I have a casting of the Chimney on my Adams O2 which I’m building and it is fully machines and ready to mount on the smoke box. However I have found it impossible to drill the base of the casting to accept 10 BA bolts to hold it on. ordinary HSS bits don’t touch it and I ordered some cobalt 1.5 mm and 2 mm drillbits but they don’t seem to want to touch it either!!
Does anybody have any suggestions about how to soften the casting enough to get a drillbit through it?