UNC tolerances
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: UNC tolerances
: UNC tolerances -- pak67	Post Reply	Top of thread	Forum

Posted by: randykimball ® 07/14/2008, 14:27:56 Author Profile eMail author Edit

Hello, You need to refer to the chapters on Screw Thread Systems & Unified Screw Threads in the Machinery's Handbook. If you do not have this book and you are asking this question, ... you need to get this one. Many (including myself) consider Machinery's Handbook to be the definitive answer on many such issues. The worst suggestion of your lifetime may be the catalyst to the grandest idea of the century, never let suggestions go unsaid nor fail to listen to them. Modified by randykimball at Mon, Jul 14, 2008, 14:29:33

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: : UNC tolerances
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Posted by: pak67 ® 07/15/2008, 06:04:49 Author Profile eMail author Edit

Thanks for the reply. I do have a copy of machinery's handbook but they do not cover the larger tpi (they only go up to 4 tpi). I have extrapolated the data out to get pitch diameter max & min and minor diameter min. but not minor Ø max, for internal threads. For external thead there is an allowance which again I cant extrapolate from the data I have. MC handook does not have formulae for any of the tolerances for UNC. Again any help appreciated Thanks

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Posted by: randykimball ® 07/15/2008, 10:27:26 Author Profile eMail author Edit

Consider this: (you are closer than you may think) If you have the pitch dia. min-max, you can calculate the rest of the values picture in the "Machinery's Handbook" based on the geometry of the thread form and the chart titled "American National Standard Unified Internal and External Thread Design Profiles", bottom figure. In this figure values are assigned based on pitch and height. The worst suggestion of your lifetime may be the catalyst to the grandest idea of the century, never let suggestions go unsaid nor fail to listen to them. Modified by randykimball at Tue, Jul 15, 2008, 10:32:17

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Posted by: pak67 ® 07/17/2008, 05:25:54 Author Profile eMail author Edit

you're partly right, what you can calculate with the threadform data is the dimensions at the maximum material condition, this wont give the actual tolerance band though. Internal threads being slightly easier as they dont have an allowance, what do they use to calculate the allowance? Thanks Paul

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Posted by: randykimball ® 07/17/2008, 08:57:24 Author Profile eMail author Edit

You say you have the min and max pitch diameters. All diameter tolerances and sizes are based on the pitch diameter. Your gap between the max and min pitch diameter is the gap for the major diameter as well. The thread form follows the pitch diameter size for that pitch through all thread sizes of that pitch. The worst suggestion of your lifetime may be the catalyst to the grandest idea of the century, never let suggestions go unsaid nor fail to listen to them.

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Posted by: pak67 ® 07/17/2008, 09:11:37 Author Profile eMail author Edit

thats what I thought, but according to machinery's handbook for a 4-4 UNC 2B the pitch Ø tolerance band is 0.0147" and the Minor Ø tolerance band is 0.038"?

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