Dimensioning for function, once and only once

Dimensions turn a picture into a specification. Each one tells a maker how big a feature is or where it sits, and, together with its tolerance, how much it may vary. A well-dimensioned drawing gives each size and position once, from a sensible origin, in a form nobody can misread. A poorly dimensioned one gives some sizes twice, others not at all, and leaves the maker to work out which numbers matter.

This chapter applies AS 1100.101 clause 8.2 to the running case housing. It covers the symbols in front of dimensions, the layout of projection lines, dimension lines and leaders, the way numbers are written, functional dimensioning, and auxiliary dimensions. Its error case is the most common dimensioning fault there is: a chain of dimensions plus an overall dimension, each with its own tolerance.

Symbols in front of dimensions

AS 1100.101 Table 8.1 defines symbols that tell the reader what kind of feature a dimension refers to. Each is placed in front of the number unless stated otherwise.

Figure 1Dimensioning symbols, AS 1100.101 Table 8.1

The symbols do more than save words. A diameter symbol lets a single view define a round feature, as Chapter 4 showed. A depth symbol after a hole size removes the need for a section to show the depth. A counterbore symbol tells the reader the feature is a stepped hole, not two separate holes.

Projection lines, dimension lines and leaders

Clause 8.2.3 sets out how the lines around a dimension are drawn:

• Projection lines (extension lines) are narrow continuous lines, projected from the feature so that the dimension can sit outside the outline. They start just clear of the outline and extend a little beyond the dimension line.
• Dimension lines are narrow continuous lines parallel to the direction of measurement, preferably outside the view. The first dimension line should be at least 3h from the outline, and each further one at least 2h from the last, where h is the character height. Arrowheads touch the projection lines.
• A centre line, an extension of a centre line or an outline must never be used as a dimension line.
• Leaders are narrow lines that show where a note or dimension applies. A leader ends in an arrowhead on a line or in a dot within an outline. It should not run parallel to nearby dimension or projection lines, and should meet the feature at no more than 45° from the normal. Long leaders should be avoided, even if that means dimensioning identical features separately or labelling them with a letter and a note.

With the 3.5 mm characters chosen in Chapter 3, the first row of dimensions on the housing sits at least 3 × 3.5 = 10.5 mm from the outline, and each further row at least 2 × 3.5 = 7 mm beyond the last. Three rows on one side of a view therefore need at least 10.5 + 7 + 7 = 24.5 mm of clear space. That is part of the reason the housing needed an A3 sheet.

Figure 2Space needed for rows of dimensions

• Formula space = 3h + 2h × (n − 1)
• h character height, mm
• n number of rows of dimension lines on one side
• Worked h = 3.5 mm, n = 3: 10.5 + 7 + 7 = 24.5 mm
• Worked h = 2.5 mm, n = 3: 7.5 + 5 + 5 = 17.5 mm

Chain, parallel and running dimensions

Where several features are positioned along one direction, there are three common arrangements, and the choice affects how tolerances add up.

• Chain dimensioning gives each feature from the previous one. It is compact, but tolerances accumulate along the chain, so the position of the last feature relative to the first varies by the sum of all the tolerances in between.
• Parallel dimensioning gives every feature from one common origin, with a separate dimension line for each. Each feature’s position varies only by its own tolerance relative to the origin. It uses more space.
• Running dimensioning also measures every feature from one origin, but along a single dimension line, with the values written near their arrowheads. AS 1100.101 clause 8.2.3.2 asks for a prominent dot at the common origin when this method is used, replaced by a datum indicator where the origin is a datum.

The choice should follow function. If two features must be accurately positioned relative to each other, dimension one from the other directly. If several features must each be accurately positioned relative to one face, dimension them all from that face. Chain dimensioning suits features whose positions relative to each other matter more than their positions relative to a distant face, such as a row of holes that must match a mating part’s row.

Aligned and unidirectional text

AS 1100.101 recognises two ways of orienting dimension text: aligned with the dimension line, readable from the bottom or right of the sheet, and unidirectional, always horizontal. Dimension lines may be broken for the number in the aligned method, and must be broken where necessary in the unidirectional method. Either can be used, but one method should be used consistently on a drawing. CAD templates usually fix the choice, and it should be the same across a business’s drawings so that readers are not switching between conventions.

Writing the numbers

Clause 8.2.4 is precise about numbers, because a misread number makes a wrong part:

• The decimal sign is a point.
• A value less than one has a zero before the point: 0.25, not .25.
• Groups of three digits on either side of the point are separated by a space, not a comma: 2 500, 2.498 5.
• Whole numbers are written without a decimal point or trailing zeros: 50, not 50.0.
• Dimensions are given to as many decimal places as the design requires, and no more.

Units are stated once, prominently: “DIMENSIONS IN MILLIMETRES”. Where more than one unit is used, the main unit is stated in a note and the others are written beside their values.

Two further conventions from clause 8.2.5 help the reader. A dimension that is deliberately not to scale, typically after a late change, is underlined with a wide continuous line, so nobody tries to check it against the geometry. And an auxiliary dimension, given for information only, is placed in parentheses, carries no tolerance and does not govern acceptance or rejection of the part.

