|A.N.P.T. (AERONAUTICAL NATIONAL PIPE TAPER)|
|Size||Threads per Inch||Plug Gage||Ring Gage|
|3-Step L-1||3-Step L-3||6-Step Plain||L-1||L-2||6-Step Plain|
In gaging internal threads, with A.N.P.T. plug gages, there are three gaging members involved:
(1) L-1 thread plug gage which checks the thread elements within hand tight engagement
(2) L-3 thread plug which checks the threads that will be engaged when wrench tight
(3) 6-Step plain plug which is used to check the minor diameter of the female fitting.
A.N.P.T. internal taper pipe threads are basically the same as the N.P.T. pipe threads except that for Army and Navy Aeronautical use they must be more carefully controlled for diameter, taper and thread form. We make our A.N.P.T. gages per SAE AS71051.
To gage Internal A.N.P.T. taper pipe threads, check first with the A.N.P.T. L-1 thread plug gage. This plug differs in dimensions from the N.P.T. plug and has minimum and maximum steps in addition to the basic step. If the face of the product is between the minimum and maximum steps of the plug, the product is in tolerance. The A.N.P.T. L-3 plug gage is used to check the three threads beyond the L-1 depth. As a check on taper the “reading” on the L-3 thread plug must be within one-half turn of the “reading” on the L-1 thread plug gage. The truncation is checked with a six-step plain taper plug gage. This plug has a minimum and maximum truncation steps for each of the three pitch diameter conditions – Minimum, Maximum and Basic. If the L-1 plug shows the product to be basic pitch diameter, the face of the product should lie between the basic minimum and the basic maximum steps of the truncation plug. If the pitch diameter is minimum, the part should fall between the MN and MNT step. If the pitch diameter is maximum, the part should fall between the MX and MXT steps. No appreciable shake or wobble is permitted. Shaky fit indicates taper error.
To gage External A.N.P.T. taper pipe threads, first check with the A.N.P.T. L-1 ring. The small end of the ring must be within one turn plus or minus of being flush with the end of the product thread. Note exactly what this “standoff” is and compare it with the “standoff” obtained with the A.N.P.T. L-2 thread ring. As a check on taper the total tolerance on standoff variation between the L-1 and L-2 rings must not exceed one-half turn. The truncation is checked with a six-step plain taper ring gage. This ring has minimum and maximum truncation steps for each of the three pitch diameter conditions – Minimum, Basic, and Maximum. If the L-1 ring shows the part to be basic pitch diameter the end of the part should fall between the B and BT steps on the six-step plain ring. If the pitch diameter is minimum, the part should fall between the MN and MNT step. If the pitch diameter is maximum, the part should fall between the MX and MXT steps. No appreciable shake or wobble should be permitted. Lead, angles and root truncation should be checked whenever possible, by projection as a check on tool wear.