What is the difference between a through-hole and a blind hole tolerance callout?

What is the difference between a through-hole and a blind hole tolerance callout?

• A. The same tolerance applies, but a projected zone may affect depth for blind holes; geometry is evaluated to the DRF
• B. The tolerance only applies to through-holes
• C. Blind holes are not toleranced
• D. Through-holes use different diameter symbols

Answer: (A)

The key idea is that the hole’s size tolerance is the same for through holes and blind holes, but the depth handling can differ. For any hole, GD&T controls the diameter and, if specified, the position relative to datums. When the hole is blind, you may add a projected tolerance zone that extends beyond the actual hole depth to a defined projection height. This projection ensures that a mating feature (like a pin or fastener) will fit properly at the portion of the assembly that projects into the hole, and that extension is evaluated in the same datum reference frame as the rest of the feature. If no projection is used, the tolerance focuses on the visible hole depth itself, but the diameter tolerance still applies to the hole’s size. So, the tolerance for the hole’s size applies regardless of whether the hole is through or blind, and a projected tolerance zone is an additional consideration used for blind holes to account for engagement depth, all evaluated against the DRF. The other statements aren’t correct because tolerances do apply to blind holes, the diameter symbol isn’t different for through holes, and the difference isn’t that tolerances only apply to through holes.

The key idea is that the hole’s size tolerance is the same for through holes and blind holes, but the depth handling can differ. For any hole, GD&T controls the diameter and, if specified, the position relative to datums. When the hole is blind, you may add a projected tolerance zone that extends beyond the actual hole depth to a defined projection height. This projection ensures that a mating feature (like a pin or fastener) will fit properly at the portion of the assembly that projects into the hole, and that extension is evaluated in the same datum reference frame as the rest of the feature. If no projection is used, the tolerance focuses on the visible hole depth itself, but the diameter tolerance still applies to the hole’s size.

So, the tolerance for the hole’s size applies regardless of whether the hole is through or blind, and a projected tolerance zone is an additional consideration used for blind holes to account for engagement depth, all evaluated against the DRF. The other statements aren’t correct because tolerances do apply to blind holes, the diameter symbol isn’t different for through holes, and the difference isn’t that tolerances only apply to through holes.