This is a great little Engineering Problem and it needs experience to solve it. The right choice of materials in the first place would have avoided this being such a frequently reported fault in all the Kitchenaid models .It's not a difficult thing to get right – it's hardly like an engine camshaft, operating at 20,000 revs!. In the whole lifetime of the unit, used a few times every day, the hinge would hardly have to operate more than 20,00 times in a ten year lifetime.

"enough friction to prevent the grubscew backing out." The pin seems to be very hard and the grub screw doesn't manage to mark it al all -so there is virtually no movement and the grub screw can't (by the nature of the geometry) work up any compression or stretch, which is what most properly tightened bolts work on.

When the screw goes slack, there is some slop in the inner hole (the one on the top half of the machine) so merely replacing the pin with one with 'ends' on it, to stop it moving out. That slop means the top of the machine rattles about and I need to stop that – just for quietness.

I like the idea of grinding a small flat on the pin and then using thread lock. I was even wondering about putting a bit of a slot / hole and grinding a point on the set screw for it to bite into. Probably a slightly softer pin would do the job and allow the grub screw to hold it better. It's a very quick job to make changes so I can experiment. I may buy some silver steel and even harden it (?) or would it be too brittle in that application?

I see where you're going but the slop is radial and I don't think shims are needed. The pin needs to be pushed radially; It's strange but any wear should be pin against the outside four holes but they are tight enough. the central hole seems to be where the slop movement is.