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Two identical screws with different thread chamfers.

Two identical screws with different thread chamfers.

Two identical screws with different thread chamfers.

  I have before me two 5/16-24UNFx 3/4 long Hex Head Cap Screws.  One has 1 to 1 1/2 threads chamfered off while the second has threads to the end (minimum chamfer).  Am I looking at two screws made to different Standards or manufacturer's discretion?

RE: Two identical screws with different thread chamfers.

Your "fully threaded" (no chamfer) part is, in general, cheaper to manufacture than a "chamfered" part.  A wide variety of part lengths can be made in a single, readily-made, relatively inexpensive "straight hole" heading die.  A chamfered point requires either a more complicated/expensive die with a "header point" (this feature also typically limits the die's use to a single fastener length), or a secondary "pointing" operation prior to thread rolling.  

If you require a point chamfer for commercial fasteners it is a very good idea to explicitly specify (and define) it up front.  Point chamfer requirements/options will depend on the specification (if any) that parts are manufactured/purchased to.

RE: Two identical screws with different thread chamfers.

I believe you're looking at manufacturer's discretion.  E.g., the first spec. I checked, which I assumed to be more or less typical, specifes a maximum chamfer of 1.25 threads, but does not specify a minimum, which means it can be far less.

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