Chip Breaking Techniques in Deep Hole Drilling: Modulated Vibration, Variable Feed, and Mechanical Chip Breakers

A manufacturer of automotive fuel injection components (316L stainless steel, Ø6 mm × 400 mm deep bore, depth-to-diameter ratio 67:1) was using conventional gun drilling at Vc = 60 m/min, f = 0.015 mm/rev, filtered oil coolant at 80 bar. The stainless steel produced long, stringy continuous chips that packed the chip flute at approximately 200–300 mm depth, causing three drill breakages per month (each costing $850 in tooling + 4 hours downtime). Switching to modulated auxiliary machining (MAM) with low-frequency axial oscillation (12 Hz, 0.08 mm amplitude, f = 0.025 mm/rev, Vc = 70 m/min) produced short, C-shaped chips of 2–4 mm consistent length. The controlled chip breaking eliminated drill breakages, improved Ra from 0.8 to 0.4 µm, extended tool life from 80 to 250 m, and increased productivity by 25%.

Chip Breaking Methods in Deep Hole Drilling ​

Comparison of Chip Breaking Techniques ​

Process Parameter Selection for Chip Breaking by Material ​

Modulated Auxiliary Machining (MAM) for Deep Hole Drilling ​

MAM Parameter Optimisation by Material ​

Effect of MAM Parameters on Chip Formation ​

Mechanical Chip Breaker Design for Gun Drills ​

Chip Breaker Geometry Parameters for Gun Drills ​

Mechanical Chip Breaker Selection Guide by Material and Feed Range ​

FAQ ​

What is the root cause of chip breaking problems in deep hole drilling, and why is chip control more critical than in conventional drilling? ​

The root cause of chip breaking problems in deep hole drilling is the confined chip evacuation path — unlike conventional drilling where the chip exits the hole freely, in deep hole drilling the chip must travel 50–300× its own width through a narrow annular clearance (0.1–0.5 mm gap between the drill OD and the bore wall) while being flushed by high-pressure coolant. If the chip does not break into short segments, a single continuous chip ribbon (which can easily reach 100–500 mm in length in ductile materials such as stainless steel, copper, or aluminium) will coil around the drill shank, pack into the flute, and block the chip evacuation path. Once the flute is packed, coolant flow is restricted, which reduces chip flushing and increases temperature at the cutting edge, leading to further chip welding and packing in a self-reinforcing cycle that typically ends with catastrophic drill breakage within 2–10 seconds of the onset of chip packing. The speed of this failure chain is why chip control is the single most critical process reliability factor in deep hole drilling — a chip packing event in a 600 mm deep bore typically requires 30–120 minutes of machine downtime for drill retrieval (which may involve EDM cutting of the broken drill, mechanical extraction, or even scrapping the component), compared to a tool change time of 2–5 minutes in conventional drilling.

The fundamental physics of chip formation in deep hole drilling is also different from conventional drilling because the cutting conditions (high coolant pressure, constrained chip space, and the guide pad burnishing action) create a different friction and temperature environment that affects chip flow. In conventional twist drilling, the chip is pushed up the flute by the drill's axial motion and the chip's own rigidity. In gun drilling, the chip is formed at the cutting edge, curls into the V-shaped flute, and is pushed along the flute by the incoming chip behind it and the coolant flow. The chip travels along the flute at a velocity of 1–3 m/s (equal to the coolant velocity in the flute, which is typically 3–8 m/s, minus the chip's sliding resistance against the flute wall). If the chip breaks into short segments (2–10 mm), each segment exits the flute independently, and the coolant flow carries it through the annular clearance and out of the bore. If the chip does not break, the continuous ribbon acts as a "plug" that blocks the flute, and the coolant flow is diverted around the plug, leaving the plug stationary while the incoming chip material piles up behind it. The pressure at the plug increases until either the coolant pressure overcomes the plug and flushes it out (causing a transient pressure spike that may damage the drill bushing seal) or the drill torque exceeds the drill's torsional strength and the drill snaps. The chip breaking techniques described in this article — MAM, CNC oscillation, mechanical chip breakers, and ultrasonic vibration — are all designed to prevent the formation of chip ribbons longer than 5–10 mm, ensuring that the chip evacuation path remains clear and that the self-reinforcing failure cycle is never initiated.

