I adjust feed rate to control chip breaking more than any other single parameter in deep hole drilling. Chip breaking is essential for deep hole drilling because the entire process depends on reliable chip evacuation through a narrow flute or inner tube. Long stringy chips pack up in the flute and cause tool breakage in seconds. Short broken chips evacuate easily, and consistent chip breaking is what separates a stable deep hole drilling process from a nightmare of broken tools and scrapped parts.
The physics are straightforward: a chip that breaks every 3-6 mm will clear the bore. A chip that runs continuously for 100 mm wraps around the drill shaft and blocks the flute. I have seen both outcomes on the same machine with the same material, and the difference was a 0.02 mm/rev change in feed rate.
How Chips Break in Deep Hole Drilling
Chips break when the bending stress at the chip curl exceeds the chip material’s fracture strength. The feed rate determines the chip thickness, and the chip thickness determines how sharply the chip must bend before it fractures. A higher feed produces a thicker chip that bends more sharply at the curl and breaks easily. A lower feed produces a thin, stringy chip that bends without reaching the fracture point.
The chip breaking frequency depends on the feed rate, cutting speed, and material ductility. For mild steel at 0.04 mm/rev feed, I get chips about 4-8 mm long. For the same feed in aluminum, the chips are 10-15 mm because aluminum is more ductile and bends further before fracturing. In stainless steel, the chips are 3-5 mm because the material work-hardens at the shear zone and fractures at a tighter bend radius.
Chip Shape Reference Table by Material and Feed Rate
Here is the reference table I use on the shop floor when setting up a new deep hole drilling job:
| Material | Feed (mm/rev) | Typical Chip Length | Chip Shape | Chip Breaking Quality |
|---|---|---|---|---|
| Mild steel 1018 | 0.04-0.08 | 4-8 mm | Short arcs, regular break | Excellent |
| Mild steel 1018 | Below 0.03 | 15-30 mm | Stringy, continuous | Poor — risk of flute packing |
| Stainless 304 | 0.02-0.05 | 3-6 mm | Tight curls, segmented | Good |
| Stainless 304 | Above 0.06 | 1-3 mm | Powder fragments | Fair — too short (reduces productivity) |
| Aluminum 6061 | 0.08-0.15 | 8-15 mm | Loose spirals | Good |
| Aluminum 6061 | Below 0.05 | 20-50 mm | Long ribbons | Dangerous — wraps around drill |
| Titanium 6Al-4V | 0.01-0.03 | 2-5 mm | Powdery segments | Fair — needs high coolant flow |
| Cast iron | 0.05-0.12 | 0.1-2 mm | Fine powder | Excellent — no chip control needed |
| Inconel 718 | 0.008-0.02 | 1-3 mm | Irregular fragments | Challenging — needs aggressive breaker |
| Cobalt Chrome | 0.008-0.015 | 0.5-2 mm | Crushed gravel | Requires high pressure and pecking |
The key insight from this table is that every material has a feed rate sweet spot for chip breaking. Operating outside that window creates chip control problems that no amount of coolant pressure can fully fix.
Chip Breaker Geometry
The chip breaker geometry on the drill tip affects chip breaking more than any other tool feature. The chip breaker creates a stress concentration — a step or groove behind the cutting edge — that forces the chip to curl and fracture at a controlled interval. A chip breaker that matches the material and feed rate produces consistent short chips reliable enough for unattended deep hole drilling.
I match the chip breaker land width to the feed rate. The land is the flat section behind the cutting edge before the chip breaker step. For a feed of 0.05 mm/rev, I use a land width of 0.3-0.5 mm. A land that is too wide allows the chip to slide out without curling against the breaker. A land that is too narrow causes the chip to jam in the flute before it can break cleanly.
The chip curl radius formula that I use for selecting chip breakers is:
rc = (wb - hb x cot θ - lc) / tan(θ/2) - tc / (2 x sin θ)
Where wb is the chip breaker width, hb is the chip breaker height, θ is the chip breaker angle, lc is the tool-chip contact length, and tc is the chip thickness. A smaller chip curl radius means the chip breaks more aggressively. I target a chip curl radius of 2-4 mm for most materials.
