Tap Limits Product Limits And Class Of Thread Engineers frequently receive a request for a Class 3B (or other class) tap. Many times, too, the customer will ask for a tap to produce a “Class 3B Fit”. Ordering taps by these specifications is incorrect, and often impractical. The following information is presented to clarify the difference between the terms Class of Thread, Tap Limits and Product Limits to make ordering taps easier and aid the customer in obtaining the tap best suited for his requirements. Class of Thread refers simply to the tolerances that control the closeness of fit between two threaded mating parts. This term should be used only in reference to a threaded assembly, as, for example, a screw and nut. Product Limits refers to the limits and tolerances of the internal or female thread. The° of tolerance is expressed by the terms Class 2, 2B, 3 and 3B. Product limits refer to the various limits and tolerances applying to nuts or internal threads and are identified by the terms Class 2, 2B, 3 or 3B. Tap Limits refers to the various sizes of taps manufactured to permit selection of a tap which will produce an internal thread within the desired product limit. Tap limits are designated as L-i, H-1 H-2, H-3, etc. In the chart on this page we have illustrated the difference between tap and product limit, using a 1/4-20 tapped hole as the example. Although ground thread taps are produced to precision tolerances under closely controlled manufacturing processes and are guaranteed for accuracy of individual elements, there is always the possibility of the presence of unknown factors which can be detrimental to good tap performance. The tap manufacturer, therefore, is not able to guarantee the size of the tapped hole. To summarize, the following points should be remembered: 1. A tap cannot produce a Class of Thread. It can produce a tapped hole within specific product limits. 2. Since it is used only in tapping a hole, or producing an internal thread, a tap has no control over the fitting properties of the mating external thread. 3. To produce what is commonly referred to as a “Class of Thread,” both the external and internal threads must be within their respective product limits. Only when both members of a threaded assembly fall within the desired class limits can the proper fit be assured. 4. The acceptability of any class of threaded hole is determined only by an accurate “Go” or “Hi” thread plug gage of the corresponding class. Similarly, the acceptability of a male part with an external thread is also determined by a corresponding “Go” or “Lo” thread ring gage. 5. When ordering taps, specify either the tap limit or the class of thread that identifies the limit of the tapped hole. 6. In special applications, supply as much information as possible. The more information our engineers have to work with, the better they can recommend a tap to do the job properly with least expense. 7. Remember, it is not always the most expensive tap which will do the job best. Our files contain many examples of special jobs which were solved with inexpensive standard taps.  link to Tap Recommendations for Classes of Thread for Unified and American National Screw Threads

Tapping and threadingmachine

Threads Per Inch Double Depth 60% Thread 65% Thread 70% Thread 75% Thread 80% Thread 85% Thread 6 7 8 9 10 11 12 13 14 16 18 20 24 27 28 30 32 36 40 44 48 56 64 72 80 0.21651 0.18558 0.16238 0.14434 0.12990 0.11809 0.10825 0.09992 0.09278 0.08119 0.07217 0.06495 0.05412 0.04811 0.04639 0.04330 0.04059 0.03608 0.03247 0.02952 0.02706 0.02319 0.02029 0.01804 0.01623 0.1300 0.1114 0.0975 0.0866 0.0779 0.0708 0.0649 0.0599 0.0556 0.0486 0.0431 0.0389 0.0326 0.0288 0.0276 0.0260 0.0243 0.0216 0.0194 0.0177 0.0161 0.0138 0.0121 0.0107 0.0097 0.1408 0.1207 0.1056 0.0939 0.0844 0.0767 0.0702 0.0649 0.0602 0.0526 0.0466 0.0421 0.0354 0.0312 0.0298 0.0282 0.0263 0.0234 0.0210 0.0192 0.0174 0.0149 0.0131 0.0115 0.0105 0.1517 0.1300 0.1138 0.1011 0.0909 0.0826 0.0755 0.0699 0.0648 0.0566 0.0501 0.0453 0.0382 0.0336 0.0324 0.0304 0.0283 0.0252 0.0226 0.0207 0.0187 0.0160 0.0141 0.0123 0.0113 0.1625 0.1393 0.1219 0.1083 0.0974 0.0885 0.0808 0.0749 0.0694 0.0606 0.0536 0.0485 0.0410 0.0360 0.0347 0.0326 0.0303 0.0270 0.0242 0.0222 0.0200 0.0171 0.0151 0.0131 0.0121 0.1733 0.1486 0.1300 0.1156 0.1039 0.0944 0.0861 0.0799 0.0740 0.0646 0.0571 0.0517 0.0438 0.0384 0.0370 0.0348 0.0323 0.0288 0.0258 0.0237 0.0213 0.0182 0.0161 0.0139 0.0129 0.1842 0.1579 0.1381 0.1228 0.1105 0.1005 0.0921 0.0850 0.0789 0.0691 0.0614 0.0553 0.0460 0.0409 0.0395 0.0368 0.0345 0.0307 0.0276 0.0251 0.0230 0.0197 0.0173 0.0153 0.0138

The descriptions, illustrations, specifications, dimensions, and part numbers are subject to change without notice.

