Hole Basis System
The Hole Basis System is a limits-and-fits method where the hole is held at the basic size, with the shaft tolerance varied to achieve clearance, transition, or interference fits. In ISO practice, the hole is designated H, meaning its lower deviation is zero. This system is foundational for standardized manufacturing, especially in CNC machining.
In a CNC cell, the hole basis system is preferred because holes are harder and costlier to vary than shafts. Standard drills, reamers, and boring bars are set for fixed hole sizes, while the mating shaft is adjusted by turning, grinding, or stock selection. Programmers often call out an H7 bore and select shaft classes like g6, h6, or p6 depending on the fit needed—slip, locational, or press. This standardization improves interchangeability across batches and reduces the need for special tools and gauges. On the shop floor, one bore can mate with multiple shaft tolerances, streamlining production and quality control while maintaining consistent assembly conditions.
Why is the hole usually chosen as the fixed element in the hole basis system?
Holes are generally more difficult and costly to adjust precisely than shafts, especially in production where reaming and boring are less flexible than turning or grinding a shaft to final size.
What does the capital letter H mean in ISO fits?
The H hole has a lower deviation of zero, so the hole’s minimum size equals the basic size; the tolerance zone lies at or above nominal.
Why is H7 so commonly referenced in CNC work?
H7 is a common standardized bore tolerance used as a practical starting point for precision fits, especially where the hole is reamed or bored and the mating part is sized to match the required assembly condition.