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HSS Pilot Counterbore End Mills

Professional bolt hole spotfacing tools with integrated pilot for perfect concentricity. Available in Standard HSS or premium HSS-Co (M35/M42) with TiN Coating. Full range M3-M20. Ideal for ensuring cap screws and bolts sit perfectly flush.

HSS / HSS-Co (M35/M42) Integrated Pilot TiN Coated M3 - M20 Bolt Hole Specialist
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HSS Pilot Counterbore End Mills
HSS Pilot Counterbore End Mills

Procurement & Commercial Terms

MOQ1 pc (sample/stock) | 20 pcs (OEM/bulk)
Lead TimeSame day dispatch (standard stock) | 7–10 days (custom/OEM)
InventoryM3–M20 full range in stock. Standard and extended lengths available.
Material & CoatingSubstrate: HSS or HSS-Co (M35 / M42) | Coating: TiN (Gold)
PackagingIndividual plastic tubes + anti-rust paper | Rigid export cartons (20/50 pcs)
ShippingDHL/FedEx/UPS for samples | Air/Sea freight for bulk. Incoterms: EXW, FOB, CIF, DDP

HSS Pilot Counterbore End Mill Specifications

Product Configuration:      Material: Standard HSS or HSS-Co (M35/M42) |      Coating: TiN (Gold) |      Type: 90° Flat Bottom with Integrated Pilot
ModelThreadPilot D (mm)Cut D (mm)Shank D (mm)Pilot L (mm)Cut L (mm)Total L (mm)Stock
HS-CB-M3M33.26.04.06.57.040● In Stock
HS-CB-M4M44.28.06.05.010.044● In Stock
HS-CB-M5M55.29.58.06.013.051● In Stock
HS-CB-M6M66.211.010.06.016.058● In Stock
HS-CB-M8M88.214.012.08.016.067● In Stock
HS-CB-M10M1010.317.512.08.018.074● In Stock
HS-CB-M12M1212.420.012.010.020.083● In Stock
HS-CB-M14M1414.424.012.012.020.090● In Stock
HS-CB-M16M1616.526.012.014.022.093● In Stock
HS-CB-M18M1818.528.016.018.024.0100● In Stock
HS-CB-M20M2021.030.016.021.026.0110● In Stock

● All sizes in stock | Pilot tolerance: h7 | Cutting Diameter tolerance: h8. Custom sizes available upon request.

Application Tip: While pilot counterbore mills ensure perfect concentricity for flat-bottomed bolt holes, they are strictly designed for spotfacing. If you need to machine conical screw head seats (like flat-head cap screws) rather than flat-bottom counterbores, consider switching to our dedicated 90° Chamfer Mills for precise angle matching and edge breaking.

Why Choose Our Pilot Counterbore End Mills?

Integrated Pilot Design

The precision-ground pilot fits snugly into the pre-drilled hole, ensuring the counterbore is perfectly concentric. Eliminates misalignment and ensures fastener heads sit perfectly flush.

HSS-Co Toughness

Counterboring often involves interrupted cuts. HSS-Co (M35/M42) provides exceptional shock resistance, preventing edge chipping when machining cast surfaces or crossing slots.

TiN Coating Benefits

Gold TiN coating reduces friction and prevents built-up edge during spotfacing. Extends tool life 2-3x compared to uncoated HSS, especially in sticky materials like aluminum or mild steel.

Cost-Effective Resharpening

Unlike carbide, HSS counterbore mills can be resharpened 5-8 times by your local tool room while maintaining the original geometry, drastically reducing long-term tooling costs.

Material Compatibility Guide

HSS-Co (M35/M42) Recommended Applications
Workpiece MaterialHardnessSuitabilityNotes
Carbon Steel (A36, 1045)< 30 HRC◎ ExcellentPrimary application. Use moderate speeds with flood coolant. HSS-Co extends tool life significantly.
Alloy Steel (4140, 4340)< 35 HRC◎ ExcellentHSS-Co essential for wear resistance. Ideal for mold base and structural component counterboring.
Cast Iron (GG20, GG30)-◎ ExcellentExcellent toughness handles abrasive graphite. Use air blast to remove dust.
Stainless Steel (304, 316)< 28 HRC◐ LimitedUse HSS-Co only. Requires low speeds (15-25 m/min), abundant coolant, conservative feeds. Not for continuous high-speed production.
Aluminum / Copper-✗ Not RecommendedHSS-Co is overkill for soft materials. Use uncoated HSS or carbide for better economics.
Standard HSS Applications (Cost-Effective Option)
Workpiece MaterialHardnessSuitabilityNotes
Mild Steel / Low Carbon Steel< 25 HRC◎ ExcellentBest cost-performance ratio. Ideal for general assembly hole spotfacing.
Cast Iron-◎ ExcellentGood toughness for interrupted cuts. Economical choice for cast iron counterboring.
Plastics / Wood-◎ ExcellentSharp edges provide clean counterbores.
Alloy Steel / Stainless> 30 HRC✗ Not RecommendedStandard HSS lacks red hardness. Will wear rapidly. Use HSS-Co instead.

