Forstner Bit

What it is and how it works

A Forstner bit is a specialized woodworking bit designed to drill clean, flat-bottom holes with crisp edges. Unlike twist bits or spade bits, it uses a rim with sharp cutting edges to score the circumference of the hole while the interior cutters remove the waste. A small center point helps position the bit, but the rim does most of the guiding. This design allows the bit to start on a smooth surface, drill partial holes, overlap existing holes, and produce holes that are square to the surface with minimal tear-out.

Forstner bits perform best in a drill press, where the tool stays perfectly vertical and speed is easy to control. They can also be used in a hand drill, especially for smaller diameters, as long as the work is clamped and you keep a steady, controlled feed.

Common uses in DIY and home improvement

• Boring 35 mm hinge cups for European-style cabinet hinges
• Creating flat-bottom recesses for hardware (tabletop fasteners, knockdown connectors, T-nuts, bolt heads, washers)
• Drilling clean dowel and plug holes in furniture parts
• Routing out waste by overlapping holes to form mortises or inlay pockets
• Making partial holes for magnets, coins, badges, or decorative inlays
• Cutting through-holes in wood with minimal tear-out when backed by scrap
• Leveling the bottom of a knot or defect before filling with epoxy

Types and variations

• Standard rim: Smooth rim with sharp scoring edges. Leaves a very clean edge in hardwood and plywood. Great for flat-bottom holes and precise work.
• Sawtooth rim: Rim has small teeth that improve chip removal and reduce heat in softwood and end grain. Edges may be slightly rougher than standard rim.
• Carbide-tipped: Hard-wearing cutting edges that stay sharp longer, especially in abrasive sheet goods like plywood, MDF, and melamine. Cost more but hold up in heavy use.
• High-speed steel (HSS): Common and affordable. Good for solid wood; will dull faster in engineered panels.
• Shank styles: Round shank fits most chucks; hex shank resists slipping in hand drills. Larger bits may have reduced shanks to fit 3/8 in or 1/2 in chucks.
• Sizes: Typically from about 1/4 in up to 2-1/2 in (and larger specialty sizes). Common cabinet size is 35 mm for hinge cups.

How to choose

• Material you drill most: Use carbide for frequent plywood or MDF work; HSS is fine for solid wood and occasional panel use.
• Rim style: Pick standard rim for the cleanest edge in hardwood and plywood. Choose sawtooth for softwood and end grain where chip clearing matters.
• Size coverage: If you only need hinge cups, buy a single 35 mm bit. For general shop use, a set covering 1/4 in to 1-3/8 in in common steps is handy. Avoid very cheap sets; poor grinding leads to burning and rough holes.
• Shank fit: Make sure the shank fits your drill chuck. Large bits may require a drill press with a 1/2 in chuck.

How to use a Forstner bit

1. Mark the center: Use a sharp pencil or awl. For hinge cups, follow the hardware template for setback from the door edge.
2. Secure the work: Clamp the workpiece firmly. Add a backer board underneath to prevent tear-out on exit.
3. Set speed: Use slower speeds for larger bits and hardwood (see guidelines below). A drill press makes this easy.
4. Start square: Hold the drill perfectly vertical. Let the center point and rim establish a shallow groove.
5. Drill in stages: Apply moderate pressure. Every 1/4 to 1/2 in of depth, lift the bit to clear chips and cool the cutting edges.
6. Stop at depth: For flat bottoms, use a depth stop or mark the shank with tape. For through-holes, continue into the backer board.
7. Clean and check: Brush away chips with the drill stopped. Test-fit hardware before moving on.

Speed guidelines (approximate)

• Bits 1/4 to 1/2 in: 1200 to 1800 rpm in softwood, 800 to 1200 rpm in hardwood
• Bits 5/8 to 1 in: 800 to 1200 rpm in softwood, 500 to 900 rpm in hardwood
• Bits 1-1/8 to 2 in: 400 to 800 rpm in softwood, 250 to 500 rpm in hardwood

Always follow the manufacturer’s chart if provided. If you see smoke or burn marks, slow down, reduce feed pressure slightly, and clear chips more often.

Care and maintenance

• Keep them sharp: Lightly hone the flat faces of the rim and inside cutters with a fine diamond card. Do not touch the outside diameter of the rim; that sets the bit’s size.
• Remove pitch: Clean resin buildup with a solvent designed for blade and bit cleaning. Pitch increases heat and dulls edges faster.
• Avoid heat: Overheating reduces edge life. Use proper speed, clear chips, and let the bit cool between deep cuts.
• Protect edges: Store bits in a case or rack so the rim does not bang against other tools.

Common mistakes to avoid

• Running too fast: High rpm causes burning, especially on larger bits. Use slower speeds and steady feed.
• Skipping a backer board: Drilling through without support leads to blowout on the exit side.
• Forcing the cut: Excess pressure overheats and can bend the shank. Let the cutters work and clear chips often.
• Using a dull or dirty bit: Dull edges tear fibers and drift off-center. Clean and hone as needed.
• Poor workholding: Drilling without clamps risks chatter, out-of-round holes, or injury.
• Expecting self-feeding: Unlike auger bits, Forstners do not have a screw point. They need steady, controlled pressure.

Related terms

• Brad-point bit: A twist-style wood bit with spurs and a sharp center point for clean entry in through-holes.
• Spade bit: Flat paddle-style bit for quick, rough holes; not ideal for flat-bottom recesses.
• Hole saw: Cuts large diameter holes with a thin kerf ring; leaves a plug, not a flat bottom.
• Auger bit: Coarse screw-tip bit for deep holes in framing lumber; aggressive feed.
• Counterbore and countersink: Cuts a recess for a fastener head; Forstners are often used to create the counterbore.
• Hinge boring bit: A dedicated 35 mm bit for cabinet hinges, often a form of Forstner.

Practical examples

• Cabinet door hinges: Use a 35 mm carbide Forstner bit in a drill press with a fence to bore consistent hinge cups at the correct depth and setback.
• Tabletop fasteners: Drill a 1 in flat-bottom recess to hide a washer and bolt head, then a smaller through-hole for the bolt shank.
• Magnet inlays: Bore a shallow 3/4 in pocket for a rare earth magnet, stopping just short of the surface on the opposite side. Epoxy the magnet in place.
• Leveling defects: Flatten a knot hole with a Forstner, then fill with tinted epoxy or inlay a contrasting wood disk cut with a plug cutter.
• Removing waste: Overlap a series of holes inside a layout line to remove most of the material before cleaning up with a chisel or router.

Used with the right speed, sharp edges, and good workholding, a Forstner bit produces crisp, accurate holes and recesses that give projects a clean, professional look.