Basics of Geometric Dimensioning and Tolerances (GD&T)¶

When ±0.005" isn't enough information and the print looks like hieroglyphics.

Why GD&T Exists¶

Traditional tolerances says "this hole is 0.500" ±0.005"". But that doesn't tell you:

Can the hole be tilted?
What if it's ovate?
How far off center can it be?
Does it matter if it's perpendicular?

GD&T answers these questions. It's a pain in the ass, but it prevents $50,000 scrap parts.

The Symbols You'll Actually See¶

Position ⊕¶

The most common and most important.

⊕ 0.010 M A B C

Means: This feature can be off position by 0.010" max relative to datums A, B, and C.

What it really means:

The center of your hole can be anywhere inside a 0.010" diameter circle
That circle is centered on the perfect location
Way more forgiving than ± tolerances

Perpendicularity ⊥¶

⊥ 0.002 A

Your feature can lean 0.002" per inch relative to datum A.

Translating it:

Check with a square and feeler gauges
Or indicate across the surface
0.002" over 1" = 0.004" over 2"

Parallelism ∥¶

∥ 0.003 A

Like perpendicularity but... Parallel.

How to check:

Dial indicator on a height stand
Zero at one end
Sweep across part
Total variation must be under 0.003"

Flatness ⏥¶

⏥ 0.001

The entire surface must fit between two parallel planes 0.001" apart.

Checking it:

Surface plate and dial indicator
Map the whole surface
High point minus low point ≤ 0.001"

Concentricity ◎¶

◎ 0.002 A

All points on the surface must be within 0.002" of the datum axis.

This one is really a bastard and has always given me a pain in the ass:

Nearly impossible to measure properly
Usually position or runout is what they really want
If you see this on a print, ask the Engineer if they really mean it

Total Runout ⟲¶

⟲ 0.003 A-B

Spin the part. Indicator can't move more than 0.003" total.

The difference from Concentricity:

This you can actually measure
Set up between centers or in chuck
Indicate while rotating
Read total indicator movement

Datums: Your Reference System¶

Datums are the features you measure from. They're labeled A, B, C, etc.

Primary, Secondary, Tertiary¶

The order matters:

A - Establishes main orientation (usually biggest flat surface)
B - Stops rotation (usually two holes or an edge)
C - Locks final degree of freedom

I think of it like this:

A = Put the part on the surface plate
B = Push it against the angle plate
C = Slide it against a stop

Setting Up Datums¶

In practice:

Clean everything (remember, one chip ruins everything!)
Datum A goes on your most stable surface
Indicate to confirm contact
Lock down before measuring

Material Condition Modifiers¶

These little letters change everything:

M - Maximum Material Condition (MMC)¶

The condition where the part weighs the most:

Largest shaft
Smallest hole

Why it matters: You get bonus tolerance as the part gets smaller/larger.

Example: 0.500" ±0.005" hole with ⊕ 0.010 M

At 0.495" (MMC): 0.010" position tolerance
At 0.500": 0.015" position tolerance
At 0.505": 0.020" position tolerance

L - Least Material Condition (LMC)¶

Opposite of MMC. Rarely used except for minimum wall thickness.

RFS - Regardless of Feature Size¶

No bonus tolerance buddy. The tolerance is the tolerance, period.

How to Actually Check This Stuff¶

Poor Man's Position Check¶

No CMM? Hey, no problem! (Coordinate Measuring Machine - A $100K+ robot that measures parts automatically)

For holes:

Make a gauge pin 0.010" smaller than MMC
Pin should fit through hole
And touch the edges of the true position
If it fits, part is good

Math check:

Measure actual X and Y location
Calculate distance from true position
Distance = √[(X error)² + (Y error)²]
Must be less than half the position tolerance

Checking Tolerances Without Expensive Tools¶

Perpendicularity:

Precision square
Feeler gauges
Do the math (0.002" per inch)

Parallelism:

Dial indicator on height gauge
Sweep the surface
Note high and low

Flatness:

Surface plate
Dial indicator
Grid pattern measurement

When You Need a CMM¶

Be honest with yourself. You need a CMM when:

Position callouts are under 0.005"
Multiple datums get complex
Profile tolerances
The Consumer requires documentation

Common GD&T Mistakes¶

In Design¶

Over-constraining: Every feature doesn't need GD&T
Impossible tolerances: 0.0001" position on a drilled hole? Get the fuck out of here!
Wrong symbols: Using Concentricity when they mean runout
No datum structure: Symbols without clear references or hierarchy

In Manufacturing¶

Ignoring bonus tolerance: That is like leaving money on the poker table
Wrong setup: Not following datum priority
Bad math: Position isn't the same as ±
Assuming RFS: When M gives you breathing room

In Inspection¶

Measuring wrong: Like using calipers for position
Setup errors: Datums not properly established
Math errors: Not doing the trig right
Over-measuring: Checking every feature when only critical ones matter

In The Real World¶

What Actually Matters¶

In order of how often it screws people:

Position - Holes not lining up
Perpendicularity - Square isn't square
Flatness - Gasket surfaces leak
Parallelism - Movement in parts assemblies where there shouldn't be any
Profile - Aerospace mostly

When to Push Back¶

Sometimes the engineer went crazy with GD&T:

Ask what the part does
Suggest loosening non-critical features
Explain cost implications
Always come to the table ready to offer functional alternatives

The Magic Words¶

"Does this need to be inspected or just made to print?"

Sometimes they just want best effort. Sometimes they're buying a CMM report (full measurement documentation from the Coordinate Measuring Machine). Know the difference.

Quick Reference Cheat Sheet¶

Symbol	Name	How to Check	Shop Reality
⊕	Position	CMM or trig	Common, learn first
⊥	Perpendic	Square+feelers	Usually 0.002-0.005"
∥	Parallel	Indicator sweep	Like flatness but rel
⏥	Flatness	Surface plate	Under 0.001" is hard
◎	Concentric	CMM only	Probably meant runout
⟲	Runout	Indicate spin	Actually measurable

Your GD&T Survival Kit¶

Must Have¶

Scientific calculator (for position math)
Surface plate
Dial indicators
Height gauge
Basic GD&T chart

Should Have¶

Gauge pins
Precision squares
Digital height gauge
GD&T textbook

Nice to Have¶

CMM (Coordinate Measuring Machine) access
GD&T software
Optical comparator
Friends in QC

Just The Basics¶

These are just the very basics of GD&T. It's a big subject, but most of the rest of it will lie in edge cases and not day-to-day encounters. Start with position and perpendicularity - in reality, those cover about 80% of all the scenarios. And remember: When in doubt, ask what the part does. Sometimes the print is wrong, and you're the only one who'll catch it.