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Drilling Technique - Where Sharp Tools Meet Metal

Good technique separates the professionals from the amateurs. Having the right tools is only the beginning - knowing how to use them effectively makes all the difference. This chapter covers the practical techniques that will help you drill clean, accurate holes consistently.

Setup Checklist

Before you pull that handle:

  1. Work secured - No exceptions
  2. Speed set - Check the chart
  3. Fluid applied - Don't run dry
  4. Safety gear - Eyes especially
  5. Clear area - No loose items

The Approach

Starting the hole

  1. Bring drill to work slowly
  2. Let center punch guide it
  3. Light pressure to start
  4. Feel it center up
  5. Increase pressure smoothly

Common mistake: Slamming into the work. Let the drill find center.

Pilot Holes - The Professional Approach

A pilot hole is a small-diameter hole drilled as a guide for larger holes or fasteners. Think of it as creating a precise starting point that eliminates drill wandering and reduces the cutting forces required for the final hole size. This technique is fundamental to achieving accurate, clean holes in metal fabrication.

What Pilot Holes Are and Their Purpose

Pilot holes serve multiple critical functions in metalworking. They provide a centering guide that prevents larger drill bits from walking across the surface, especially on curved or angled surfaces. They also reduce the web thickness that the final drill must cut through, significantly decreasing the torque and thrust forces required. This reduction in cutting forces translates to longer tool life, better surface finish, and more accurate hole placement.

The pilot hole also creates a stress concentration that helps the main drill bit start cutting immediately rather than deforming material. This is particularly important in work-hardening materials like stainless steel, where any rubbing without cutting can create hardened zones that destroy drill bits.

When to Use Pilot Holes

Use pilot holes whenever drilling holes larger than 1/4 inch in metal, especially in materials over 1/8 inch thick. They are essential when:

  • Drilling hardened materials like stainless steel or tool steel
  • Working with thin sheet metal where breakthrough might cause distortion
  • Creating holes in round stock or angled surfaces where drill walking is likely
  • Using portable drills where control is more difficult than on a drill press
  • Drilling large holes (over 1/2 inch) in any material
  • Working with materials that work harden, such as stainless steel or inconel

In aluminum and mild steel, pilot holes become less critical for smaller holes but still improve accuracy and surface finish. For hardened steel or cast iron, pilot holes are nearly mandatory for any hole larger than the drill's web thickness.

Determining Correct Pilot Hole Size

The optimal pilot hole size depends on the material and final hole diameter. For most applications, use these guidelines:

Standard Rule: Pilot hole diameter should equal the web thickness of the final drill bit. Web thickness typically measures 10-15% of the drill diameter for standard twist drills.

Quick Pilot Hole Sizing:

Final Hole Size Pilot Hole Size
1/4" to 1/2" 1/8"
1/2" to 3/4" 3/16"
3/4" to 1" 1/4"
Over 1" ~25% of final diameter

Material Considerations:

  • Soft materials (aluminum): Can use slightly larger pilots
  • Hard materials (stainless): Use smaller pilots to maintain drill control
  • Thin sheet (under 1/8"): Pilot should be 50% of material thickness

Step-by-Step Pilot Hole Technique

Step 1: Layout and Center Punch Mark your hole location precisely and create a center punch dimple. The punch should be deep enough to capture the pilot drill tip but not so deep as to cause stress concentrations in the finished part.

Step 2: Drill the Pilot Hole Use a 135-degree split-point drill bit for the pilot hole. These bits self-center better and cut more aggressively at the tip. Set your drill press to approximately 1.5x the speed you would use for the final hole size in the same material. Apply steady, moderate pressure - enough to produce continuous chips but not so much that the drill flexes.

Step 3: Check Pilot Hole Quality The pilot hole should be round, properly sized, and free of burrs. Any wandering in the pilot hole will be magnified in the final hole. If the pilot hole position is incorrect, it's better to fill it with weld and start over than to attempt correction with the final drill.

Step 4: Drill to Final Size Switch to your final size drill bit. For best results, use a 118-degree point drill for the final pass, as the longer cutting edges remove material more efficiently when guided by the pilot hole. The 135-degree bit did its job of starting accurately; now the 118-degree bit can optimize material removal.

Run the final drill at normal speeds for the hole size and material. The pilot hole eliminates the need for the drill to cut through the web, so cutting forces are significantly reduced. Feed pressure should be steady and moderate - the pilot hole guides the drill, so excessive force is unnecessary and can cause problems.

Pro Tips from the Field

The Sound Tell

Experienced operators drill by ear:

  • Pitch changes = problem
  • Steady sound = good cut
  • Learn your machine's song

The One-Shot Rule

Try to drill in one pass when possible:

  • Better finish
  • Less work hardening
  • Faster overall
  • Less wear

The 90% Rule

At 90% through, ease up:

  • Prevents grab
  • Cleaner exit
  • Saves drills
  • Safer

Breaking Through

When you're almost through the material:

  1. Feel for reduced resistance
  2. Back off pressure
  3. Let drill finish naturally
  4. Don't force the last bit

This prevents:

  • Drill bit grabbing
  • Material tearing
  • Poor hole finish
  • Broken drills

De-burring - The Professional Finish

After drilling any hole, burrs need removal. The easiest method I have found uses a countersink in a cordless drill. Keep one handy by your machine. Even when not visible, I consider it good craftsmanship to always de-burr and add a slight chamfer to holes. This provides a clean look and prevents cuts.

Countersink Method

My favorite for most work:

Process:

  1. Countersink in cordless drill
  2. Light pressure
  3. Just break the edge
  4. Both sides of hole
  5. Consistent chamfer

Results:

  • Clean appearance
  • No sharp edges
  • Professional look
  • Prevents cuts

Deburring Knife

A de-burring knife also works and belongs in every metal working shop. While effective at removing burrs, it does not achieve quite the same finish as a countersink, though it works especially well on aluminum.

Technique:

  1. Angle blade to edge
  2. Rotate around hole
  3. Light pressure
  4. Remove burr only
  5. Don't enlarge hole

Remember: Good technique beats expensive tools every time. Take your time, follow the process, and let the drill do the work.