OpenSpecEngineering

The symbols used in feature control frames on a drawing, from ASME Y14.5-2009 [1]. For each of the 14 geometric characteristics the page says what type of tolerance it is, whether it takes a datum, whether it can carry a material condition modifier, and whether that earns bonus tolerance. Designers use it to write a callout, and inspectors and machinists use it to read one.

What the page covers

• Geometric Characteristic Symbols, Y14.5-2009 Fig. 3-1: form, profile, orientation, location and runout tolerances.
• Modifying Symbols, Fig. 3-11: material conditions, projected zone, free state, diameter and radius, all around and the rest.

Reading the tables

Datum reference is Never for a form tolerance, Optional for profile, and Required for orientation and runout. Position usually takes datums. Ⓜ is maximum material condition (MMC) and Ⓛ least material condition (LMC). With no modifier, Rule #2 applies: the tolerance holds regardless of feature size (RFS) [1, para. 2.8]. “No (RFS only)” means the characteristic can never take Ⓜ or Ⓛ. Bonus tolerance is the extra tolerance a feature of size gains as it departs from its MMC or LMC. Y14.5 para. is the paragraph that defines the control. Unicode is the code point to type the symbol in CAD or a document; “none” marks a symbol Unicode lacks, which the page draws instead.

Limits and common mistakes

• Ⓜ or Ⓛ applies only to a feature of size, such as a hole, pin or slot. Flatness of a plain face is always RFS.
• Circularity, cylindricity, profile, concentricity, symmetry and runout cannot take a modifier.
• Y14.5-2018 dropped concentricity and symmetry. Use the edition the drawing names.
• The page lists symbols. It does not cover datum feature symbols, frame layout or tolerance values.

Worked example

A steel bracket bolts to a frame with four M6 screws through clearance holes of ⌀6.6–6.8 mm. The holes must be located to datums A, B and C, and the tolerance should grow as the holes come out larger.

1. In Geometric Characteristic Symbols, read the Position row: ⌖ Position · Location · Usually (may be unrelated)
   Ⓜ or Ⓛ on a feature of size · Yes, with Ⓜ or Ⓛ · 7.2 · U+2316
2. In Modifying Symbols, read the Ⓜ row (Maximum material condition, 3.3.5, U+24C2) and the ⌀ row (Diameter, 3.3.7, U+2300).
3. Write the frame for a ⌀0.4 mm cylindrical zone at MMC: ⌖ | ⌀0.4 Ⓜ | A | B | C

Check the fit. At MMC (⌀6.6) the hole gets ⌀0.4; at LMC (⌀6.8) it gets 0.4 + 0.2 bonus = ⌀0.6. The hole’s virtual condition is 6.6 − 0.4 = ⌀6.2 mm, which still clears a 6.0 mm screw.