Nominal thread size

Where on the threads would I need to measure to find the nominal size of a bolt?
If I measure the major diameter of an external thread (bolt), the size
is always considerably smaller than the nominal size. I.e. if I measure an M12 bolt, it will measure around 11.8 mm or so. Similarly, if I measure a 1/2-inch bolt, it will measure around 12.5 or 12.6 mm, where 12.700 would be exactly half an inch.
The problem occurs when I want to make an external thread on the lathe. So far, I have always turned the stock to the nominal size (12 mm for an M12 thread), and then cut the treads. This consistently produces terrible results, where the nut will not fit until the thread is cut way too deep.
So, I suspected that maybe the nominal thread diameter would be referenced from the theoretical point where the flanks of the treads would meet to a sharp point. To check this, I drew a quick thread in Fusion 360 (it has built-in threads), and did some measurements there. Alas, it still does not work out. In this case, the size comes out too large, by about the same amount it was small before.
--
RoRo

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typed in rec.crafts.metalworking the following:

    The nominal size is what the bolt started out as. E.G., the M12 bolt started as 12 mm diameter shaft. (Same with wood sizes - a 2"x4" board _started_ life as a 2x4 inch board, but then was faced, and reduced in size).     The problem is, that 12mm bolt has to fit into a 12mm dia hole. Which leads to the question of "class of fit" (sloppy, normal, or too tight), maximum material conditions and is the hole "in spec" but too small for a bolt also "in spec". (also known as "2+2=5 for large values of 2 and small values of 5.")
    So, measuring across the threads gives you a number "close" to the actual nominal size, and round up. Always up - unless you're measuring the inside of the nut. Then you round down.
tschus pyotr
-- pyotr filipivich "With Age comes Wisdom. Although far too often, Age travels alone."
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    Are you using a threading tool which comes to a sharp point? Normal threads (especially commercial ones, which are rolled rather than cut) have a rounded bottom to the V, and a rounded or squared off crest to clear the rounded bottom of the V in the nut's thread.
    Do you have a copy of _Machinery's Handbook_? (Just about any edition will work for this). It will have a drawing of the threads, showing the rounded crests, and the square crests from different styles and means of production. (E.G. thread turning vs thread rolling.)
    If you buy inserts for insert-type threading tools, you can get either sharp pointed (will work down to a certain number of TPI (or up to a certain thread pitch for metric threads), but you have to make up for how much extra your sharp point will take out. If you're going to be turning a lot of a given thread, get the "full form" inserts, which have the right truncation at the point (bottom of the thread), and will also cut the truncation on the crest to the right shape.
    Also -- you need a different feed in for the toolpost at 20 degrees (or 19-1/2 degrees) than for feeding in square.
    The drawings in _Machinery's Handbook_ give the formulas for calculating all of the features of the thread.

    Missing the crest truncation -- either rounded or square.
    Good Luck,         DoN.
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On 9/2/2018 7:17 PM, DoN. Nichols wrote:

is done with the compound .
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    A mistype --- the 29-1/2 degrees is what I intended, or 30 degrees.
    Sorry,         DoN.
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On 9/3/2018 8:37 PM, DoN. Nichols wrote:


mistake .
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Here's a table that lists the max and min major diameters: http://www.fasnetdirect.com/refguide/Machinescrewthreads.pdf
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