I see this discussion is still going on, so let me ask for a little help. I've had time to go back and reread the book with the critique of Marx's value theory that I tried to defend and want someone to show me where the flaw is. The book is Gavin Kitching, Karl Marx and the Philosophy of Praxis. After summarizing Marx's representation of production as a physical process in which machines and labor are combined to produce commodities and then Marx's explanation of the relations of price and value in these commodities, Kitching quotes Marx: "Therefore the means of production can never add more value to the product than they themselves possess independently of the process which they assist. However useful a given kind of raw material, or a machine, or other means of production may be, though it cost 150 Pounds, or say, 500 days' labour, yet it cannot, under any circumstances, add to the product more than 150 Pounds." p. 205, Capital I, Progress Publishers
Kitching then quotes Marx on labor: "The action of labour power, therefore, not only reproduces its own value, but produces value over and above it. This surplus value is the difference between the value of the product and the value of the products consumed in the formation of the product, in other words of the means of production and the labor power." p. 208
Kitching's claim is that Marx just asserts this difference between machines and labor and can only sustain it as a proposition because he doesn't apply the same accounting procedure to labor as he does to machinery. Taking the 500 days SNL in the machine and spread it over 500,000 commodities and you get 1,24 minutes of SNL per commodity. Now let's say that the SNL for the sustenance of a laborer is also 500 days of SNL and over their working life they produce 500,000 commodities. Presented like this, we can say the laborer simply transferred 1.24 minutes of SNL to each commodity but added no new value. Marx's reply is that only living labor power can work in excess of the SNL required for its production.
Kitching answers: "But this reply is no reply at all, because it is not true. Machines can do this too. Indeed, if a capitalist could not run machines productively (i.e. at average or above average levels of productivity) for a time well exceeding the labor time embodied in them, it is doubtful whether he or she would want to install such machines at all."
I said in a reply to Tom Walker that a stick in Marx's tensegrity model was broken. This is that stick. Either neither machines or labor add value or both do. I think both do, but not value in Marx's sense; only value in the use value sense.