Profondeur de Trou Borgne vs Trou Débouchant
Engineering drawings specify hole depths in two distinct ways: usable cylindrical depth or full tip penetration depth. Here is how to program your Z-axis to prevent bottoming out or leaving an incomplete cylindrical hole.
1. Blind Hole Depth Rule: Cylindrical vs. Tip
Unless explicitly noted as "depth to point", engineering drawing callouts for blind holes (e.g. ⌀.250 ↧ 1.000) specify the depth of full cylindrical diameter.
Formula: l = (D / 2) / tan(θ / 2). For a 118° drill, l ≈ 0.300 × D. For a 135° drill, l ≈ 0.207 × D.
Tapping Clearance Rule: If you are subsequently tapping the blind hole, leave extra depth for tap lead-in chamfers:
- Taper Tap (7–10 threads chamfer): Add 4× to 5× pitch extra depth.
- Plug Tap (3–5 threads chamfer): Add 3× pitch extra depth.
- Bottoming Tap (1.5–2 threads chamfer): Add 1.5× to 2× pitch extra depth.
2. Through Hole Breakthrough Allowance
When drilling through a plate or tube wall, the drill must extend past the bottom face by the full point length plus a safety breakthrough margin (usually 1.0 mm – 2.0 mm or 0.040" – 0.080").
If the drill stops the moment the tip pierces through, the full diameter has not exited. This leaves a severe, razor-sharp circular burr that grips the drill margin during rapid retract, frequently chipping carbide corners or wedging the drill in place!
Calculate Tip Lengths & Penetration Offsets
Interactive calculator with live SVG cross-section for 90°, 118°, 135°, and 140° points.