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Two identical screws with different thread chamfers.

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wh

Mechanical
Feb 22, 2002
7
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?
 
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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.
 
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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