GD&T Tutoring

Most engineers meet GD&T twice: once in a single lecture in a design or drafting course, and again on the job, when a drawing full of feature control frames lands on their desk. Neither is enough time to learn to read it properly. STEM Tutor 360 teaches GD&T the way design work uses it: what each control actually limits, how the datums set up the part, and how tolerances add up when parts go together.

Sessions are one-on-one and online with a shared whiteboard. Bring your course material, a practice drawing, or a drawing from work that you're allowed to share, and we'll work through it one control at a time. Your tutor is a Georgia Tech mechanical engineer with more than 8 years of experience at Fortune 500 companies designing components and creating their drawings to the latest GD&T standards.

A position tolerance at MMC, worked

A hole is dimensioned Ø10.0 +0.2/-0.0 mm and located with a position tolerance of Ø0.1 at MMC relative to datums A, B, and C. The inspected hole measures Ø10.15, and its center sits 0.06 mm from true position in x and 0.08 mm in y. Does it pass?

Start with the size limits. For a hole, MMC is the smallest size, Ø10.0, and LMC is the largest, Ø10.2. The actual hole is Ø10.15, which is 0.15 mm away from MMC, so the MMC modifier adds 0.15 mm of bonus tolerance: the allowed position zone grows from Ø0.1 to Ø0.25.

Now the location. The center is off by $\sqrt{0.06^{2} + 0.08^{2}} = 0.10\ \text{mm}$, and a position zone is a diameter, so this hole needs a zone of Ø0.20. That fits inside Ø0.25, so the hole passes. Without the MMC modifier the tolerance would stay at Ø0.1, and the same hole would be rejected.

The number that ties it together is the virtual condition: MMC minus the position tolerance, Ø10.0 minus 0.1, or Ø9.9. A fixed Ø9.9 gauge pin at true position enters every acceptable hole. That's the point of MMC: a bigger hole can sit further off location and the parts still assemble.

Where students get stuck

• Reading the datum letters as a list instead of an order. The primary, secondary, and tertiary datums constrain different degrees of freedom.
• Forgetting that MMC is the smallest hole but the largest pin.
• Picturing a position tolerance as a plus-or-minus square instead of a cylindrical zone.
• Confusing profile of a surface with flatness, or runout with circularity.
• Adding bonus tolerance when the feature control frame has no modifier.

Bring a drawing

GD&T is fastest to learn on a real print. Send a drawing or problem set before the session and we'll go through every feature control frame on it. If it's a company drawing, check that you're allowed to share it; a redrawn or simplified version works just as well.

Training a whole team?

For design, manufacturing, and quality teams, there's a 2-day on-site GD&T course built on your company's own drawings, plus a half-day refresher and drawing review.