Appearance
A deep hole drilling operation producing 50 mm × 3,000 mm bores in medium-carbon alloy steel experiences recurring tool failure due to long stringy chips packing in the BTA drill tube. Analysis reveals that the chip breaker geometry is mismatched to the feed rate — the 1.5 mm groove width produces chip strain insufficient for fracture at 0.12 mm/rev feed, creating continuous ribbon chips that jam the chip evacuation channel. Installing inserts with a 1.0 mm chip breaker groove at the same feed rate reduces chip curvature radius from 8 mm to 3 mm, producing small C-shaped chips that evacuate reliably.
Chip Formation Mechanics in Deep Hole Drilling
Chip formation in deep hole drilling follows the same fundamental mechanics as general metal cutting but with critical differences driven by the enclosed cutting environment. The chip is formed in the primary deformation zone where the cutting edge shears material from the workpiece. It then passes through the secondary deformation zone on the rake face, where extrusion friction creates a bending moment that curls the chip toward the chip breaker.
In BTA drilling with staggered teeth, the cutting forces are distributed across central, intermediate, and external teeth. Each tooth experiences different cutting conditions — the central tooth operates at the lowest cutting speed (near the drill centreline) and produces the thickest chip, while the external tooth cuts at the highest speed with the thinnest chip. The chip thickness ratio across central : intermediate : external teeth is approximately 1.53 : 1.17 : 1. This means each tooth requires a chip breaker geometry tailored to its specific chip load.
In gun drilling, the single cutting lip produces a ribbon chip that flows along the V-shaped flute. The chip formation is influenced by the approach angle and the dub-off angle on the drill flank, which affect chip curling and evacuation.
Chip Morphology Types
| Chip Type | Description | Cause | Consequence |
|---|---|---|---|
| C-shaped (ideal) | Short, curved segments 3–10 mm long | Correct chip breaker + feed | Reliable evacuation; stable process |
| Spiral / conical | Continuous 3D spiral | Insufficient chip strain | Wraps around tool; causes blockage |
| Ribbon / stringy | Long straight ribbon | Very low feed or no chip breaker | Packs in drill tube; tool failure |
| Segmented / saw-tooth | Periodic variation in chip thickness | High-speed cutting of hard materials | Acceptable if segments separate |
| Needle / powder | Very small fragments | Excessive chip strain or brittle material | Indicates overly aggressive breaker |
| Laminated / cracked | Chips with transverse cracks | Intermittent chip flow | Causes vibration; poor surface finish |
Theoretical Models of Chip Breaking
Nakayama Deformation Criterion
The fundamental model for chip breaking was developed by Nakayama (1972). The strain increment on the free surface of the chip is:
Δεc = (tc / 2) × (1 / rc − 1 / rf)
Where:
- Δεc = strain increment on chip free surface
- tc = chip thickness
- rc = curvature radius after passing the chip breaker
- rf = final curvature radius after elastic recovery
- K = rf / rc (ratio of final to initial curvature, typically 0.7–0.9)
Chip fracture occurs when Δεc exceeds the material's fracture strain. This model demonstrates that chip breaking is controlled by three factors: chip thickness (tc), chip curvature radius after the breaker (rc), and material ductility.
Chip Curvature Radius
The chip curvature radius after passing through the chip breaker is governed by:
rc = wb − hb · cot θ − lc
Where:
- wb = chip breaker groove width
- hb = chip breaker groove depth
- θ = chip breaker angle
- lc = tool-chip contact length on the rake face
This equation shows that chip curvature decreases (more bending) as:
- Chip breaker width (wb) decreases
- Chip breaker depth (hb) increases
- Tool-chip contact length (lc) decreases
Tool-Chip Contact Length
In staggered teeth BTA drilling, the tool-chip contact length is approximately 1.65 times the chip thickness. Contact length increases with:
- Higher feed rates (thicker chips = longer contact)
- Higher material strength
- Lower cutting speeds
- Increased tool wear (flank wear increases contact friction)
As tool wear progresses, the contact length grows, altering chip curl and potentially causing chip breaking to fail even though the insert geometry has not changed.
