Appearance
The chip does not lie — its shape, size, and surface tell you exactly what is happening at the cutting edge, and reading that signal correctly is the most powerful diagnostic tool in deep hole drilling.
Overview
Chip formation in deep hole drilling is fundamentally different from chip formation in conventional drilling. In standard drilling, chips travel a short distance up a fluted drill and are discarded. In deep hole drilling, chips must travel the entire hole depth — meters in many cases — through a narrow evacuation path. If chip formation is not precisely controlled, the evacuation path becomes blocked, and tool failure follows within seconds.
Two distinct chip evacuation systems exist:
| Process | Evacuation Path | Chip Size Constraint |
|---|---|---|
| Gun drilling | V-shaped flute along the drill shank | Chips must fit within the flute cross-section |
| BTA drilling | Through the drill tube interior | Chips must pass through the tube bore |
Both methods require chips that are small, consistent, and free-flowing. The formation of such chips depends on cutting parameters, tool geometry, material properties, and coolant delivery.
Chip Formation Mechanics
Shear Zone and Chip Thickening
In deep hole drilling, chip formation follows the same basic mechanics as other machining processes. Material ahead of the cutting edge undergoes plastic deformation in the primary shear zone, then flows across the rake face as a chip. However, the chip thickness ratio (cut chip thickness / uncut chip thickness) is typically higher in deep hole drilling due to:
- Restricted chip flow space in the V-flute or drill tube
- Higher friction along the rake face from constrained evacuation
- Secondary deformation from chip curl against the bore wall or tool flank
The tool-chip contact length in BTA drilling has been measured at approximately 1.65 times the chip thickness. As drilling depth increases and tool wear progresses, friction at the chip-tool interface increases, further thickening the chip and lengthening the contact zone.
Chip Curl
Chip curl is determined by the bending moment at the cutting edge as the chip exits the shear zone. Two factors control curl:
- Rake angle — more positive rake produces tighter curls
- Chip breaker geometry — groove or step-type breakers impose a bending radius on the chip
The chip's natural curl radius must be smaller than the available space in the evacuation path. If the curl radius is too large, the chip does not break and becomes a long, continuous ribbon that blocks the flute or tube.
Ideal Chip Forms
Gun Drilling
The ideal gun drilling chip is a compact, well-broken form that moves freely through the V-flute without sticking or bridging.
| Chip Form | Description | Diagnostic Meaning |
|---|---|---|
| C-shape / comma-shape | Curled chip that breaks cleanly at each end | Correct feed, proper geometry, good chip breaking |
| Tight conical spiral | Small cone-shaped spiral from inner cutting edge | Normal inner edge chip, indicates proper inner angle |
| Short helical "pigtail" | Small, tight helix, 2–3 mm long | Acceptable if not entangling |
BTA Drilling
In BTA drilling, chips exit through the drill tube center bore and are carried by the coolant stream. The ideal chips are:
| Chip Form | Description | Diagnostic Meaning |
|---|---|---|
| Segmented chips | Short, individual segments 2–8 mm long | Proper chip breaking, stable process |
| Well-curled arcs | Arc-shaped chips that do not interlock | Good coolant flow, correct feed |
| Fine helical chips | Small, loose helices that flow freely | Normal for some material-geometry combinations |
Problematic Chip Forms
Chip Form Troubleshooting Reference
| Problematic Chip | Visual Description | Root Cause | Corrective Action |
|---|---|---|---|
| Long continuous spiral | Unbroken helical ribbon, meters long | Feed too low; outer angle too small; inadequate chip breaker | Increase feed; increase outer angle; verify chip breaker geometry |
| Bird's nest / stringy mass | Tangled, intertwined mass of chips | Gummy material (low-carbon steel, stainless); low feed; worn cutting edge | Increase feed; check edge sharpness; increase coolant pressure |
| Rod-shaped / needle chips | Thin, needle-like chips that pack in the flute | Feed too high; excessive wear on outer corner; poor chip breaker | Reduce feed; regrind drill; optimize chip breaker |
| Entangled multi-strand | Chips of different widths tangled together | Chipped or worn cutting edge; vibration; uneven wear | Regrind or replace tool; check TIR and alignment |
| Rigid continuous spiral rolls | Stiff, inflexible long spirals | Mismatch of feed and geometry for material | Adjust feed; verify outer/inner angles for material |
| Powder / dust chips | Fine, powder-like chips | Feed too low; tool rubbing instead of cutting | Increase feed to establish proper chip thickness |
| Scratched chip surface | Scoring marks visible on chip face | Feed too high; worn cutting edge; poor geometry | Reduce feed; regrind; verify drill geometry |
When in doubt, increase feed
The most common chip formation problem in gun drilling is chips that are too long. Feed rate is the primary control — increasing feed increases chip thickness, which promotes chip breaking. If chips are long and stringy, increase feed within the recommended range before making any other adjustment. Cutting speed primarily affects tool life, not chip form.
