1/2"长度单位乘25.4等于12.7mm
G1/2" 英制直管螺纹标准,不可换算
NPT1/2"美制锥管螺纹标准,不可换算
UNC1/2"美制粗牙螺纹,不可换算
公制螺纹用螺距来表示，美英制螺纹用每英寸内的螺纹牙数来表示；
公制螺纹是60度等边牙型，英制螺纹是等腰55度牙型，美制螺纹为等腰60度牙型；
公制螺纹用公制单位（如mm），美英制螺纹用英制单位（如英寸）；
“行内人”通常用“分”来称呼螺纹尺寸，一英寸等于8分，1/4英寸就是2分，以此类推。
另外还有：ISO—公制螺纹标准60度；UN—统一螺纹标准60度；API—美国石油管螺纹标准60度；W—英国惠氏螺纹标准55度。
G1/4外径13.158mm,内径11.446mm，螺距1.337mm,19牙/英寸。
G1/2外径20.956mm,内径18.632mm，螺距1.814mm,14牙/英寸。
G1/4是英制直管螺纹，4分管牙一般说的是英制锥管螺纹，就是ZG1/4.
G1/2螺纹是非螺纹密封的管螺纹，4分管牙标作R1/2，是螺纹密封的锥管外螺纹，这两组螺纹的牙型角都是55°，每英寸14牙（4分管即4/8英寸口径的管子，标作1/2".怎么跑到1/4去了？）。
半寸管是Φ20毫米的管，管螺纹G1/2和M20x1.5是相近的，它们不是3/4的。
你师傅那是经验派的，那是老手了。管螺纹的标称是管子的内径。G1/2指的是内径是1/2英寸的管子的外面的螺纹！但像我们没经验的实际保险起见，还是老实的用游标卡尺卡一下外圆。看看怎么样先，要是还是不能分辨就老老实实的量螺距，对照手册，绝对不会错。
经验是:
1:首先在常见的管螺纹里,只有G螺纹是直螺纹.
2:凭经验,和M10螺栓差不多粗的是G1/8,M12是G1/4,M16是G3/8,M20是G1/2,.....
7/16-20表示为：统一螺纹，大径：0.4375英寸，每英寸牙数：20，细牙螺纹系列，*UNF，基本中经：0.4050英寸，外螺纹小径：0.3762英寸，内螺纹小径：0.3834英寸，适用于低强度材料上
G1/2":中国GB管牙标准 圆柱直管牙、管子公称尺寸1/2英吋、螺距14牙/吋、螺丝锥度55度
PS 1/2-14：日本管牙 圆柱直管内螺、管径公称1/2英吋、 螺距14/吋、螺丝锥度55度
RP1/2-14:ISO提出英制密封管螺纹标准（ISO R 7）、圆柱直管内螺、管子公称尺寸1/2英吋、 螺距14/吋、螺丝锥度55度
英制密封圆柱内螺纹的特征代号为：RP；
英制密封圆锥内螺纹的特征代号为：RC；
管螺纹有如下几种分类,大致如下:
管螺纹(英制PT)
牙深=
0.6403*(25.4/每吋牙数)
(牙角55度)
PT 3/4-14 (锥度管螺纹) 锥度管螺纹,锥度比1/16
3/4英吋管用,每英吋14牙
管螺纹
(PS直螺纹)(PF细牙)
牙深=
0.6403*(25.4/每吋牙数)
(牙角55度)
PS 3/4-14 (直形管螺纹)
PF1 1/8-16 (直形管螺纹)
(细牙)
直形管螺纹
3/4英吋管用,每英吋14牙
1 1/8英吋管用,每英吋16牙
公制粗螺纹对照表
直径X螺距(MM) | 底孔直径(MM) | 直径X螺距(MM) | 底孔直径(MM) | |
M1X0.25 | 0.75 | M7X1 | 6 | |
M1.2X0.25 | 0.95 | M8X1.25 | 6.75 | |
M1.4X0.3 | 1.1 | M9X1.25 | 7.75 | |
M1.5X0.35 | 1.15 | M10X1.5 | 8.5 | |
M1.6X0.35 | 1.25 | M11X1.5 | 9.5 | |
M1.8X0.35 | 1.45 | M12X1.75 | 10.2 | |
M12X0.4 | 10.6 | M14X2 | 12 | |
M2.2X0.45 | 1.75 | M16X2 | 14 | |
M2.5X0.45 | 2.05 | M18X2.5 | 15.5 | |
M3X0.5 | 2.5 | M20X2.5 | 17.5 | |
M3.5X0.6 | 2.9 | M22X2.5 | 19.5 | |
M4X0.7 | 3.3 | M24X3 | 21 | |
M4.5X0.75 | 3.75 | M27X3 | 24 | |
M5X0.8 | 4.2 | M30X3.5 | 20.5 | |
M6X1 | 5 | | |
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英制螺纹、公制螺纹对照表
单位(Inch:MM)
英制 | 公制 | | 英制 | 公制 | | 英制 | 公制 |
4# | 2.9 | 1-5/8 | 41.275 | 4-5/8 | 117.475 |
5# | 3.2 | 1-3/4 | 44.45 | 4-3/4 | 120.65 |
6# | 3.5 | 1-7/8 | 47.625 | 4-7/8 | 123.825 |
