Ok i'll break down your points Timelord.You state that softening the tang does not really help absorb shock,then explain very nicely why this is so
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Yes the force is concentrated in the tang,and as you rightly said.
"1Also having a softer tang (as you should) does not really help to absorb the shockwave as you suggest. The purpose of softening the tang is threefold. 1) So that it is not brittle in the area the shockwaves focus into after travelling down the length of the blade."
Well yes if the tang wasn't ductile enough to absorb all of that percussive shock then it is a focal point for the sword to fail.As you stated "So that it is not brittle in the area the shockwaves focus into"
"2 as most historical swords were made out of steels that would "work harden", then the tang started off soft when the sword was new. this does not alleviate reason 1)"
Yes work hardening is a possibility(a very unlikely one to my mind),but it would take a lot of repeated forceful blows over an extended period of time,sword's aren't designed for that kind of impact though.
To me that would mean a lot of edge on edge strikes,or edge on armour strikes for that to even be a likely reason for faliure.The sword would be destroyed long before that due to micro fractures in the blade.
If you're parrying you would always try and use the flat of the blade (as you're well aware) to deflect a strike rather than using the sword to absorb it. Though edge on edge strikes did happen it was something to be avoided.
Even so the tang wasn't made softer to take into account work hardening,it was made softer because if it wasn't the sword would very likely fail and snap. As you stated in reason 1.
"3) So that the peen was soft & "peenable". Otherwise if just softening the immediate area of the peen, then the area immediately in front of the peen would be tempered hard & brittle. Therefore the whole mass of the tang down to and just in front of the shoulders had to be softened. This does not alleviate reasons 1) & 2)."
It wouldn't matter with regard to peening if the entire tang was hardened and tempered. As soon as you heat the end of the tang up to forging temperature it would be soft enough to peen anyway,regardless of the hardness of the rest of the tang,unless you mean cold peening which is inferior to a hot peen.
And the tang/or blade for that matter would only be brittle if the sword wasn't tempered correctly.As you know the sword is hardened,then it is tempered to alleviate the brittleness yet still leave it capable of holding a cutting edge.The tang is still left softer than the blade to help absorb the shock (it would bend rather than fracture)
As far as i'm concerned if the pommel is keyed to the tang (IE a nice snug fit on the tang without any play) then a keyed pommel with a recessed nut is more than strong enough for the purpose.
If this isn't the case (as with cheaper manufactured swords) Or if the pommel is held on with a normal nut (as apposed to a broached extended nut that has a tight tolerance in the pommel) then yes it's a bad mechanical fix and could likely cause problems.
I'm not disagreeing with the hot peen assembly being the superior,i'm just saying that a correctly made mechanical fix will certainly stand up to the use that a sword would be expected to do.Any lateral stress would be concentrated on the portion of the tang that is keyed into the pommel,and not concentrated on the threads.
At least it's been that way in my experience and I've done a fair amount of cutting with both hot peen and correctly done mechanical peens,the most I've had to do is give the nut a tweak after a heavy cutting session. A minor thing and offset by the easy of customization on this type of assembly.
I welcome your comments Time lord as this is a fascinating discussion