Thread addendum

Thread Addendum

Thread Addendum is simply the distance from the thread crest to the pitch line in an external thread.
Addendum is a shorter distance than the Thread Height, it is designated with has. (Internal threads don’t have an addendum).

How To Inspect the Thread Addendum?

1) How To Inspect the Thread Addendum?

All casing and tubing (API 5B 60° V threads: 8 Round & 10 Round) falling under the 5B specification manufactured by an API licensee.

All casing threads need to be inspected. Non-upset tubing (NUE), regular thread external upset tubing (EUE), and integral joint tubing must all be inspected. For casing and tubing, the inspection tolerance is ± 0.0015 in for both the coupling and pin.

2) How do I inspect it?

Inspection should be done with an indicator style gage with contact points, like our TA-3002 Thread Addendum Gage. Choose a contact point based on the Threads Per Inch (TPI). In Table 20, 5B provides the options for you:

Threads Per Inch (TPI) Contact Point Diameter Gage maker Model Number
8 0.072″/1.83mm T072
10 0.057″/1.45mm T057
Note: Tolerance is 0.002 in/ 0.05mm

Set the gage with the proper standard. For both 8 and 10 Round threads, Gagemaker separately offers the TAS-1014 addendum standard for use with any of its addendum gages.

Recommended Gages (Gage maker):

  • TA-3002: External Thread Addendum Gage For 8 & 10 Round API
  • TAS-1014: Thread Addendum Standard for 8 & 10 Round

    • Note: If the dedicated gauge is not available, you should measure the pitch and major diameters and calculate the addendum according to the below formula.

    These formulas are valid for 60° symmetrical straight threads (ISO metric and Unified Inch standards)

    \( \large h_{as} = \frac {\left (Major Diameter\,-\,Pitch Diameter\right )}{2} = \frac {\left (d – d_2\right )}{2} \)
    \( \large d_2 = d – \frac{3}{8}\times\sqrt{3} \times P \)
    \( \large h_{as} = 0.5\,\times\,\left (d – \frac{3}{8}\times\sqrt{3} \times P \right )= 0.3247595\times P \)

    Where:
    has -Thread Addendum.
    d2 – Pitch diameter of external thread.
    d– Major diameter of external thread.
    P – Pitch

    Note:from the formula, the thread Addendum depends solely on the pitch.