On the south bend lathe I always use Ridgid dark cutting oil on my taper shank bits. They mostly come out of the work looking just like they did going in. If you note which cutting edge turns out the heaviest shaving you can knock it down a little to get an even cut and a round and accurate diameter. Most of these bits are 60 years old. On smaller bits the nitrided black ones seem best ! the old machinist would sharpen the big ones by measuring with a pocket scale he always carried !
