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Why GD&T Is as Much a Design Tool as It Is a Tolerancing System

Understanding geometry as intent — not just inspection.

“Why do we need GD&T? Can’t we just do ±0.10 everywhere?”

Sure — if you want to pay to “heat the whole room.” At Kotatsu we like the kotatsu approach: apply precision where people and parts actually interact. Geometric Dimensioning & Tolerancing (GD&T) is the same idea in engineering. It’s not just a fancy inspection language; it’s a design tool that forces you to think about how a part will be used, assembled, and checked.

When you apply GD&T from the start, you end up with drawings that match the real assembly, stackups that make sense, and inspection that’s fast and repeatable. Let’s walk through how.

1. Datums Make You Design to the Real Assembly

The first big shift with GD&T is datum selection. A|B|C isn’t a CAD formality — it’s your statement of “this is how the world will grab this part.”

  • Primary (A): the surface that actually sits, seals, or carries load.
  • Secondary (B): the feature that aligns the part laterally (pin, boss, rail).
  • Tertiary (C): the feature that kills the last bit of rotation or shift.

By doing this, you’re encoding the assembly’s contact strategy right in the drawing. Design, manufacturing, and QA now share the same reality.

2. Functional Stackups Beat Absolute Stackups

Traditional “add up all the ±” stackups treat every dimension like it matters equally. GD&T lets you tolerate based on function. For example, instead of saying two holes must be ±0.05 from some arbitrary corner, you say, “this hole pattern must line up to the mating part,” and you control it with position to A|B|C.

That’s powerful because it lets you:

  • Control the relationship that matters (hole to datum, boss to panel) instead of random edges.
  • Use profile to wrap complex surfaces into one clear requirement.
  • Use material modifiers (like MMC) to get bonus tolerance and better yield.

Result: stackups reflect how the component is actually used, not just how the CAD model is dimensioned.

3. It Forces “Design-for-Inspection” Early

This is the piece lots of teams leave until the end: how are we going to secure this part to check it in an accurate and repeatable way? GD&T makes you answer that on day one.

If your datums are functional, the check fixture practically designs itself — plate for A, pin for B, slot for C. That gives you:

  • Repeatable QA: less “wiggle” in measurement, better R&R.
  • Faster acceptance: you can build a functional gage instead of measuring everything manually.
  • Cleaner supplier conversations: they know exactly how you will judge the part.

4. GD&T Is a Cost Control Knob

Because you tighten tolerances only where function demands it, you stop over-controlling nonfunctional surfaces. That means fewer rejects for the wrong reasons. Tactics we use a lot:

  • Call out a profile just on a sealing land — leave the rest looser.
  • Use position @ MMC on clearance holes to get bonus tolerance.
  • Use composite position on patterns to keep holes together but relaxed to the part.

You end up paying for precision only where it buys you performance.

5. A Quick Case Study: Sensor Bracket the “GD&T Way”

Say we have a small bracket holding a sensor behind a fascia. The assembly cares about pointing and connector fit — not about some random edge.

  • Make the mounting face datum A (that’s how it sits).
  • Make the locating hole datum B and a slot datum C (that’s how it aligns).
  • Call the hole pattern position 0.20 to A|B(MMC)|C(MMC) (that’s how it mates).
  • Control the sensor boss with profile to A|B|C (that’s how it points).

Now the drawing, fixture, and assembly are all telling the same story.

6. A 10-Step GD&T Design Checklist

Drop this into your workflow:

  • State the function in one sentence.
  • Mark the contact map — where does the assembly actually touch?
  • Pick A|B|C to lock all 6 DOF.
  • Identify the few critical relationships (holes, seals, sliding faces).
  • Use position for features of size, profile for faces.
  • Add [MMC] wherever clearance exists.
  • Sketch the check fixture from your datums.
  • Plan inspection: gage for production, CMM for diagnostics/PPAP.
  • Run a quick functional stackup.
  • Delete any dimension nobody will ever use.

7. Why This Fits the Kotatsu Ethos

Like the Japanese heated table that warms what the person is actually using, GD&T lets you “heat” the functional geometry, not the entire drawing. It’s precision on purpose.

Let’s Make Your Drawings Functional