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The primary problem that most people encounter when trying to cut pipe threads is determining the correct Major or Minor diameter, which is necessary in order to program the tool path.
Unfortunately, the Machinery’s Handbook does not supply these numbers. It provides the pitch diameter, and the major or minor diameters must be calculated accordingly. This becomes tricky due to the fact that all of these diameters are at an angle; therefore, these values will change depending upon the horizontal Z value.
Step by step instructions will be provided for programming both a 2.5"-8 NPT external and a 2.5"-8 NPT internal thread to show the actual process required to determine the minor and major diameters.
2.5" - 8 NPT External Pipe Thread
2.5" - 8 NPT Internal Pipe Thread
American National Standard Taper Pipe Thread (NPT) Chart
First, a given horizontal value must be established to act as a gauge point. Since the Machinery Handbook supplies the pitch diameter at the start of the thread, the horizontal value most commonly used is Z0 (the face of the part). The system also assumes this value for the major and minor diameters, and will calculate the major and minor diameters at the start and end of the toolpath based on this assumption. The advantage of this is that only one value needs to be calculated; in the case of external pipe threads, only the minor diameter at the face of the part is needed, and with internal pipe threads only the major diameter at the face of the part is needed.
Find the Pitch Diameter at Beginning of External Thread (E0) from Machinery Handbook: American Pipe Threads: Table 3 (Basic Dimensions, American National Standard Taper Pipe Threads). For a 2.5" - 8 NPT external thread this value is 2.71953.
Find the nominal truncated Height of Pipe Thread (h) from Machinery Handbook: American Pipe Threads: Table 1 (Limits on Crest and Root of American National Standard Taper Pipe Threads). This value is given as a max/min dimension; add the minimum and maximum height and divide by two to obtain the nominal thread height. For a 2.5" - 8 NPT external thread this would be (.1000+.09275)/2 or 0.096375.
Find the Minor diameter at the start of the thread. To calculate this value, simply subtract the nominal thread height from the Pitch diameter (E0). For a 2.5" - 8 NPT external thread this would be 2.71953 - 0.096375 or 2.623155.
Find the Pitch Diameter at Beginning of External Thread (E1) from Machinery Handbook: American Pipe Threads: Table 3 (Basic Dimensions, American National Standard Taper Pipe Threads). For a 2.5" - 8 NPT internal thread this value is 2.76216.
Find the nominal truncated Height of Pipe Thread. This value does not change for external and internal threads and is the same as the 2.5" - 8 NPT external thread above (0.096375).
Find the Major diameter at the start of the thread. To calculate this value, simply add the nominal thread height to the Pitch diameter (E1). For a 2.5" - 8 NPT internal thread this would be 2.76216 + 0.096375, or 2.858535.
This is a simple chart containing the values for the Standard NPT Pipe Thread sizes. For an external thread, enter the Minor diameter as given on the chart, and for an internal thread, enter the Major diameter as given on the chart.
| PIPE SIZE | EXTERNAL THREADS | INTERNAL THREADS | |||
|
Nominal Pipe Size |
TPI |
Minor |
Major |
Minor |
Major |
|
1/16” |
27 |
0.2439 |
0.2985 |
0.2539 |
0.3085 |
|
1/8” |
27 |
0.3362 |
0.3908 |
0.3463 |
0.4009 |
|
1/4” |
18 |
0.4360 |
0.5188 |
0.4502 |
0.5330 |
|
3/8” |
18 |
0.5706 |
0.6534 |
0.5856 |
0.6684 |
|
1/2” |
14 |
0.7045 |
0.8124 |
0.7245 |
0.8324 |
|
3/4” |
14 |
0.9138 |
1.0216 |
0.9349 |
1.0428 |
|
1" |
11 1/2 |
1.1475 |
1.2797 |
1.1725 |
1.3047 |
|
1 1/4” |
11 1/2 |
1.4910 |
1.6232 |
1.5173 |
1.6495 |
|
1 1/2” |
11 1/2 |
1.7300 |
1.8622 |
1.7563 |
1.8884 |
|
2" |
11 1/2 |
2.2029 |
2.3351 |
2.2302 |
2.3624 |
|
2 1/2” |
8 |
2.6232 |
2.8159 |
2.6658 |
2.8585 |
|
3" |
8 |
3.2442 |
3.4370 |
3.2921 |
3.4849 |
|
3 1/2” |
8 |
3.7411 |
3.9339 |
3.7924 |
3.9852 |
|
4" |
8 |
4.2380 |
4.4308 |
4.2908 |
4.4835 |
|
5" |
8 |
5.2944 |
5.4871 |
5.3529 |
5.5457 |
|
6" |
8 |
6.3497 |
6.5425 |
6.4096 |
6.6023 |
|
8" |
8 |
8.3372 |
8.5300 |
8.4037 |
8.5964 |
|
10" |
8 |
10.4489 |
10.6417 |
10.5246 |
10.7173 |
|
12" |
8 |
12.4364 |
12.7286 |
12.6208 |
12.7142 |
|
14" OD |
8 |
13.6786 |
13.8714 |
13.7763 |
13.9690 |
|
16" OD |
8 |
15.6661 |
15.8589 |
15.7794 |
15.9721 |
|
18" OD |
8 |
17.6536 |
17.8464 |
17.7786 |
17.9714 |
|
20" OD |
8 |
19.6411 |
19.8339 |
19.7739 |
19.9667 |
|
24" OD |
8 |
23.6161 |
23.8089 |
23.7646 |
23.9573 |
|
