GD&T feature control frame builder

What a feature control frame says

A feature control frame is read from left to right. The first cell holds the characteristic: what is being controlled, such as flatness or position. The second holds the tolerance: the full width of the zone, not a ± value, with Ø in front when the zone is a cylinder and a circled M or L behind when the tolerance depends on the feature's size. The remaining cells name the datums in the order the part is set up.

Two rows

A composite frame has one symbol across two rows. The upper row locates a whole pattern on the part; the lower row is a tighter control inside the pattern, and its datums control orientation only. Two single frames stacked each have their own symbol, and each is a complete requirement from its own datums. The two look alike and mean different things, so the page reads them differently.

Other modifiers

• Circled P, projected zone: the zone stands outside the part, at the height given, where a screw or pin sticks out.
• Circled F, free state: checked with the part unrestrained.
• Circled T, tangent plane: the requirement applies to the plane touching the high points of the surface.
• Unequal profile: the profile zone is not centred on the true profile. ASME Y14.5 writes a circled U and the amount of the zone outside the material; ISO 1101 writes UZ and a signed offset of the middle of the zone, and also allows UZ with position of a flat surface.
• SØ: a spherical zone, for the centre point of a sphere.
• CZ, combined zone (ISO 1101, chosen beside the tolerance value): one requirement for several features, with their zones locked to each other by the TEDs, such as coplanar faces under one flatness or a hole pattern under one position. Show the number of features with n× (the count in the size) or with several leader lines, not both. SZ, separate zones, is the default and usually only makes it explicit. For position on several features that the datums do not lock fully, ISO 5458:2018 rule A requires CZ or SZ to be written, and SZ on a position with no datum is meaningless. Drawings to ISO 5458:1998 read a frame without CZ as a pattern.

Size and virtual condition

When the frame controls a feature of size, you can enter its size: a ± value, an upper and a lower deviation, or an ISO 286 tolerance class such as H7 (capitals for a hole) or g6 (small letters for a shaft). The size is drawn above the frame, with the count in front for a pattern, such as 4× Ø10. The page then works out the limits of size, the maximum and least material size, and the tolerance at each.

It also gives the inner and outer boundaries: the perfect forms the surface never crosses when the size and the frame are both met. With a circled M the one on the material side is the virtual condition (MMVS in ISO 2692): MMS − t for a hole, MMS + t for a shaft, the size of the pin or hole on a functional gauge. With a circled L it is LMS + t for a hole, LMS − t for a shaft (LMVS), which guarantees the least wall. ASME Y14.5 calls the other boundary the resultant condition. Without M or L the boundaries are only the worst case; in ISO the size and the frame are then checked separately (ISO 8015). A table shows the bonus at five sizes from MMS to LMS. Datum shift is not included: it depends on the size of the datum feature.

Datum targets

A datum target symbol is a circle cut in half. The lower half names the target, such as A1; the upper half gives the size of the contact area, and stays empty for a point or a line. Targets are used when a datum should come from chosen places on a surface, as on castings and forgings, not from the whole surface.

What this page checks

• Form tolerances take no datum; orientation, runout, concentricity and symmetry need one.
• Ø only when an axis or a point is controlled, SØ only for the position of a centre point.
• A circled M or L only for a feature of size, and never with circularity, cylindricity, profile or runout.
• Datum letters: none skipped, none repeated, I, O and Q not used (ASME), and I, O, Q and X avoided (ISO 5459). Up to two letters in ASME (AA to AZ), up to three repeated letters in ISO (BB, CCC).
• Composite frames: position or profile only, a smaller value in the lower row, and lower datums that repeat the upper ones in order.
• Each extra modifier with the characteristics it belongs to.
• Differences between the standards: ASME Y14.5 does not allow a circled M or L with concentricity and symmetry, and removed both symbols in its 2018 edition; ISO 1101 keeps them. ISO 1101 also allows position on a flat surface, symmetry of an axis, a point or a flat surface, and concentricity of a point in a cross-section (it calls the axis case coaxiality), none of which ASME Y14.5 allows. In ISO 1101 the circled T needs a datum, and the modifiers follow the order CZ or SZ, UZ, T, P, M or L, F. ASME Y14.5 has no CZ or SZ. The ISO rules were checked against ISO 1101:2017.

The checks cover what can be judged from the callout alone. They cannot see the drawing, so they cannot tell whether a datum really is a feature of size, whether the basic dimensions are there, or whether the tolerance suits the function of the part. Where the two standards differ and the rule is not certain, the page asks you to check instead of calling it an error.

What it does not cover

Frames with three rows or more, a different material condition on each letter of a common datum, movable datum targets, the datum feature symbol, and the newer ISO modifiers such as UF, OZ and SIM.

Using the image

"Copy as an image" puts a PNG on the clipboard, ready to paste into a document, a spreadsheet or a chat. Browsers that do not allow that download the file instead. The SVG stays sharp at any size and suits documents that will be printed. The text form, such as Position | Ø0.1 (M) | A | B, is for places that only take text. The form with symbols gives the same line with the Unicode GD&T symbols; it reads like the real frame where the symbols display, mostly in documents on a computer, and breaks where they do not.