Chip Breaker Geometry Design
| Parameter | Effect on Chip Breaking | Guideline |
|---|---|---|
| Groove width (wb) | Smaller width = tighter curl = better breaking | 0.8–2.0 mm depending on feed and material |
| Groove depth (hb) | Deeper groove = more chip deflection | 0.3–0.8 mm; deeper for ductile materials |
| Backwall height | Higher backwall = more chip impact | 0.2–0.5 mm above rake face |
| Rake angle | Positive rake reduces cutting forces | 0–6° positive for deep hole drilling |
| Land width | Narrower land = less contact area | 0.1–0.3 mm before chip breaker |
The chip breaker geometry must be matched to both the feed rate and the material being cut. For a given material, each feed rate has an optimal groove width — too wide and chips do not curl enough to break, too narrow and chips jam in the groove.
Chip Breaker Selection by Material and Feed
| Material Group | Feed (mm/rev) | Recommended Groove Width | Recommended Groove Depth |
|---|---|---|---|
| Low-carbon steel | 0.05–0.12 | 0.8–1.2 mm | 0.3–0.5 mm |
| Low-carbon steel | 0.12–0.25 | 1.2–1.8 mm | 0.4–0.6 mm |
| Alloy steel (300 HB) | 0.05–0.12 | 1.0–1.4 mm | 0.4–0.5 mm |
| Alloy steel (300 HB) | 0.12–0.25 | 1.4–2.0 mm | 0.5–0.7 mm |
| Stainless steel (austenitic) | 0.05–0.12 | 1.2–1.6 mm | 0.4–0.6 mm |
| Stainless steel (austenitic) | 0.12–0.20 | 1.6–2.2 mm | 0.5–0.8 mm |
| Titanium / superalloy | 0.03–0.08 | 0.8–1.2 mm | 0.3–0.5 mm |
| Titanium / superalloy | 0.08–0.15 | 1.2–1.6 mm | 0.4–0.6 mm |
TIP
The most common chip breaking error in deep hole drilling is using a chip breaker that is too wide for the feed rate. A groove width of 1.5 mm may produce excellent C-shaped chips at 0.20 mm/rev feed but produce stringy ribbon chips at 0.10 mm/rev feed because the chip does not curl enough to contact the backwall with sufficient force to fracture. Rule of thumb: the feed rate must produce a chip thick enough that the chip breaker groove width is no more than 8–12 times the chip thickness. For a 0.12 mm/rev feed in steel, the chip thickness is approximately 0.18–0.24 mm (chip thickness ratio ~1.5–2.0), requiring a groove width of 1.0–1.4 mm. Materials with higher ductility require tighter chip breaker geometry (smaller width, deeper groove) to produce the higher strain needed for fracture.
Process Parameters Affecting Chip Breaking
Feed Rate (Most Influential Parameter)
Feed rate has a greater impact on chip breaking than cutting speed. Increasing feed rate:
- Increases chip thickness
- Increases tool-chip contact length
- Produces higher chip strain at the chip breaker
- Improves chip breaking (up to a point)
Feed rate is the primary adjustment for controlling chip form. If chips are stringy, increase feed. If chips are too fragmented or causing vibration, decrease feed.
Cutting Speed
Cutting speed has a secondary but significant effect:
- Higher speed reduces chip thickness slightly
- Higher speed increases cutting temperature, softening the material
- Higher speed can make chip breaking more difficult in ductile materials
- Lower speeds improve chip breaking but reduce productivity
Material Properties
Material ductility is the primary material property affecting chip breaking:
- High-ductility materials (low-carbon steel, copper, aluminium) are difficult to break — require aggressive chip breaker geometry
- Brittle materials (hardened steel, cast iron) break naturally — require less aggressive geometry
- Work-hardening materials (stainless steel, superalloys) produce hard chips that can be easier to break but require sharp edges
Chip Breaking in Staggered Teeth BTA Drilling
BTA drills with staggered teeth present a unique chip breaking challenge because each tooth operates at different radial positions with different cutting speeds and chip loads. The central tooth (innermost position) operates at the lowest cutting speed and produces the thickest, most difficult chip. The external tooth operates at the highest speed with the thinnest chip.
