CNC Knowledge

Geometric Tolerances: A Complete Collection of the 14 Symbols

A working reference for engineers who need to call out form, orientation, location and runout on machined parts. This geometric tolerances complete collection covers what each symbol controls, which ones a mill or lathe can hold, and when a tighter callout buys nothing. Read it before you release a drawing.

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14 symbols±0.005 mmDatum strategy100% inspection

Measuring the geometric angle of a turned part, part of this geometric tolerances complete collection

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In this geometric tolerances complete collection

1
Why a size tolerance leaves the part undefined
2
The complete collection by category
3
Datum reference frames decide whether the callout means anything
4
What the machine can hold in different materials
5
How each control is verified on the shop floor
6
Which control to use, and when to skip it

Fundamentals

Why a size tolerance leaves the part undefined

A size tolerance controls how far a dimension may drift. It says nothing about the shape of the surface that sits between those limits. A shaft can measure 19.98 mm at every cross-section and still be bent, lobed or tapered. Push it into a bearing and the fit changes even though every measurement passed. Geometric tolerances exist to close that gap.

The split is simple. Dimensional tolerance handles size. Geometric tolerance handles form, orientation, location and runout. A drawing that only carries plus and minus limits tells the shop what to hit but not what the surface must look like once it gets there.

Two parts made to the same size limits can behave differently. One assembles cleanly. The other binds, leaks or vibrates. The difference is usually straightness, perpendicularity or position, not diameter. That is why a datum reference frame belongs on any drawing where parts mate.

1
Size vs. geometryLimits of size bound the dimension; geometric controls bound the surface.
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Rule #1At maximum material condition, the envelope of perfect form must not be violated.
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Bonus toleranceWith MMC modifiers, departure from MMC adds usable geometric tolerance.

The 14 symbols

The complete collection by category

Fourteen symbols sit in the standard, grouped into five families. Form covers straightness, flatness, circularity and cylindricity. Orientation covers perpendicularity, parallelism and angularity. Location covers position, concentricity and symmetry. Profile covers profile of a line and profile of a surface. Runout covers circular runout and total runout.

Form controls stand alone. They need no datum because they describe the surface against itself. Everything else needs at least one datum. That single fact explains most drawing errors: a perpendicularity callout with no datum letter is not a valid callout.

Profile is the most flexible tool in the set. Profile of a surface can control size, form, orientation and location in one frame, which is why it shows up on castings, complex brackets and free-form housings. It is also the hardest to inspect without a CMM.

1
FormStraightness, flatness, circularity, cylindricity. No datum required.
2
OrientationPerpendicularity, parallelism, angularity. Datum axis or plane required.
3
LocationPosition, concentricity, symmetry. Usually tied to a feature of size.
4
Profile and runoutLine, surface, circular runout, total runout.

Datums

Datum reference frames decide whether the callout means anything

A datum is a theoretically exact plane, axis or point derived from real features on the part. The order matters. Primary datum removes three degrees of freedom, secondary removes two, tertiary removes one. Swap the order and you measure a different part.

Choose datums that match how the part sits in the assembly. If a housing bolts flat against a machined face, that face is your primary datum, not the largest outside diameter. Inspectors can only reproduce what the drawing states.

On a 5-axis part, datum targets on a casting allow the same setup to be used for machining and inspection. We often mark three target points on the raw surface and machine everything from them. That keeps the tolerance stack honest from blank to finished part.

1
Order is not cosmeticPrimary, secondary, tertiary each remove specific degrees of freedom.
2
Match the assemblyUse the faces and bores that actually locate the part in service.
3
Casting targetsDatum targets let one setup serve both machining and inspection.

Materials and process

What the machine can hold in different materials

Aluminium 6061 and 7075 hold tight geometric callouts well. Thin walls move after clamping, so flatness on a 1.5 mm web is a different problem from flatness on a 12 mm plate. Stress relief before finish cuts helps more than a tighter tolerance does.

Stainless 304 and 17-4PH work-harden. Circularity and cylindricity on a long bore drift when the tool rubs instead of cuts. Lighter radial engagement and a sharp edge keep the bore round. Titanium TC4 (Ti-6Al-4V) and Inconel move more under heat, so total runout on a long shaft often needs a warm-up pass and a final spring pass.

