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jwagh17-Peridot
17-Peridot
June 24, 2021
Solved

Which of your flat head screw dimensions are wrong?

  • June 24, 2021
  • 1 reply
  • 13511 views

We don't model the threads of a screw for various reasons. When modeling a flat-head (non-countersunk) screw, the geometry of the head (height, dia, angle) automatically determine the 'body' of the screw. Therefore, if you want an accurate 'thread body' size, you would need to have an inaccurate head angle, height or diameter. We had chosen to keep all head dimensions accurate (angle, max dia, max height) and use the body as-is. Some users are getting confused. I checked a random screw from the 'Intelligent Fastener' system and it seems like they chose to not have an accurate head diameter. I would assume a head diameter is more critical to a thread body size, if the part name is saying it is an "M5". What is everyone's thoughts? How do you approach this conundrum. 

jwagh_0-1624553854782.png

Thanks!

    Best answer by jwagh

    OK. After looking at my assembly I posted earlier, I realized that I never tested a scenario with an incorrect head height. It turns out, an incorrect head height doesn't impact the height placement of the screw in the hole. The only factors are angle and the theoretical sharp max head diameter. Therefore, it seems like McMaster's approach was the best one after all. 

    jwagh_0-1624632852887.png

     

    In your last post, since the model doesn't have a sharp edge, then the theoretical sharp is actually larger than 9.2, which is larger than the max in the spec and will raise the screw higher in the hole. This is screw 'E' in my earlier assembly.

    jwagh_1-1624632998521.png

     

    Thank you for going back and forth with me on this. I believe based on the assembly testing I've done the best approach is to have an incorrect head height, which doesn't impact placement and allows for the thread body to be nominal.

     

    1 reply

    T
    23-Emerald IV
    TomU23-Emerald IV
    23-Emerald IV
    June 24, 2021

    Real fasteners are never sharp like that.  We typically show a radius or a flat and then set the diameter equal to the actual size as measured on our actual fasteners, somewhere between the 'theoretical sharp' and 'absolute minimum'.

     

    https://www.holo-krome.com/uploads/7/2/7/5/72754089/hk_consolidated_tech_manual_full_2015.pdf

    Page 18:

    TomU_0-1624555500888.png

    KenFarley
    KenFarley21-Topaz II
    21-Topaz II
    June 24, 2021

    One thing that recently stung me about this is for some of the short screws (i.e. 1/4-20 X 1/4 long) the head is actually undercut. Not having the undercut made the geometry goofy.

    For me, the FHCS has to have the proper angle (to go with the countersink in the mating part) and I also define the head height and head diameter. The "flat" cylindrical portion of the head just "falls out" based on the height and diameter (the diameter is less than the theoretical sharp).

    Flat heads are my least favorite type of screw - I've stripped out the hex on them more than any other. Button heads are almost as bad, but you can usually get vise grips around their heads to "convince" them to exit.

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