CAD foundations · Lesson 9 · Approx. 7 minutes including practice
Tolerances and fits
A tolerance defines permitted variation around a requirement. It affects manufacturing, inspection and assembly. Drafters communicate approved tolerances; they should not choose a tighter fit to make a drawing look complete.
Step 1: Function source
A documentation workflow, not an equipment operating sequence. Animation starts only when you choose Play.
Communicate permitted variation
Tolerances describe acceptable variation and must reflect functional requirements and the chosen manufacturing and inspection approach. A drafter’s role is to represent approved requirements clearly. A tighter tolerance is not automatically a better instruction: it may be unnecessary, difficult to measure or inconsistent with the rest of the assembly. Identify the applicable general tolerances and any feature-specific requirement. Resolve contradictions between notes, model annotations and detail dimensions before issue.
Prepare and check the work
For each important feature, check where the tolerance originated, which feature it controls and how the reader identifies it. Maintain the approved notation and avoid converting between systems without competent review. In a tolerance discussion, distinguish a dimensional requirement from a measurement result and from an assembly observation. Do not infer fit from the fact that one sample happened to assemble. Record queries about stack-up, mating components or inspection feasibility for the design authority.
Worked example · demonstration only
A hole note from an older drawing conflicts with the fit specified for the current shaft. Preserve the evidence, identify the configuration and ask the responsible engineer to resolve the requirement. The documentation team can then update the relevant drawings and BOM references as one controlled change.
Guided practice and review evidence
Use only a demonstration dataset for this exercise: List dimensions requiring engineering clarification on a sample component. Before starting, write down the question the finished record should answer and identify the source information you would need on a real job. Keep a separate list of missing inputs. Prepare a first draft, then compare it with the topic-specific checks above. Ask another person to follow your references without explaining them verbally. Record where they hesitate, what information they cannot locate and any different interpretation. Keep a brief review log with four columns: observation, proposed action, responsible reviewer and closure evidence. Revise the example and repeat the information-finding check. Your evidence of completion is the revised demonstration record plus that log, not simply time spent reading. Do not upload client information to the academy or practise on operating equipment.
What good completion looks like
A useful result can be understood by someone who did not prepare it. They can identify its purpose, applicable item, source and review status, and they can distinguish confirmed information from unresolved questions. Pay particular attention to this failure mode: Never infer an interference fit solely because two model surfaces touch. Check the output in the format the recipient will use. A native file, spreadsheet and exported PDF can expose different problems. Keep the source revision and the issued output linked, and record any limitations of the handover. On a real project, follow the customer's authorised checking and approval arrangements before the information is used. The knowledge check below tests understanding of this lesson; it does not assess practical competence or authorise regulated work.
Practical workflow
- Establish the source: identify the approved input, document revision and person responsible for its technical content. Record missing information as an open question.
- Prepare a small representative example before applying the approach to a whole project. Check identifiers, units, references and the intended reader’s needs.
- Ask the nominated reviewer to check technical meaning. Capture the response, resolve comments and issue only under the agreed document-control process.
Review checklist
- Can another reader identify the source and applicable revision?
- Are assumptions and unresolved decisions clearly recorded?
- Is there evidence of the required review before issue?
Educational material and demonstration examples. Not accredited training, practical competency assessment or authority to perform regulated work. Use current approved sources and your responsible technical authority on real projects.