The Complete Guide to Drill Bit Sets: What to Actually Buy

Walk into any industrial supplier and you will find drill bit sets ranging from $18 to $1,800. The sets look similar in photos — a metal case, rows of drills arranged smallest to largest — but what is inside varies enormously in coverage, quality, and value for any given shop. Buying the wrong set wastes money on drills you will never use. Not buying a set when you should means constant individual orders at high per-unit cost.

Here is how drill index systems work and how to decide what to actually buy.

The Three Index Systems: Fractional, Number, and Letter

American drill bit sizing uses three overlapping systems that together cover the full range from tiny instrument work down to large structural holes.

Fractional inch drills are the most familiar. Sizes run from 1/64" up to 4" or larger, in 1/64" increments for small sizes and 1/32" increments for larger ones. A standard fractional set covers 1/16" through 1/2" in 1/64" steps — 29 drills. A more complete fractional set covers 1/16" through 1" in both 1/64" and 1/32" steps. Fractional drills are what most shops use for the majority of work: clearance holes, counterbores, through holes for bolts and screws.

Wire gauge (number) drills use a numbering system from #1 (largest, 0.2280") down to #80 (smallest, 0.0135"). The sizes do not correspond to any simple mathematical progression — the system evolved historically and the increments are irregular. Number drills are essential for tapping: tap drill charts for unified thread series specify number drills for most thread sizes under 1/4". A #7 drill (0.201") is the tap drill for 1/4-20; a #29 (0.136") is tap drill for #8-32. Without number drills, you cannot tap correctly in this size range.

Letter drills run from A (0.234") to Z (0.413"), filling gaps between fractional sizes in the 1/4" to 7/16" range. Like number drills, letters are specified in tap drill charts — an R drill (0.339") is the tap drill for 3/8-16 UNC. Letter drills also come up in reaming work where you need a drill 0.005–0.010" undersized from the reamer diameter for a specific hole.

A complete 115-piece set contains all three: number drills 1–60, letter drills A–Z, and fractional drills 1/16" through 1/2". A 135-piece set adds fractions to 3/4". These are the standard "all-in-one" sets sold by most suppliers.

What Each System Is Actually For

Fractional for clearance and general holes. Most structural holes — bolt clearances, cable passthroughs, mounting holes — use fractional dimensions. If you are drilling for a 1/2-13 bolt, you probably want a 17/32" or 9/16" clearance hole. Fractional drills are the daily workhorses.

Number drills for tapping small threads. Any shop that taps #4 through #12 screws uses number drills constantly. Without them, you either drill oversize (threads strip) or undersize (taps break). A number drill set is not optional for tap work in this range.

Letter drills for tapping large threads and close-tolerance work. Less frequently needed than number drills, but when you need a Q or an S drill and don't have it, no fractional drill will substitute cleanly. Letter sets are important for shops tapping 5/16" and larger threads frequently.

Which Sets Are Worth Buying

The 115-piece or 135-piece combination index: For general job shops, this is the right starting point. It covers all three systems in one case, costs $80–$200 for a decent M2 HSS set from a name supplier (Cleveland, Norseman, Irwin industrial grade). The combination index handles probably 90% of drilling situations without hunting for individual drills. Buy one good set rather than three cheap sets.

Jobber length vs. screw machine length: Standard jobber drills are the length included in most sets. Screw machine (stub) length drills are shorter and stiffer — better for production work where rigidity matters. If your shop runs production on a specific set of sizes, consider buying those specific sizes in screw machine length and keeping the jobber set for general use.

Cobalt (M42) vs. standard M2: M42 cobalt sets cost roughly 2–3× more than M2 sets. For a combination index that will see general use — mild steel, aluminum, soft alloys — M2 is adequate and more economical. For shops drilling stainless, heat-treated alloy steels, or titanium regularly, a cobalt set pays back in tool life. Some shops keep an M2 combination index for general work and a cobalt set just for the tap drill sizes they use in hard materials.

Fractional-only sets for large sizes: Once you are past 1/2" diameter, number and letter drills are no longer relevant. A fractional set from 1/2" to 1" (or to 1-1/2") in 1/32" steps is the right format for large-hole work. These are sold as separate cases and are worth having if your shop regularly drills in this range.

When Not to Buy a Set

When you have a known high-consumption drill. A 29-piece fractional set includes one #3/8 drill. A production shop drilling 3/8" holes in 4140 all day burns through that single drill quickly and is left with 28 rarely-used others. Buy the production sizes individually in quantity. Sets are for general shops that need every size occasionally, not for shops that hammer on two or three sizes constantly.

When your work is dominated by a specific thread family. A fastener shop tapping almost entirely #8-32, #10-32, and 1/4-20 needs exactly three tap drills: #29, #21, and #7. Buying 115 drills to get those three is wasteful. Buy the tap drill sizes you actually use, in quantity, in the grade you need.

When quality matters more than coverage. A $180 combination index from a reputable manufacturer will have better geometry, tighter tolerances, and last longer than a $35 set from an import supplier. If budget is constrained, buy a partial set of good drills rather than a full set of poor ones. The cheap sets often have inconsistent point geometry and poor web thickness — they drill oversize and wear fast.

Managing Set Wear in Production

The biggest weakness of combination index sets in production is that frequently-used sizes wear down quickly while rarely-used sizes sit untouched for years. A well-used 115-piece set might have 12 drills that are dull or missing and 103 that look new.

Two approaches work well here. First, designate specific sizes in the set as "consumable" and track them — when a drill in the set gets dull, pull it out, send it for resharpening (if 1/4" or larger), and put the resharpened drill back. The set stays functional and you are not replacing the full set each time a few sizes wear out. Second, for the most-used sizes, carry spare individual drills outside the set case. When a size in the set is dull, swap in a fresh spare and send the dull one to resharpening without disrupting the set.

Resharpening HSS drills from a combination index is economically sound for sizes 1/4" and larger. Below 1/4", replacement is usually cheaper than resharpening. Know which category each size falls into and manage accordingly.

The Short Answer

Buy a quality 115-piece combination index (M2 HSS for general work, cobalt if you run hard materials) from a name supplier. Supplement it with individual drills in your highest-consumption sizes. Do not buy cheap import sets — the geometry inconsistency costs more in ruined holes and tap breaks than you saved on the set. If you are a production shop drilling primarily two or three sizes, skip the set and buy those sizes individually in the grade and quantity that fits your actual usage.