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  • Countersinks

    I was re-reading an HSM serial article from late 2003 - June 2004, by Robert Bailey : a precision Ball Maker he made for his 9" SB lathe. It remains to this date one of the best articles or projects, in my opinion; though there have certainly been many great ones.

    For several bolts/screws he specifies 100deg counter sunk flat head screws. Now I was aware that several different angles are used for countersinking, but most commonly I usually see 82deg.

    I've been wondering about his reason for the 100deg. I can see where that wider footprint would spread the force, and since this was 1/4" material (steel) perhaps his concern was having enough material left under the screw head.

    ...or perhaps I've just been under the wrong impression all along and 100deg is more common for metal applications (as opposed to wood, plastic, etc..)

    I would appreciate any comments from all those better informed in this area than I. ...which is pretty much everybody!

  • #2
    The most common are 82 degree for inch based flathead screws, 90 degree for metric based flathead screws and 100 degree for flathead aircraft rivets. There are lots of other combinations though.

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    • #3
      Originally posted by Toolguy View Post
      The most common are 82 degree for inch based flathead screws, 90 degree for metric based flathead screws and 100 degree for flathead aircraft rivets. There are lots of other combinations though.
      Yes, and 100 was probably the only C/S he had ?

      When you use the wrong C/S , like using a 100 C/S with a 82 degree screw ( SAE for example ) the screw
      puts all the pressure near the hole center which can allow the hole to be upset/deformed
      It is always good practice to match taper angles for proper retention

      Rich
      Last edited by Rich Carlstedt; 12-27-2013, 05:58 PM. Reason: added comment

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      • #4
        I've only used 100° screws in my designs twice. Both times were because the stock was thin and a normal fastener would bottom out before seating in the countersink. They were brackets that needed to be as low profile as possible.

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        • #5
          Toolguy got it.
          100 Degree Countersinks are mainly used for Aircraft Rivet heads.

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          • #6
            I think the reason behind using 100 degree for aircraft rivets is because most of them are holding sheet metal skins to the underlying structure. A smaller angle head would "bottom out" sooner and would need the hole in the underlying rib to be at least partially countersunk as well as the sheet metal skin. Although the other angles could be used, the larger holes in the sheet metal skin would result in a weaker bond.
            Paul A.

            Make it fit.
            You can't win and there is a penalty for trying!

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            • #7
              Originally posted by Toolguy View Post
              The most common are 82 degree for inch based flathead screws, 90 degree for metric based flathead screws and 100 degree for flathead aircraft rivets. There are lots of other combinations though.
              I never knew metric flathead screws were 90؛. Thanks for that. (I used 82؛ for both and never realized they were different.)

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              • #8
                the difference between 90° and 100° escapes me. i have sofar only owned 90° and 120° (for deburring) countersinks. unfortunately all of them chatter, unles you go 30 rpm.

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                • #9
                  This Robert Bailey, who designed this ball maker was a design engineer with over 50 years experience at General Electric; I suspect he had some specific, technical reasons for using 50 degree CS screws.

                  For the same size screws, say 1/4-20, would the head of a 100deg CS be wider, and/or thinner (i.e. from shank to top of flat head) than an 82deg screw (same manufacturer)?

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                  • #10
                    A 100 degree head would be thinner. Maybe not wider. I have not run across any bolts with a 100 degree head that I know of or seen any in a catalog. I'm not sure where one would find them. The 100 degree rivets are going through thin sheet metal so they need a thin head.

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                    • #11
                      Originally posted by Toolguy View Post
                      ....I have not run across any bolts with a 100 degree head that I know of or seen any in a catalog. .
                      Hmmm, so they're not common then. His bill of material listed them under 'Purchased Parts', but maybe he just had some on hand and wanted to use them up. It just caught my attention in reading the article recently. I don't think I noticed that spec. when the article was first published.

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