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SHCS and material thickness?

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  • SHCS and material thickness?

    Is there a general rule of thumb for matching minimum material thickness to a socket head cap screw size?

    I understand that design and application will dictate a lot of this.

    Is there a simple formula like X times bolt diameter equals minimum material thickness used?

  • #2
    No.

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    • #3
      There are too many factors to take into consideration for any hard and fast rules. Just a modicum of common sense.

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      • #4
        I would think that there is a recommended number of threads needed to secure properly?
        Glenn Bird

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        • #5
          Thread engagement = 1-1/2 times diameter of screw has always been my minimum formula. Absolute minimum is about 5 threads.

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          • #6
            Does this help: https://www.nord-lock.com/insights/b...a-tapped-hole/
            or: https://www.engineersedge.com/wwwboard/posts/3626.html

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            • #7
              Usually the type of fastener and size to be used are determined by what the part is, load forces, etc.

              JL.............

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              • #8
                Measure the thickness of the nut.
                John b. SW Chicago burbs.

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                • #9
                  Originally posted by oxford View Post
                  Is there a general rule of thumb for matching minimum material thickness to a socket head cap screw size?

                  I understand that design and application will dictate a lot of this.

                  Is there a simple formula like X times bolt diameter equals minimum material thickness used?
                  Yes, There is a general rule that the tapped hole depth be 1-1/2 times diameter But that assumes the "General Rule" for ordinary applications

                  In a perfect world the screw and the tapped hole should be the same material. That is if a steel screw is put into a steel hole and they are the same steels (!) the general rule of thumb is that the depth of the tapped hole should equal the diameter. In other words, a ½-13 only needs to go in one half inch.

                  Generally, because screws have rolled threads, in most cases they are superior in strength than general materials that are tapped. As such most manufacturers call out hole Depth at 1 ½ times thread diameter.
                  If we look at a grade 5 Hex head bolt- which is 120,000 pounds tensile strength approximately, and steel plate being 60,000 pounds, we realize that the bolt should go in twice its diameter depth which would mean 2 x diameter for maximum strength in that application . Materials selected are of the utmost importance when determining a proper fastening joint. Your HSCS is 170,000 # Minimum if American made , or a Metric 12.9 class

                  Rich

                  Green Bay, WI

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                  • #10
                    Not clear whether you really mean threaded length or what. You reference screws and bolts both. Screws thread into material, bolts are screws used with a nut.

                    If bolting, I'd suggest that the needed strength is the main issue, unless there is a particular reason to use a larger size. If bolting sheet metal, that may suggest more smaller bolts to preserve tensile area while retaining bolt shear area and decent bearing areas of bolt to sheet metal.

                    For screws, usually 3/4 of the screw diameter is generally a decent minimum for strength and ability to actually screw it in. More usually thickness same as diameter or a bit over. That generally works out to the "right" number of threads.

                    Other than that, "as-needed". Electric motors are generally held together by very long bolts, for instance. Why? Because it is what is needed for a simple assembly.
                    CNC machines only go through the motions

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                    • #11
                      Sorry maybe first post is a little confusing. I am not talking about thread engagement in a piece.

                      Let’s say you have a piece of 1/2” material that you are counter boring for a SHCS. If bored so the bolt sits flat to the top, a 1/4” bolt will give 1/4” left of material under the bolt head, 5/16” bolt will leave 3/16”, and a 3/8” bolt will leave 1/8”.

                      I was asking if there was a basic rule of thumb for minimum material thickness vs the bolt.

                      I figured the first 2 posts were the answer I was going to get but was still wondering.

                      On edit, yes I am using “bolt” and “screw” interchangeably. This is the type of fastener I am talking about which have always seen called a socket head cap screw (SHCS). They may be a bolt , you can call them what you want.



                      Attached Files
                      Last edited by oxford; 01-30-2021, 12:53 PM.

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                      • #12
                        There's a neat trick when you bolt is going into thin material that doesn't meet the length of engagement standard. Drill the tap hole undersize and use a form tap. The form tap will mushroom the material out on the back side giving a greater length of engagement. Of course, this won't work with thick or high strength materials.

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                        • #13
                          Originally posted by oxford View Post
                          Sorry maybe first post is a little confusing. I am not talking about thread engagement in a piece.

                          Let’s say you have a piece of 1/2” material that you are counter boring for a SHCS. If bored so the bolt sits flat to the top, a 1/4” bolt will give 1/4” left of material under the bolt head, 5/16” bolt will leave 3/16”, and a 3/8” bolt will leave 1/8”.

                          I was asking if there was a basic rule of thumb for minimum material thickness vs the bolt.

                          It will come down to strength.

                          To have the maximum strength, you want the shear strength of the remaining material to not be less than the tensile strength of the screw. So the remaining thickness x the circumference of the screw head gives the area, and the material then gives the strength. Yes, there is some secondary stuff that affects is also, but it is just that, secondary.

                          Shear strength is often about half the tensile, so you would want, if screw and plate material were identical material, the thickness to come out with twice the area of the screw core. If they are different, then proportion it accordingly.
                          CNC machines only go through the motions

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                          • #14
                            So you're looking for material thickness under the head. Not the threaded amount.

                            I'd want to leave a minimum of one bolt diameter worth of thickness below the counterbore if there's a structural load to this use. If the use is only cosmetic such as a light duty cover I might go with a little less down to perhaps 1/2 the bolt diameter. But for anything actually load bearing or for a cover that needs to seal something with a gasket then I'd be back to the idea of 1 bolt diameter of material. Or if thicker isn't an option I'd use flat head socket screws if the values look "iffy". For greater structural loads I'd want to see more like 1.5x the diameter or more.

                            At least that's my own in shop standards.
                            Chilliwack BC, Canada

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                            • #15
                              Originally posted by BCRider View Post
                              ...I'd want to leave a minimum of one bolt diameter worth of thickness below the counterbore
                              if there's a structural load to this use...
                              That's probably a good rule of thumb but you should also consider how many fasteners you're going
                              to use. If you're concerned about strength as the material thickness beneath the screw head becomes
                              thinner just pop in a few more holes to compensate. I'm sure that the "engineers" on here will find ways
                              to confuse us with numbers and myriad calculations but in the real world where I dwell you quickly get a
                              sense of how many fasteners of a given size are required to hold something together...

                              Keith
                              __________________________
                              Just one project too many--that's what finally got him...

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