quick and easy question about turning diamaters on the lathe for threading...

Pcmaker

Pcmaker

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I want to turn something to make a 5/8-27 bolt

What diamater do I turn down a 3/4" rod to? 5/8 is .625 in inches. Do I turn it down to that size? And for future reference, do I turn to SAE and metric to their inch equivalent EXACTLY when turning or boring for the purpose of threading?
 
Generally speaking, the major diameter should be the nominal size, in your example, it would be .625", but a small amount undersize is acceptable as well; We aim for the nominal, and after cutting the thread, we have burrs and use a file to eliminate them, the result is a smaller major diameter, that generally does not hurt anything. We can easily get overly technical over questions like this one, for most work, all that is necessary is that the parts fit together with an appropriate amount of slop, except where a tight fit is required, as with studs where the fit needs to be tight, or an interference fit.
 
I have found it best to turn the major diameter a little smaller than spec. When you get down to trial fitting having a sharp point on the thread creates problems with getting a good test fit. Plus if you turn to a sharp point, you will roll over that edge and have to file it off anyway. Same goes for internal threads, bore a little large will save work at the end.

Plus the strength in a thread is most around the area that the pitch diameters meet and very little at the points.
 
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