7# | 3.9 | 2 | 50.8 | 5 | 127 |
8# | 4.2 | 2-1/8 | 53.975 | 5-1/4 | 133.35 |
10# | 4.8 | 2-1/4 | 57.15 | 5-3/8 | 136.525 |
12# | 5.5 | 2-3/8 | 60.325 | 5-1/2 | 139.7 |
14#/1/4 | 6.35 | 2-1/2 | 63.5 | 5-5/8 | 142.875 |
1/8 | 3.175 | 2-5/8 | 66.675 | 5-3/4 | 146.05 |
1/4 | 6.35 | 2-3/4 | 69.85 | 5-7/8 | 149.225 |
5/16 | 7.936 | 2-7/8 | 73.025 | 6 | 152.4 |
3/8 | 9.525 | 3 | 76.2 | 6-1/4 | 158.75 |
7/16 | 11.113 | 3-1/8 | 79.375 | 6-3/8 | 161.925 |
1/2 | 12.7 | 3-1/4 | 82.55 | 6-1/2 | 168.275 |
9/16 | 14.288 | 3-3/8 | 85.725 | 6-5/8 | 171.45 |
5/8 | 15.875 | 3-1/2 | 88.9 | 6-3/4 | 174.625 |
3/4 | 19.05 | 3-5/8 | 92.025 | 6-7/8 | 177.8 |
7/8 | 22.225 | 3-3/4 | 95.25 | 7 | 184.15 |
1 | 25.4 | 3-7/8 | 98.125 | 7-1/4 | 187.325 |
1-1/8 | 28.575 | 4 | 101.6 | 7-3/8 | 190.5 |
1-1/4 | 31.75 | 4-1/4 | 107.95 | 7-1/2 | 193.675 |
1-3/8 | 34.925 | 4-3/8 | 111.125 | 7-5/8 | 196.85 |
1-1/2 | 38.1 | 4-1/2 | 114.3 | 7-3/4 | 200.025 |
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英制公制對照表
英吋(inch) / 1"=25.4mm | 英制 (分數) | 公制 (小數) | 0" | 1" | 2" | 3" | 4" | 5" | 6" | 7" | 8" | 9" | | 0 | | 25.4000 | 50.8000 | 76.2000 | 101.6000 | 127.0000 | 152.4000 | 177.8000 | 203.2000 | 228.600 | 1/64 | .015625 | 0.3969 | 25.7969 | 51.1969 | 76.5969 | 101.9969 | 127.3969 | 152.7969 | 178.1969 | 203.5969 | 228.997 | 1/32 | .031250 | 0.7938 | 26.1938 | 51.5938 | 76.9938 | 102.3938 | 127.7938 | 153.1938 | 178.5938 | 203.9938 | 229.394 | 3/64 | .046875 | 1.1906 | 26.5906 | 51.9906 | 77.3906 | 102.7906 | 128.1906 | 153.5906 | 178.9906 | 204.3906 | 229.791 | 1/16 | .062500 | 1.5875 | 26.9875 | 52.3875 | 77.7875 | 103.1875 | 128.5875 | 153.9875 | 179.3875 | 204.7875 | 230.188 | 5/64 | .078125 | 1.9844 | 27.3844 | 52.7844 | 78.1844 | 103.5844 | 128.9844 | 154.3844 | 179.7844 | 205.1844 | 230.584 | 3/32 | .093750 | 2.3812 | 27.7812 | 53.1812 | 78.5812 | 103.9812 | 129.3812 | 154.7812 | 180.1812 | 205.5812 | 230.981 | 7/64 | .109375 | 2.7781 | 28.1781 | 53.5781 | 78.9781 | 104.3781 | 129.7781 | 155.1781 | 180.5781 | 205.9781 | 231.378 | 1/8 | .125000 | 3.1750 | 28.5750 | 