broaching

[eotrfish]

You could drill out most of the waste material then farm it out to a shop with a decent ram EDM. Should be able to hold +/- .002” with perfect alignment and dead square corners.

 

[historicalarms]

I think two 1/2" HSS square cutters ( 6 or 7 bucks each) to make a couple of "usable" broaches and then used side by each in the slot that needs to be cut in the alignment/guide shaft would work..at least that's how I would "frugally" attack a one-off that I may never use again.

Another "usable in a pinch" method I can come up with is to drill/ mill all the excess metal away as was previously mentioned but I would drill the pilot hole a bit deeper in the work-piece and mill past the 1" requirement of the opening then install the 1" key or whatever is being used in the slot and them tightly shim the backside of the cutout to tighten the slot to size required.

 

[Dabbler]

Asking 5 machinists how to do something is likely to yield 10 or more ways to do it... If you ask an expert, the likely response is 'it depends'.

 

[Everett]

Where do you live MHW20? There are a few people with shaper/slotting machines around, you might find a local with one who you could horse trade favors with. If I was to do something of a square hole that size that would be my first thought for machinery, but like a number of people have already said more information about material/purpose/etc would be helpful.