Page 1931 - Master Catalog 2017, Inch

Technical Information
Ground Thread Tap Limits

In addition to the nominal size and pitch of a tap, there is Figure 1 Illustrates the numbering system and the .0005" diameter
another important dimensional factor to be considered when increment separation between successive limits. Because
selecting a ground thread tap for a given job. This factor is the the starting point is basic pitch diameter, dividing the limit
pitch diameter tap limit, “H” and “L”. “H” represents (high) number by two establishes, in thousandths of an inch, the
above basic pitch diameter; “L” (low) is below basic pitch amount the maximum tap pitch diameter is above basic in
diameter. Tap limits have been established to provide a choice the “H” series and the amount the minimum tap pitch
in the selection of the tap size best suited to produce the class diameter is under basic in the “L” series.
of thread desired.
Figure 2 Illustrates the positioning of the tap limits in relation to the
various classes of threads for a 1/4–20 size.

Figure 1
.0050 .0050
H10
This chart shows the
.0045 .0045
method of designating limits H9
and does not indicate
.0040 .0040
available taps in any
particular size. H8
.0035 .0035
H7
.0030 .0030
H6
.0025 H6 .0025
H5
.0020 H5 .0020
H4
.0015 H4 .0015
H3
.0010 .0010
H2
.0005 .0005
H1
basic basic
P.D. For sizes from 1–1-1/2" P.D.
L1 inclusive, 6 TPI and fi ner
.0005 For taps through 1" inclusive, 8 TPI and fi ner

Figure 2 Class of Thread – 1/4–20 UNC and NC
.2255 .2255

.2245 .2248 .2245
tap limits tapping hole limits
.2235 .2235

.2225 .2225
.2248
.2215 .2215 Technical Information
1B
.2211 .2211
.2205 .2205
H5
2B
TOL.0005 .2201
.2195 .2195
H3 2 3B
3
H2 TOL.0005
.2185 .2185
H1 TOL.0005 TOL TOL TOL TOL TOL
.0073 .0049 .0036 .0036 .0026
basic TOL.0005
.2175 .2175
P.D. TOL.0005
L1
.2165 .2165

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