Id say there is just as good a chance that an oversize hole is caused by run out on the chuck/drill as it is by the grind on the drill.
One can grind a drill to purposefully go oversize, and even undersize in some cases. Ive fought that a lot with the drills Ive been getting lately for the acrylic drills I have ground. Its pretty obvious to the guy I have grinding them for me. (He walks me through the happenings, so I have info to go back to the drill supplier with). and its amazing how often the drills are not ground properly.... well.. I cant say that... they are ground properly for the accuracy intended for a twist drill. But for the uses many call makers are needing them for... it gets kinda "dramafied"
Technically (at least this is what I am told and have read)... a twist drill (jobber type) are considered "within spec" if they drill +8% of the diameter / -0 - so in the case of a 5/8" (.625") drill... 8% is 0.050" :-O so in the eyes of many of the drill makers, unless youre paying big bucks for a drill, you can easiliy expect to see .005-.010" oversize which is enough to mess with a guy and his mandrel - though .005 and even .010 can be handled by the mandrels I carry, but its on the edge of comfort.
Drills are NOT a high accuracy method of hole making. And it shows in the manufacture of them. Asymmetrical flutes, bowed from internal stresses, run out, axial misalighment, improper feeds and speeds all contribute to an oversized hole. And this has really been in the spot light with our acrylic drills. Which is why I have my grinder check the holes produced, and I have pretty tight tolerances on him... so our drills end up doing quite well in acrylic... but they arent really all that well suited for wood. So far, on .750 and under drills, we've been able to keep hole tolerances to +.003/-0.000... but there is no way those drills would do that until my guy has repointed them and corrected the inconsistencies.
Reamers CAN work in acrylic... but it aint easy and not pleasant... and quite messy. Not to mention you need a lathe or drill press that will go VERY slow and have a 39/64 drill that goes over size a bit, but not quite to .625". I used to use em all the time, but to keep the crazing and heat down you had to be super slow and careful... to the point, I spend my money on a metal lathe to avoid using the reamers on acrylic anymore. A boring bar on a metal lathe, with acrylic... is your friend. And a perfect excuse for more toys :D But not something everyone can get... which is why I started the research on the drills I sell now.... for acrylic. If youre doing a bunch of acrylic bores, get a metal lathe and boring bar... youll thank yourself. But if you dont do much in the way of acrylic barrels, its probably not worth it unless you already have one.
Anyway... thought Id throw some of that out there in hopes it would help explain a bit of what people are seeing with drills.
Cheers
Wade