How does modulated auxiliary machining (MAM) achieve chip breaking in deep hole drilling, and what hardware is required to implement it? ​

Modulated auxiliary machining (MAM) achieves chip breaking by superimposing a controlled axial oscillation on the cutting tool's feed motion, creating a periodic variation in the uncut chip thickness. In standard deep hole drilling, the feed motion is continuous — the tool advances at a constant feed rate, producing a chip of constant thickness. In MAM, the feed axis oscillates at a frequency f_osc and amplitude A_osc, such that the instantaneous feed rate varies sinusoidally: f_inst(t) = f_nom + A_osc · 2π · f_osc · cos(2π · f_osc · t). When the instantaneous feed rate reaches zero or becomes negative (the tool briefly moves backward relative to the workpiece), the chip thickness goes to zero, creating a separation in the chip. Each oscillation cycle produces one chip segment, and the chip length L_chip is determined by the relative velocity between the tool and workpiece during the cutting portion of the cycle: L_chip = V_cutting / (f_osc · k), where k is a factor that depends on the fraction of the oscillation cycle during which cutting occurs. For a standard sinusoidal oscillation with amplitude sufficient to create chip separation, k ≈ 2–3, meaning the chip length is 2–3 revolutions of the workpiece per chip segment. For 316L stainless steel at Vc = 70 m/min on a Ø6 mm bore (spindle speed 3700 rpm), with f_osc = 12 Hz and sufficient amplitude (A_osc > 0.06 mm), each chip segment corresponds to approximately 3700/12 = 308 revolutions, and the chip length is approximately 308 × 0.025 mm/rev / 2.5 = 3.1 mm — consistent with the experimental result of 2–4 mm chip length.

The hardware required for MAM depends on the oscillation frequency range required. For low-frequency oscillation (< 20 Hz), the standard CNC feed servo can be programmed to execute the oscillation trajectory directly — this is called CNC oscillating feed and requires no additional hardware beyond a CNC control with the capability to execute a real-time oscillating feed command (available on Siemens 840D sl, Heidenhain TNC 640, Fanuc 30i, and Mitsubishi M80 platforms). The limitation of CNC oscillating feed is the servo bandwidth — standard AC servo drives have a closed-loop bandwidth of 50–100 Hz, meaning the practical oscillation frequency is limited to 10–20 Hz (30–60 Hz for linear motor drives) before the servo cannot accurately track the commanded trajectory. For higher-frequency oscillation (20–100 Hz), a dedicated oscillation drive module is required — typically a piezo-actuator stack or a secondary linear motor mounted between the spindle and the feed axis. These modules add $15 000–40 000 to the machine cost and require a separate controller that synchronises the oscillation with the spindle rotation (for applications where oscillation phase relative to spindle angle matters). The higher-frequency MAM systems (20–100 Hz) produce shorter chip segments at a given feed rate and are particularly effective for difficult materials such as copper and stainless steel at high cutting speeds. The key process control parameter in MAM is the ratio of the oscillation amplitude to the nominal feed per revolution (A_osc / f_nom). When A_osc / f_nom > 0.5–1.0, the instantaneous feed rate goes to zero during part of the oscillation cycle, ensuring complete chip separation. When A_osc / f_nom < 0.5, the chip thickness varies but never reaches zero, and chip breaking may be incomplete (the chip becomes wavy but does not separate into segments). The practical recommendation is to maintain A_osc / f_nom > 0.8 for reliable chip breaking across all materials, with A_osc / f_nom > 1.2 preferred for difficult materials such as copper and stainless steel.

What is the difference between modulated auxiliary machining (MAM), CNC oscillating feed, and ultrasonic vibration for chip breaking, and which is most suitable for production deep hole drilling? ​

