How To Sharpen Drill Bits On A Bench Grinder Like A Professional Machinist

Sharpening drill bits on a bench grinder requires precise control of the standard 59-degree point angle, 8- to 12-degree lip relief angle, and symmetrical cutting edge lengths to restore factory-fresh drilling performance. By manually guiding the bit through a controlled two-axis motion—matching the heel-to-tip clearance—you can salvage dull high-speed steel and cobalt twist drills in less than two minutes per tool.

Pre-Operation & Equipment Checklist

Restoring twist drill bits requires an understanding of bit geometry, specifically the chisel edge, the primary relief surface, and the cutting lips. Attempting to freehand grind without proper safety equipment or wheel preparation will result in burned tool steel, uneven cutting paths, and structural bit failure.

• Essential gear, tools, and materials:
  
  
  • 6-inch or 8-inch bench grinder equipped with a fine-grit aluminum oxide grinding wheel (60-grit or 80-grit) and a dressing tool for wheel trueness.
  • Eye protection (ANSI-approved safety glasses and a full-face shield).
  • Heavy leather shop gloves (remove when actively touching the spinning wheel).
  • Quenching container filled with clean water or light cutting oil.
  • Precision drill gauge or protractor to verify the 118-degree or 135-degree split-point angle.
• Mandatory prerequisite knowledge and standards:
  
  
  • Understanding of high-speed steel (HSS) heat-sensitivity; overheating past straw-yellow or blue oxidation ruins the temper of the steel.
  • Familiarity with the shank, body, flute, land, margin, lip, heel, and chisel edge nomenclatures of twist drills.
• Estimated budget and duration benchmarks:
  
  
  • Time investment: 5 to 10 minutes for initial wheel dressing and setup, followed by 90 seconds per bit.
  • Budget: Zero additional cost if a bench grinder is already available; $15 to $30 for a replacement aluminum oxide wheel and drill gauge if needed.

Step-by-Step Drill Bit Sharpening Workflow

Step 1: Prepare the Bench Grinder and Dress the Wheel

Before bringing any metal to the machine, inspect the grinding wheel for cracks, chips, or grooving. Turn on the grinder and use a star wheel or diamond dresser across the face of the aluminum oxide wheel to expose fresh, flat abrasive material. An uneven or grooved wheel makes it impossible to achieve the flat, consistent relief surfaces required on a drill bit. Adjust the tool rest so it sits approximately one-eighth of an inch away from the wheel face and slightly below the centerline of the spindle.

Warning: Never use a wire wheel or a coarse silicon carbide green wheel meant for tungsten carbide to sharpen high-speed steel twist drills, as this causes excessive thermal shock and micro-fractures.

Step 2: Grasp the Drill Bit and Position Your Hands

Hold the drill bit with both hands to maintain total stability. Grip the shank end of the bit with your dominant hand, placing your thumb along the body for guidance. Grasp the front portion of the bit near the flutes with your non-dominant hand, using your index finger as a pivot point resting directly on the tool rest. Align the shaft of the drill bit at roughly a 59-degree angle relative to the face of the grinding wheel (which equates to an angle of about 30 degrees between the drill bit axis and the face of the wheel rest).

Pro-Tip: Rest the index finger of your lower hand securely against the tool rest edge to act as a pivot hinge, ensuring your hands move as a single locked unit during the grinding stroke.

Step 3: Establish the Cutting Edge Contact

With the grinder running at full RPM, gently touch the cutting lip of the drill bit flat against the face of the wheel. Ensure that the cutting lip is parallel to the horizontal plane of the tool rest. The entire surface of the lip should touch the wheel simultaneously to prevent rounding over the sharp edge. Apply very light downward pressure against the wheel face for no more than one or two seconds.

Step 4: Execute the Downward Sweeping Motion

As soon as the lip makes contact with the wheel, execute the classic machinist's motion: rotate your wrist downward while simultaneously sweeping the shank of the drill bit downward and away from the wheel. This complex motion grinds the primary relief angle behind the cutting edge, removing material progressively from the lip down to the heel. The motion should resemble scooping water out of a bucket with a spoon—a smooth, continuous arc ending with the shank dropped lower than the tip.

