GD&T and machining tolerances, in plain words

What GD&T is

A size on a drawing says how big a feature can be. GD&T adds limits on shape, tilt and position, so parts fit and work together. It stands for geometric dimensioning and tolerancing.

Two families of standards use it:

• ASME Y14.5: the current edition is Y14.5-2018
• ISO GPS: ISO 1101 covers geometric tolerances

The marks look alike, but some rules differ. Name the standard and its edition on the drawing.

The symbols, group by group

Form: shape only, no datum

• Straightness (a short straight line): how much a line along the surface, or a centre line, can bend
• Flatness (a slanted parallelogram): how far a face can rise or dip from a perfect plane
• Circularity (a circle): how out of round each cross section can be, one at a time
• Cylindricity (a circle between two slanted lines): roundness, straightness and taper of the whole cylinder at once

Orientation: tilt against a datum

• Parallelism (two slanted lines): how far a face or axis can tilt from parallel to the datum
• Perpendicularity (an upside down T): how far it can tilt from square to the datum
• Angularity (an angle mark): how far it can tilt from the angle shown on the drawing

Location: where a feature sits

• Position (a circle with a cross): how far the centre of a hole, axis or slot can sit from its exact spot

Profile: any shape

• Profile of a line (an open arc): how far an outline can stray from its true shape, one section at a time
• Profile of a surface (an arc closed at the bottom): the same for a whole surface. With datums, it also sets where it sits.

Runout: parts that turn

• Circular runout (one slanted arrow): wobble at each section as the part turns on its datum axis
• Total runout (two slanted arrows): wobble across the whole surface at once

Retired in 2018

ASME Y14.5 dropped two symbols in 2018. Concentricity is two circles, one inside the other. Symmetry is three short lines, the middle one longest. Older drawings still use them. Ask the shop how it will check them.

How to read a feature control frame

The feature control frame is the row of boxes next to a feature. Read it left to right:

1. The symbol: which control applies
2. The tolerance: how much variation is allowed, with any modifier
3. The datums: the references it's measured from, most important first

A datum is a perfect reference: a plane, a line or a point. The datum feature is the real face or hole it comes from.

MMC (maximum material condition) is the size with the most metal: the smallest hole or the biggest pin. With an M in a circle in the frame, a hole that comes out bigger gets extra position tolerance. That extra is called bonus tolerance.

Tolerances when the drawing has none

ISO 2768 sets limits by size. Two rows from the Hubs table:

A default is not a promise of perfect parts. Put a tolerance on every size that matters, and ask which default applies to your quote.

What tight tolerances cost

Protolabs warns that precision you don't need adds steps, cost and lead time. Tighter limits can mean grinding, slower cuts and more measuring.

Canadian shops advertise much tighter work:

• Aaron Machine Shop, Calgary: up to ±0.0005 inch
• G&M Automation, Brampton: up to ±0.0005 inch
• EM Dynamics, Toronto: as tight as 0.0005 inch

That's what a shop can hold on some features, not a promise for every size. Put tight limits only where parts mate, seal or spin.

How shops check it

• CMM: a coordinate measuring machine probes the part and compares it with the drawing. Xometry offers CMM inspection with a dimensional report.
• First article inspection: a full report on the first part made. Aerospace work uses AS9102.
• Ask before you order: which report you'll get. A dimensional report and a first article report are not the same thing.

When you need GD&T

• You don't for a simple bracket or plate where only the sizes matter
• You do when holes must line up with a mating part, a face must seal, or a shaft must spin true
• Not sure? Send the drawing and say what the part does. The shop can tell you where limits matter.

More help: what to send a shop, STEP and DXF files and what CNC machining costs. Ready for a price? Send one request to machine shops near you.

Sources

• ASME Y14.5
• ISO 1101
• Redlux GD&T reference, with pages on flatness, symmetry, profile of a surface and total runout
• KEYENCE: orientation tolerance, the feature control frame and datums
• Fictiv: GD&T basics, CNC specs and standard tolerances
• Xometry: CNC part tolerances and its quality lab
• Protolabs: precision machining tolerances and fine tuning tolerances
• Protolabs Network (Hubs): ISO based tolerances
• Aaron Machine Shop, Calgary
• G&M Automation, Brampton
• EM Dynamics CNC machining, Toronto
• SAE AS9102C