Imagine you are trying on a new pair of pants. Your waist size dictates the class of fit. You are the bolt. Unfortunately, the likelihood that you find a pair that fits perfectly is rare. Too tight and too loose are immediately obvious. (I know it’s a stretch from making threads in metal, but stay with me.) Now imagine that tight is the “Go” gage, and loose is the “No-Go” gage. Anywhere between the two, is acceptable. Maybe not completely comfortable, but acceptable. Thankfully, you have the ability to “fine-tune” the fit. You buy a belt. The notches in the belt are the “H-limit’s”. The first notch is very close to your waist size. The last notch barely stops your pants from falling down. Every notch in between changes the way you, and your pants, fit together.
When a threaded part is designed, a Class-of-Fit is assigned to the threads of the mating parts. Thread “fit” is dictated by a combination of allowances and tolerances, and the measure of tightness or looseness between them. Industry world-wide has assigned standard thread tolerances to both Imperial Unified and Metric threads. Within those tolerances are “standard” Classes-of-Fit to assure consistency in manufacturing and functionality. We typically see, as we manufacture taps, Classes like 2B and 3B for Internal Unified Thread, and 6H and 4H for Internal Metric Thread. 2B and 6H are by far the most common. 2B or not 2B, that is the question. Allowing a general-purpose fit between mating parts. 3B and 4H provide a closer tolerance. Each of these classes have the same “basic” dimension (Go), but the allowance between Go and No-Go is smaller..
Threads also have a tolerance. Class-of-Fit defines the overall dimensions allowed for proper fitment of mating parts. It is has a Minimum (demonstrated by the Go gage), and a maximum (demonstrated by the No-Go gage). H-limits, or D-limits for those going metric, are a method to define “fine-tuning” of the actual size to create the desired mating of the two parts.
H-limits for Unified Thread and D-limits for Metric Thread, in increasing increments, allow the Pitch-diameter of the tap to influence the fit of the mating parts.
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