Calculator Inputs
Example Data Table
| Case | Nominal Size | Head Diameter | Head Height | Pilot Hole | Recommended Counterbore | Recommended Depth |
|---|---|---|---|---|---|---|
| Metric socket screw | M6 | 10.00 mm | 6.00 mm | 6.60 mm | 10.45 mm | 6.40 mm |
| Metric socket screw | M8 | 13.00 mm | 8.00 mm | 9.00 mm | 13.50 mm | 8.45 mm |
| Metric socket screw | M10 | 16.00 mm | 10.00 mm | 11.00 mm | 16.65 mm | 10.55 mm |
| Imperial socket screw | 1/2 in | 0.750 in | 0.500 in | 0.531 in | 0.775 in | 0.520 in |
These examples illustrate typical outputs for flush-seating layouts. Always confirm standards, production capability, and field tolerances before release.
Formula Used
- Recommended counterbore diameter:
Dcb = Dh + C + 2F - Recommended counterbore depth:
Tcb = Hh + A + R + F - Diameter tolerance band:
Dmin = Dcb - TdandDmax = Dcb + Td - Depth tolerance band:
Tmin = Tcb - TtandTmax = Tcb + Tt - Annular wall thickness:
Wall = (Dcb - Dpilot) / 2 - Annular removal area:
A = π × (Dcb² - Dpilot²) / 4 - Material removed:
V = A × Tcb - Suggested edge distance:
E = 1.5 × Dcb
Where Dh is head diameter, Hh is head height, C is diametral clearance,
F is finish allowance, A is depth allowance, R is corner relief,
Td is diameter tolerance, and Tt is depth tolerance.
How to Use This Calculator
- Select metric or imperial units.
- Choose a fit class to control default clearance and tolerance levels.
- Load a preset fastener or enter custom head and shank dimensions.
- Enter the through-hole size, finish stock, depth allowance, and any corner relief.
- Add workpiece thickness and an optional maximum tool diameter.
- Press the calculate button to see the result above the form.
- Review the tolerance band, remaining floor thickness, edge distance, and tool suggestion.
- Export the calculated report as CSV or PDF for drawings, reviews, or field coordination.
FAQs
1) What is a counterbore?
A counterbore is a flat-bottomed cylindrical recess cut above a smaller hole. It lets a bolt head, screw head, or washer seat below or flush with the surrounding surface.
2) How is a counterbore different from a countersink?
A counterbore has straight walls and a flat base. A countersink has a conical shape for flat-head fasteners. They solve different seating conditions and should not be interchanged casually.
3) Why does fit class matter?
Fit class changes default clearance and tolerance. Close fits improve alignment, while loose fits reduce assembly difficulty in field conditions where drilling drift, coating buildup, or installation tolerance is expected.
4) How much clearance should I add around the head?
Use enough clearance for easy seating, coating thickness, and realistic machining capability. Tight shop-made parts can use smaller values. Site-installed structural parts often need more forgiving allowances.
5) When should I use a spotface instead?
Use a spotface when you only need a small flat seating surface rather than full head embedment. It is common when thickness is limited or when only washer or nut seating must improve.
6) Why is corner relief included?
Many fasteners have underside fillets or rounded transitions. Corner relief prevents interference at the recess bottom edge and helps the head seat fully without rocking or false tightening.
7) What does remaining floor thickness tell me?
It shows how much material stays below the recess. Very small values may reduce strength, stiffness, pull-through resistance, or durability, especially in thin plates and heavily loaded connections.
8) Why does the tool suggestion matter?
It checks whether a standard cutter can produce the required diameter inside your machine or tooling limit. If no tool fits, you may need a larger cutter, revised dimensions, or a different process.