About Drill Point & Blind-Hole Depth Calculator
A blind hole has at least two depths: the distance to the deepest point and the distance over which the full drill diameter exists. A pointed drill leaves a cone below its cylindrical cutting envelope. If a drawing requires 20 mm of full-diameter hole, stopping the drill tip at 20 mm leaves the cylindrical portion short. This calculator separates those conventions and labels them on a cross-section.
The point allowance follows from an ideal cone. Split the included angle in half, draw a right triangle from the axis to the cutting corner, and divide the drill radius by the tangent of that half-angle. Increasing the included angle shortens the point. A 10 mm drill with a 118-degree point adds about 3.0043 mm below the full-diameter portion; a 135-degree point adds about 2.0711 mm. Neither value is a universal machining allowance: they describe geometry for the angle and diameter entered.
Stock thickness adds another useful distinction. The tip can emerge from the underside while the full drill diameter is still inside the material. Conversely, keeping a positive web of material below a blind hole means checking tip depth, not just its cylindrical depth. Use the remaining-material result to inspect that geometry, then add whatever process-specific allowances the toolmaker, drawing, and machine setup require. Actual drills have chisel edges, split points, corner treatments, and regrind variation that an ideal cone does not represent.
How It Works
- Enter the drill diameter and included point angle. The preset angles are geometry examples, not material-selection recommendations.
- Choose which depth you know: required full-diameter depth or actual tip depth measured from the work surface.
- Enter stock thickness to see whether the tip or full diameter reaches the underside. The thickness is a geometric comparison, not a safe clamping or clearance limit.
- Read the cone allowance and converted depth. When the tip has entered less than one complete point length, cylindrical depth is zero and the calculator reports the smaller ideal entry diameter.
- Export the labeled quantities or share the setup. Do not paste the tip-depth value directly into a machine program without accounting for your datum, tool length, offsets, and travel direction.
Worked Example
For a 10 mm drill with a 118° included point angle, h = 10 / [2 tan(59°)] = 3.004303 mm. A required 20 mm full-diameter blind hole therefore puts the ideal tip 23.004303 mm below the surface. In 25 mm stock, about 1.995697 mm remains below that tip. Keeping the same diameter and changing to a 135° point gives h = 2.071068 mm and tip depth 22.071068 mm. The difference comes from cone geometry, not from a change in the requested cylindrical depth.
Formulas
- Ideal conical point allowance
h = D / [2 tan(θ/2)]- Tip depth from full-diameter depth
z_tip = z_full + h- Full-diameter depth from tip depth
z_full = max(0, z_tip − h)- Entry diameter before the full cone enters
D_entry = min(D, 2z_tip tan(θ/2))
Standards & References
- Ideal cone/right-triangle geometry; manufacturer-specific point details are outside this model.
- Harvey Performance Company, Spot Drilling: The First Step to Precision Drilling, updated October 18, 2023: included drill point angles and the distinction between spotting and drilling geometry — https://www.harveyperformance.com/in-the-loupe/choosing-spot-drill/
- NIST SI Units — Length: 1 inch = exactly 25.4 mm — https://www.nist.gov/pml/owm/si-units-length
Frequently Asked Questions
Is blind-hole depth measured to the drill tip?
It depends on the drawing or process instruction. A specified full-diameter depth excludes the conical bottom, while a tip depth includes it. Check the actual depth convention instead of assuming the two are interchangeable.
Why does a 135-degree drill need less point allowance than a 118-degree drill?
For the same diameter, the larger included angle makes a flatter cone. Its axial length is D divided by twice the tangent of half the angle. The half-angle tangent is larger at 135 degrees, so the point is shorter.
Does tip breakthrough mean the hole is drilled through at full size?
No. The tip reaches the far surface before the outside cutting corners do. To bring the full nominal diameter to that surface, the tip must travel an additional ideal point length beyond the stock thickness, before any extra process allowance.
What happens when the entered tip depth is shorter than the point?
Only part of the cone has entered the stock. There is no full-diameter cylindrical section yet. The calculator sets cylindrical depth to zero and reports the smaller opening predicted by the ideal cone.
Can this set a blind tapping depth or CNC Z value?
No. A tap has its own lead, chip space, and usable-thread requirements. A CNC program also depends on datum location, tool compensation, and sign conventions. This calculator reports geometric distances from the surface, not executable machining instructions.