Blind holes vs thru holes
This picture of a part cut in half shows the difference between a thru hole and a blind hole. The three holes on the left are blind holes which do not break through to the other surface.
The thru hole on the right goes all the way through the part.
Symbols used to dimension holes
The diameter of the hole is the size. The diameter is the distance across the hole.
Less often, holes are specified with a radial dimension instead of a diameter. The radius is the distance from the center of the hole to the edge of the hole. This is equal to 1/2 of the diameter.
The depth of the hole is the distance that it is drilled to.
Features associated with holes
Spotfaces are functionally no different than a counterbore. They are machined on a part to allow a fastener to sit flush, though not always completely recessed.
Holes with a spotface will specify the size and depth of the holes as well as the size and depth of the spotface.
Note: spotfaces are not always specified with the symbol shown. At times, they will only use the counterbore symbol and other times they will use the SF notation only.
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