It depends on the diameter of the hole and how deep it is. I have done that at work a couple times and made it work but here is some problems you might run into 1. When you heat metal up to melting point, all the carbon in the surrounding are rushes to the weld making it stronger and harder. This will affect all the surrounding material as well because now you have different carbon levels in your part so in the future cracks may start to appear in the part( could be months or years) 2. Machining through welded material sucks balls. just turn your RPM down 1/3 of what your were previously running. tapping it is even worse. taps break very easily. i don't know how you plan on tapping them but i would do it by hand. a tap-o-matic would just mess it all up. 3. depending in the part you are making the surface of the part may have to be machined again to compensate for the part expanding, contracting, warping, etc. P.S. in case you didn't know the formula for calculating RPM on a lathe, mill, or drill press is CS X 4 divide by the diameter of the tool. The CS stands for cutting speed which you will have to get a chart that says the cutting speeds for different materials. i happen to remember that the cutting speed for 1020 steel is 100. so 100 x 4 = 400. now lets say its a .187 drill bit. so you would go 400 divide by .187 giving you 2139. and bam you have the maximum rpm for that material.