square holes - reciprocating file??
If there is a need for a fully internal hole with sharp (square, acute or obtuse) corners, the router will not work (only rounded corners)
Has anyone seen a reciprocating file alternative in their travels.
It shouldn't be that hard to make if push comes to shove!
Most filing machines use draw files, they have the cut running towards the tang not away from it!
The alternative would be broaching the corners square! It's a practice I've used frequently over the years! A hardened drill blank makes the best broaching tools, but they need to be really sharp and everything must be extreemly ridgid! Aluminum broaches real easily too!
The last shop I worked in would broach keyways and corners right in their CNC mills!
do you have a specific project? easiest way to make a square hole is to not have to - is a design change possible?
failing that, imo if its small, broach, if its big, hand file. there are plans and castings for die filers, and somewhere i've seen made that clamps on the lathe bed and is driven by the lathe. not too hard to make, challenge is the bushings wear and a subjected to the chips (so design for easy replacement) and you need die files, as mentioned cut towards the tang and have a straight tang so they can held in the chuck
here's pic of some homemade broaches i made & like eric says you can use the mill or lathe as a shaper or slotting machine to cut internal shapes