Tap H-Limits & D-Limits Explained

What it is

Ground thread taps are made with the pitch diameter slightly larger than the basic size. The amount is shown by a limit marking on the shank. Inch taps use H limits (high) and L limits (low, below basic). Metric taps use D limits (over basic) and DU limits (under basic). The limit system lets a shop pick a tap that cuts a thread inside the class it needs.

How to read it

Inch H limits step in units of 0.0005 in above basic pitch diameter. Metric D limits step in units of 0.013 mm (about 0.0005 in) above basic pitch diameter. The number is the step. For example, an H3 tap cuts a pitch diameter between 0.0010 in and 0.0015 in over basic.

Limit steps

Each step follows the same rule: the upper limit is the step number times the unit, and the lower limit is one step less. This one-step band applies to the common small and medium tap sizes. Larger taps (inch taps over 1 in) keep limit numbers in the same increments but have a wider pitch diameter tolerance, so check ASME B94.9 or the maker's chart for large sizes.

Key points

• The limit is a tap tolerance, not the thread class. The thread class (2B, 3B, 6H) is checked with gauges on the finished part.
• A tap usually cuts slightly oversize. Material, coolant and tap condition all change the result. Gauge the first parts.
• Higher limits are used when parts will be plated or coated, or when a free-running fit is wanted.
• Common stock limits differ by size and maker. Check the maker's chart for the limit that suits 2B or 3B in each size.
• Thread-forming taps are often made to higher limits than cutting taps of the same size, because formed threads spring back.

Common mistakes

• Reading "H3" as a thread class.
• Buying a high limit tap for a 3B job, which can cut threads out of tolerance.
• Forgetting plating build-up and tapping to a standard limit on parts that will be plated.
• Mixing up the metric "6H" thread class with the inch "H" limit system.

Governing standards: ASME B94.9 (tap limits); ASME B1.1 (inch thread classes); ASME B1.13M (metric thread classes).

Reference only. Verify against the current governing code or standard, the manufacturer's data and the project documents before use. Apps for Distributors disclaims liability arising from reliance on this reference.