Research by Li et al. (2018, 2019) shows:
- The central tooth requires the most aggressive chip breaker geometry
- The external tooth can use a wider groove due to thinner chips
- Feed rate of 0.12–0.30 mm/rev produces optimal chip shapes in alloy steels
- Chip thickness increases by 9.75–18.34% as tool wear progresses
Chip breaker design for BTA inserts must balance the requirements of all teeth. Modern BTA heads use different insert geometries for each tooth position, optimised for their specific cutting conditions.
Chip Evacuation Dynamics
Chip breaking alone is insufficient — the broken chips must be transported out of the bore through the chip evacuation system. In BTA drilling, chips are carried by the returning coolant flow through the hollow drill tube. The chip transport depends on:
| Factor | Effect | Control |
|---|---|---|
| Coolant flow velocity | Carries chips upward through tube | Minimum 3–5 L/min per mm of bore diameter |
| Chip size and shape | C-shaped 3–10 mm chips transport best | Correct chip breaker geometry |
| Chip density | Steel chips (7.8 g/cm³) sink in oil | Velocity must overcome settling velocity |
| Drill tube inner diameter | Larger ID improves transport | 0.5–0.7 × bore diameter |
| Coolant viscosity | Higher viscosity suspends chips better | 10–40 cSt at operating temperature |
Troubleshooting Chip Problems
| Problem | Likely Cause | Corrective Action |
|---|---|---|
| Long stringy chips | Feed too low for groove width | Increase feed or install narrower groove insert |
| Spiral/conical chips | Insufficient chip strain | Decrease groove width or increase groove depth |
| Chip packing in drill tube | Inadequate coolant flow | Increase flow; check filtration; clear blockage |
| Chip jamming at drill head | Chip breaker too aggressive | Reduce feed or increase groove width |
| Needle/powder chips | Feed too low; excessive strain | Increase feed; reduce groove depth |
| Chip colour changes (blue) | Excessive heat | Increase coolant pressure or flow |
| Chips welding to insert | Insufficient lubrication | Increase coolant concentration or pressure |
| Intermittent chip breaking | Tool wear or material variation | Replace insert; check material consistency |
| Vibration during chip formation | Chip thickness variation | Adjust speed/feed for stable chip formation |
| Chips not entering flute (gun drill) | Dub-off angle incorrect | Adjust drill geometry per material |
FAQ
Why is chip breaking critical in deep hole drilling?
Chip breaking is the most important process control factor in deep hole drilling because the cutting zone is enclosed within the bore, with no direct access for chip removal. Unlike conventional drilling where chips can exit freely, deep hole drilling requires chips to travel up to 12,000 mm through the drill tube or flute. If chips are not broken into small, regular shapes, they pack in the chip evacuation channel, causing coolant pressure to spike, cutting torque to increase, and ultimately tool failure. In BTA drilling, chip packing is the leading cause of scrapped components.
What is the ideal chip shape in deep hole drilling?
The ideal chip in deep hole drilling is a small C-shaped or comma-shaped chip 3–10 mm long with a consistent curvature radius. These chips flow freely in the coolant stream, do not tangle with each other, and evacuate reliably through the drill tube or flute. Spiral chips longer than 20 mm are acceptable if they are consistent and do not tangle, but they carry a higher risk of packing. Ribbon or stringy chips longer than 50 mm are unacceptable and indicate inadequate chip breaking.
How does feed rate affect chip breaking?
Feed rate is the most influential parameter for chip breaking. Higher feed rates produce thicker chips that undergo greater strain when passing through the chip breaker, making fracture more likely. The chip thickness ratio (chip thickness / feed) is typically 1.5–3.0 depending on material and tool geometry. For a given chip breaker geometry, there is a minimum feed rate below which chips will not break. If chips are stringy, increasing feed by 20–30% often resolves the problem. However, excessive feed can cause chip breaker jamming or excessive cutting forces.
How does chip breaker geometry affect chip breaking?
The chip breaker groove width and depth directly determine the chip curvature radius. A narrower groove produces tighter chip curl and higher strain on the chip surface, promoting fracture. A deeper groove increases chip deflection against the backwall. The groove width must be matched to the feed rate — as a rule of thumb, the groove width should be 8–12 times the chip thickness. For a feed of 0.15 mm/rev in steel (chip thickness ~0.25 mm), the groove width should be approximately 1.2–1.5 mm. The groove depth should be 0.4–0.6 mm for most steel applications.
What is the Nakayama criterion for chip breaking?
