Pilot drilling
How to use the pilot drilling calculator
Enter main drill diameter and depth, select material, drill type and machining conditions - the calculator will determine the optimal strategy, pilot diameter range, pilot hole depth, spot-drill Ø and recommended CNC cycle (G81/G73/G83).
Three strategies:
- Spot only—alignment only without a pilot. Suitable for carbide drills with self-centering or 4-facet geometry with L/D ≤ 10 and rigid system.
- Pilot + drill—pilot hole in front of the main one. Reduces the load on the chisel edge, reduces slip with large L/D or weak stiffness.
- Step (step) - rough + finishing drill. Recommended for Ø ≥ 25 mm, deep or intermittent drilling.
Why is a pilot needed and why should it not be overestimated?
The pilot unloads the jumper of the main drill - the “softest” part of the geometry, which does not cut, but presses. Without a pilot, with a large diameter or unstable entry, the drill moves to the side, runout increases and position accuracy decreases.
Too large a pilot is dangerous: if its diameter exceeds the optimal range (~0.18–0.45·D), the cutting edge of the main drill enters the finished hole off-center, which leads to chipping and even greater drift. The optimum is a narrow range, which depends on the drill type, material and L/D.
CNC cycle: G81, G73 or G83
G81 is a simple cycle without output. For shallow holes (L/D ≤ 5) in stable conditions with flood coolant.
G73—chip-break: periodic chip breaking without complete removal. For moderately deep holes and steels with tough chips.
G83—full output at every step. Mandatory for L/D ≥ 8–10, blind holes, stainless steel, heat-resistant alloys and dry cutting - where chips do not evacuate themselves.
Frequently Asked Questions
What angle should the spot-drill have? The tip angle of the spot-drill should be equal to or greater than the tip angle of the main drill. If the spot-drill is sharper, the cutting edge will enter the chamfer and crumble on the first pass.
Is a pilot needed when drilling with carbide? Modern carbide drills with self-centering geometry self-center without a pilot. But spot-drilling at the right angle is still recommended - it sets an accurate position and protects the edge when entering an uneven surface.
What to do with L/D > 10? Mandatory G83 with full output, runout control (< 0.02 mm), overhang reduction and internal coolant supply (through-tool). Without these conditions, the pilot will not save you from being diverted.
Do you need drills for deep or precision drilling? We deliver from the catalogue, select the material and L/D.
How to use the pilot and alignment calculator
Enter the final drill diameter and L/D ratio (hole depth divided by diameter), select drill type, material and alignment method - the calculator will calculate the Ø and depth of the pilot hole, the minimum Ø spot-drill and the target runout for a stable entry.
Three alignment methods:
- Spot only - centering without a pilot. Suitable for carbide drills with short L/D in rigid system.
- Center drill is a combination tool (type A or B). Forms a chamfer and centers in one pass. Faster, but limited by diameter.
- Spot + pilot—maximum stability. Spot sets the position, the pilot stabilizes the final drill to a depth of 0.6–1.5×D.
Why runout is critical when deep drilling
When L/D > 8, tool runout accumulates along the length of the hole. A runout of 0.05 mm on a short drill is insignificant. The same runout on a drill with L/D = 15 gives a slip at the bottom of the hole of 0.5–1 mm and ellipse. For monolithic carbide, the permissible runout is ≤ 0.01–0.02 mm, for HSS — ≤ 0.03–0.05 mm.
Rule: check the runout in the chuck with an indicator before each clamp when changing tools - not when installing the machine.
Spot-drill: what Ø and angle to choose
The minimum Ø spot-drill is calculated as 0.75–0.90×D drill (more for deep holes and drills with indexable inserts). A smaller spot does not create a sufficient support chamfer - the drill moves away when cutting into the hole.
The point angle of the spot-drill must be equal to or greater than the angle of the main drill. If the spot is sharper, the cutting edge of the main drill cuts “in a chamfer” and is chipped during the first pass. Standard safe choice: spot 120° for 118° drill, spot 140° for 135° drill.
Do you need a pilot for a carbide drill? Modern short carbide drills often do not require a pilot with L/D ≤ 5 and a rigid system. But a spot-drill is always needed - it protects the edge from hitting an uneven surface and sets the exact position.
Why do you need a pilot when drilling with indexable inserts? Core drills with indexable inserts do not have a jumper - they cut from the first touch. Without a rigid coaxial entry, the plate experiences an impact load and will chip. The pilot and spot are required here, especially with L/D > 3.
What is the correct pilot hole depth? Enough for the final drill to sit securely - usually 0.6–1.5×D. No need to go deeper: the pilot does not improve direction outside the impact zone, but increases cycle time.
Do you need special drills for deep drilling or machining heat-resistant alloys? We supply from the catalog or select it for the task.