Plastics are the hardest case. POM and PEEK creep after machining, so a cylindricity callout that passes at the bench can fail a week later. If geometry must hold on plastic, specify a stress-relief soak and inspect after it, or move the part to aluminium.

1
AluminiumStable and predictable. Watch thin walls and clamping distortion.
2
Stainless and titaniumHeat and work hardening drive roundness and runout errors.
3
PlasticsCreep changes geometry after inspection. Plan a relaxation step.

Inspection

How each control is verified on the shop floor

Flatness and straightness get checked on a surface plate with an indicator or on a CMM. Circularity and cylindricity need a roundness tester or a rotary table with a probe. A micrometer cannot measure either, no matter how many points you take.

Position is verified against the datum frame with a CMM or a functional gage. Functional gaging is faster and matches assembly reality, so we use it on production runs where the same hole pattern repeats. Total runout needs the part supported on its own datum axis, not on centers chosen for convenience.

We inspect 100% of parts before shipment, with raw material checks, in-process monitoring and final inspection. Reports are available on request. On a ±0.005 mm callout, the measurement uncertainty of the gage has to be a fraction of the tolerance, or the number on the report is noise.

1
FormSurface plate, indicator, roundness tester, CMM.
2
PositionCMM against the datum frame, or a functional gage.
3
RunoutPart mounted on its own datum axis, dial indicator or probe.

Selection guide

Which control to use, and when to skip it

Pick the loosest control that still guarantees function.

Control	Use when	Typical value	Skip when
Flatness	Sealing face or bolted joint	0.02–0.05 mm	Surface is cosmetic only
Parallelism	Two faces must stay aligned	0.03 mm	Only one face is functional
Perpendicularity	Bore meets a mounting face	0.02 mm	Parts are pinned by dowels anyway
Position	Hole patterns that must assemble	Ø0.1 mm at MMC	Single hole, loose clearance
Circularity	Bearing seat or rotating fit	0.01 mm	Static fit with light load
Cylindricity	Long bore with a moving piston	0.015 mm	Short bore, low travel
Total runout	Balanced rotating assembly	0.02 mm	Part never spins
Profile of a surface	Free-form housing or casting	0.2 mm	Simple prismatic part

The callout that costs the most is the one nobody can inspect

If a feature must assemble, use position at MMC and let bonus tolerance work for you. If a surface must seal or slide, use flatness or cylindricity and accept a tighter number. If the part never rotates, drop runout entirely and spend the money on position instead.

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FAQs

Questions engineers ask about geometric tolerances

What is the difference between circularity and cylindricity?

Circularity is a 2D check. It looks at one cross-section and asks how close that circle is to a perfect circle.

Cylindricity is 3D. It checks every cross-section along the length at once, plus straightness of the axis. A bore can pass circularity everywhere and still fail cylindricity if it tapers or bows.

When should I use MMC instead of RFS?

Use maximum material condition when the feature's main job is assembly. As the hole grows away from its smallest size, the allowed position error grows with it. That is bonus tolerance.

Use regardless of feature size when the fit depends on the axis location itself, such as a rotating shaft in a bearing. There is no bonus to give.

Can a 3-axis mill hold a 0.02 mm perpendicularity callout?

Yes, if the datum face and the machined face are cut in the same setup. Once the part is re-fixtured, the error stack includes the fixture, not just the machine.

On parts where two sides must relate, a 4-axis or 5-axis setup removes one re-clamp and usually improves the result more than tightening the tolerance would.

Why does my plastic part fail cylindricity days after inspection?

POM, PA and PEEK relieve internal stress slowly after machining. A bore that measured round on Tuesday can be oval by Friday as the wall relaxes.

Either anneal the blank before finishing, or inspect after a defined relaxation period. If the geometry is critical and the part sees load, switch the material.

How tight can GreatLight hold geometric callouts?

Our standard machining tolerance is ±0.005 mm (±0.0002 in), with surface finish down to Ra 0.2–0.8 μm when the drawing calls for it.

We run 127 high-precision CNC machines, including 16 simultaneous 5-axis centers and 16 mill-turn centers, up to 4,000 mm in maximum processing size. Whether a specific callout is realistic depends on the feature, the material and the datum scheme, so send the drawing and we will say so.

Do I need a datum on every geometric callout?

No. The four form controls work without a datum because they compare a surface to itself.

Every other symbol in the set needs at least one datum, and the order of datums changes the measurement. A location callout without a datum frame is not a valid requirement.