53.9750 | 79.3750 | 104.7750 | 130.1750 | 155.5750 | 180.9750 | 206.3750 | 231.775 | 9/64 | .140625 | 3.5719 | 28.9719 | 54.3719 | 79.7719 | 105.1719 | 130.5719 | 155.9719 | 181.3719 | 206.7719 | 232.172 | 5/32 | .156250 | 3.9688 | 29.3688 | 54.7688 | 80.1688 | 105.5688 | 130.9688 | 156.3688 | 181.7688 | 207.1688 | 232.569 | 11/64 | .171875 | 4.3656 | 29.7656 | 55.1656 | 80.5656 | 105.9656 | 131.3656 | 156.7656 | 182.1656 | 207.5656 | 232.966 | 3/16 | .187500 | 4.7625 | 30.1625 | 55.5625 | 80.9625 | 106.3625 | 131.7625 | 157.1625 | 182.5625 | 207.9625 | 233.362 | 13/64 | .203125 | 5.1594 | 30.5594 | 55.9594 | 81.3594 | 106.7594 | 132.1594 | 157.5594 | 182.9594 | 208.3594 | 233.759 | 7/32 | .218750 | 5.5562 | 30.9562 | 56.3562 | 81.7562 | 107.1562 | 132.5562 | 157.9562 | 183.3562 | 208.7562 | 234.156 | 15/64 | .234375 | 5.9531 | 31.3531 | 56.7531 | 82.1531 | 107.5531 | 132.9531 | 158.3531 | 183.7531 | 209.1531 | 234.553 | 1/4 | .250000 | 6.3500 | 31.7500 | 57.1500 | 82.5500 | 107.9500 | 133.3500 | 158.7500 | 184.1500 | 209.5500 | 234.950 | 17/64 | .265625 | 6.7469 | 32.1469 | 57.5469 | 82.9469 | 108.3469 | 133.7469 | 159.1469 | 184.5469 | 209.9469 | 235.347 | 9/32 | .281250 | 7.1438 | 32.5438 | 57.9438 | 83.3438 | 108.7438 | 134.1438 | 159.5438 | 184.9438 | 210.3438 | 235.744 | 19/64 | .296875 | 7.5406 | 32.9406 | 58.3406 | 83.7406 | 109.1406 | 134.5406 | 159.9406 | 185.3406 | 210.7406 | 236.141 | 5/16 | .312500 | 7.9375 | 33.3375 | 58.7375 | 84.1375 | 109.5375 | 134.9375 | 160.3375 | 185.7375 | 211.1375 | 236.538 | 21/64 | .328125 | 8.3344 | 33.7344 | 59.1344 | 84.5344 | 109.9344 | 135.3344 | 160.7344 | 186.1344 | 211.5344 | 236.934 | 11/32 | .343750 | 8.7312 | 34.1312 | 59.5312 | 84.9312 | 110.3312 | 135.7312 | 161.1312 | 186.5312 | 211.9312 | 237.331 | 23/64 | .359375 | 9.1281 | 34.5281 | 59.9281 | 85.3281 | 110.7281 | 136.1281 | 161.5281 | 186.9281 | 212.3281 | 237.728 | 3/8 | .375000 | 9.5250 | 34.9250 | 60.3250 | 85.7250 | 111.1250 | 136.5250 | 161.9250 | 187.3250 | 212.7250 | 238.125 | 25/64 | .390625 | 9.9219 | 35.3219 | 60.7219 | 86.1219 | 111.5219 | 136.9219 | 162.3219 | 187.7219 | 213.1219 | 238.522 | 13/32 | .406250 | 10.3188 | 35.7188 | 61.1188 | 86.5188 | 111.9188 | 137.3188 | 162.7188 | 188.1188 | 213.5188 | 238.919 | 27/64 | .421875 | 10.7156 | 36.1151 | 61.5156 | 86.9156 | 112.3156 | 137.7156 | 163.1156 | 188.5156 | 213.9156 | 239.316 | 7/16 | .437500 | 11.1125 | 36.5125 | 61.9125 | 87.3125 | 112.7125 | 138.1125 | 163.5125 | 188.9125 | 214.3125 | 239.712 | 29/64 | .453125 | 11.5094 | 36.9094 | 62.3094 | 87.7094 | 113.1094 | 138.5094 | 163.9094 | 189.3094 | 214.7094 | 240.109 | 15/32 | .468750 | 11.9062 | 37.3062 | 62.7062 | 88.1062 | 113.5062 | 138.9062 | 164.3062 | 189.7062 | 215.1062 | 240.506 | 31/64 | .484375 | 12.3031 | 37.7031 | 63.1031 | 88.5031 | 113.9031 | 139.3031 | 164.7031 | 190.1031 | 215.5031 | 240.903 | 1/2 | .500000 | 12.7000 | 38.1000 | 63.5000 | 88.9000 | 114.3000 | 139.7000 | 165.1000 | 190.5000 | 215.9000 | 241.300 | 33/64 | .515625 | 13.0969 | 38.4969 | 63.8969 | 89.2969 | 114.6969 | 140.0969 | 165.4969 | 190.8969 | 216.2969 | 241.697 | 17/32 | .531250 | 13.4938 | 38.9839 | 64.2938 | 89.6938 | 115.0938 | 140.4938 | 165.8938 | 191.2938 | 216.6938 | 242.094 | 35/64 | .546875 | 13.8906 | 39.2906 | 64.6906 | 90.0906 | 115.4906 | 140.8906 | 166.2906 | 191.6906 | 217.0906 | 242.491 | 9/16 | .562500 | 14.2875 | 39.6875 | 65.0875 | 90.4875 | 115.8875 | 141.2875 | 166.6875 | 192.0875 | 217.04875 | 242.888 | 37/64 | .578125 | 14.6844 | 40.0844 | 65.4844 | 90.8844 | 116.2844 | 141.6844 | 167.0844 | 192.4844 | 217.8844 | 243.284 | 19/32 | .593750 | 15.0812 | 40.4812 | 65.8812 | 91.2812 | 116.6812 | 142.0812 | 167.4812 | 192.8812 | 218.2812 | 243.681 | 39/64 | .609375 | 15.4781 | 40.8781 | 66.2781 | 91.6781 | 117.0781 | 142.4781 | 167.8781 | 193.2781 | 218.6781 | 244.078 | 5/8 | .625000 | 15.8750 | 41.2750 | 66.6750 | 92.0750 | 117.4750 | 142.8750 | 168.2750 | 193.6750 | 219.0750 | 244.475 | 41/64 | .640625 | 16.2719 | 41.6719 | 67.0719 | 92.4719 | 117.8719 | 143.2719 | 168.6719 | 194.0719 | 219.4719 | 244.872 | 21/32 | .656250 | 16.6688 | 42.0688 | 67.4688 | 92.8688 | 118.2688 | 