Which Drill Bit Is Best For Drilling Glass ?

Precision and Patience: The Expert Guide to Selecting and Using Drill Bits for Glass

Working with glass is one of the most nerve-wracking tasks for any contractor or serious DIY enthusiast. Unlike wood, which yields to force, or metal, which curls away in ribbons, glass is an amorphous solid that does not "cut" in the traditional sense. Instead, drilling glass is an exercise in controlled abrasion. One wrong move, one spike in temperature, or the wrong choice of hardware can result in a catastrophic "spiderweb" crack, ruining an expensive piece of material—like a custom-tempered shower door—in a fraction of a second.

At Mutual Screw & Supply, we understand that professional results require professional-grade tooling. This starts with selecting brands that have a proven track record in industrial applications. For professionals who cannot afford a mistake, we almost exclusively recommend Relton products. Known for their industry-leading engineering and "Groo-V Tip" technology, Relton bits are the gold standard for anyone serious about glass and masonry work.

1. The Cardinal Rule: Know Your Glass

Before you even reach for your drill, you must identify the type of glass you are working with. This is the most critical step in the entire process.

Annealed Glass

Commonly found in window panes, mirrors, and picture frames, annealed glass can be drilled. It is designed to be cut and shaped after it is manufactured. If you make a mistake, it will crack into large, sharp shards. For these applications, a precision bit like the Relton GRT-3-3 is ideal for creating clean, accurate pilot holes or mounting points.

Tempered Glass

Never attempt to drill tempered glass. Tempered glass is "safety glass" that has been heat-treated to increase its strength. However, this process creates massive internal tension. If you penetrate the surface "skin" of tempered glass with a drill bit, the entire sheet will instantaneously shatter into thousands of small pebbles.

Most shower doors, patio doors, and glass tabletops are tempered. If you are installing a shower door and realize you need an extra hole, you cannot drill it yourself. You must order a new pane from the manufacturer with the holes pre-drilled before the tempering process. Look for a small etched "bug" or logo in the corner of the glass to confirm if it is tempered.

2. Choosing the Right Tool: Premium vs. Value

When browsing our full collection of drill bits, you will notice a significant range in price and performance. Selecting the right one depends on your project frequency and the value of the material you are drilling.

The Premium Standard: Relton Groo-V Tip Bits

For high-stakes projects like custom shower installations or commercial glass work, we recommend the Relton Groo-V Tip series. These bits feature a specialized serrated carbide tip that penetrates materials up to 50% faster than standard bits.

  • The Relton GRT-3-3 (3/16" x 3"): This is our most popular bit for small-diameter precision work. Its 3-inch length provides excellent stability for mirrors and standard glass panes.

  • The Relton GRT-3-4 (3/16" x 4"): For those requiring a bit more reach or working with thicker glass assemblies, the 4-inch version offers the same legendary Groo-V performance with added length.

  • The Relton GRT-4-4 (1/4" x 4"): When your hardware requires a slightly larger bore—common in many modern shower door hinge assemblies—the 1/4" GRT-4-4 is the professional's choice.

The Budget-Friendly Option: VG Series

If you are a homeowner tackling a one-time project and need a reliable but less expensive solution, the VG-3002 (3/16" x 3") is an excellent entry-level bit. While it lacks the high-speed "Groo-V" serrations of the Relton line, it is a dependable carbide-tipped bit that will get the job done on a budget, provided you follow proper cooling techniques.

3. The Physics of the Process: Speed, Pressure, and Heat

To drill glass like an expert, you must manage three variables: RPM (speed), Feed (pressure), and Temperature.

Variable 1: Rotational Speed (RPM)

Glass drilling requires a slow, steady hand. High speeds generate friction, and friction generates heat. Heat causes the glass to expand locally, while the rest of the pane remains cool. This "thermal shock" is the primary cause of cracking.

  • Small bits (3/16"): Aim for 400–600 RPM.

  • Large bores: Aim for 100–200 RPM. If you see smoke or smell burning, you are going too fast. High-quality bits like the Relton GRT series are designed to handle these lower speeds with maximum efficiency.

Variable 2: Pressure

Let the bit do the work. If you find yourself leaning your body weight into the drill, you are asking for a "blowout." Apply only enough pressure to keep the bit in contact with the material. The specialized tip on the GRT-4-4 is specifically engineered to "bite" the glass without requiring excessive force.

Variable 3: Lubrication and Cooling

You should never drill glass dry. Water is the most common coolant.

  • The Dam Technique: For horizontal surfaces, create a ring around your marks using plumber's putty. Fill this "dam" with water to keep the bit submerged.

  • The Spray Technique: For vertical surfaces (like a mirror already on a wall), have an assistant mist the site constantly. This is vital whether you are using a premium Relton bit or a value bit like the VG-3002.

4. Step-by-Step: Drilling a Shower Door or Glass Pane

If you have confirmed your glass is not tempered and you have your Mutual Screw drill bits ready, follow this expert workflow:

Step 1: Stabilization

Place the glass on a perfectly flat surface. Any void under the glass creates a "flex point." A piece of rigid foam insulation or a flat plywood scrap makes an excellent sacrificial backing.

Step 2: Marking

Glass is slippery. A standard pencil won't work. Use a wax pencil or a piece of masking tape to mark your center point. For high-precision layout work, using a bit like the GRT-3-3 is advantageous because its tip geometry is designed to prevent "skating" or wandering across the glass surface.

Step 3: The Start

Do not start the drill at full speed. Begin with a very slow rotation to allow the carbide tip to "score" the surface. Once a small divot is established, you can slightly increase the speed to the recommended RPM range.

Step 4: The Halfway Flip (Expert Tip)

For the cleanest possible hole, professionals often drill halfway through from one side, then flip the glass over and finish the cut from the other side. This ensures that the "exit" of the hole is clean on both faces, eliminating the risk of rear-side chipping. 

5. Maintenance: Extending the Life of Your Bits

High-quality tools like those from Relton are an investment. To ensure they last:

  1. Clear the Slurry: As you drill, a white "slurry" of glass dust and water will form. Periodically pause and wipe this away or flush it with fresh water.

  2. Check for "Glazing": If the bit seems to stop cutting, the diamonds or carbide may be "glazed" over with glass residue. You can "dress" the bit by drilling for a few seconds into an abrasive dressing stone or a soft brick to expose fresh cutting surface.

  3. Inspect the Tip: Before starting a new hole, inspect the carbide tip of your GRT-3-4 for any microscopic chipping. A damaged tip will create uneven heat and likely crack the glass.

6. Safety First

When drilling glass, you aren't just dealing with sharp edges; you are dealing with glass dust.

  • Eye Protection: Safety glasses are non-negotiable.

  • Respiratory Protection: If you are drilling for an extended period, wear a mask. Inhaling fine glass dust (silica) can cause long-term health issues.

  • Hand Protection: Wear thin, cut-resistant gloves that allow for tactile feedback while protecting you from raw edges.

Why Source from Mutual Screw & Supply?

In the world of glass fabrication, there is no margin for error. Using a sub-par drill bit from a "big box" bargain bin is a recipe for a costly mistake. Whether you choose the industry-leading performance of a Relton GRT-4-4 or the economical VG-3002, you can trust that Mutual Screw & Supply only stocks tools that meet professional standards.

When you choose the right tools, manage your heat, and exercise patience, drilling glass moves from a stressful gamble to a routine part of your professional repertoire. Respect the material, invest in quality, and always use the right bit for the job.

Explore our full range of Relton and Professional Drill Bits here.