Standard solution: specification of a geometric tolerance
A fixed way of thinking that makes GD&T unambiguous and functional
Geometric tolerances are closely related to how the part is inspected. Roundness and cylindricity are often measured on a roundness tester or coordinate measuring machine (CMM) while runout can be easily checked with a dial indicator while the part is rotated between centers. Because the tolerances refer to datums the measurement setup must mirror the datum sequence the drawing specifies. Understand the symbols and you know both what the part should be able to do and how to prove it can.
§1. Determine the function
- 01What must the part be able to do — fit, center, rotate, seal, align?
- 02Which property is critical: form, orientation, location or runout?
- 03What happens if this particular property deviates — how bad does it go?
§2. Choose dates by function
- 01Clamp the part up 'in your mind' as it sits in reality.
- 02Let the load-bearing or centering surface be the primary datum.
- 03Add secondary and tertiary datum if the part is otherwise not uniquely locked.
§3. Choose property and tolerance zone
- 01Choose the right symbol for the property (flatness, position, runout, etc.).
- 02Specify the tolerance zone – width or diameter zone – according to the function's requirements.
- 03Only link a material condition if you understand its full meaning.
§4. Check unambiguity and measurability
- 01Can the specification be interpreted in only one way?
- 02Can it be checked with realistic measuring equipment based on the selected dates?
- 03Is the tolerance as loose as possible and only as tight as necessary?