Reading a feature control frame
Answer questions about the frame. Get 10 right to finish. ASME practice is in inches, ISO in millimetres.
Which is the secondary datum?
How to read it
- Characteristic: the first box says what is controlled. Here, position.
- Tolerance: Ø0.2 means a cylindrical zone 0.2 across. No Ø would mean two parallel planes 0.2 apart.
- Modifier: the circled M means maximum material condition. A circled L means least material condition. None means the tolerance applies at any size.
- Datums: A is primary, B secondary, C tertiary. The part is set up on A first, then B, then C.
Read as a sentence: the axis of this hole must lie in a Ø0.2 cylinder at MMC, located from datums A, B and C.
Common mistakes
- Reading the value as a radius. With Ø it is the full diameter of the zone.
- Going by the letters instead of the order. B can be primary if it comes first.
Questions
- How do you read a feature control frame?
- Left to right: the characteristic symbol, then the tolerance compartment (diameter symbol, value, modifier), then the datums in order: primary, secondary, tertiary.
- What does the diameter symbol in a feature control frame mean?
- The tolerance zone is a cylinder and the value is its diameter. Without it, the zone is the space between two parallel planes and the value is the width.
- Does the order of the datums matter?
- Yes. The first datum is primary and sets up the part first, then the secondary, then the tertiary. A|B|C and B|A|C are different requirements.
Explained in our own words and checked against several GD&T references. For the full rules, see ASME Y14.5-2018 and ISO 1101:2017.