Every physical object is three-dimensional. A hole is a void in a three-dimensional object and may be of any size or shape. A perfect round hole is a cylindrical surface that extends between and is normal to the front and back surface of a substrate sheet. Further, a round hole generates a circular void in the front surface and a circular void in the back surface of the substrate sheet. There are many parameters to consider when specifying or describing a hole.
For the sake of discussion, let us reflect upon the characteristics of a round hole, as described above. In the micro-dimensional real world, the absolutely perfect round hole is difficult to achieve. Thus, we must consider how the round hole may deviate from perfection. The ends of a cylinder describe flat surfaces that are a circular, parallel to each other and normal to the axis. The hole entrance and exit apertures may not be circular, they might be oval or irregular in shape.
- If the front and back surfaces of the substrate sheet are not parallel to each other, the cross section of the hole cylinder is trapezoidal in shape.
- If the hole is not drilled at the normal to the surface of a parallel surface substrate sheet, the cross-section of the hole cylinder describes a parallelogram.
- If the hole entrance circle diameter is different than the hole exit circle diameter, the surface extending between them (the hole wall) is a section of a cone.
Also, the surface extending between the entrance and exit aperture may be totally irregular and may contain particles. The edges of the circular entrance and exit aperture may be beveled or rounded and may contain particles or burrs known as ablation. The descriptive geometrics of dimensioning and tolerances of a round hole include roundness or circularity, cylindrically or deviation from a perfect cylinder, perpendicularity or how much the cylindrical axis deviates from the normal to the substrate sheet surface.