Drilling

Standard twist drill bits

Drilling is a fundamental machining operation used to create round holes in a variety of materials using a rotating cutting tool called a drill bit. It is performed using tools such as drill presses, milling machines, or lathes with tailstock drills. This page covers drill bit types, material considerations, marking techniques, best practices, and common mistakes to avoid.

Drill Bit Types

There are many types of drill bits, each suited for specific hole geometries or materials.

Standard Twist Drills

• Imperial Sizes: Measured in inches (e.g., 1/4", 3/8")

• 

  Types of drill bits

  Metric Sizes: Measured in millimeters (e.g., 6mm, 10mm)

Letter Gauge Bits

• Labeled A–Z, commonly used in aerospace and precision applications.
• Example: Size "F" = 0.257" (6.53 mm)

Wire Gauge Bits

• Numbered #1 (largest) to #80 (smallest).
• Useful for small holes and electronics work.

Tapered Shank Bits

• Used in drill presses or lathes with matching taper sockets (e.g., Morse taper).
• Provide better concentricity and torque transmission.

Reduced Shank Bits

• Larger diameter bits with a shank turned down to 1/2" or 3/8"
• Useful when your chuck cannot fit the full-size shank.

Hole Saws

• Circular cutters for large-diameter holes in sheet material.
• Not suited for high-precision or deep holes.

Step Drill Bits

• Cone-shaped bits with incrementally larger steps.
• Excellent for enlarging holes in thin sheet metal.

Center Drill Bits

• Short and stiff bits used to start holes precisely.
• Often used in lathes to create a pilot hole for tailstock drilling.

Forstner Bits

• Flat-bottomed hole cutters for woodworking.
• Provide clean edges and flat holes.

Drill Bit Materials

Drill bits are made of various materials depending on the application:

Material	Properties & Use
High-Speed Steel (HSS)	General-purpose. Affordable. Good for mild steel, wood, and plastic.
Cobalt Steel (M35/M42)	Withstands higher temps. Better for stainless steel and hard metals.
Carbide	Very hard and brittle. Ideal for production environments or composites.
Titanium Coated	Reduces friction and wear. Not meant to be resharpened.
Black Oxide Coated	Increases lubricity and corrosion resistance.

Center Punch used for marking hole centers for drilling.

Marking and Starting Holes

Accurate hole positioning is critical.

Marking Out

• Use a scribe and steel rule or layout dye for precision.
• Mark the hole with a center punch to guide the drill bit and prevent wandering.
• For very small bits, use a spring-loaded automatic center punch.

Starting Techniques

• Always spot drill or use a center drill to create a pilot feature before using twist drills, especially in metal.
• When using a large bit (over 1/2"), consider pilot drilling with a smaller bit first.

Drilling Techniques

General Process

1. Clamp work securely.
2. Mark and punch the hole location.
3. Use cutting fluid as appropriate for the material.
4. Drill at the recommended speed and feed.
5. Clear chips frequently (peck drilling if needed).
6. Deburr hole edges after drilling.

Peck Drilling

• Drill in small increments, backing out between each peck to clear chips and reduce heat.
• Essential for deep holes or materials like aluminum that tend to clog flutes.

Backing Material

• When drilling through workpieces (especially sheet), place a backing board underneath to reduce burrs and protect the table.

Best Practices

✅ DO's

• Use the correct speed and feed for your material and bit.
• Always clamp the workpiece securely.
• Apply cutting fluid when drilling metal (except cast iron).
• Start with a center drill or spot drill for accuracy.
• Use sharp bits—dull tools cause heat and inaccuracy.

❌ DON'Ts

• Don’t drill without marking or center punching.
• Don’t use excessive pressure—let the bit do the work.
• Don’t use wood bits on metal or vice versa.
• Don’t force a dull or undersized bit into a hole to "open it up"—use a reamer or the correct size instead.
• Don’t forget to clear chips—especially on deep holes.