Step 5: Check and Mirror the Opposite Lip

Withdraw the bit from the wheel and dip the tip into the water quench container to cool the steel. Inspect the ground surface under good lighting; the grind should extend cleanly from the cutting edge to the trailing heel, leaving a clean, silvery finish without blue heat discoloration. Flip the drill bit 180 degrees and repeat the identical sweeping motion on the second cutting lip. Both lips must be ground equally to ensure the web center remains centered and the bit drills a hole matching its nominal diameter.

Twist Drill Geometry and Grinding Specifications

Common Sharpening Failures and Field Fixes

• Symptom: The drill bit wanders, chatters, or drills an oversized hole.
  
  
  • Root Cause: The two cutting lips are unequal in length or ground at different angles, causing the drill to orbit off-center.
  • Actionable Fix: Grind the longer lip back to match the shorter one using a drill point gauge to verify symmetry, ensuring both lips share the exact same relief angle.
• Symptom: The cutting edge turns dark blue or black during the grinding process.
  
  
  • Root Cause: Excessive pressure applied to the grinding wheel or holding the bit against the stone for too long without water quenching, destroying the heat treat hardness.
  • Actionable Fix: Grind away the discolored, softened steel by taking multiple shallow, quick passes and quenching the bit in water frequently.
• Symptom: The back of the cutting edge (the heel) rubs against the material without cutting.
  
  
  • Root Cause: Insufficient lip relief angle, meaning the heel was left higher than the cutting lip.
  • Actionable Fix: Re-position the drill bit on the tool rest and lower the shank more aggressively during the sweeping motion to drop the heel away from the cutting plane.
• Symptom: The drill bit instantly smokes and burns when touching metal.
  
  
  • Root Cause: The chisel edge is too wide or the web of the bit is too thick due to multiple sharpenings.
  • Actionable Fix: Thin the web of the drill bit by carefully grinding a small notch on both sides of the center web using the corner of the grinding wheel.

Frequently Asked Questions

What is the correct angle for sharpening standard twist drill bits?

The standard point angle for general-purpose high-speed steel twist drill bits is 118 degrees included angle (59 degrees from the centerline). For harder materials like stainless steel, a 135-degree split point is often preferred because it provides a stronger cutting edge and prevents walking on the workpiece surface.

Why does my drill bit turn blue when sharpening on a bench grinder?

The blue discoloration indicates thermal breakdown caused by friction, which removes the heat treatment (temper) of the high-speed steel and softens the tip. To prevent this, apply minimal pressure against the wheel, avoid lingering in one spot, and quench the bit in water every two seconds.

How do I know if both cutting lips are even?

You can verify lip symmetry by using a specialized V-shaped drill point gauge or by measuring the distance from the center chisel edge to the outer margin of each lip using calipers. If the hole drilled is larger than the bit size, the lips are uneven and the longest lip must be ground down.

Can masonry bits be sharpened on a standard bench grinder?

Yes, masonry bits featuring carbide tips can be sharpened on a bench grinder, but you must use a green silicon carbide wheel instead of a standard aluminum oxide wheel. Grind only the top rake face of the carbide insert rather than the relief angle to maintain the chisel-like scraping geometry.

How often should a drill bit be sharpened?

A drill bit should be sharpened as soon as you notice a drop in performance, such as requiring more feed pressure, producing powdery chips instead of continuous spirals, or emitting a squealing sound. Waiting until the bit is completely dull requires removing excessive metal to restore the edge.

Master Precision Metalworking and Tool Maintenance

Equipping yourself with the skills to accurately freehand sharpen twist drill bits eliminates workflow bottlenecks and ensures cleaner, faster, and safer hole-making operations in your shop. Explore our advanced machinery guides to further optimize your bench grinder setup and prolong the service life of your cutting tools.