Chip Breaker Design
Groove-Type Chip Breakers
Groove-type chip breakers are the most common chip control feature on gun drills and BTA inserts. A groove ground behind the cutting edge imposes a bending radius on the chip as it flows across the rake face.
| Parameter | Effect on Chip Breaking | Recommendation |
|---|---|---|
| Groove width | Wider groove = chip bends less → harder to break | Reduce groove width for stringy materials |
| Groove depth | Deeper groove = sharper bend → easier to break | Increase depth for tough materials |
| Groove radius | Smaller radius = tighter curl → better breaking | Match to material ductility |
| Land width | Narrower land = chip contacts breaker earlier | Reduce for earlier chip engagement |
Research on groove-type chip breakers in deep hole drilling of 42CrMo steel found that decreasing groove width and increasing groove height produced higher chip strain and more reliable chip breaking.
Step-Type Chip Breakers
Step-type breakers create an abrupt change in the rake face plane, forcing the chip to bend sharply. These are common on indexable BTA inserts. The step height and distance from the cutting edge determine the breaking force applied to the chip.
Staggered Teeth BTA Tools
In staggered-teeth BTA heads, chip breaking is influenced by:
- Cutting radius of each tooth — determines individual chip thickness
- Feed per tooth — the most significant parameter affecting chip deformation
- Tooth wear state — worn teeth increase friction, thickening chips and compromising breaking
Feed rate has a greater impact on chip deformation and breaking than cutting speed in BTA drilling. Increasing drilling depth and tooth wear both aggravate friction at the chip-tool interface, increasing chip thickness and tool-chip contact length.
Material-Specific Chip Behavior
Carbon and Alloy Steels
| Material | Chip Tendency | Recommended Approach |
|---|---|---|
| Low-carbon steel (< 0.25% C) | Long, stretchy, stringy chips | Higher feed; outer angle 30–35°; aggressive chip breaker |
| Medium-carbon steel (1045) | Moderate curl, breaks well at correct feed | Standard parameters; adjust feed for C-shaped chips |
| Alloy steel (4140, 4340) | More brittle chip, easier breaking | Lower speeds; reduced feed range |
| Q&T steel (30–40 HRC) | Segmented, easily broken chips | Low feed; high coolant pressure |
Chip color in steel drilling provides an immediate diagnostic:
- Straw to light blue — correct cutting temperature (600–700°C at the cutting zone)
- Dark blue to purple — speed too high
- Silver — speed too low, risk of built-up edge
Stainless Steels
Austenitic stainless steels (304, 316) produce tough, stringy chips that resist breaking. The material's high ductility and work-hardening rate mean that chips become harder and more difficult to break as they form.
| Challenge | Cause | Solution |
|---|---|---|
| Stringy, gummy chips | High ductility, work hardening | Deep chip breaker groove; sharp cutting edge |
| Chip packing in flute | Sticky chip surface | Coolant pressure ≥ 70 bar; sulfurized oil preferred |
| Edge build-up | Adhesion to carbide | Maintain steady feed; never dwell |
Titanium Alloys
Titanium's low thermal conductivity concentrates cutting heat in the tool and chip, producing chips that are difficult to break. Titanium chips also present a fire risk when dry.
- Chips tend to form long, stringy spirals
- Coolant pressure must be ≥ 80 bar to flush chips effectively
- Sharp cutting edges are critical — dull tools produce thicker, unbreakable chips
- Chip breaker geometry must be optimized specifically for titanium
Titanium chip fires
Titanium chips can ignite at high cutting temperatures, particularly above 40 m/min with inadequate coolant. Maintain uninterrupted coolant flow at all times. Never allow titanium chips to accumulate dry on the machine or in the chip tray. A small chip fire can escalate rapidly if magnesium or other reactive materials are present.