◎ = Excellent | ◐ = Limited (Requires parameter adjustment) | ✗ = Not Recommended

Typical Applications for Pilot Counterbore Mills

Mold & Die Manufacturing

Counterboring of mold base plates, ejector pin holes, and guide pin holes. HSS toughness handles interrupted cuts in pre-hardened mold steels.

Structural Steel Fabrication

Bolt hole spotfacing for steel frames, flanges, and structural connections. TiN coating extends tool life in carbon steel and alloy steel.

Machine Assembly

Precision counterboring for cap screw holes in machine components, ensuring flush fastener seating for proper assembly and aesthetics.

CAM Setup & Cutting Parameters Guidance

Recommended Cutting Data (HSS-Co + TiN)
Workpiece MaterialCutting Speed (Vc)Feed per Revolution (f)Coolant Recommendation
Carbon Steel (< 30 HRC)40 - 60 m/min0.08 - 0.15 mm/revFlood Coolant
Alloy Steel (4140, 4340)30 - 50 m/min0.06 - 0.12 mm/revFlood Coolant
Cast Iron35 - 55 m/min0.10 - 0.18 mm/revAir Blast (Dry)
Stainless Steel (304, 316)15 - 25 m/min0.05 - 0.10 mm/revAbundant Flood Coolant

*For standard HSS, reduce Cutting Speed (Vc) by approximately 20-30%. Counterbore mills are not designed for high-speed machining.

Best Practices for Counterboring
  • ⚠️ Pilot Fit is Critical - Ensure the pre-drilled hole matches the pilot diameter (h7 tolerance). A loose pilot will cause misalignment and chatter.
  • ⚠️ Use Low RPM - Counterbore mills have large diameters. Calculate RPM carefully: N = (Vc × 1000) / (π × D)
  • ⚠️ Peck Cycle for Deep Holes - For deep counterbores, use peck cycle to clear chips and prevent clogging around the pilot.
  • ⚠️ Abundant Coolant for Steel - Flood coolant is essential for steel counterboring to manage heat and flush chips.
  • ⚠️ Avoid Dwell - Never pause the tool in the cut. This causes work hardening and rapid tool wear, especially in stainless steel.
  • ⚠️ Check Pilot Wear - If the pilot wears undersized, the tool will lose concentricity. Replace or regrind promptly.

Factory Production & Quality Control

HSS-Co counterbore mill grinding and quality inspection

Manufacturing Process & QA Steps

  • 🔹 Raw Material: Premium HSS and HSS-Co (M35/M42) steel blanks sourced from certified mills.
  • 🔹 Grinding: CNC tool grinders ensure precise 90° flat bottom accuracy and pilot concentricity.
  • 🔹 Heat Treatment: Vacuum heat treatment with triple tempering to achieve optimal hardness (HRC 64-66) and toughness.
  • 🔹 Coating: TiN coating applied via PVD process for uniform gold finish and adhesion.
  • 🔹 Inspection: 100% visual inspection, pilot runout verification, and cutting tests.

Frequently Asked Questions

The pilot (guide pin) fits into the pre-drilled through hole. It ensures the counterbore is perfectly concentric with the bolt hole, preventing misalignment and ensuring the cap screw head sits perfectly flush with the workpiece surface.

Standard HSS offers excellent toughness and cost-effectiveness for carbon steel and cast iron. HSS-Co (M35/M42) provides superior red hardness and wear resistance, making it essential for alloy steels and extending tool life in continuous production.

Yes, but we strongly recommend the HSS-Co version. Due to 304's tendency to work-harden, you must use low cutting speeds (15-25 m/min), abundant flood coolant, and a continuous feed to prevent edge rubbing and rapid wear.

A counterbore mill has a flat bottom and an integrated pilot for machining flat-bottomed bolt holes with perfect concentricity. A chamfer mill has an angled cutting edge (e.g., 90°) for edge breaking and deburring. While a 90° chamfer mill can do light countersinking, it lacks the pilot for strict depth control and concentricity required for precise bolt hole spotfacing.

Request Quote for HSS Pilot Counterbore End Mills

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