What Is A Number 9 Drill Bit

The term Number 9 originates from the old fractional drill size chart used in the United States. It represents the ninth size in the series that starts at 1/64 inch and increases in increments of 1/64 inch. In practice, a Number 9 bit measures 0.1625 inches in diameter, placing it between the more common 5/32 (0.15625 in) and 11/64 (0.1719 in) sizes. Knowing this helps you quickly match the bit to a drawing or a specified hole size without converting between systems.

A Number 9 bit’s diameter of 0.1625 in translates to 4.13 mm. While the metric system doesn’t have an exact 4.13 mm standard, the nearest common size is 4 mm, which is slightly smaller, and 4.2 mm, which is marginally larger. When precision is critical, you’ll want to stick with the exact 4.13 mm (or 0.1625 in) size to avoid tolerance issues. This dual‑reference knowledge is essential when working on projects that mix imperial and metric specifications.

Most Number 9 drill bits are forged from high‑speed steel (HSS), offering a good balance of hardness and toughness for general‑purpose drilling. For harder materials like stainless steel, you’ll find cobalt‑alloyed HSS (often labeled M35 or M42) or even carbide tips. The choice of material directly influences chip evacuation, heat resistance, and overall lifespan, so selecting the right composition based on your target material is a key early decision.

The nominal diameter of a Number 9 bit is 0.1625 in, which equals 4.13 mm. This precise measurement is critical when drilling holes that must fit standard hardware such as #10–32 screws or 4 mm dowels. Always verify the diameter with a caliper before beginning a project, as manufacturing tolerances can cause slight variations that affect fit.

Number 9 bits are available with shank lengths ranging from 1 inch to 6 inches, and overall lengths from 2 inches up to 12 inches. Shorter shanks provide better rigidity for hand‑drilling, while longer shanks are preferred for drill presses or deep‑hole applications. Selecting the appropriate length reduces flex, improves accuracy, and helps you reach the required depth without changing tools.

Standard Number 9 bits feature a straight shank with a 118‑degree split‑point tip. This geometry minimizes walking on the workpiece and provides clean entry. For specialty applications, you may encounter a 135‑degree point for softer materials or a 90‑degree point for metal‑working where a larger chisel angle reduces thrust. Understanding these angles helps you select a bit that starts smoothly and reduces exit burrs.

HSS is the workhorse material, suitable for wood, plastic, and most metals up to 4140 steel. Cobalt‑alloyed HSS (typically 5‑8% cobalt) retains hardness at higher temperatures, making it ideal for stainless steel and titanium. Carbide‑tipped bits offer the highest wear resistance and can drill abrasive composites, but they are brittle and require careful handling. Choose the base material based on the hardness and abrasiveness of the material you’ll be drilling.

A Titanium Nitride (TiN) coating gives the bit a golden‑gray finish and reduces friction, extending tool life by up to 25 %. TiN also provides a slight increase in surface hardness, which helps when drilling high‑speed steel at elevated RPMs. However, the coating can wear off if you frequently sharpen the bit, so it’s best suited for jobs where you expect to use the bit many times without re‑sharpening.

Black oxide offers modest corrosion resistance and a matte finish that reduces glare, making it a good budget option for general‑purpose drilling. Diamond coating is reserved for ultra‑hard materials such as glass‑filled polymers, ceramics, or hardened steel. While expensive, a diamond‑coated Number 9 bit can drill through these substrates without excessive heat buildup, delivering a clean, burr‑free hole.

In woodworking, a Number 9 bit is perfect for drilling pilot holes for #10–32 wood screws, creating clearance holes for dowels, and making precise holes for small hardware like hinges. Its size provides a tight fit without splitting delicate grain, especially when paired with a sharp split‑point tip and a moderate drilling speed (around 2,500 RPM for hardwood).

Mechanics often use Number 9 bits to drill holes for oil‑filter bolts, small mounting screws, and sensor installations. When working with stainless steel or aluminum, opt for a cobalt‑alloyed HSS or TiN‑coated bit and keep the RPM lower (1,000‑1,500 RPM) while applying steady feed pressure. Using cutting fluid further extends bit life and improves surface finish.

