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  • Screw and Glue, or just Screw

    Still designing my CNC router.

    Modern airplanes are held together with a combination of mechanical fasteners and adhesives. I am looking at this component and wondering if there is any benefit to using both screws and adhesive to build this component. It is the cradle for the water cooled router. There are more features to be added, but let's just focus on the strength and rigidity of the aluminum to aluminum joint shown.

    The goal is to create a part which will deform very little under the application of a transient sideways load. Would a modern adhesive actually help, or would I just be wasting time and money?


  • #2
    Personally i cant say id see any adhesive offering anything the screws dont, but im also an amateur hackjob so make of that what you will. Rather than adhesive, have you considered adding some pins? Screws for attachment, pins for positioning and to lock down movement

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    • #3
      I would say no to glue but I like the idea of say 4 dowel pins to positively lock it in place.

      Comment


      • #4
        My question would be; Why is that part two pieces? Trying to save a buck on thinner stock or it's necessary for the design because something is held between the two parts we can't see?

        Eliminate the glue and make the cradle walls thicker. The "glue" won't help where you're going to have the sideways load. And why six fasteners?

        What's the scale of this part?

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        • #5
          With the screws fitted in through clearance holes you are dependent on the friction between the two surfaces to resist motion. Dowel pins eliminate that as the way to resist the motion but adhesive will do the same thing and perhaps make an easier assembly but much harder to take apart again. You rely on the screws to keep the parts together but the adhesive will be in sheer mode and will take a lot to break that.

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          • #6
            Originally posted by CCWKen View Post
            My question would be; Why is that part two pieces? Trying to save a buck on thinner stock or it's necessary for the design because something is held between the two parts we can't see?

            Eliminate the glue and make the cradle walls thicker. The "glue" won't help where you're going to have the sideways load. And why six fasteners?

            What's the scale of this part?
            Scale: 1/4-20 bolts. The diameter of the cradle is 3.25 inches.
            Why six: Seemed like a good number.
            Why two pieces: I get my aluminum from an industrial recycler who gets drops and cutoffs and so I tend to get smaller bits. I have a supply of 2" thick by 4" wide to make the cradle, and 3/4" thick plate is relatively easy to come by. I don't have much that is thicker than 2"


            Originally posted by RMinMN View Post
            Dowel pins eliminate that as the way to resist the motion but adhesive will do the same thing and perhaps make an easier assembly but much harder to take apart again.
            Originally posted by bandsawguy View Post
            I would say no to glue but I like the idea of say 4 dowel pins to positively lock it in place.
            Dowel pins: Yes, good idea. That's going to be incorporated.

            Comment


            • #7
              Originally posted by bandsawguy View Post
              I would say no to glue but I like the idea of say 4 dowel pins to positively lock it in place.
              If it's to prevent movement, two dowels is enough. If repositioning to close tolerance is important they should be as far apart as possible. Two diagonal dowels is traditional and placing them not symmetrically will insure the parts can't be reassembled in more than one position.
              .
              "People will occasionally stumble over the truth, but most of the time they will pick themselves up and carry on" : Winston Churchill

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              • #8
                While a rather different application, my results are good. I built a water tank to fit under the boiler of my 2" scale model steam roller .
                I used 1/4 thick plastic, forget exactly what now, it was grey.
                It is screwed together with 4/40 brass screws at about 1" centres. Once I had done a trial assembly I dismantled it , ran epoxy on all jointing surfaces and reassembled. It has been in service for over 20 yrs, has been hit on gateposts, dropped on kerbs and even has a small vice bolted to it to help in " field" repairs and it still does not leak os show signs of problems. Hope this is encouraging David Powell.

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                • #9
                  Two dowels will have the shear resistance of two dowels, which may or may not be enough.

                  The shear resistance of two dowels may be less than the shear resistance provided by the clamping pressure of six screws.

                  There is nothing magical about the fixity provided by dowels, any more than the fixity provided by fitted bolts or rivets.

                  Generally speaking, dowels are used to provide initial location, and resistance to external forces is provided by screws, bolts, clamps, adhesives or other means.

                  Two dowels is enough to provide location in one plane. Sometimes, as in fixture design for instance, two round dowels is too much, and one dowel is oval or has two sides relieved. The round dowel locates the part at one point and the other prevents rotation around that point.

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                  • #10
                    Use only 4, 1/4 - 28 screws and 2 dowel pins :-) (see the coarse vs fine thread post) :-)
                    ...lew...

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                    • #11
                      It should be one piece as mentioned. There's no need that I can see to take it apart. But if you're using material you can get easily and for free/cheap and have these sizes on hand then that's more than reason enough to do it this way. While it's fine as is I can see what the others are saying about dowel pins. Especially if it'll never be taken apart. If that's the case then sure, stick some form of industrial adhesive intended to work well on aluminium between the parts. It would take the place of the dowel pins being suggested. Just pay attention to the adhesive maker's instructions on use for aluminium. It can be tricky material at times after all. And due to that you may want to consider a caustic dip to lightly etch the surface and alodine as a resistance to corrosion. But again whatever the adhesive maker recommends for surface prep.

                      If that's too much bother than for the size of it and given what sounds like fairly light loads I'm thinking four screws and two pins to ensure it stays. But 6 certainly won't take much longer or break the bank for cost of the project.
                      Chilliwack BC, Canada

                      Comment


                      • #12
                        Originally posted by Lew Hartswick View Post
                        Use only 4, 1/4 - 28 screws and 2 dowel pins :-) (see the coarse vs fine thread post) :-)
                        ...lew...
                        +1 on this design.
                        OPEN EYES, OPEN EARS, OPEN MIND

                        THINK HARDER

                        BETTER TO HAVE TOOLS YOU DON'T NEED THAN TO NEED TOOLS YOU DON'T HAVE

                        MY NAME IS BRIAN AND I AM A TOOLOHOLIC

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                        • #13
                          Use a single piece of steel, stiffer, cheaper.

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                          • #14
                            A brittle epoxy alone on bendy metal will be prone to a crack suddenly letting the whole thing go. Fixings alone in a vibrating or repeatedly varying load might progressively fret their holes and end up loose. A belt and braces approach does more than double the resilience.

                            Comment


                            • #15
                              Depends.
                              In theory and practice good glue and good surfaces will be stronger than needed.
                              Much.

                              2 drilled/reamed dowel pin holes later on cannot hurt.

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