ctrlPlanner GD&T reference ASME Y14.5-2018

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Is that feature control frame actually legal?

Every GD&T chart draws the twelve symbols. This one knows the rules that go with them: which controls need a datum, which cannot have one, and which two the 2018 standard removed.

Why a chart is not enough

A symbol chart tells you that the upside-down T means perpendicularity. It does not tell you that a perpendicularity with no datum reference is an incomplete callout, that a flatness with a datum is equally wrong the other way, or that a runout can never carry a maximum-material-condition modifier. Those are the rules that decide whether a drawing survives a review, and they are the ones that get left out.

The three that catch people

Orientation controls always need a datum. Perpendicularity, parallelism and angularity are all statements about a relationship, so a frame with one of those symbols and an empty datum compartment does not say anything yet.

Form controls never take one. Straightness, flatness, circularity and cylindricity are checked against the feature itself. A datum on them is not extra information, it is a contradiction.

Some controls are always RFS. Runout, circularity, cylindricity and both profiles apply regardless of feature size. A material-condition modifier means "at maximum or least material condition", which is a statement about a size that can vary, and these controls have no size for it to attach to.

True position is a diametral zone

The most consistently misread callout on a shop floor. A position tolerance of .010 on a hole does not mean the hole can be five thou out in X and five thou out in Y. It means the axis has to fall inside a cylinder .010 across, which at forty-five degrees is a good deal tighter than the coordinate reading suggests. Machinists who came up on plus-and-minus tolerancing check it the coordinate way and pass parts that are out.

Concentricity and symmetry are gone

Both were removed in the 2018 revision. They controlled derived median points, which are expensive to measure and were almost never what the design actually needed. If you are holding a drawing that cites Y14.5-2018 in the title block and carries one of them anyway, the practical answer is usually position for coaxiality of a feature of size, or profile of a surface where the requirement is really about the surface. The tool above will tell you the same thing if you try to build one.

Getting these onto a control plan

A frame that is legal still has to reach the floor, with a method that can actually verify it and a station that owns it. That is what ctrlPlanner does: balloon the print, key the characteristics with their frames intact, and the control plan and the AS9102 forms come out of data already entered.

Reading a feature control frame, and the Y14.5 rules that decide whether it is legal

What the compartments mean

A feature control frame reads left to right in three parts. The first compartment carries the geometric characteristic: the symbol that says what is being controlled. The second carries the tolerance zone, which may be preceded by a diameter symbol if the zone is cylindrical, and may be followed by a material-condition modifier. The third and any that follow carry the datum references, in order of precedence, each of which may carry a modifier of its own.

Everything after the first compartment is optional in the sense that the frame will still draw without it, and not optional at all in the sense that the standard requires it. A frame that draws is not a frame that says something.

Which controls need a datum, and which cannot have one

This is the rule most often broken, in both directions. Orientation controls -- perpendicularity, parallelism, angularity -- and location and runout controls are statements about a relationship, so they are meaningless without a datum reference frame to be related to. A perpendicularity with an empty datum compartment has not said anything yet.

Form controls run the other way. Straightness, flatness, circularity and cylindricity are checked against the feature itself, so a datum on one of them is not extra rigor, it is a contradiction. Profile of a line and profile of a surface sit in between: they take datums when the profile is located and stand alone when it is not, which is why a profile callout is the one worth reading twice.

Which controls can carry a material-condition modifier

A material-condition modifier means at maximum material condition or at least material condition, which is a statement about a size that is allowed to vary. Only a feature of size has such a size. Position and the orientation controls apply to features of size and can carry one; runout, circularity, cylindricity and both profiles apply regardless of feature size and cannot.

This is a rule people break by analogy, having seen a position tolerance at MMC and reasonably assumed the modifier is available generally. A runout with a modifier on it looks entirely ordinary on a drawing and is non-conformant.

True position is a diametral zone, not a square one

The most consistently misread callout in production. A position tolerance of .010 on a hole does not permit five thousandths of error in X and another five in Y. It says the axis has to fall inside a cylindrical zone .010 across, centered on true position. The corner of the square a coordinate reading describes sits about forty percent further out than the cylinder allows, which means checking position by reading X and Y deviation passes parts that are out of tolerance.

The diameter symbol in the second compartment is what makes the zone cylindrical, and it is not decorative. A position tolerance without it describes a different requirement.

Concentricity and symmetry no longer exist

ASME Y14.5-2018 removed both as geometric characteristics. They controlled derived median points -- the middle of the material rather than the surfaces -- which is expensive to measure, hard to interpret consistently, and in most cases not what the design needed. Drawings that called concentricity usually meant runout or position, and drawings that called symmetry usually meant position or profile.

This matters because drawings outlive standards. A print released in 2012 and revised in 2024 can carry a 2018-dated title block over a 2009-era callout, and the shop is left holding a symbol the cited standard does not define. The practical answer is position where the requirement is coaxiality of a feature of size, and profile of a surface where it is really about the surface. Ask the customer to confirm rather than deciding for them, and put the answer in writing.

Reading the flag rather than the frame

Two things sit outside the frame and change what it means. A leader pointing at a surface controls that surface; a leader pointing at a dimension or at a feature of size controls the feature. And an all-around or all-over symbol on the leader extends the control past the profile it appears to touch. A frame read correctly and applied to the wrong feature is exactly as wrong as an illegal frame, and much harder to spot.

Getting these onto a control plan

A legal frame still has to reach the floor. Somebody has to decide which operation owns the characteristic, what method can actually verify it, and how often it gets checked. That is what ctrlPlanner does: balloon the print, key the characteristics with their frames intact, and the control plan and the AS9102 forms come out of data already entered rather than being transcribed a second time.

Common questions

Does perpendicularity need a datum?

Yes. Perpendicularity, parallelism and angularity are all relationships, so a frame carrying one of them with no datum reference is incomplete. Straightness, flatness, circularity and cylindricity are the opposite: they take no datum at all.

Can I put an MMC modifier on a runout?

No. Runout applies regardless of feature size, as do circularity, cylindricity and both profile controls. A material-condition modifier only has meaning on a feature of size, which those controls do not have.

What replaced concentricity in Y14.5-2018?

Position, where the requirement is coaxiality of a feature of size, or profile of a surface where the requirement is really about the surface. The same answer covers symmetry.

Is a position tolerance a square zone or a round one?

Round, when the diameter symbol precedes the value, which is the normal case for a hole. Reading it as a square by checking X and Y deviation separately is the single most common way an out-of-tolerance hole gets passed.

What does it mean when there is no modifier at all?

Regardless of feature size. Y14.5-2018 treats RFS as the default, so there is no symbol for it and no need to state it.

Does this page store anything I type?

No. The frame builder runs in your browser, nothing is stored, and closing the tab is the end of it. There is a separate optional endpoint for automation, at /api/mcp, which takes a symbol and a tolerance from a closed list of values and has no free-text field at all.

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