Cast iron, for the most part, is pretty easy to machine, the main annoyance being it is very dirty due to the large amounts of graphite. Old cast iron can really suck to machine: large chunks of carbide that are only grind-able (they destroy endmills and files skate on them) and are fragile/brittle. A friend of mine was a machinist “back in the day” (WWII?) and said they had to be vigilant because sometimes foundries would use porcelain cast iron bathtubs in their melts, resulting in chunks of porcelain in the cast parts that destroyed tooling.
Stanley seems to have used two different types of CI in their planes: the bodies and frogs seem OK (probably because they had to machine the castings) but all the cap irons (3?) I’ve modified have had carbides in them.
This casting (Ohio?) was pretty rough, lots of flash, sprues sort of ground off, etc. While cleaning it up, I knocked it out of the vise, it bounced a time or two on the bench then hit the concrete floor. Oh sh!t. Oh sh!t indeed as it did break. I brass brazed it back together (you can see a line of brass in the photo, the break wanders over to the middle of the part) but what a pain. Cast iron is soooo dirty that the flux had a really difficult time cleaning so the brass could stick. As it was cooling, I was waiting for the dreaded “ping” when the CI breaks again. Got lucky. Clean up was a lot of work to get all the brass out of the corners and clean up the file scratches.
I added the plate with the cupids bow to cover the cut out (in both the metal and wood) where the old frog sat. Strangely enough, silver brazing the part went quite smoothly. But, as I was fitting it to the back of the aluminum bed, I ran into a big carbide. Out came the die grinder, which is not a precision tool. Luckily, the result is not visible.