Threads are described by the major diameter of the thread followed by the number of threads per inch (TPI).
1/4-20: .250” basic major diameter with 20 TPI
Each Machine Screw Number = .013” larger than .060”.
#0 tap = .060 + .000 = .060” major diameter
#1 tap = .060 + .013 = .073” major diameter
#2 tap = .073 + .013 = .086” major diameter
Threads are described by the major diameter of the thread followed by the pitch or spacing of the thread.
M6 x 1.0: 6mm basic major diameter with a pitch of 1.0mm.
The basic reference dimension and is not the actual dimension of the tap or the screw going in the hole.
Thread pitch = 1
The imaginary line that is half way up the height of the thread.
See Figure 3 in ANSI B1.13M.
Basic Pitch Diameter = minimum PD for an internal thread (Go Gage)
English = Major Dia. - .64952/TPI
#2-56 Basic Pitch Dia. = .086 - .64952/56 = .0744”
1/4-20 Basic Pitch Dia. = .250 - .64952/20 = .2175”
Metric = Major Dia. - .64952 x Pitch
M6 x 1.0 Basic Pitch Dia. = 6 - .64952 x (1.0) = 5.350mm
Gaging Threads & Classes of fit:
There is only one thread specification. This is for cutting threads and forming threads.
Pin Gages: These inspect the Minor diameter.
Thread Plug gages: These inspect the Pitch Diameter and Major Diameter.
”Go Gage” = Minimum Pitch Diameter (Most of the time this is the Basic Pitch Diameter)
”No Go Gage” = Minimum Pitch Diameter + Tolerance (See Figure 6 in ANSI B1.13M)
“No Go" thread gage limits are established by classes of fit. Tolerances for classes of fit are established by formulas based on the diameter of the thread and the pitch. Fine pitch threads have tighter tolerances than coarse pitch threads.
English Sizes: 2B and 3B are most common.
2B is a loose class of fit (standard)
3B is a tight class of fit
Metric Sizes: 6H and 4H are most common.
6H is a loose class of fit (standard) – comparable to a 2B
4H is a tight class of fit – comparable to a 3B
6g is for external threads
6G is an oversized internal thread
Unified Screw Thread series tables provide all this information.
English taps are measured in "H" numbers. 1 H = .0005 in.
Metric taps are measured in "D" numbers. 1 D = .0005 in.
The tap tolerance is expressed as a multiple of "H” or "D” numbers and is measured as an increase above the Basic Pitch Diameter (GO GAGE) of the thread size:
H1 or D1 = basic pitch diameter + .0005
H2 or D2 = .0005 to .001 over basic
H3 or D3 = .001 to .0015 over basic
H4 or D4 = .0015 to .002 over basic
H5 or D5 = .002 to .0025 over basic
H6 or D6 = .0025 to .003 over basic ****(H6 is not the same as 6H)****
¼-20 2B: Class of fit
¼-20 H6: Tap Size
M6 x 1 6H: Class of fit
M6 x 1 D5: Tap Size
M6 x 1 H5: Tap Size (Some customers specify metric taps with H limits)
“H” Size Selection:
Cutting taps cut oversize and Forming taps form threads undersize due to springback.
Therefore, forming tap H and D#'s are always 2-3 numbers higher than the same size cutting tap.
Cutting taps will screw into the tapped hole by hand, and forming taps will not screw into the tapped hole by hand.
This is also why as long as the forming tap is smaller than the NO GO Gage, there is almost no chance for the tap to make oversize threads.
Use our Balax recommended H numbers and hole size chart.\
The competition makes their taps too small. Consequences: poor tap life.
For soft materials or parts with thin walls that will have larger amount of springback, use higher recommended “H” numbers.
For hard materials, use lower “H” numbers to reduce the amount of cold forming work.
If a part gages tight, increase the “H” number. If the part gages loose, reduce the H”” number.
Plating of parts after tapping requires a larger pretap H or D number in order to adjust for the buildup on the threads after tapping. The formula to adjust the tap size is as follows:
Add the additional H or D# to the standard tap H or D#.
Additional H#'s = 4 x plating thickness
Most plating problems are caused by excessive buildup in the threads after plating which ruins the thread gaging of the parts. Small diameter threads and fine thread pitches are more difficult to plate because the threads tend to fill with material. If customers are unsure about plating thickness they should try a range of H numbers to evaluate actual results.