Free Counterbore & Pilot Hole Calculator
Look up socket head cap screw dimensions, calculate counterbore diameter, counterbore depth, and pilot through-hole clearance for imperial and metric fasteners.
⚙️ Fastener Selection & Seating Style
M6 SHCS Head: 10.0mm Dia × 6.0mm Height
• 1. Spot Drill: 90° spot drill to center pilot hole.
• 2. Drill Through-Hole: Drill clearance hole completely through workpiece.
• 3. Counterbore: Plunge piloted counterbore cutter with cutting fluid to depth stop.
Sub-Flush Depth Rule: Always counterbore socket head cap screws 0.015" to 0.020" (0.4mm to 0.5mm) deeper than the nominal head height. This prevents screw heads from protruding above mating reference surfaces due to manufacturing tolerances or corner burrs.
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Standard Counterbore Geometry for SHCS
Socket Head Cap Screws (SHCS) feature cylindrical heads designed to sit flush or sub-flush inside recessed counterbores. Standard machining conventions provide:
- Counterbore Diameter: Approximately 0.030" to 0.060" (0.8mm to 1.5mm) larger than the nominal head diameter to allow room for the socket wrench or ball-end Allen driver.
- Counterbore Depth: Nominal head height + 0.015" (0.4mm) sub-flush margin.
- Through-Hole Clearance: Sized per ASME B18.2.8 or ISO 273 (Normal, Close, or Loose fit) to allow bolt passage without binding while maintaining alignment.
Frequently Asked Questions
Why does a piloted counterbore cutter produce cleaner holes than an end mill?
A piloted counterbore has an unthreaded solid cylinder at its tip that slips into the pre-drilled through-hole. This mechanical pilot guides the cutting flutes rigidly, eliminating chatter and guaranteeing that the counterbore is 100% concentric with the bolt hole.
What is the difference between a counterbore and a countersink?
A counterbore has a flat, 90° bottom designed for flat-bottomed cylindrical screw heads (socket head cap screws). A countersink has an angled cone bottom (typically 82° for imperial, 90° for metric) designed for flat-head conical screws.
Can I counterbore on a manual drill press?
Yes, provided you clamp the workpiece securely! Piloted counterbores can grab aggressively in thin material or aluminum if hand-held. Always run at low RPM with firm downfeed.