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Calculators / Plug and ring gauge sizes

Calculators

Plug and ring gauge sizes

Find the new GO size, the wear limit and the NO GO size of plain limit gauges for holes, slots, shafts and keys, from an ISO 286 class or deviations: ISO 1938-1 gauge types A to K, or ASME B89.1.5 plug gauges, classes XXX to ZZ.

Tolerance class
Workpiece (mm)—
GO, new (mm)—
GO, wear limit (mm)—
NO GO (mm)—

How the sizes are found

The gauge sizes follow ISO 1938-1:2026 (the values of Tables 6 to 11 are the same as in the 2015 edition). Each gauge has a size tolerance H, a fine IT grade set by the gauge type and the workpiece grade (Table 6). The GO gauge sits inside the workpiece tolerance by z, so it can wear, and may wear to y beyond the limit. Over 180 mm the safety allowance α moves both gauges further in. z, y and α come from Tables 7 to 11 by size and grade.

Hole or slot (plug gauge), from the lower limit LSL and upper limit USL of the workpiece:

GO new=LSL+z±H2\text{GO new} = \text{LSL} + z \pm \tfrac{H}{2}GO wear limit=LSL+α−y\text{GO wear limit} = \text{LSL} + \alpha - yNO GO=USL−α±H2\text{NO GO} = \text{USL} - \alpha \pm \tfrac{H}{2}

Shaft or key (ring or gap gauge):

GO new=USL−z1±H2\text{GO new} = \text{USL} - z_1 \pm \tfrac{H}{2}GO wear limit=USL+y1−α1\text{GO wear limit} = \text{USL} + y_1 - \alpha_1NO GO=LSL+α1±H2\text{NO GO} = \text{LSL} + \alpha_1 \pm \tfrac{H}{2}
  • Tolerance not given as an ISO code: the grade is the coarsest one whose tolerance interval T is not larger than the workpiece tolerance in the same size range. For 20 ± 0.02 (40 µm) that is IT8 (33 µm).
  • 6N, 7N, 8N: y and α are zero for the GO gauge, so it may only wear down to the workpiece limit. The NO GO gauge is unchanged.
  • Form: the gauge's form tolerance is half of the 2 × F value of Table 6. Type G has none.
  • The tables cover workpiece grades IT6 to IT18 and sizes up to 500 mm; IT14 to IT18 are not given up to 1 mm. The GO gauge of types A, B and H must be at least as long as the feature when the envelope requirement applies, unless agreed otherwise.

Check against the edition of ISO 1938-1 agreed with the customer before ordering or accepting gauges.

ASME B89.1.5, Absolute policy

The American system puts both plug gauges inside the hole tolerance with no wear allowance (the Absolute policy of ASME Y14.43, and the default of ASME B89.1.5): the GO gauge is plus only from the smallest hole size, the NO GO minus only from the largest. Once the GO gauge wears below the smallest hole size it is outside its own tolerance. The gauge tolerance per gauge comes from the class, XXX the finest to ZZ the widest (B89.1.5 Table 3, converted from inches).

GO=LSL  0+tNO GO=USL −t 0\text{GO} = \text{LSL}\ {}^{+t}_{\ 0} \qquad \text{NO GO} = \text{USL}\ {}^{\ 0}_{-t}

By the usual 10 % rule the two gauge tolerances together should take no more than 10 % of the hole tolerance; the calculator picks the widest class that does. B89.1.5 covers plug gauges only; ring gauges are in B89.1.6, which is not included.