Hole defects
How to use the Defect Expert
Select the defect group (steer / taper / ovality / scuffing / burr), indicate the specific symptom, type of tool and hole depth (L/D) - the calculator will determine the probable cause, what to check first, the level of risk and a specific action. For a burr at the exit, an extended mode opens: drill diameter, material, exit conditions, edge condition, coolant and priority - the calculator estimates the size of the burr and gives a correction at the exit.
Why hole defects are a systemic problem
Drilling is the most common operation on a machine and at the same time the most vulnerable. The tool works inside a closed space: it is not visible, the chips are not visible, the temperature is not visible. Any error appears later - when the hole does not pass inspection.
Hole defects are divided into five groups: axis misalignment (the hole is not where it should be), taper (the diameter changes with depth), ovality (the hole is not round), scoring and scratches (bad surface inside), burr at the exit (plastic deformation during a breakthrough). Each group has its own reasons and its own logic of elimination. The calculator links the symptom to the cause and gives specific steps - without guessing.
What Causes Major Defects?
Axle slip. Three reasons: tool/chuck runout (runout > 0.02 mm), poor centering at the entrance (no spot drill) and long reach without a pilot. At L/D ≥ 5 without a preliminary hole, drift is almost inevitable. Indexable inserts are more sensitive to runout than solid carbide.
Taper: The hole is wider at entry than at depth—or vice versa. The main reason is tool deflection: at the entrance, the drill “breaks” the wall due to impact contact (bell-mouthing), and at depth the diameter decreases due to compression. Spot drill and rigid clamp solve the entry problem. For depth - pilot hole and peck cycle.
Ovality. The hole is not round - the difference in diameters along two axes exceeds the tolerance. Almost always the cause is chuck/collet runout or vibration (chatter). Measure the runout: if > 0.02 mm, change the chuck or collet. If runout is normal, look for resonance (shift RPM by 10–15%).
Scratches and risks. Spiral marks - from high feed or worn drill guide bands. Galling - from poor chip removal: the chips re-contact the wall. Traces of vibration - from low rigidity or resonance. To ensure cleanliness of H7–H8, a finishing operation (countersink or reamer) is always necessary after drilling.
Burr at the exit. When the drill breaks through the material, the last layer is deformed plastically, rather than being cut off - a burr is formed. Its magnitude depends on the material (stainless steel and aluminum give the largest), the condition of the edge (worn = larger), the exit conditions (thin wall = worse) and the exit feed. Reducing the feed to 30–50% of the nominal value in the last 0.5–1.0 mm is a universal solution.
Frequently Asked Questions
The hole is moving away from the nominal position - where to start? Checking the runout (runout). If the tool runout in the chuck is > 0.02 mm, this is the main reason. The second step is to check if there is a spot drill or alignment at the inlet. The third is to evaluate L/D: if ≥ 5, a 1–2×D pilot is needed. The order is exactly this: first the mechanics, then the modes.
Why is the hole oval with a new drill? A new drill does not guarantee the absence of runout. Runout depends on the chuck, collet, spindle taper and cleanliness of the seating surfaces. Measure the runout with an indicator on the reach of the drill - this is a quick test that often reveals the problem in a minute.
How to reduce the output burr without a secondary operation? Reduce the output feed (fn × 0.3–0.5 in the last 0.5–1.0 mm) - this is programmed in G-code as a separate block. If the exit is in a thin wall, use sacrificial backing. For stainless steel and aluminum - pre-chamfer / countersink on the outlet side, if accessible.
At what L/D depth is it necessary to make a pilot hole? General recommendation: for L/D ≥ 5 a pilot hole is desirable, for L/D ≥ 8 it is mandatory. The pilot is made to a depth of 1–2×D with a drill of the same diameter or slightly larger. This eliminates entry drift and stabilizes further drilling.
What is the difference between scoring marks and feed marks? Feed marks are spiral lines with a pitch equal to feed per revolution. This is a normal tool mark that can be removed with a reamer. Galling is chaotic rough scratches from chips that are stuck between the drill and the wall. Scuffing is a sign of poor chip removal, and not just of operating conditions.
Need help troubleshooting hole defects or selecting tools? Contact our specialists - we will diagnose and find a solution.