The Nakayama deformation criterion states that chip fracture occurs when the strain increment on the free surface of the chip exceeds the material's fracture strain. The strain increment depends on chip thickness and the change in curvature radius as the chip passes through the breaker. Materials with higher ductility (higher fracture strain) require either thicker chips (higher feed) or tighter curvature (smaller groove width) to achieve fracture. This theoretical framework explains why ductile materials like low-carbon steel and copper are more difficult to chip-break than brittle materials like hardened steel.
How does tool wear affect chip breaking?
As the BTA insert wears, the tool-chip contact length increases due to flank wear. This shifts the effective chip breaker position relative to the cutting edge, altering the chip curl radius. Research shows that chip thickness increases by 9.75–18.34% as the external tooth wears to VB = 0.25 mm. A worn tool that produced excellent C-shaped chips when new may begin producing stringy or spiral chips after 50 m of drilling, even though the chip breaker geometry has not physically changed. This is a common cause of intermittent chip problems — the solution is to replace inserts at the first sign of chip form deterioration.
How does cutting speed affect chip breaking?
Cutting speed has a secondary effect on chip breaking compared to feed rate. Higher cutting speeds increase cutting temperature, which can soften the chip and make it more ductile, potentially making chip breaking more difficult. Lower cutting speeds improve chip breaking but reduce productivity. For most alloy steels, the optimal speed for chip breaking is in the mid-range of the recommended cutting speed for tool life. Very low speeds (below 20 m/min) can cause chatter, while very high speeds (above 120 m/min) can make chip control unpredictable.
How does material ductility affect chip breaker selection?
Material ductility determines the chip strain required for fracture. Materials with high elongation (low-carbon steel, copper, aluminium at 30–60% elongation) require aggressive chip breaker geometry — narrow groove, deep profile — to produce the high strain needed for fracture. Materials with low elongation (hardened tool steel, cast iron at 2–10% elongation) break naturally and require less aggressive geometry. Austenitic stainless steels are particularly challenging because they combine high ductility with work-hardening — they require chip breakers designed specifically for stainless steel grades.
How is chip evacuation achieved in BTA drilling?
In BTA drilling, coolant is pumped under high pressure through the annular gap between the drill tube outer wall and the bore surface. The coolant flows to the cutting zone, where it cools the inserts and guide pads, then reverses direction and flows back through the hollow interior of the drill tube, carrying the broken chips with it. The Venturi effect at the drill head creates suction that draws chips into the tube. The chip-laden coolant flows to a chip collection box, where chips settle and coolant is filtered and recirculated. Adequate flow velocity (3–5 L/min per mm of bore diameter) is essential to keep chips suspended in the coolant stream.
What is the most common chip breaking mistake in deep hole drilling?
The most common mistake is using a chip breaker geometry that is too wide for the actual feed rate. Tooling catalogues often recommend chip breaker geometries based on nominal feed ranges, but operators tend to run at the lower end of the feed range for safety, producing chip loads that are too low to activate the chip breaker effectively. The result is stringy chips that cause packing. The solution is either to increase feed to match the chip breaker geometry, or to install inserts with a narrower chip breaker groove matched to the actual operating feed. The second most common mistake is not adjusting chip breaker selection when changing materials — a chip breaker that works well in 4140 steel will likely produce stringy chips in 316L stainless due to the higher ductility.
Summary
Chip breaking theory in deep hole drilling is governed by the Nakayama deformation criterion, which relates chip fracture to the strain increment produced when the chip passes through the chip breaker. The chip curvature radius is controlled by groove width, groove depth, and tool-chip contact length. Feed rate is the most influential process parameter — higher feeds produce thicker chips that break more readily. The ideal chip form is a C-shaped segment 3–10 mm long, produced when the chip breaker geometry is correctly matched to the feed rate and material. Chip breaker selection must account for tooth position in staggered BTA heads (central tooth requires the most aggressive geometry), material ductility (ductile materials need tighter breakers), and tool wear progression (contact length changes alter chip curl). Adequate coolant flow velocity of 3–5 L/min per mm of bore diameter is essential for chip transport. Chip breaker geometry matching to feed rate and material is the key process control factor that distinguishes successful deep hole drilling from operations plagued by chip packing and tool failure.