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A diamond core drill bit set is the right tool for drilling clean holes in porcelain, ceramic tile, glass, granite, marble and stone, and the best set for most people is a vacuum-brazed set covering roughly 6 mm to 68 mm (1/4 inch to about 2-5/8 inches) with the shank type that matches the tool you own. Bond type, shank and cooling method matter far more than piece count.
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What we cover
The Three Diamond Bond Types
All diamond core bits cut by abrasion, not by slicing, but the way the diamond grit is attached determines speed, life and whether you need water.
| Bond type | How it is made | Cutting speed | Life (relative) | Best for | Typical set price |
|---|---|---|---|---|---|
| Electroplated | Single layer of grit plated on the rim | Fast at first, slows as grit wears | Short: a handful to a few dozen holes | Glass, ceramic wall tile, occasional jobs | $15-$40 |
| Vacuum-brazed | Grit brazed in a strong metal bond, exposed edges | Fast, can cut dry | Medium: dozens of holes in porcelain | Porcelain, ceramic, granite, DIY and trade | $40-$120 |
| Sintered segmented | Diamond mixed through a metal matrix | Slower, steady | Long: grit renews as matrix wears | Concrete, hard stone, professional rigs | $100-$300+ |
For home renovation, vacuum-brazed is the most versatile. Sintered core bits are usually bought individually for rotary core rigs rather than as small sets.
Match the Shank to Your Tool
- Round or hex shank: fits a cordless drill or drill press. Hex shanks lock into quick-change chucks and some impact-ready holders, but use a drill, not an impact driver.
- 5/8-11 thread (US) or M14 thread (metric): screws directly onto an angle grinder spindle. Grinders spin much faster and suit dry-cutting vacuum-brazed bits in porcelain.
- SDS-plus: available but use rotation-only mode; hammer action will crack tile and chip the diamond rim.
Mixing a grinder-thread set with a drill (or the reverse) is the most common buying mistake, so check before ordering.
Sizes That Cover Real Jobs
| Diameter | Typical job |
|---|---|
| 6 mm (1/4 in) | Wall plugs for towel rails, small anchors |
| 8-10 mm (5/16-3/8 in) | Larger anchors, shower rail fixings |
| 12-14 mm (1/2 in) | Toilet and basin fixings |
| 20-25 mm (3/4-1 in) | Water supply pipes |
| 32-35 mm (1-1/4-1-3/8 in) | Mixer taps, waste pipes |
| 40-45 mm (1-5/8-1-3/4 in) | Shower valves, larger waste |
| 51-68 mm (2-2-5/8 in) | Electrical boxes, downlights, shower drains |
A 5- to 7-piece set spanning 6 mm to 35 mm handles most bathrooms and kitchens. Add a single 68 mm bit if you fit electrical back boxes in tiled walls.
Speed and Cooling
Diamond bits fail mostly from heat. A glazed rim or a blue discoloration means the bit has been run too fast or too dry. The general rule is that larger diameters need lower RPM.
| Bit diameter | Cordless drill speed (wet or wax-cooled) | Cooling method |
|---|---|---|
| 6-10 mm | About 1,500-3,000 RPM | Water spray or wax-filled core |
| 12-25 mm | About 800-1,500 RPM | Water dam or continuous trickle |
| 32-45 mm | About 500-1,000 RPM | Water dam; pause to cool |
| 51-68 mm | About 400-800 RPM | Water essential with a drill |
Grinder-mounted vacuum-brazed bits are designed for higher speed and dry cutting, but even these need short pauses so the rim cools. Some bits include a wax or coolant core that melts as they heat, which helps when water is impractical, such as on walls near electrics.
Step-by-Step: Drilling Porcelain Without Cracking It
- Mark and protect: apply masking tape and mark the center; it helps the bit stay put and reduces chipping.
- Use a guide: a suction-cup or plastic guide jig stops the bit wandering on glazed surfaces. Without one, start at an angle so only part of the rim touches, then bring the drill upright once a crescent groove forms.
- Cool: build a small putty dam filled with water, or keep a spray bottle on the hole.
- Light pressure, gentle orbit: let the diamond do the work; a slight circular rocking motion clears slurry and speeds cutting.
- Turn off hammer mode. Always.
- Back off before breakthrough: reduce pressure in the final millimeters to avoid blowing out the back face.
- Switch bits for the wall behind: once through the tile, use a masonry bit for brick or block to save the diamond rim.
Reputable Brands and What to Expect
Recognizable makers of diamond core bits include Bosch, DeWalt, Milwaukee, Rubi, Montolit and Makita, along with many general-brand sets of variable quality. Professional-brand bits tend to have more consistent brazing and better rim concentricity, which reduces chipping. Budget sets can be fine for a single bathroom; for a tiling business, buying individual pro-grade bits in your most-used sizes is usually more economical per hole.
Common Mistakes and What Wears First
- Running dry with a drill: overheats electroplated and brazed rims quickly.
- Too much pressure: polishes the diamonds and slows cutting rather than speeding it up.
- Hammer mode: chips the rim and cracks tiles.
- Stuck cores: clear the slug after each hole; a packed core increases friction and heat.
- Glazed bit: if cutting slows, a few seconds drilling into an abrasive dressing stone or soft brick can re-expose fresh diamond on brazed and sintered bits.
Bottom Line
Buy a vacuum-brazed diamond core drill bit set in 6 to 35 mm (plus a 68 mm if you fit electrical boxes), choose the shank that matches your drill or grinder, and keep the rim cool. Choose electroplated only for occasional glass and ceramic work, and sintered only for professional stone and concrete coring.