How unit 7 is examined
This unit covers preparing an AutoCAD drawing (page, units, limits, standards, snaps) and drawing with it; the marks sit in circle and ellipse drawing methods, the other three topics are unasked recently.
Drawing page, printer, scale, units and limits
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Definition. <mark>Drawing setup fixes the units, the limits (the drawing area) and the plot scale before drawing, so that the sheet prints at the correct size.</mark>
Key points.
- Units are set with the UNITS command, where the type (decimal for engineering) and the precision are chosen; one drawing unit is normally taken as 1 mm.
- Limits are set with the LIMITS command by giving the lower-left and upper-right corners, for example 0,0 and 420,297 for an A3 sheet.
- Grid is turned on and ZOOM All is used so that the limits fill the screen.
- In Page Setup or Plot, the printer or plotter, paper size, plot area and plot scale are chosen; plot scale = paper units : drawing units.
- A drawing made full size (1:1) is plotted at the required scale, for example 1:2 to reduce it by half, so the model is never redrawn to a smaller size.
- Limits only mark the working area; with LIMITS ON, AutoCAD refuses points outside them, which stops the student drawing off the sheet.
Example. For an A3 sheet in millimetres: UNITS, decimal, precision 0.00; LIMITS 0,0 to 420,297; ZOOM All; then Plot with paper A3, area Limits, scale 1:1.
Steps.
Step 1: Type UNITS and choose decimal type and precision.
Step 2: Type LIMITS; give lower-left 0,0 and upper-right 420,297.
Step 3: Turn on GRID and type ZOOM, then All.
Step 4: Choose Page Setup, select the plotter and paper size.
Step 5: Set plot area to Limits and plot scale to 1:1 (or the required ratio).
ISO and ANSI dimensioning and tolerancing standards
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Definition. <mark>ISO (international) and ANSI (American) standards fix how dimensions, tolerances and lettering are shown on a CAD drawing, so every reader interprets it the same way.</mark>
Key points.
- AutoCAD dimension styles (DIMSTYLE) store the standard's arrow size, text height, extension line gap and units.
- ISO uses first-angle projection by default and the metric system; ANSI uses third-angle projection and, traditionally, inches.
- Coordinate dimensioning gives every feature as X and Y distances from one datum origin, avoiding chain errors.
- A tolerance is the permitted variation of a size, shown as a limit form (25.05 / 24.95) or a plus-minus form (25 ±0.05).
- Geometric tolerancing (GD&T) controls form, orientation and position using symbols in a feature control frame.
- A feature control frame is a rectangle read left to right: the geometric symbol, the tolerance value, and the datum letters the feature is measured from.
| Aspect | ISO | ANSI |
|---|---|---|
| Origin | International (Europe and most countries) | United States |
| Default projection | First angle | Third angle |
| Units | Millimetres | Inches (mm also allowed) |
| Dimension style | Arrows closed, text aligned with the line | Text usually horizontal |
Example. In coordinate dimensioning a hole is written as X = 40, Y = 25 from the datum corner; its size carries a tolerance such as 12 +0.02 / 0, so the hole may measure 12.00 to 12.02 mm.
Orthographic constraints and object snap
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Definition. <mark>Ortho mode restricts the cursor to horizontal and vertical movement, and Object Snap (OSNAP) makes the cursor lock onto exact geometric points of existing objects.</mark>
Key points.
- Ortho is toggled with F8 and forces lines to be exactly horizontal or vertical.
- Running (automatic) object snap is toggled with F3 and set in Drafting Settings, so chosen snap points are picked up whenever the cursor is near them.
- Manual (override) snap is chosen for a single pick with Shift plus right-click or by typing its name, for example END or MID.
- Common snaps are Endpoint, Midpoint, Center, Intersection, Perpendicular, Tangent, Quadrant and Nearest.
- Snap gives exact accuracy, since clicking by eye can never hit an exact endpoint or centre.
- Ortho and OSNAP work together: Ortho fixes the direction of a line, and OSNAP fixes exactly where it starts and ends.
Example. To draw a line from the midpoint of a slanted line to the centre of a circle, run LINE, type MID and pick the slanted line, then type CEN and pick the circle.
| Snap | Locks onto |
|---|---|
| END | end of a line or arc |
| MID | middle of a line or arc |
| CEN | centre of a circle or arc |
| INT | crossing of two objects |
| PER | foot of a perpendicular |
| TAN | tangent point on a circle |
Coordinate input methods for lines and ways of drawing circles
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Definition. <mark>Lines are drawn by giving points through absolute, relative or polar coordinates or direct distance entry, and circles and ellipses are drawn by the CIRCLE and ELLIPSE commands using several option methods.</mark>
Key points.