Tapping and threadingprocess

Constants For Finding Pitch Diameter And Minor Diameter Of Screw Threads Constants for Finding Basic Pitch Diameter Constants for Finding Basic Minor Diameter Threads Per Inch Pitch In Inches National Thread Whitworth Thread Theoretical V National Thread Whitworth Thread Theoretical V 80 72 64 60 56 50 48 44 40 36 32 30 28 27 26 24 22 20 18 16 14 13 12 11-1/2 11 10 9 8 7 6 5-1/2 5 4-1/2 4 3-1/2 3-1/4 3 0.012500 0.013888 0.015625 0.016666 0.017857 0.020000 0.020833 0.022727 0.025000 0.027777 0.031250 0.033333 0.035714 0.035461 0.037037 0.041666 0.045454 0.050000 0.055555 0.062500 0.071428 0.076923 0.083333 0.086956 0.090909 0.010000 0.111111 0.125000 0.142857 0.166666 0.181818 0.200000 0.222222 0.250000 0.285711 0.307692 0.333333 0.00812 0.00902 0.01015 0.01083 0.01160 0.01299 0.01353 0.01476 0.01624 0.01804 0.02030 0.02165 0.02320 0.02406 0.02498 0.02706 0.02952 0.03248 0.03608 0.04059 0.04639 0.04996 0.05413 0.05648 0.05905 0.06495 0.07217 0.08119 0.09279 0.10825 0.11809 0.12990 0.14434 0.16238 0.18558 0.19985 0.21651 0.00800 0.00889 0.01000 0.01067 0.01144 0.01281 0.01334 0.01455 0.01601 0.01779 0.02001 0.02134 0.02287 0.02372 0.02463 0.02668 0.02911 0.03202 0.03557 0.04002 0.04574 0.04926 0.05336 0.05568 0.05821 0.06403 0.07115 0.08004 0.09148 0.10672 0.11642 0.12807 0.14230 0.16008 0.18295 0.19702 0.21344 0.01083 0.01203 0.01353 0.01443 0.01546 0.01732 0.01804 0.01968 0.02165 0.02406 0.02706 0.02887 0.03093 0.03208 0.03331 0.03608 0.03936 0.04330 0.04811 0.05413 0.06186 0.06662 0.07217 0.07531 0.07873 0.08660 0.09623 0.10825 0.12372 0.14434 0.15746 0.17321 0.19245 0.21651 0.24744 0.26647 0.28868 0.01624 0.01804 0.02030 0.02165 0.02320 0.02598 0.02706 0.02952 0.03248 0.03608 0.04059 0.04330 0.04639 0.04812 0.04996 0.05413 0.05905 0.06495 0.07217 0.08119 0.09279 0.09993 0.10825 0.11296 0.11809 0.12990 0.14434 0.16238 0.18558 0.21651 0.23619 0.25981 0.28868 0.32479 0.37115 0.39970 0.43301 0.01601 0.01786 0.02001 0.02134 0.02286 0.02562 0.02668 0.02910 0.03202 0.03558 0.04002 0.04268 0.04574 0.04742 0.04926 0.05336 0.05821 0.06403 0.07114 0.08004 0.09147 0.09851 0.10672 0.11132 0.11642 0.12806 0.14230 0.16008 0.18295 0.21344 0.23284 0.25613 0.28458 0.32017 0.36590 0.39404 0.42689 0.02165 0.02406 0.02706 0.02887 0.03093 0.03464 0.03608 0.03936 0.04330 0.04811 0.05413 0.05773 0.06186 0.06416 0.06662 0.07217 0.07873 0.08660 0.09623 0.10825 0.12372 0.13323 0.14434 0.15062 0.15746 0.17321 0.19245 0.21651 0.24744 0.28868 0.31492 0.34641 0.38490 0.43301 0.49487 0.53294 0.57733

NC (National Coarse) and UNC (Unified National Coarse) are the same. NF (National Fine) and UNF (Unified National Fine) are the same

Threaded hole Inserts

£9.99 flat shipping is applicable on orders with an aggregate weight less than 10kg. Orders exceeding 10kg will be charged default shipping rates

Tappinghole size

Servicing the Canadian provinces and territories of Newfoundland, NF, NL, Nova Scotia, NS, New Brunswick, NB, Prince Edward Island, PEI, PE, Quebec, PQ, QC, Ontario, ON, Manitoba, MB, Saskatchewan, SK, Alberta, AB, British Columbia, BC, Northwest Territories, NT, Nunavut, NU, Yukon, YT Servicing Canadian cities, Toronto, Mississauga, Scarborough, Laval , Montreal, Montréal, Vancouver, Ottawa, Gatineau, Calgary, Edmonton, Québec City, Winnipeg, Hamilton, Kitchener, Cambridge, Waterloo, London, St. Catharines , Niagara, Halifax, Oshawa, Victoria, Windsor, Saskatoon, Regina, Sherbrooke, St. John's, Barrie, Kelowna, Abbotsford, Mission, Sudbury Kingston, Saguenay, Trois-Rivieres, Trois Rivieres, Trois-Rivières, Guelph, Moncton, Brantford, Saint John, Thunder Bay, Peterborough, Ajax, Aurora, Brampton, Brock, Burlington, Caledon, Clarington, Durham Region, Oshawa, Pickering, Whitby, Halton Hills, Milton, Oakville, Mississauga, New market, Richmond Hill, Vaughn, Orangeville Servicing US 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Formula for Obtaining Tap Drill Sizes (Select nearest commercial stock drill) (Outside Diameter of Thread) - (0.01299 X Amount of Percentage of Full Thread / Number of Threads per Inch) = Drilled Hole Size (Number of Threads per Inch) X (Outside Diameter of Thread - Selected Drill Diameter / 0.01299) = Percentage of Full Thread Figures in table show amount to subtract from O.D. of screw to obtain specific percentages of thread.

American tap manufacturers use a series of tap pitch diameter limits. These limits feature a .0005” tolerance in tap sizes #0 through 1 inch, and a .001 inch or greater tolerance in tap sizes above 1 inch through 1-1/2 inch diameter, inclusive. The chart shows the relationship between tap pitch diameter limits and basic (nominal) pitch diameter.

The pitch can be found by looking at a chart for either NC or NF and changes depending on the diameter. (See tables below)

Tapped hole vs threaded hole

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