Chip Breaker Selection Guide
Standard gun drill inserts come in three chip breaker configurations that I keep on hand:
| Chip Breaker Type | Feed Range (mm/rev) | Best For | Land Width |
|---|---|---|---|
| Small breaker | Below 0.03 | Hard materials, superalloys, titanium | 0.15-0.25 mm |
| Medium breaker | 0.03 to 0.08 | Steels, stainless, general purpose | 0.30-0.50 mm |
| Large breaker | Above 0.08 | Soft materials, aluminum, brass | 0.50-0.80 mm |
I keep all three chip breaker types on hand and swap them when I change materials. The time to change an insert is 30 seconds. The time to clear a packed flute is 30 minutes — and that is if the drill has not already snapped. The chip breaker selection directly determines whether the chips exit the hole or pack inside it.
How to Adjust Feed for Chip Control
When I see stringy chips on a deep hole drilling job, my first adjustment is to increase the feed rate. Here is my step-by-step approach:
- Identify the chip problem: Look at the chips coming out of the bore. Stringy chips longer than 10 mm mean the feed is too low for the chip breaker geometry.
- Increase feed by 10-20%: A small feed increase thickens the chip enough to promote breaking. I never jump more than 20% to avoid overload.
- Check the chip length: After the adjustment, observe chip length for 3-5 holes. Target is 3-6 mm chips.
- If chips are still stringy: The chip breaker geometry may be wrong. Switch to a smaller breaker or a breaker with a narrower land width.
- If chips are too short (powder): The feed may be too high or the breaker too aggressive. Reduce feed or switch to a larger breaker.
- If chips jam despite good shape: The coolant flow rate may be insufficient to transport the chips. Increase pressure or check for blockages in the return line.
For more on the chip transport side of the equation — how coolant velocity and flow rate move chips out of the bore — see the chip management systems article.
Coolant Pressure Effect
High-pressure coolant forces the chip to curl more tightly as it exits the cutting zone. The tighter curl increases the bending stress at the chip breaker and helps the chip fracture at a shorter length. I have seen a 500 psi increase in coolant pressure improve chip breaking in stainless steel enough to eliminate stringy chips entirely — without changing the feed rate or chip breaker.
The coolant nozzle position also matters. The nozzle should direct the stream directly at the chip-tool interface, not at the workpiece surface. I check the nozzle alignment by running the coolant at low pressure and watching the stream direction relative to the cutting edge. A misaligned nozzle by even 5 degrees reduces the chip breaking effectiveness by pushing the chip against the bore wall instead of curling it.
For BTA drilling, the coolant enters through the annulus between the drill shaft and the bore wall, then exits through the inner tube carrying chips with it. The coolant velocity determines the chip transport capacity. I maintain a minimum velocity of 10 m/s in the annulus and 15 m/s in the inner tube for reliable chip transport. A drop below those velocities means chips settle in the tube and cause blockages.
I aim to produce chips 3-6 mm long in most materials. That size evacuates reliably through the flute without bridging across the clearance gap. The cutting forces article has more detail on how chip load affects thrust and torque — see cutting forces in gun drilling for that relationship.
For superalloy and cobalt chrome chip control specifically, see the cobalt chrome drilling article, where the chip breaking challenges are more extreme.
Key Takeaways
- Feed rate is the primary control for chip breaking in deep hole drilling — increase feed to break chips shorter.
- Target chip length of 3-6 mm for reliable evacuation in most deep hole drilling setups.
- Match the chip breaker land width to the feed rate: land width should be approximately 6-10x the feed per revolution.
- Keep small, medium, and large chip breaker inserts available for material changes — swapping takes 30 seconds versus 30 minutes to clear a packed flute.
- High-pressure coolant above 1500 psi helps break chips by increasing the chip curl stress at the breaker.
- To fix stringy chips: increase feed 10-20% first, then change chip breaker if needed.
- Every material has a feed rate sweet spot for chip breaking — operating outside it creates problems that coolant alone cannot fix.
- Monitor chip shape as a real-time indicator of process health — change in chip shape signals tool wear or material variation.