I used to think chip breaking was entirely about feed rate. If the chips were too long, I increased the feed until they broke. That works up to a point, but the drill’s chip breaker geometry sets the baseline for what feed rate will work.
Understanding chip breaker geometry helps you choose the right drill for the material and know whether a regrind is maintaining the correct geometry.
What a Chip Breaker Does
A chip breaker is a groove or step ground into the rake face of the gun drill tip, just behind the cutting edge. As the chip slides across the rake face, it hits the chip breaker and curls. The curl creates bending stress in the chip, and when that stress exceeds the material’s fracture limit, the chip breaks.
Without a chip breaker, the chip slides in a straight ribbon across the rake face and exits the flute as one continuous strand. In most materials, that strand will be too long to evacuate reliably.
The chip breaker does not change how much material the edge removes. It changes the shape of the chip after it leaves the cutting zone.
Chip Breaker Depth and Feed Rate
The most important relationship to understand is between chip breaker depth and feed rate. The chip breaker groove must be deeper than the chip thickness for it to work effectively.
| Feed Rate | Chip Thickness | Minimum Chip Breaker Depth |
|---|---|---|
| 0.010 mm/rev | ~0.010mm | 0.020mm |
| 0.020 mm/rev | ~0.020mm | 0.035mm |
| 0.030 mm/rev | ~0.030mm | 0.050mm |
| 0.040 mm/rev | ~0.040mm | 0.065mm |
If the chip breaker is shallower than the chip thickness, the chip bridges across the groove without curling. The chip breaker becomes ineffective and the chip forms a continuous ribbon.
This is why running a low feed rate on a drill designed for high feed can produce stringy chips — the chip is too thin to engage the breaker properly.
Chip Breaker Width and Curl Radius
The width of the chip breaker groove determines how tightly the chip curls. A narrow groove produces a tight curl and short chips. A wide groove produces a larger curl radius and longer chips before breaking.
| Chip Breaker Width | Typical Chip Length | Best For |
|---|---|---|
| 0.3-0.5mm | 2-5mm | Hard materials, high feed |
| 0.5-0.8mm | 5-10mm | General purpose steel |
| 0.8-1.2mm | 10-20mm | Soft materials, low feed |
I use narrow chip breakers for alloy steels and stainless where I want aggressive chip breaking. I use wider breakers for soft materials like aluminum where the chip needs room to curl without packing.
The chip breaker width on a reground drill can change if the regrind service removes too much material from the rake face. A chip breaker that was originally 0.5mm wide can become 0.8mm wide after several regrinds, shifting the chip breaking behavior.
Chip Breaker Position Relative to the Cutting Edge
The chip breaker must be positioned close enough to the cutting edge that the chip hits it while still hot and ductile. If the chip breaker is too far back, the chip cools before it curls and becomes harder to break.
The distance from the cutting edge to the chip breaker — called the retracted distance — should be 0.2-0.5mm for most gun drilling applications. A retracted distance over 0.5mm reduces chip breaking effectiveness noticeably.
I check this distance with a microscope when evaluating reground drills. If the retracted distance has increased by more than 0.1mm from the original specification, I expect chip breaking to degrade.
Material-Specific Chip Breaker Considerations
Not all materials respond to chip breakers the same way:
Steel (4140, 1045). Chip breakers work well. Standard geometry with 0.5-0.8mm width and 0.03-0.05mm depth works for most applications.
Stainless steel (304, 316). Needs aggressive chip breaking. Narrower width (0.3-0.5mm) and deeper groove (0.05-0.08mm) to overcome the material’s toughness.
Aluminum. Soft and ductile. Chips do not break cleanly at chip breakers. Wider groove (0.8-1.0mm) and higher feed rates help, but aluminum will always produce longer chips than steel.
Brass and copper. Chips break naturally without much help. Chip breaker geometry is less critical.
Titanium. Needs a balance. Too aggressive a chip breaker creates a stress concentration that can chip the cutting edge. I use moderate width (0.5-0.7mm) with a smooth radius at the bottom of the groove.
I cover the relationship between chip shape and parameters in Chip Formation in Deep Hole Drilling.
How Chip Breaker Geometry Changes With Regrinds
Every regrind removes material from the rake face, which changes the chip breaker geometry. The groove becomes wider and shallower with each regrind.
After 5-6 regrinds on a typical gun drill, I have measured the chip breaker width increase by 30-50% and the depth decrease by 20-30%. At that point, the chip breaking performance is noticeably different from a new drill.
I track this by recording chip breaker dimensions after each regrind. When the width exceeds 1.0mm or the depth drops below 0.02mm, I retire the drill from production work and move it to roughing or less demanding jobs.
I cover regrind inspection in Gun Drill Regrind Inspection.
Key Takeaways
- Chip breaker depth must exceed the chip thickness to work. At low feed rates, the chip may be too thin to engage the breaker.
- Narrow chip breakers produce tighter curls and shorter chips. Wide breakers produce longer chips.
- The chip breaker should be positioned 0.2-0.5mm from the cutting edge. Greater distance reduces effectiveness.
- Regrinds change chip breaker geometry. Track width and depth after each regrind.
- Different materials need different chip breaker geometries. What works for steel will not work optimally for aluminum or stainless.
- If chip breaking degrades on a drill that used to work well, the chip breaker has likely worn or been altered by regrinding.