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Drill Bit Basics for Metal Fabrication

Understanding your tools is the first step to drilling success. While it might seem like all drill bits are created equal, the differences between types, materials, and designs can make or break your drilling operation. This chapter covers everything you need to know about selecting, using, and maintaining drill bits for metal fabrication work.

Understanding Drill Bits

Let's start by discussing the drill itself, specifically the drill bit. The twist drill is what we will use for all drilling covered here. It is the common drill that most people think of when purchasing drill bits, but it is good to be aware that there are other types.

An annular cutter, for example, does not remove material from the center. You typically use it in a mag-drill (magnetic) to make your way through heavy plate quickly. A step drill is excellent for hand drilling through sheet metal. Not only do you get a variety of different sizes, but it also limits the load of your cut in each step up through it. Step-drills should never be used for materials thicker than about 3mm (1/8th inch), though you can push to about 4 mm in aluminum if using cutting fluid and low RPM — beyond that, the flutes tend to clog and the step shoulders lose effectiveness.

Twist Drill Variations - What Really Matters

These twist drills might all look the same at face value, but they are not. The point angle differs between types. The most common is a 118-degree point, found at your regular hardware store. The other common angle is 135 degrees.

The 118-degree drill has a longer cutting edge, allowing it to remove more material at once. This makes it great for wood, plastics, aluminum, and even mild steel. However, it has a standard point where the center lacks any cutting edge and must deform material first in order to cut it. This is why these drills tend to wander, and they suck for metal drilling in most cases.

Most 135-degree bits have a split point with four cutting edges. Those extra cutting edges at the tip that extend to the center prevent walking on metal. The 135-degree point has shorter cutting edges that will not get loaded as heavily, making them my preferred choice for nearly all my metal work.

The Split Point Difference

This is what separates good drills from hardware store specials:

Standard Point (118°):

     \     /
      \   /
       \_/

No cutting edge
at center
  • Center doesn't cut
  • Has to deform material before it can start cutting!
  • Why drills walk around
  • Why your hole in that steel plate took 20 minutes and looks like an oval.

Split Point (135°):

    \ | /
     \|/
      V

Cutting edges
go to center
  • Four cutting edges
  • Cuts immediately on center
  • Self-centering
  • Worth the extra cost

Step Drills - Sheet Metal Champions

Step drills are great for sheet metal by hand because they provide multiple sizes in one bit and limit cut load per step. However, they're not for thick or hardened materials! Keep them under 3mm (1/8") thickness, though you can push to 4mm in aluminum with cutting fluid and low RPM.

Drill Material and Coatings

Different materials and coatings affect drill performance. High-speed steel with a bright polish is standard. The polish helps chips evacuate freely without sticking. Cobalt drills (actually steel alloy with cobalt) can be a good option for harder materials like stainless steel or cast iron.

High-Speed Steel (HSS) - Your Daily Driver

Bright Polish:

  • Polished so chips don't stick
  • Good chip evacuation
  • This is the standard choice

Black Oxide:

  • Reduces friction, but only slightly
  • Looks cool (doesn't make it better)
  • Don't get conned and pay extra for it

Titanium Nitride (TiN) Coated:

  • That gold coating you see
  • NOT made of titanium (Watch out for marketing BS)
  • Lasts slightly longer
  • Again, don't pay extra. Generally no better than HSS bits.

Cobalt Drills - For the Tough Stuff

Actually steel with cobalt alloy:

  • Use these for stainless steel
  • Use these for hardened materials
  • More expensive
  • Only buy when you need them

Various coatings exist, including titanium nitride (those gold-colored "titanium" drill bits at the hardware store are not made of titanium but coated with it), black oxide (which can reduce friction), and combination coatings. Generally, I find coatings overrated for most job work and would not spend extra for them on regular drills.

What to Buy - Drill Bit Selection

For General Fabrication

Starter Set:

  • 135° split point HSS
  • Fractional sizes 1/16" to 1/2" by 64ths
  • Bright finish is fine
  • Name brand matters (And USA made if possible)

Add Later:

  • Number drills - are for tap sizes
  • Letter drills - are for reaming
  • Cobalt drills - are for stainless
  • Larger sizes as you need them

Skip These

  • Titanium coated sets from Uncle Jeff's website or big box stores
  • Super cheap imports (100% false economy)
  • Fancy coatings for general work
  • Specialty grind unless you need it (you probably don't)

Drill Bit Care and Maintenance

Storage

Do:

  • Keep in index (those metal cases with sizes)
  • Store clean and dry
  • Keep cutting edges protected
  • Sort by size

Don't:

  • Throw in drawer loose
  • Let them bang together
  • Store wet or oily
  • Mix bit materials (eg: HSS with cobalt)

Maintenance

After each use:

  • Wipe clean with oiled rag
  • Check cutting edges
  • Light oil if storing long term
  • Put back in correct slot

Sharpening (advanced):

  • Drill doctor for small sizes
  • Bench grinder for larger
  • Keep both edges equal
  • Maintain proper angles
  • Or just buy new (time vs money)

Remember: A sharp drill bit in good condition is worth ten dull ones. Take care of your tools and they'll take care of you.