GD&T Flashcards
How do you specify counterbore
ex)
ø 0.25 THRU ALL (diameter of hole)
⌴ ø 0.50 ↧ 0.25 (diameter of counterbore and counterbore to a certain depth)
How do you specify spotface
Spotface is a shallow cutout similar to counterbore but specifically only meant to provide a flat surface for the head of a bolt to sit on
ex)
ø 0.25 THRU ALL (diameter of hole)
⌴ ø 0.50 (diameter of counterbore and but no depth)
How do you specify countersink
ex)
ø 0.25 THRU ALL (diameter of hole)
⌵ ø 0.50 X 90° (diameter of countersink and the angle of the countersink)
What is a feature control frame
A place where GD&T symbols are placed. Used for specific features. Consist of the GD&T symbol, the tolerance, the datums and the modifiers (i.e. MMC or LMC)
What is ⌖
The true position (or just position) tolerance. Specifies how much a feature can deviate in distance from its theoretical position
What are basic dimensions
The perfect and ideal dimensions of a part (as specified in CAD). Enclosed in a box. Must be used alongside other GD&T symbols. For example, if using a position tolerance, you must use basic dimensions since regular ones would double up the tolerances.
What is ⏥
The flatness tolerance. Imagine two parallel planes offset to the tolerance. If all the points in the surface fall between the planes, the tolerance is met. Can also be applied to features of size like a cutout. The midpoints of opposite points of the cutout form a median plane, which you then see if it falls in the flatness tolerance. Measured using a dial indicator that is swept across the whole part, and if the difference between the highest and lowest point falls in the tolerance, it is good
How are datum planes defined
Using 3 jacks (points) that can be adjusted up or down so that a dial indicator reads 0 on all three points
What is —
The straightness tolerance. The same as the flatness tolerance but only considers the deviation across a line rather than the whole surface. These lines are measured many times for the whole width of a surface, but tolerance is only considered for each line instead of the whole surface. If on a feature of size (typically cylinders), the derived median line indicates the straightness of the axis.
What is ◯
Circularity tolerance. Indicates the allowable deviation between the most outer and the most inner part of a circle. Circularity is independent of orientation and of diameter size, just tells you how circular something is. A probe is rotated around the part and mapped in a polar graph to see if any value is outside of tolerance zone.
