[TheForge] engineering question

[marilyn traber 011221]

> Maybe the whole point here is to use a square tenon, not to find the
> strongest joint. But I gotta go with Rick. The problem with a square
> tenon as I see it is that regardless of its orientation it introduces
> sharp angles where stresses can concentrate. In m&t joints where the
> stress is rotational like in the right-angle joints in a parallellogram
> table or compression-tension like in a diagonal  brace--which is most
> of them--the breaking force attempts to tear the mortised piece apart
> transverse to its axis. A square tenon gives sharp angles on this 
> axis that need to hold up against the entire moment arm of the tenon 
> piece
> (rotational) or one or both of the moment arms on either side of the 
> joint in compression/tension (there's a vector problem there....)
>  Either way, they're a nice place for the metal to tear.
> 
> But like I say, maybe the whole point is to use a square tenon.
> 
> For what it's worth then. Physics geeks?
> 
> JRF
> 
> ps. who's got a press? 

Now, maybe I'm just dumb, but I thought we were all blacksmiths here. If in 
fact that's the case, why couldn't the smith get some 1/2 inch (or whatever 
size you want the hole) square stock, and shape it into a drift that starts 
out oval, goes to round, then ultimately squares out, with possibly leaving 
the corners a bit rounded, if the concern is the stress at the sharp corners? 
Certainly, anyone who knows how to change round to square and back again, or 
vice versa (just put my apprentice to doing that last time I worked with him) 
could handle something like that, and there certainly shouldn't be a 
difficulty shaping whatever steel you're using the make the square bit going 
through the hole just a bit to fit.

As far as the stress on the steel you're making the hole in, it seems to me, 
depending on your application, you could either upset the section to make it 
a bit thicker, or start with a thicker piece to begin with, if you have a 
concern about structural strength.

Yes, a thinner section, wherever it is, is going to be weaker, whether you 
drift squared or on the diagonal. Either way you do this, you're thinning the 
diameter of the steel. If that's a problem, then thicken it. But think about 
it before you thin everything so much that you thin it to a (weak)thread.

Saint Phlip,
CoD






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