Machinist exam: GD&T basics

Key points

• A feature control frame reads: symbol, tolerance, modifiers, then datums in order.
• Form tolerances (flatness, straightness) need no datum.
• Position controls location and usually references datums.
• MMC (circled M) allows bonus tolerance as the feature moves from its maximum material size.
• Runout is measured with a dial as the part rotates about a datum axis.

Study tip

Datums are listed in priority order: primary, secondary, tertiary.

Try these

1. Which control needs no datum?

   1. Position
   2. Flatness
   3. Perpendicularity
   4. Runout
   Show answer

   B. Flatness Form controls stand alone.

2. A circled M means:

   1. Least material condition
   2. Maximum material condition
   3. Regardless of feature size
   4. Datum
   Show answer

   B. Maximum material condition MMC.

3. The first datum letter in a frame is the:

   1. Tertiary
   2. Secondary
   3. Primary
   4. Optional
   Show answer

   C. Primary Order shows priority.

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