When I started working in metal I did purchase a good set of drill bits ( but now I realize they are the 'wood worker set' (17 in the set. they are the ones that have been the worst as far as longevity of use.

My personal opinion of the 'TiN' (Titanium Nitride, I think) coated drill bits are a joke.

In the past I have purchased drill bit sets that are inexpensive enough for me to afford and found that the drill bits sometimes cannot drill one hole in soft wood without being ruined by the act of using them. Now that I am into 'Metal working' I need drill bit sets designed for the metal worker and in this realm one needs to drill both clearance holes and holes to be tapped. All the Sets of drill bits I have ever purchased were designed for the Woodworker! Thus most 'Woodworkers' need only bits that make bold clearance holes and bolts come in standard sizes. When working with wood one normally does not drill holes that are to be tapped with threads, so one just uses drills that make clearance holes for bolts (yes, sometimes one needs to remove just enough material so a wood screw won't split the wood, but that in an in-exact selection process based on the piece of wood itself and so a drill bit size that is 'close' is usually good enough).

Then I turned to the table of thread sizes (page 598) and what drill bit to use and compared the numbered dimensions to the recommended 'tap drill' to use and noticed that the Numbered drills are the correct sizes for drilling holes to be tapped. Glover Pocket Ref.' book and looked up the sizes (pgs 450-459). I was just now reading the Harbor Freight flyer and noticed a drill bit set for sale that is the 60 'Numbered' bits and wondered what sizes those were. Sometimes I have to alter my design to use a different sized bolt so that I can drill a hole small enough to have decent threads in it for bolting something to the part with the hole. (It does most times make it easier to cut the threads to have an over sized hole, but it sometimes leaves me worried about the strength of the bolted joint!) so I just kind of muddle along picking the next larger size and hope there is enough material left in the hole to form threads when I am done. I kind of figured out that I would need to purchase one of the 115 piece sets to finally get all the possible sizes, but, to get quality bits it would be too expensive to do so. I purchased the best 'small' set of drill bits I could afford and it did not contain the correct drill bits for drilling holes to be 'tapped'. I just figured they were a hold over from some earlier time before some Standards Bureau settled on the fractional sizes.Ī few years ago, when I started to 'PLAY' in metal working (bought a Lathe and Mill), I began to need to drill holes that I wanted to put (tap) screw (bolt) threads into and found I never had the correct size drill for the size of bolt I wanted to make a mating hole for. I did not understand why the numbered and lettered sizes existed. For years and years I never understood why drill bits came in the standard fraction inch (1/8, 1/4, etc.) and metric sizes as well as the 'numbered' and 'lettered' sizes.

Actual size drill bit chart