While buying just the sizes you need is OK, drilling the larger holes for the larger diameter tubes is best done in stepped increments. I recommend you buy a 1/16 to 1/2 set, then add the odd sizes, that is metric and Letter, as you need them. With your new lathe you can use the drill bits to start your hole and then bore it out to precisely the right size. I have them from a 1/16 to 1/2 by 32nds, then up to 3/4 by 16ths, plus the letter set. I use almost all of them occsionally. I fill in on the lower end with individual numbered drills. OF course I tend towards overkill.