48610 Chap.2 Dimensioning and Tolerancing to AS 1100
Dimensioning and Tolerancing to AS 1100
Views say what the shape is; dimensions say how big it is and where its features sit. Week 2 separates those two jobs, because the commonest failure in the drawing assignment is giving one of them twice and the other not at all.
The rules are specific and all of them are checkable: dimension each feature once and only once, never on two views; note the number of occurrences for repeated identical features rather than repeating the dimension; keep dimension lines outside the outline and stop them crossing; dimension on the view that shows true length; avoid dimensioning to hidden lines; put small dimensions nearest the object and overall dimensions furthest out; leave at least 6 mm between parallel dimensions and no less than 10 mm from the outline.
Tolerance follows from a premise the lecture states flatly: perfection is not available, no stated size can be hit exactly, and two parts off the same machine will differ.
A tolerance says how much variation the design can absorb, which makes it a design decision rather than a manufacturing one. It can be written symmetrically, as a nominal size with an allowance either way, or as two limits.
The manufacturing guide adds the consequence that matters most on an assembly: chain dimensioning accumulates tolerance from feature to feature, while dimensioning everything from a common reference does not, and a drawing carrying dimensions but no tolerances is incomplete because no measurement can then accept or reject the part.
What this chapter covers
- 01
Size dimensions against location dimensions
- 02
Dimension each feature once, and note repeated features rather than repeating them
- 03
Dimension lines outside the outline, not crossing, aligned and grouped
- 04
The view that shows true length, and avoiding hidden lines
- 05
Spacing: 6 mm between parallel dimensions, 10 mm clear of the outline
- 06
Extension line gap of about 1.5 mm and overrun of about 3 mm
- 07
Over dimensioning as conflict, under dimensioning as ambiguity
- 08
Three schemes for a row of identical holes
- 09
Diameter for a full circle, radius for an arc
- 10
Symmetric and limit tolerance forms, and the general tolerance note
- 11
Tolerance stacking from chain dimensioning, and the datum fix
Where the third hole really ends up
- +1Write what the chain permits. The position of hole three measured from the left face is the sum of three dimensioned distances, so its worst case is 25 plus or minus 0.1, three times over, giving 75 plus or minus 0.3 mm. The extremes add because nothing on the drawing ties hole three back to the left face directly.
- +1Compare with what the mating part needs. The plate allows plus or minus 0.15 mm and the drawing permits plus or minus 0.3 mm, so a fully conforming bar can still fail to assemble. The drawing is wrong even though no individual tolerance on it is unreasonable.
- +1Re-dimension from a common reference. Give all three holes from the left face at 25, 50 and 75 mm, each plus or minus 0.1 mm. Hole three is now specified directly, so its position is 75 plus or minus 0.1 mm, comfortably inside the plate's requirement.
- +1Notice what was given up. Datum dimensioning moves tolerance rather than removing it: the gap between holes one and two is now the difference of two dimensions and is 25 plus or minus 0.2 mm, looser than before. If neighbour spacing were the functional requirement, the chain would have been correct and the overall position the thing to relax.
Key terms
- Size dimension
- A dimension stating how large a feature is, such as the diameter of a hole or the width of a slot. Every feature needs one, given once and on the view that shows the feature at true length.
- Location dimension
- A dimension stating where a feature sits relative to something else, such as 25 mm from the left face. A feature with a size but no location cannot be made in the right place.
- Redundant dimension
- A dimension that repeats information already given, such as three step lengths plus the overall length. Each carries its own tolerance, so the requirements conflict and the maker cannot tell which governs.
- Datum dimensioning
- Dimensioning every feature from one common reference face rather than from its neighbour, so that each feature carries only its own tolerance and no accumulation occurs along the part.
- General tolerance
- A tolerance stated in the notes that applies to any dimension without an explicitly defined tolerance of its own. Without it, every unmarked dimension on the drawing is uninspectable.
- Limit dimension
- A tolerance written as the two extreme acceptable sizes rather than as a nominal size with an allowance, for example 24.9 and 25.1 instead of 25.0 plus or minus 0.1.
- Extension line
- A thin line projecting from the object to carry a dimension clear of the outline. It starts about 1.5 mm off the edge and runs about 3 mm past the dimension line.
Dimensioning and Tolerancing to AS 1100 FAQ
How do I know whether I have dimensioned too much or too little?
Run two passes. For completeness, point at every feature in turn and ask whether both its size and its location are stated somewhere on the drawing. For redundancy, add up each chain of dimensions along an edge and compare the total with any overall dimension on that edge; if both are given, one is redundant and should go.
Over dimensioning creates a conflict because each dimension carries its own tolerance, and under dimensioning forces the maker to scale off the drawing, which is exactly what dimensions exist to prevent.
Why is a diameter preferred to a radius for a hole?
Because you can measure it. A calliper can be placed across a hole to read its diameter directly, whereas the centre of an arc on a real part is a construction that no instrument can sit on. The convention follows the inspection method: full circles take a diameter with the diameter symbol, and arcs less than a full circle take a radius.
Is tolerance stacking really a problem at the scale of a student prototype?
Yes, and it shows up as parts that will not go together. Five 50 mm steps marked out one from the next, each half a millimetre out in the same direction, put the last feature two and a half millimetres from where the drawing says. That is enough to miss a bolt hole. The fix costs nothing at the drawing stage: choose one face as the datum and dimension every feature from it.
How tight should I make a tolerance?
As loose as the function allows. The published guidance is to avoid specifying tolerances tighter than absolutely necessary for the part to work properly, because every halving of a tolerance pushes the job toward a slower process, a better machine and a more careful inspection, and all three cost money.
Ask what the feature has to do, such as clear a bolt or locate a bearing, and tolerance it for that rather than for tidiness.
Assessment move
Work on other people's drawings before your own. Take any dimensioned drawing, including the pictorial view supplied with the assignment, and audit it against the four passes: completeness, redundancy, placement on the true length view, and appearance.
The assignment sheet explicitly warns that the pictorial it gives you may contain redundant dimensions or ones that do not follow the standard, so treating it as a source of sizes rather than a model of practice is the point. Then dimension one object of your own and hand it to someone else with the instruction to build it from the sheet alone; the questions they ask are your missing dimensions.
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