automatically the first time a dimension is placed, and has the unusual property of being visible on screen but never plotted — which is why viewport frames are so often parked there.

Keep to the seven standard colour indices — 1 red, 2 yellow, 3 green, 4 cyan, 5 blue, 6 magenta, 7 white — wherever the pen table maps colour to plotted weight. Colours 8 to 255 exist, and every one of them is an opportunity for a plot to come out wrong because nobody mapped it.

§5Dimension styles set to AS 1100

A dimension style has more than fifty variables behind it. A handful of them decide whether the drawing reads as professional work, and they are the ones the standard actually specifies.

The standard also governs where dimensions go, and these four rules resolve most disputes on a checking print:

• A feature is dimensioned once only, on the view that shows it most clearly. A second dimension for the same feature is not redundancy, it is a contradiction waiting for a revision.
• Dimensions are distributed among the views rather than banked on one.
• Hidden detail is dimensioned only if no other view shows it as a visible outline.
• Shorter dimensions sit inside longer ones, so extension lines never cross a dimension line.

§6The model-space route — insertion scale

On this route one number governs everything downstream. Choose the scale before annotating, and apply it consistently to every element that is not geometry.

1. Start from the template, never from scratch — the layers, styles and units come with it.
2. Draw the views at full size. Real dimensions, no scaling of geometry, ever.
3. Insert the sheet block at the plot scale N. For 1:10, the A3 border comes in at scale 10.
4. Insert symbols at the same factor N — welding symbols, surface texture symbols, section arrows. Appliances and library parts that represent real objects go in at 1.
5. Set the dimension style's overall scale to N, changing it from 1.
6. Add text at its nominal height × N. A 3.5 mm note on a 1:10 drawing is drawn 35 units high.

The plot is then taken at the scale the sheet was inserted at, which reduces the enlarged border back to true A3. The weakness is visible from the sequence: every step is tied to a single N, so a second scale on the same sheet means a second set of dimension styles, a second text height and a second insertion factor, all live at once.

§7The layout route — viewports and XP

A layout is the sheet of paper, represented at 1:1. Viewports are holes cut in it through which the model is seen, each at whatever magnification suits the view.

Viewports are paper-space objects, created and modified there, though the model inside one is worked on by switching into model space and clicking inside it — only one is active at a time, and overlapping viewports make that pick worth watching. The frame itself is usually unwanted on the plot: create viewports on a layer that gets frozen, or on Defpoints, which never plots.

Scale is set by zooming the viewport contents by an XP factor — the ratio of model units to paper units — entered as nXP at the zoom scale prompt. Because the layout is 1:1 and the sheet is at true size, the XP factor is the drawing scale.

Set the scale first, then pan to compose the view — panning does not disturb the zoom factor, and a viewport that has been scrolled with the wheel afterwards is a viewport whose scale is no longer what the title block claims. Lock the viewport once it is right.

Three further behaviours are what make this route worth the setup:

• Dimensions scale themselves. Set the dimension style to scale to layout and the annotation sizes itself to each viewport's factor automatically. The multiple-dimension-style problem of the model-space route disappears.
• Layers freeze per viewport. The same model region can appear twice on one sheet with different content — a contour plan and a site plan from one drawing, differing only in whether the contour layer is thawed.
• Views align by construction. Move the viewport together with its dimensions and notes as one selection, pick a base point on a centreline endpoint or midpoint, and use the perpendicular snap to bring it into projection with its neighbour. Multileaders and free notation do not always follow a viewport — include them in the selection deliberately.

§8Scaled details without drawing twice

The governing rule of any CAD package is that the object is never drawn twice. An enlarged detail of a groove or an edge indent is the same geometry, seen closer.

On the layout route this is almost free: open a second viewport over the same model region and zoom it to a larger XP factor. The geometry is shared, the dimensions size themselves, and nothing needs correcting.

On the model-space route it takes more care. Copy the entities forming the detail, scale the copy up, and the drawn size is now wrong by that factor — the geometry is correct because it was copied, but the dimension text will report the enlarged figure. Create a dimension style whose measurement scale factor is the reciprocal of the enlargement and apply it to that detail only.

Any dimension placed with the wrong style before the mistake was noticed can be brought onto the correct one with a dimension update rather than deleted and replaced. Label the detail and its scale on the sheet regardless of route — DETAIL A — SCALE 5:1 — because the reader has no other way to know.

§9Hatching and sectional views

A section replaces hidden-line clutter with visible outline. The hatch is what tells the reader which of the resulting areas is solid material.

Steel is conventionally indicated by parallel lines at 45°, spaced about 3 mm; other materials carry their own patterns, and separate components meeting in one assembly are distinguished by changing the angle or the spacing between them, never by leaving one blank. Any area left unhatched reads as a hole or an opening — a forgotten boundary is not a cosmetic defect, it is a change to what the drawing says the part is.

A full section removes half the object along a cutting plane; a half section removes a quarter of a symmetrical part, showing internal detail on one side and external form on the other, with the hidden lines in the unsectioned half omitted rather than drawn. In both cases, inherit properties from an existing hatch when adding to a drawing already in progress — it copies pattern, angle and scale in one operation, and keeps a sheet visually consistent when it has been worked on by more than one person.

§10Plotting and issue

The plot is what leaves the office. It goes to checking and approval, then to estimating, ordering, job planning and the workshop — and by then nobody is looking at the file.

• Plot style: monochrome. Layer colours exist to organise the drawing on screen and, through the pen table, to control plotted lineweight. They should never reach paper as colour.
• Scale. From a layout, 1:1. From model space, the scale the sheet block was inserted at. Fit to paper is not a scale and has no place on an issued drawing.
• Use standard scales only — those recognised by AS 1100. Anything printed off-scale must be marked NOT TO SCALE, or CHECKING PRINT where that is what it is.
• Watch the printable area. The dashed boundary in the layout is device-specific and moves when the output device changes. Preview before committing; overlap is flagged at the edges.
• Plot area. Layout for a finished sheet, Extents for everything drawn, Window for a checking print of the region that changed — which saves plotting a full sheet to review one revision.

Contents

§11Quick reference