143.6688 | 169.0688 | 194.4688 | 219.8688 | 245.269 | 43/64 | .671875 | 17.0656 | 42.4656 | 67.8656 | 93.2656 | 118.6656 | 144.0659 | 169.4656 | 194.8656 | 220.2656 | 245.666 | 11/16 | .687500 | 17.4625 | 42.8625 | 68.2625 | 93.6625 | 119.0625 | 144.4625 | 169.8625 | 195.2625 | 220.6625 | 246.062 | 45/64 | .703125 | 17.8594 | 43.2594 | 68.6594 | 94.0594 | 119.4594 | 144.8594 | 170.2594 | 195.6594 | 221.0594 | 246.459 | 23/32 | .718750 | 18.2562 | 43.6562 | 69.0562 | 94.4562 | 119.8562 | 145.2562 | 170.6562 | 196.0562 | 221.4562 | 246.856 | 47/64 | .734375 | 18.6531 | 44.0531 | 69.4531 | 94.8531 | 120.2531 | 145.6531 | 171.0531 | 196.4531 | 221.8531 | 247.253 | 3/4 | .750000 | 19.0500 | 44.4500 | 69.8500 | 95.2500 | 120.6500 | 146.0500 | 171.4500 | 196.8500 | 222.2500 | 247.650 | 49/64 | .765625 | 19.4469 | 14.8469 | 70.2469 | 95.6469 | 121.0469 | 146.4469 | 171.8469 | 197.2469 | 222.6469 | 248.047 | 25/32 | .781250 | 19.8438 | 45.2438 | 70.6438 | 96.0438 | 121.4438 | 146.8438 | 172.2438 | 197.6438 | 223.0438 | 248.444 | 51/64 | .796875 | 20.2406 | 45.6406 | 71.0406 | 96.4406 | 121.8406 | 147.2406 | 172.6406 | 198.0406 | 223.4405 | 248.841 | 13/16 | .812500 | 20.6375 | 46.0375 | 71.4375 | 96.8375 | 122.2375 | 147.6375 | 173.0375 | 198.4375 | 223.8375 | 249.238 | 53/64 | .828125 | 21.0344 | 46.4344 | 71.8344 | 97.2344 | 122.6344 | 148.0344 | 173.4344 | 198.8344 | 224.2344 | 249.634 | 27/32 | .843750 | 21.4312 | 46.8312 | 72.2312 | 97.6312 | 123.0312 | 148.4312 | 173.8312 | 199.2312 | 224.6312 | 250.031 | 55/64 | .859375 | 21.8281 | 47.2281 | 72.6281 | 98.0281 | 123.4281 | 148.8281 | 174.2281 | 199.6281 | 225.0281 | 250.428 | 7/8 | .875000 | 22.2250 | 47.6250 | 73.0250 | 98.4250 | 123.8250 | 149.2250 | 174.6250 | 200.0250 | 225.4250 | 250.825 | 57/64 | .890625 | 22.6219 | 48.0219 | 73.4219 | 98.8219 | 124.2219 | 149.6219 | 175.0219 | 200.4219 | 225.8219 | 251.222 | 29/32 | .906250 | 23.0188 | 48.4188 | 73.8188 | 99.2188 | 124.6188 | 150.0188 | 175.4188 | 200.8188 | 226.2188 | 251.619 | 59/64 | .921875 | 23.4156 | 48.8156 | 74.2156 | 99.6156 | 125.0156 | 150.4156 | 175.8156 | 201.2156 | 226.6156 | 252.016 | 15/16 | .937500 | 23.8125 | 49.2125 | 74.6125 | 100.0125 | 125.4125 | 150.8125 | 176.2125 | 201.6125 | 227.0125 | 252.412 | 61/64 | .953125 | 24.2094 | 49.6094 | 75.0094 | 100.4094 | 125.8094 | 151.2094 | 176.6094 | 202.0094 | 227.4094 | 252.809 | 31/32 | .968759 | 24.6062 | 50.0062 | 75.4062 | 100.8062 | 126.2062 | 151.6062 | 177.0062 | 202.4062 | 227.8062 | 253.206 | 63/64 | .984375 | 25.0031 | 50.4031 | 75.8031 | 101.2031 | 126.6031 | 152.0031 | 177.4031 | 202.8031 | 228.2031 | 253.603 | | | | | | | | | | | | |
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公制、美制和英制螺纹标准
NPT，PT，G 都是管螺纹．
NPT 是 National (American) Pipe Thread 的縮寫，屬於美国標准的 60 度锥管螺纹，用於北美地區．国家標准可查阅 GB/T12716-1991
PT 是 Pipe Thread 的縮寫，是 55 度密封圓锥管螺纹，屬惠氏螺紋家族，多用於欧洲及英联邦国家．常用於水及煤气管行业，锥度規定为 1:16． 国家標准可查阅 GB/T7306-2000
G 是 55 度非螺纹密封管螺纹，屬惠氏螺紋家族．标记为 G 代表圓柱螺紋．国家標准可查阅 GB/T7307-2001
另外螺纹中的1/4、1/2、1/8 标记是指螺紋尺寸的直徑，單位是英吋．
行內人通常用分來稱呼螺紋尺寸，一吋等於８分，1/4 吋就是２分，如此類推．
G 好像就是管螺纹的统称(Guan)，55，60度的划分属于功能性的，俗称管圆。即螺纹由一圆柱面加工而成。
ZG俗称管锥，即螺纹由一圆锥面加工而成，一般的水管接头都是这样的，老国标标注为Rc
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公制螺纹用螺距来表示，美英制螺纹用每英寸内的螺纹牙数来表示，这是它们最大的区别，
公制螺纹是60度等边牙型，英制螺纹是等腰55度牙型，美制螺纹60度。
公制螺纹用公制单位， 美英制螺纹用英制单位。
管螺纹主要用来进行管道的连接，其内外螺纹的配合紧密，有直管与锥管两种。公称直径是指所连接的管道直径，显然螺纹大径比公称直径大。
1/4，1/2，1/8是英制螺纹的公称直径，单位是英寸。转载请注明出自六西格玛品质论坛 /cdb/,本贴地址:/cdb/viewthread.php?tid=45420
公制、美制和英制螺纹标准 /cdb/viewthread.php?tid=45420 NPT，PT，
G 都是管螺纹． NPT 是 National (American) Pipe Thread 的缩写，属於美国标准的 60 度锥管螺纹，用於北美地区．国家标准可查阅 GB/T12716-1991 PT 是 Pipe Thread 的缩写，是 55 度密封圆锥管螺纹，属惠氏螺纹家族，多用於欧洲及英联邦国家．常用於水及煤气管行业，锥度规定为 1:16． 国家标准可查阅 GB/T7306-2000 G 是 55 度非螺纹密封管螺纹，属惠氏螺纹家族．标记为 G 代表圆柱螺纹．国家标准可查阅 GB/T7307-2001 另外螺纹中的1/4、1/2、1/8 标记是指螺纹尺寸的直径，单位是英吋．行内人通常用分来称呼螺纹尺寸，一吋等於8分，1/4 吋就是2分，如此类推． G 好像就是管螺纹的统称(Guan)，55，60度的划分属于功能性的，俗称管圆。即螺纹由一圆柱面加工而成。 ZG俗称管锥，即螺纹由一圆锥面加工而成，一般的水管接头都是这样的，老国标标注为Rc 公制螺纹用螺距来表示，美英制螺纹用每英寸内的螺纹牙数来表示，这是它们最大的区别，公制螺纹是60度等边牙型，英制螺纹是等腰55度牙型，美制螺纹60度。公制螺纹用公制单位， 美英制螺纹用英制单位。 管螺纹主要用来进行管道的连接，其内外螺纹的配合紧密，有直管与锥管两种。公称直径是指所连接的管道直径，显然螺纹大径比公称直径大。 1/4，1/2，1/8是英制螺纹的公称直径，单位是英寸。
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