Free Drill Point Depth Calculator
Calculate drill point tip height, CNC program Z-depth to reach full cylindrical diameter, and breakthrough clearance past the bottom surface for through-holes.
⚙️ Drill Geometry & Hole Parameters
Full Diameter Depth (1.250") + Tip Height (0.150")
CNC Tip Compensation Rule: Print blueprints dimension blind holes to the full cylindrical shoulder, NOT to the bottom of the drill point. Programming Z to the print depth leaves an undersized hole that will bind taps or pins.
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The Trigonometry of Drill Tip Height
A twist drill tip forms a cone whose apex angle is designated as the point angle $ heta$. The vertical height $h$ from the drill's cylindrical shoulder to its sharp chisel edge is calculated using basic trigonometry:
For 118° Drill: h ≈ D / (2 × tan(59°)) ≈ 0.3001 × D
For 135° Drill: h ≈ D / (2 × tan(67.5°)) ≈ 0.2071 × D
For 90° Spot Drill: h = D / (2 × tan(45°)) = 0.5000 × D
118° Standard vs. 135° Split Point Drills
- 118° Point Angle: The classic general-purpose angle for mild steel, brass, aluminum, and cast iron. It centers easily in softer metals but creates a longer tip cone (requiring more Z-depth to clear).
- 135° Split Point Angle: The industry standard for tough alloys, 304/316 stainless steel, and aerospace superalloys. The flatter point engages the material sooner, reduces thrust force by 30%, and prevents "walking" or wander without requiring a center punch.
Through-Hole Breakthrough Clearance
When drilling through a plate or tube, the drill must plunge beyond the material thickness by the full tip height plus an extra clearance margin (typically 0.030" to 0.080" or 1 to 2 mm). If the feed stops right as the tip breaks through, the outer cutting corners will not clear the back face, leaving an uncut burr ring that can catch reamers or damage fixtures.
Frequently Asked Questions
Why do drawings specify blind hole depth to the shoulder instead of the tip?
Engineers dimension to the cylindrical shoulder because that is the only usable length for mating dowel pins, bolts, or fluid ports. The conical point cavity left by the drill tip provides necessary chip and fluid relief.
How much deeper do I need to drill for tap thread clearance?
For blind tapped holes, standard practice requires drilling 3 to 5 pitches deeper than the required full thread length when using a plug tap, or 1.5 to 2 pitches for a bottoming tap, in addition to the drill point cone height.
What is the quick rule of thumb for 118° drills?
The quick machinist rule of thumb for a 118° drill is: tip height equals 30% of the drill diameter (e.g. 1/2" drill tip is 0.500 * 0.300 = 0.150"). For 135° drills, it is approximately 21% of the diameter.