All worktop jigs have the ability to cut both 45 and 22.5 degree
internal joints, but remember that those angles are not
throughout the joint, they are only at the leading (front) edge. The joint is otherwise a simple butt joint, held together with bolts in routed slots - I prefer to also use a single (28mm top) or double (40mm top) row of biscuits to help with alignment and glue-up.
Cutting a joint using the 45 degree option is for a 90 degree internal joint, the 22.5 degree option used for what is often referred to as a "corner solution" or a "cooker solution" where a portion of the worktop is fitted diagonally across a corner. If that's
still not clear, check
here for pictures.
The original enquiry was about fitting an
external 90 degree corner - in other words, a situation where the rolled (post-formed) edge of the worktop goes around the
outside of the corner,
not the inside. A worktop jig is no use in that situation, because the joint is
not a conventional butt joint.
In order to achieve an
external 90 degree joint, as Red realised, the worktop must be accurately mitred over its entire mating surface, with the mitre bisecting the required 90 degree angle and thus forming the tip, or point, of the external corner - the same as an external corner joint in ceiling coving (only flat!).
The difficulty in completing this joint both accurately and cleanly arise from the inclusion of a post-formed edge in the worktop, coupled with the direction of cut of a router. Router cuts need to
start at the rolled edge. If you exit the cut at the rolled edge, chipping of the laminate
will occur, so - for obvious reasons, you cannot cut two at the same time as suggested by
tombo above (because one would be against the direction of cut and the resultant chattering this would cause has the potential to ruin both edges. Anyway - just don't even think about doing it - it's not practical and it's certainly not safe either).
What must be done is to cut one side face up (if stood inside the corner, the right-hand side), with the other side being cut face down in order to orient the cutter correctly. This introduces another variable - transferring mark-up lines to an opposing face - but that one is simple enough to deal with.
The routed edges will be cut using a straight-edge clamped across the worktop at 45 degrees (the straight outside edge of the worktop jig may be suitable for this if it's long enough), then cut with a pattern-following bit or using a guide bush with the straight-edge clamped correspondingly further away from the cut line.
None of this is impossible, but it needs attention to detail, accurate marking and absolutely rigid clamping of the straight-edge. I would cut most of the waste with a saw first (circular, jig saw or hand saw, doesn't matter as long as it's sharp), then clean with a router as Red originally suggested. Once I'd got the mating faces
absolutely right, I'd then rout the butterfly slots, clamp the tops together dry, offer the assembly up and scribe it in if required. Depending upon the size of the clamped assembly, the offering-up will probably be a two-person job and needs tackling safely.
Apologies for the length of this post, but I felt that it was going astray in a way that was not going to help Red complete the project.
Ray.