Unless specifically requested; NPT NPTF and ANPT Pipe Ring Gages made in the USA are made with no step. Pipe Ring Gages made to (BSPT) BS-21 System-A are made with no step. Use the gage by counting turns of the threaded part. Count plus or minus one turn of the part from the small end of the gage.
This one is a little trickier. Be careful because a two-step gage could be confused with a one-step gage. Unless specifically requested; NPT NPTF and ANPT Pipe Plug Gages made in the USA are made with one-step. The step could marked “BASIC” but likely is not. Use the gage by counting turns of the threaded part. Count plus or minus one turn of the part from the step machined into the gage thread.
The two-step gage design is common for pipe thread gages made outside of the USA. In (BSPT) BS-21 the two-step gages are designated as System-B. In (BSPT) ISO-7 the two-step gage is the specified design for Gage-1, Gage-2. Gage-3 and Gage-4. Basically the two-step gage is a GO/NOGO gage in that the part must GO past the first step and NOGO past the end of the gage which is the second step. The gage steps could be marked with “+” and “-” or “MIN” and “MAX”. The two-step gage is not common in the USA where the three-step is more often used. The two-step gage has the advantage of removing a lot of guesswork from the machine operator in the use of the gage, but really; how hard is it to count to one?
The three-step gage is basically a two-step gage with the addition of the BASIC step. The three-step gage is common in USA, but if shipped to other parts of the world they ask why the BASIC step is included because it seems useless. As a plug gage; when a three-step is requested a gage maker can pull a standard one-step plug gage from stock and add the other two steps. The three-step is preferred by the USA gage maker and consumer because the cost for modifying the stocked gage is much less and faster shipped than the cost of making a new two-step or four-step gage. As a ring gage the three-step is less common. Ring gages can be made as three-step but that is much more expensive because the gage maker cannot easily modify a standard no-step ring gage. One gage maker has developed a method of adhering additional material onto the gage so that the steps can be machined onto an existing stock ring gage. All the other gage makers make the three-step ring gage as a special using specially made gage blanks. The cost of making the gage special can be over double the cost of a production no-step ring gage with the lead-time extending to 6 to 8 weeks. The three steps may be marked: “MAX”; “BASIC”; and “MIN”. Like the two-step; the three-step is a GO/NOGO gage. The part must GO past the first step and NOGO past the end of the gage which is the second step. The BASIC step should be the target for a part which is in the middle of the tolerance. The small additional cost of the three-step plug gage pays for itself on the production floor by removing a lot of guesswork from the machine operator in the use of the gage. As for the three-step ring gage; it may be better to get the four-step version.
We see the four-step gage specified in ANSI/ASME B1.20.5 where it states: To facilitate use of 6-step crest and root check gages, qualification of the product thread into one of [three] ranges is necessary and is best determined with the four-step L1 gage. The four-step is likely only used for ANPT or NPTF where the additional six-step gages (Root-Check; Crest-Check) are required. The use of the six-step requires the gage user to have knowledge of the reading of the L1 gage by thirds of the product tolerance. The four-step gage design makes the thirds of tolerance easily identifiable so reading of the additional gages much more accurate. The steps are to be numbered: 1; 2; 3; 4. The gage use instructions in B1.20.5 state that: Between Step-1 and Step-2 is the Maximum Range; Between Step-2 and Step-3 is the Basic Range; Between Step-3 and Step-4 is the Minimum Range. I strongly suggest the use of the L1 four-step when testing ANPT and NPTF parts. Purchase of the L2 or the L3 in the four-step style may be extravagant; but if done it would make harmonization of gage use much better for checking NPTF or ANPT parts.
How many options do you want? Five-step is not available, so let’s just move on to the six-step.
The six-step is used for ANPT and NPTF when testing the root (root-check gage) or crest (crest-check gage) of the product threads. The six steps are needed because the three zones of the gage are overlapping. That overlap requires the two additional steps to identify the same three zones as found in the four-step gages. Checking the root and the crest of the product threads is critical for NPTF because the thread accomplishes a dry-seal by crushing the root and crest of the mating threads together in the wrench-tight assembly. If the root or the crest is not within tolerance then the dry-seal will not occur and some potentially hazardous leakage could occur. The six steps of the gages are marked: MX; MXT; B; BT; MN; MNT. Simply stated; after gaining the test result of the L1 gage; If the part falls in the Minimum Range it must fall between MN and MNT on the six-step gage; If the part falls in the Basic Range it must fall between B and BT on the six-step gage; If the part falls in the Maximum Range it must fall between MX and MXT on the six-step gage.
This page has just been a brief summary of the differences between the allowed steps on pipe gages. For official usage instructions consult the industrial specification related to the part which you are making. This data is provided for general information only. The intention is to provide accurate information; regardless; errors may exist in the supplied information. If accuracy is critical, base your final decisions on the data provided in the root documents which are copyrighted. To purchase a copies visit an Authorized Reseller.
Original Posting: 8/26/2014
Last Revision: 11/13/2015
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