Aluminum Alloys
| Aluminum Type | Chip Behavior | Control Strategy |
|---|---|---|
| Wrought (6061, 7075) | Good chip breaking at high feeds; risk of BUE at low speed | Maintain Vc > 100 m/min; polished flute gun drills |
| Cast (A356, 319) | Short, broken chips naturally | Standard parameters |
| High-silicon (> 12%) | Abrasive, short chips | PCD or fine-grain carbide; high feed |
| Pure aluminum (< 0.13% Si) | Long, hard spiral chips at feed > 0.05 mm/rev | Reduce feed; avoid small inner/outer angles |
Cast Iron
Cast iron produces short, discontinuous chips (often called "chip dust" or "powder" in gray iron) that evacuate easily. The primary concern is not chip breaking but:
- Abrasive wear from graphite and carbide particles
- Fine chip particles that pack in filters
- Coolant contamination from graphite fines
In-Process Chip Monitoring
Coolant Pressure Signal Analysis
Coolant pressure provides a real-time window into chip evacuation health. Research by Chin, Wu, and Young (1993) established two phenomenological models linking coolant pressure signals to chip state:
| Chip State | Pressure Signal | Model |
|---|---|---|
| Normal chip flow | Stable pressure at expected value | Slug flow model — chips move as discrete plugs in the coolant stream |
| Developing congestion | Gradual pressure rise of 2–5 bar above baseline | Diametral effect — chip diameter increases relative to bore, creating restriction |
| Partial blockage | Oscillating pressure, 5–10 bar swings | Chips alternately accumulate and clear |
| Complete blockage | Sudden pressure spike of 10–20 bar then drop | Flow stopped, tool overheating imminent |
Pressure-Diagnosed Chip State
| Observation | Interpretation | Action |
|---|---|---|
| Stable pressure, expected value | Normal chip evacuation | Continue |
| Gradual upward drift across multiple holes | Tool wear progression affecting chip formation | Plan tool change |
| Sudden pressure rise > 5 bar | Chip accumulation at restriction | Stop feed, retract 50 mm, resume at lower feed |
| Pressure oscillation ±3 bar | Intermittent chip congestion | Reduce feed 10–15% to improve evacuation |
| Pressure drop > 10% from baseline | Coolant leak or pump issue | Stop and inspect system |
| High-frequency pressure fluctuation | Tool chipping or vibration at cutting edge | Reduce speed, inspect tool |
Chip Geometry Monitoring
Beyond pressure signals, direct chip inspection remains the most accessible diagnostic. A structured inspection process:
- Collect chips from one complete hole — do not mix with previous chips
- Examine chip size distribution — most chips should be 2–10 mm in length
- Check for consistency — uniform chip shape indicates stable cutting
- Look for outliers — a few very long chips signal an incipient problem
- Inspect chip color — applies to steel only; stable color indicates stable temperature
Structured Troubleshooting Process
Step 1: Inspect the Chips
Drill one hole, stop, and examine the chips before changing any parameter. Chip form is the primary diagnostic.
Step 2: Listen to the Process
| Sound | Indication |
|---|---|
| Steady, consistent cutting sound | Normal operation |
| Crunching or rumbling | Chip evacuation difficulty — risk of seizure |
| High-pitched squeal | Vibration or incipient tool failure |
| Intermittent cracking | Chip breaking occurring at tool |
Step 3: Adjust Feed Rate
Feed rate is the most effective control for chip form:
- Chips too long → increase feed
- Chips too thick or scratching → reduce feed
- Powder or dust → increase feed significantly
Step 4: Verify Coolant Parameters
| Parameter | Target Range | Diagnostic Relevance |
|---|---|---|
| Pressure | 40–120 bar (gun drilling); 20–80 bar (BTA) | Chip transport capacity |
| Flow rate | 2–6 L/min per mm of diameter | Chip clearing from cutting zone |
| Filtration | ≤ 10 µm | Prevents chip recirculation and pad wear |
| Temperature | 20–40°C | Coolant viscosity and heat removal |
Step 5: Check Mechanical Setup
- Guide bush clearance: +0.003 to +0.008 mm for gun drilling
- Spindle-to-bush concentricity: < 0.013 mm
- Whip guide position and condition
- Clamping rigidity
Summary
| Aspect | Gun Drilling | BTA Drilling |
|---|---|---|
| Ideal chip form | C-shape, tight conical spiral, short pigtails | Segmented chips, well-curled arcs, 2–8 mm |
| Primary chip control | Feed rate | Feed per tooth |
| Chip breaker type | Groove on drill tip | Groove on insert or step-type |
| Evacuation path | V-flute along shank | Through drill tube bore |
| Coolant pressure for chip transport | 40–120 bar | 20–80 bar |
| Key diagnostic signal | Chip form inspection | Coolant pressure + chip form |
| Most common chip problem | Long continuous spiral | Chip packing in return tube |
| Feed adjustment for long chips | Increase feed | Increase feed per tooth |
FAQ
What does a C-shaped chip mean in gun drilling?
