Originally posted by joe shmoWell I liked it! Of course the bearing stress would have a limit because if you used say, a 20 mm dowel (3/4 inch) you would start running into the limitation of the wood of the strut, now you would only have 5 mm spacing minimum to the edge of the strut. So 13 sounds like a good number, about 1/2 inch. Looks like I will be going with that.
Out of the dials you have, the optimum size is the 13 mm dial...
Three parameters can nail it down in my opinion.
Normal stress-Tension or compression from axial loading
Bearing stress -from transverse loading
Stress concentration factor - an experimentally obtained factor that is dependent on the geometry of the pieces to be mated by the pin.
...[text shortened]... g stress ~ .16* (load)/(length_pin)
man was that overkill, but good practice none the less...
One thing I saw on the old dowels, they are fluted, like ancient roman columns, presumably to make for higher surface area for the glue to contact.
Originally posted by sonhouseI think it (the fluting) is mainly to add a bit of friction-fit and to allow the glue and air to escape as the dowel is inserted.
Well I liked it! Of course the bearing stress would have a limit because if you used say, a 20 mm dowel (3/4 inch) you would start running into the limitation of the wood of the strut, now you would only have 5 mm spacing minimum to the edge of the strut. So 13 sounds like a good number, about 1/2 inch. Looks like I will be going with that.
One thing I ...[text shortened]... like ancient roman columns, presumably to make for higher surface area for the glue to contact.
Since you're inserting the dowel into a round hole, you would actually get maximum contact area from a smooth surface.
Originally posted by forkedknightIf both the hole and the dowel had the same matching cuts, there would be increased area for both friction and gluing but I only see the flutes on the dowel. I guess it would just be for friction like you said.
I think it (the fluting) is mainly to add a bit of friction-fit and to allow the glue and air to escape as the dowel is inserted.
Since you're inserting the dowel into a round hole, you would actually get maximum contact area from a smooth surface.
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Is it perhaps theoretically best to use holes of different depths? (i.e. pair a 1cm hole with a 2cm hole, and then a 0.5 cm hole with a 2.5 cm hole etc) Then the dowels don't all "end" at the same depth in the wood, which would cause a weakness there.
The reason I used the area of the circle at the end of the dowel in my calculation was it seemed that keeping the "thinness" of wood left constant at the plane of the join was fair to the different dowel widths.
As a practical matter, woods have very different physical properties, and the reality is that some woods will do better than others. For example, certain woods are much more easily steamed and bent into shape than others. Others, like metals, are more elastic or more brittle. Osage Orange dowels may be superior for this application. Of course, aesthetics are a consideration. Further, beveling the holes and rods to fit will increase the surface area to be glued, and along with the good idea of differing the hole depths, will effectively eliminate stress concentration as a problem during remotely normal loading. Such joints will be stronger than the wood itself.