- Absolute coordinates give a point from the origin as X,Y, for example 50,30.
- Relative coordinates give a point from the last point as @X,Y, for example @40,20.
- Polar coordinates give distance and angle from the last point as @distance<angle, for example @50<30.
- Direct distance entry: with Ortho on, point the cursor in a direction and type the length, which is the fastest way to draw horizontal and vertical lines.
- Dynamic input shows the length and angle in a box near the cursor, so a polar line is drawn by typing the length, pressing Tab and typing the angle.
- Absolute input is used for the first point, relative or polar input for every point after it.
Example. A rectangle 60 by 40: LINE, 0,0, then @60,0, @0,40, @-60,0, and C to close.
Example (polar). A line 50 long at 30 degrees from point 20,20: LINE, 20,20, then @50<30.
Circle methods (CIRCLE command).
| No. | Method | Input |
|---|---|---|
| 1 | Center, Radius | centre point, then radius value |
| 2 | Center, Diameter | centre point, then diameter value |
| 3 | 2-Point (2P) | two end points of the diameter |
| 4 | 3-Point (3P) | three points on the circumference |
| 5 | Tan, Tan, Radius (TTR) | select two tangent objects, then radius |
| 6 | Tan, Tan, Tan (TTT) | select three tangent objects |
Ellipse methods (ELLIPSE command).
| No. | Method | Input |
|---|---|---|
| 1 | Center | centre point, end of first axis, distance to other axis |
| 2 | Axis, End | two end points of first axis, then distance to other axis |
| 3 | Elliptical Arc | ellipse as above, then start and end angles for a partial ellipse |
| 4 | Isocircle | in isometric snap mode, centre and radius give an ellipse on an isometric plane |
Steps (circle by 3P).
Step 1: Type CIRCLE (or C) and press Enter.
Step 2: Choose the 3P option.
Step 3: Pick the first, second and third points on the circumference.
Steps (ellipse by Axis, End).
Step 1: Type ELLIPSE and press Enter.
Step 2: Pick the two end points of the first axis.
Step 3: Pick or type the distance to the other axis.
Answer frame. Open with "In AutoCAD, circles are drawn with the CIRCLE command and ellipses with the ELLIPSE command"; draw a small sketch beside each method (2P diameter, 3P points, TTR touching two lines); develop the methods in the table order, giving the input sequence of each; close with "Choice of method depends on the data given, for example TTR when tangents are known".
Asked: [7 marks] (Jun 2022) Prepare an ellipse using four different methods in AutoCAD. Asked: [7 marks] (Nov 2022) Explain the different methods used for drawing a circle in AutoCAD. Pitfall: Do not stop at Center-Radius; the examiner expects all six circle methods (including TTR and TTT) and all four ellipse methods.
Last-minute revision
- Circle methods: Center-Radius, Center-Diameter, 2P, 3P, TTR, TTT (six in all).
- Ellipse methods: Center, Axis-End, Elliptical Arc, Isocircle (four).
- Absolute is X,Y; relative is @X,Y; polar is @distance<angle.
- F8 is Ortho, F3 is running Object Snap.
- LIMITS takes the lower-left and upper-right corners; A3 is 420,297.
- UNITS sets type and precision; plot scale = paper units : drawing units.
- ISO uses first-angle projection; ANSI uses third-angle projection.
- Coordinate dimensioning measures everything from one datum origin.
- GD&T symbols sit in a feature control frame with the tolerance value and datum letters.
- 2P takes the two ends of a diameter; 3P takes three points on the circumference; TTR needs two tangent objects and a radius.
- Isocircle appears in the ELLIPSE command only when isometric snap style is on.
- Dimensions in coordinate dimensioning are ordinate, from a single datum, which avoids accumulating errors.
Memory hooks
- Circle: "CD 2 3 TT" gives Centre-radius, Diameter, 2P, 3P, TTR, TTT.
- Ellipse: "C-A-A-I" gives Center, Axis-end, Arc, Isocircle.
- Coordinates: absolute A from the origin, relative @ from the last point, polar < for angle.
- F8 is "eight, straight"; F3 is "three, three-point snaps".
Coverage checklist
- set up of the drawing page and the printer, including scale settings, Setting up of units and drawing limits: no past question.
- ISO and ANSI standards for coordinate dimensioning and tolerancing: no past question.
- Orthographic constraints, Snap to objects manually and automatically: no past question.
- Producing drawings by using various coordinate input entry methods to draw straight lines, Applying various ways of drawing circles: Jun 2022 ellipse methods, Nov 2022 circle methods.