A C-shaped or comma-shaped chip indicates correct cutting parameters — feed rate is appropriate for the material and tool geometry, the chip breaker is engaging properly, and the chip is curling enough to break under its own weight. This is the target chip form for most gun drilling operations.
How do I fix long continuous spiral chips in gun drilling?
Increase feed rate first — this is the most common and effective fix. If chips remain long after increasing feed, check the outer cutting angle (increase by 2–5°), verify the chip breaker groove geometry (may need narrower or deeper groove), and confirm coolant pressure is adequate (minimum 40 bar for steel).
What causes chip packing in BTA drilling?
Chip packing occurs when chips are too large or too numerous to pass through the drill tube bore. Root causes: feed rate too high producing oversized chips, inadequate coolant flow reducing transport velocity, rough surfaces inside the drill tube catching chip edges, or a partial blockage creating a dam behind which more chips accumulate. Coolant pressure monitoring catches packing early — a rise of 5–15 bar above normal indicates a forming blockage.
Can coolant pressure really tell me about chip condition?
Yes. Coolant pressure responds to flow resistance in the chip return path. Under stable conditions, pressure is steady. As chips accumulate, flow resistance increases and pressure rises. This relationship has been validated experimentally — pressure signal analysis can distinguish between normal slug flow (stable pressure), developing congestion (gradual rise), and complete blockage (sharp spike). For BTA drilling especially, pressure monitoring is the most practical in-process chip diagnostic.
What chip shape indicates a worn gun drill?
Several chip shape changes indicate wear: entangled chips of different widths (uneven wear between inner and outer cutting edges), scratched chip surfaces (worn or chipped edge), a shift from C-shaped chips to longer spirals (increasing edge radius reduces effective chip breaking), and a gradual increase in chip thickness over successive holes (pad wear allowing more deflection).
Why does my aluminum drilling produce bird's nest chips?
Bird's nest chips in aluminum are caused by built-up edge (BUE) at low cutting speeds or inadequate chip breaking geometry. Increase cutting speed above 100 m/min to reduce BUE formation, use polished flute gun drills to reduce chip friction in the V-flute, and verify the chip breaker geometry is appropriate for aluminum. If using pure aluminum, reduce feed below 0.05 mm/rev to avoid the hard long spiral chip formation common in this material.
How many times should I expect to regrind between chip-related failures?
With proper parameters, chip-related failures should be rare. If chip form problems recur between regrinds (typically 500–3,000 holes for steel gun drills), the root cause is likely in the cutting parameters or setup rather than tool wear. If chip problems appear only after many holes and correlate with increasing feed force, normal flank wear is the cause and regrinding restores proper chip formation.
What is the difference between chip formation in gun drilling vs BTA?
In gun drilling, the single cutting edge produces one chip that must fit within the V-flute. Chip size is constrained by the flute cross-section. In BTA drilling, multiple cutting edges (typically 2–5 inserts) produce multiple chips that travel through the drill tube bore. BTA chips are generally larger and thicker because the multi-edge design distributes the chip load. BTA also allows higher feed rates (0.10–0.25 mm/rev) compared to gun drilling (0.01–0.06 mm/rev), producing thicker chips that break more readily.
Chip formation depends on tool geometry, cutting parameters, coolant conditions, and workpiece material. The values in this article represent typical production ranges. Consult tool suppliers for application-specific chip control recommendations. This article reflects industry knowledge as of 2026.