Functional dimensioning

AS 1100.101 defines a functional dimension as one that directly affects the functioning of the product. The practical rule follows: dimension functional features directly, from the surfaces that locate them in use, so that their tolerance applies to the relationship that matters.

This is an illustration. The housing locates on the pump through its spigot face and spigot diameter. The bearing is pushed into the bore from the rear until it meets the shoulder, and the end cap clamps it there. The designer chose the functional origins accordingly:

• Axial dimensions for features that relate to the pump, such as the spigot length and flange thickness, are taken from the spigot face, because that face sits against the pump.
• The bore depth to the shoulder is taken from the rear face, because that is the face the end cap clamps against, and the clamping depends on the distance from the rear face to the shoulder.
• Diameters and hole positions are referred to the axis, which is established by the spigot and the bore together (Chapter 7 adds the geometric tolerance that makes this explicit).

Figure 3Axial dimensions on the housing, chosen by function

Dimensioning for the inspector

Every dimension is a measurement someone must make. A dimension to a point that cannot be reached, such as the intersection of two chamfers or the centre of a hole hidden behind a flange, forces the inspector to measure something else and calculate. That adds error and argument. Where possible, dimension to surfaces and edges that exist on the part and can be reached with a gauge or probe. Where an imaginary point is unavoidable, AS 1100.101 asks for the projection lines to pass through it and allows it to be emphasised with a dot, so that at least the definition is unambiguous.

The error: a chain and an overall, both toleranced

The housing’s first issue showed the spigot length 5, the flange thickness 12 and the body length 60, and also the overall length 77, all as ordinary dimensions covered by the general tolerance note. Under the medium grade the business used (Chapter 7 explains the grades), those dimensions carry permissible deviations of:

• 5 mm: ±0.1
• 12 mm: ±0.2
• 60 mm: ±0.3
• 77 mm: ±0.3

The chain of three segments can vary by up to 0.1 + 0.2 + 0.3 = ±0.6 mm in total, but the overall length is limited to ±0.3 mm. A part can have every segment within tolerance and still fail the overall dimension, or meet the overall dimension only if the segments are held tighter than their own tolerances allow. The drawing contradicts itself.

The machine shop’s programmer noticed and asked which dimension governed. That was the right question, and it exposed the error. The designer’s fix was to decide which relationship mattered. Here, the overall length matters only for packaging, while the spigot length, flange thickness and bearing seat depth matter for fit. The overall length became an auxiliary dimension, (77), in parentheses, untoleranced and not used for acceptance. AS 1100.101 clause 8.2.5.5 describes exactly this: where all the intermediate dimensions are given, the overall dimension should generally be auxiliary.

There was a second valid fix: keep the overall length as a real, toleranced dimension and remove the body length 60, leaving it to be the remainder. That would be the right choice if the overall length mattered more than the body length, for example because the housing had to fit between two fixed faces. The point is not which dimension is removed but that the designer decides, on the basis of function, rather than leaving the maker to choose.

The programmer found the conflict while preparing his in-process inspection sheet. Listing every dimension with its upper and lower limits, he noticed that the same length appeared twice: once as 77 ± 0.3 and once as the sum of three toleranced segments whose limits ran from 76.4 to 77.6. The nominal values agreed; the limits did not. Not every shop builds that list, and not every programmer would have stopped to ask. A designer who runs a simple stack, adding the tolerances along each chain and comparing them with any overall dimension, finds the same problem before release.

The general rule is that every dimension on a drawing is a requirement. Two requirements for the same distance are either identical, in which case one is redundant, or different, in which case one is wrong.

Dimensioning hole patterns and threads

The housing’s holes illustrate several conventions at once. The four flange holes are given as a count and size, “4 × Ø9”, with their pitch circle diameter, Ø115, dimensioned on the end view, and their angular positions shown by centre lines at 90°. The four tapped holes are given as “4 × M6”, with a thread depth and a drill depth using the depth symbol, on a Ø78 pitch circle, with their 45° offset from the flange holes dimensioned explicitly so that the relationship between the two patterns is not left to inference. The lubrication hole carries its angle in words as well as by dimension, for the reason Chapter 4 explained.

Repeated features may be shown once in full and the rest by centre lines only, under AS 1100.101 clause 9.3.1, which keeps the end views uncluttered. The count in front of the size, such as “4 ×”, is what tells the reader how many there are.

Exercise: fix the shaft dimensions

A drawing of a stepped shaft shows four diameters with lengths of 20, 35, 40 and 25 mm along the axis, all dimensioned in a chain from the left end, and also an overall length of 120 mm. The bearing seats are the 35 mm and 25 mm steps, and their axial positions control the bearing preload. What is wrong, and how should it be dimensioned?

Worked answer. The chain adds up to 20 + 35 + 40 + 25 = 120 mm, so the overall length duplicates it. With general tolerances on all five dimensions, the drawing over-defines the length, exactly as on the housing.

Choose the functional origin first. The bearing seats’ positions control preload, so the distance between the bearing shoulders is the critical dimension, and it should be given directly with its own tolerance rather than built up from several chained dimensions. A sound scheme is to dimension both bearing shoulders from one origin, usually one shoulder or one end face that locates the shaft, and to make the overall length auxiliary, (120), or to omit one non-functional segment instead. The result gives each functional distance once, directly, with a tolerance that applies to the relationship that matters.