Plastics such as acrylic, polycarbonate, and composite panels benefit from the clean entry of a split‑point Number 9 bit. Because plastics melt at high heat, run the drill at higher speeds (3,000‑4,000 RPM) but with light feed pressure, and consider a lubricating spray or water mist to keep temperatures down. This approach prevents burr formation and cracking.

For soft woods and plastics, a standard HSS bit is sufficient. When drilling medium‑hard metals like mild steel, upgrade to a cobalt‑alloyed HSS or TiN‑coated bit to resist heat. For extremely hard or abrasive materials, consider a carbide‑tipped or diamond‑coated variant. Selecting the correct hardness rating prevents premature dulling and reduces the risk of breakage.

A drill press provides consistent spindle speed, vertical alignment, and depth control, making it ideal for precision holes in metal or stacked workpieces. A hand drill offers flexibility for on‑site repairs or large‑area woodworking but may introduce wobble. If you anticipate frequent use on metal, invest in a drill press or a high‑torque cordless drill with a variable‑speed trigger for better control.

Entry‑level HSS bits cost as little as $2‑$3 per piece, suitable for occasional hobby work. Mid‑range TiN‑coated or cobalt bits range from $5‑$10 and provide longer life for regular use. Premium carbide or diamond‑coated bits can exceed $20 but are justified for high‑volume production or specialty materials. Weigh the frequency of use, material hardness, and tolerance requirements to decide where to invest.

Use the formula RPM = (Cutting Speed × 4) / Diameter to calculate appropriate speeds. For a 0.1625‑in (4.13 mm) bit in mild steel (cutting speed ≈ 30 SFPM), aim for ~740 RPM. In wood, you can increase to 2,500‑3,000 RPM. Feed rate should be steady but not forced; a good rule is to advance the bit about 0.001 in per revolution for metal, and slightly faster for wood.

When drilling deep holes (>1 in), employ the peck drilling technique: drill a short distance, withdraw to clear chips, then repeat. This reduces heat buildup and prevents bit binding. For metal, apply a few drops of cutting oil or a mist of coolant at each peck to further lower temperature and extend tool life.

Start each hole with a small pilot indentation using a center punch or a low‑speed start‑stop sequence. Keep the drill perpendicular to the workpiece; even a few degrees of tilt can cause the bit to wander. For brittle bits like carbide, limit the feed pressure and avoid sudden impacts, as these increase the risk of fracture.

After drilling, wipe the bit with a lint‑free cloth soaked in mineral spirits or a mild degreaser to remove metal shavings and oil residues. For carbide or diamond‑coated bits, use a soft brush to avoid scratching the coating. Allow the bit to air‑dry completely before storage to prevent rust on HSS or cobalt bits.

Inspect the cutting edge regularly. If the tip is dull, rounded, or chipped, a professional sharpening service can restore HSS and cobalt bits up to three times before they become too thin. Carbide and diamond bits are generally not sharpenable; replace them when wear exceeds 15 % of the original diameter. Keeping a log of usage hours helps you predict when a replacement is due.

Store bits in a dry, magnetic bit holder or a padded case to protect the tips from impact. Separate coated bits from uncoated ones to avoid cross‑contamination of debris. For long‑term storage, consider a silica‑gel packet in the container to absorb moisture, extending the life of HSS and cobalt steel bits.

Always wear safety glasses or a face shield to guard against flying chips. Use hearing protection when operating high‑speed drills for extended periods. For metal drilling, wear gloves to protect hands from sharp burrs and hot surfaces, and consider a dust mask when drilling wood or plastics to avoid inhaling fine particles.

Chatter appears as a vibrating, uneven surface finish; it often results from excessive feed rate or insufficient rigidity. Reduce feed pressure and increase spindle speed slightly to cure it. Overheating manifests as discoloration or loss of hardness; mitigate by using coolant, lowering RPM, or switching to a harder‑grade bit such as cobalt.

If a hole is too small, you can gently enlarge it with a reamer of the correct size. For broken bits lodged in the workpiece, try extracting with a left‑hand drill bit or a screw extractor. When the bit repeatedly dulls after a few uses, it’s time to reassess material compatibility or upgrade to a higher‑grade bit rather than continuing to force a sub‑optimal tool.