I have milled bores as close as +/-.0005 on a 15 year old Johnford and as close as +/-.0001 on my Hardinges.
I folow some rules when I need to be that close
1:use an endmill that less than 1/2 the hole dia.
2:arc on and off
3:do 3 or more passes
4:if its a helical bore feed the endmill the depth per rotation past the hole |