Carbide Rod Sizes Explained: Diameter, Length, Tolerance & Coolant Holes

When sourcing tungsten carbide rods (also called carbide round bars or blanks), "size" is not just diameter × length. For CNC toolmakers, the right size definition directly decides grinding stock, tool rigidity, coolant delivery, and batch scrap rate. Below is how professional buyers should read carbide rod sizes.

1. Diameter (Ø): The Most Critical Dimension

• Standard range: Ø0.1 mm (micro-drill) up to Ø50 mm+ (heavy wear bars). Most common commercial range is Ø1.0–Ø32 mm.

• Small dia (Ø0.1–Ø3 mm): PCB micro-drills, engraving bits, dental burrs. Requires ultrafine grain (0.2–0.4 µm) and tight tolerance.

• Medium dia (Ø3–Ø16 mm): Most universal—solid carbide end mills, twist drills, reamers.

• Large dia (Ø16–Ø50 mm): Heavy roughing mills, punch cores, guide rollers; usually higher Co grade for shock resistance.

• Diameter tolerance classes:

  • Unground / sintered blank: +0.2 to +0.5 mm allowance (needs centerless grinding).

  • Precision ground: h6 or h5 per ISO 286-1, e.g. Ø6 h6 = 6.000 / 5.994 mm; roundness ≤ 0.005 mm.

  • Micro-rod (PCB): tolerance can be ±0.05 mm or tighter on request.

2. Length: Standard Bars vs Cut-to-Length

• Standard stock lengths: 310 mm and 330 mm are the global default for tool grinding lines (allows multiple tools per bar with cutoff loss).

• Short bars: 50 / 75 / 100 / 150 / 200 mm for specific tool programs or sample runs.

• Custom long bars: Up to 500 mm or 2000 mm for special wear parts (not for flute grinding).

• Length tolerance: Ground bars typically 330 mm ±1.5 mm; sintered blanks 330 mm +0/+5 mm.

3. Surface Condition: Unground vs Ground

• Sintered blank (unground): Cheaper per kg, leaves 0.2–0.5 mm grinding stock. Good for large-batch standard drills.

• Ground & polished: Ready for grinder chuck, straightness ≤ 0.5 mm/1000 mm, Ra ≤ 0.2 µm, reduces setup time and scrap in aerospace/medical tooling.

4. Coolant Hole Configuration (Internal Structure)

This is where "size" gets misunderstood. The hole geometry is part of the rod specification:

• Solid (no hole): Standard end mills, reamers, rotary files.

• Single straight hole (Ød): Central lube hole, e.g. Ø6.3 × ID1.0 × 330 mm. For straight-flute drills.

• Double straight hole: Two parallel holes, used in larger deep-hole drills (Ø8+).

• Helical hole (30° / 40°): Coolant follows flute twist, best for spiral drills and titanium/aero alloys; hole concentricity ≤ 0.05 mm.

• Hole size rule: ID usually 0.3–10 mm, tolerance ±0.10 to ±0.20 mm; hole deviation (offset from center) ≤ 0.15–0.50 mm depending on OD.

5. How to Read a Rod Code

Example: B Ø3.3 × 2Ø0.4 × 1.7 × 330

• Ø3.3 = outer diameter 3.3 mm

• 2Ø0.4 = two coolant holes, each Ø0.4 mm

• 1.7 = distance between hole centers

• 330 = length 330 mm

• "B" prefix often denotes blank/solid variant per maker's internal code.

6. Size-to-Application Quick Map

7. Buyer Checklist for RFQ

Always state in your inquiry:

1. OD and tolerance class (h6 blank +0.3)

2. Length and cutoff allowance

3. Solid / straight-hole / helical / twin-hole (+ hole Ø and offset)

4. Ground or unground

5. Grade (Co%, grain size) — size alone is not enough

Key takeaway: A "Ø10×330 carbide rod" can mean 10 totally different products. Define OD tolerance, hole geometry, and ground state, or you will compare prices on apples vs oranges.