Reducing Burr Formation When Drilling

Burrs are an accepted nuisance in most shops — something to deal with after the fact rather than prevent at the source. But in high-volume production, the deburring labor adds up fast, and in some precision applications a burr in the wrong place is a nonconformance. Understanding where burrs form and what controls them lets you make intelligent process choices that reduce burr size and deburring time.

Entry Burr vs Exit Burr

Every drilled hole can produce burrs at both ends, but they form through different mechanisms and require different countermeasures.

The entry burr forms as the drill point pushes material aside before cutting begins. In soft, ductile materials like aluminum and mild steel, the material deforms plastically before the cutting edge reaches it and piles up around the entry. Entry burrs are generally small and consistent when the drill is sharp. A dull drill with high point thrust force produces significantly larger entry burrs because it deforms more material before cutting.

The exit burr forms as the drill breaks through the far side of the material. As the drill point approaches breakthrough, the remaining material becomes thin enough to deflect rather than cut cleanly. The material peels away from the underside of the hole rather than shearing, leaving a crown-shaped or flap burr on the exit face. Exit burrs are almost always larger than entry burrs and harder to remove without surface damage.

Exit burr size is controlled by three main factors: material ductility (ductile materials produce larger exit burrs), drill sharpness (dull drills produce massive exit burrs), and feed rate at breakthrough (high feed at breakthrough dramatically increases exit burr size).

Feed Rate at Breakthrough

The single highest-impact control for exit burr reduction is feed rate at breakthrough. As the drill nears the far side of the material — typically within the last 10-20% of hole depth — reducing the feed rate by 50-70% dramatically reduces exit burr size. The slower feed gives the cutting edges time to cut rather than push, shearing the material cleanly rather than peeling it.

In CNC operations, this is implemented with a feed rate override in the drilling cycle. A G83 peck cycle with a reduced final peck depth and feed is one approach. Alternatively, split the drilling into two operations: rough drill at full feed, then a final pass at reduced feed for the breakthrough. In practice, for production applications where cycle time matters, the feed ramp-down is controlled at the cycle level — not a manual operation.

Backup Plate Technique

Using a backup plate (also called a backer board) is the most reliable method for eliminating or drastically reducing exit burrs. The drill exits into a softer sacrificial material rather than into open air. With material supporting the exit face, the thin wall at breakthrough has something to press against rather than peel away from, and the drill cuts cleanly through the supported interface.

Backup plates are standard practice in sheet metal fabrication, PCB drilling, and aerospace composite drilling. The plate material must be softer than the workpiece to avoid affecting the drill geometry on exit. Common choices: MDF or phenolic for steel and aluminum work, nylon or Delrin for soft metals.

The limitation is fixture complexity. Backup plates work easily in flat, single-layer workpieces but become complicated in complex 3D workholding. They also must be replaced as they accumulate holes — a consideration in high-volume production.

Drill Sharpness and Burr Size

A dull drill increases burr size at both entry and exit, but the effect at exit is dramatic. As the cutting edges lose their sharpness, they push more material than they cut. At breakthrough, this pushing action peels a much larger flap of material off the exit face. In production environments where burr size suddenly increases, a worn drill is the most common cause — check drill sharpness before investigating other variables.

Deburring ROI

For low volumes, manual deburring with a countersink, file, or deburring blade is the standard approach. For high volumes, the math often supports automation. Robotic deburring cells, vibratory deburring, and automated countersinking can process hundreds of parts per hour unattended. The break-even on automated deburring equipment depends on labor rates, volume, and cycle time — but for shops spending more than 20 minutes per shift on manual deburring, the analysis is worth running. Alternatively, investing in process control (sharp drills, backup plates, breakthrough feed control) to reduce burr size often has a better payback than adding deburring equipment, because it solves the source rather than the symptom.