The three techniques differ in frequency range, amplitude, hardware requirements, and the physical mechanism that creates chip separation. CNC oscillating feed uses the standard machine tool feed servo to oscillate the drill axially at frequencies of 0.5–20 Hz (limited by servo bandwidth) with amplitudes of 0.02–0.50 mm. No additional hardware is required — the oscillation trajectory is programmed in the CNC. This is the lowest-cost implementation and is suitable for production applications where the required oscillation frequency is below 20 Hz. The limitation is that at higher cutting speeds (> 100 m/min) or with small-diameter drills (< 6 mm), the chip segments may become longer than optimum because the oscillation frequency cannot keep pace with the spindle speed (chip segments grow longer as spindle speed increases while oscillation frequency is fixed). Modulated auxiliary machining (MAM), in the strict sense, refers to the use of a dedicated oscillation drive module (piezo or linear motor) that can operate at higher frequencies (up to 200 Hz with linear motors, 1000 Hz with piezo actuators) and with independent amplitude control. The dedicated module provides faster frequency response, more precise amplitude control, and the ability to maintain chip breaking at higher cutting speeds. MAM is suitable for production applications where the chip breaking requirement exceeds CNC servo capability (frequency > 20 Hz, or amplitude accuracy required at low amplitudes < 0.05 mm). The additional hardware cost ($15 000–40 000) is typically justified for high-volume production of difficult materials (stainless steel, copper, aluminium) where reliable chip breaking is critical to process uptime.

Ultrasonic vibration operates at 18–24 kHz with very low amplitudes (5–30 µm peak-to-peak) — fundamentally different from the low-frequency, high-amplitude oscillation used in MAM and CNC oscillating feed. At ultrasonic frequencies, chip breaking occurs through a different mechanism: the high-frequency vibration creates cyclic stress in the chip at the cutting edge that causes the chip to fracture in a brittle manner, even in ductile materials. The chip segments produced by ultrasonic vibration are extremely short (0.1–1.0 mm) and are effectively "powder-like" in form, providing the best chip evacuation characteristics. However, ultrasonic vibration requires a completely different machine architecture — an ultrasonic spindle with a piezoelectric transducer, booster, and horn that vibrates the entire tool at ultrasonic frequency. The ultrasonic spindle adds $30 000–80 000 to the machine cost, the tool must be designed to resonate at the ultrasonic frequency (limiting tool length and diameter options), and the ultrasonic energy can generate heat in the tool that may cause thermal damage to coatings or brazed joints. For production deep hole drilling, CNC oscillating feed is the preferred method for materials that break chips at low frequency (steels, alloy steels), MAM with a dedicated oscillation module is preferred for difficult materials requiring higher frequency (stainless steel, copper, aluminium), and ultrasonic vibration is reserved for specialised applications where the highest chip control is required (micro-drilling < 3 mm diameter, ceramic and glass drilling, or materials that are extremely difficult to break by other methods). The choice is ultimately driven by the material's chip-breaking difficulty, the required chip length, and the available machine capability. A practical guideline is: if the material produces reliable chip breaking with CNC oscillating feed at < 15 Hz, no additional hardware investment is needed. If chip breaking requires > 15 Hz, consider a MAM module. If the material still produces uncontrolled chips with MAM at 100 Hz, consider ultrasonic vibration.

How does feed rate selection interact with chip breaking in deep hole drilling, and what are the risks of reducing feed to extend tool life? ​

Feed rate selection has a direct and often underestimated effect on chip breaking in deep hole drilling. The feed rate determines the chip thickness — higher feed produces thicker, stiffer chips that are easier to break; lower feed produces thin, flexible chips that are difficult to break regardless of the chip breaking method used. The relationship is well-established: for a given material and tool geometry, there exists a minimum feed per revolution below which reliable chip breaking cannot be achieved, even with MAM or mechanical chip breakers. For 316L stainless steel with a standard mechanical chip breaker, the minimum feed for reliable chip breaking is approximately 0.02 mm/rev. Below this feed, the chip is too thin (< 0.03 mm after accounting for chip thickness ratio) to develop the bending stress required for fracture at the chip breaker groove, and the chip forms a continuous ribbon regardless of the chip breaker geometry. For copper, the minimum feed is higher — approximately 0.03 mm/rev — because of the material's extreme ductility (the chip must be thicker to generate enough bending stress to overcome the material's work hardening and fracture). For titanium alloys, the minimum feed for reliable chip breaking is approximately 0.02 mm/rev, though titanium's natural shear localisation (adiabatic shear banding) produces shorter chips than the feed-per-revolution relationship would predict — a unique characteristic that makes titanium less dependent on chip breaking assistance than stainless steel or copper at equivalent feeds.

The risk of reducing feed to extend tool life is that the chip breaks less reliably, and the probability of a chip packing event increases as feed decreases. A tool life extension of 10–20% achieved by reducing feed from 0.025 to 0.018 mm/rev can be completely negated by a single drill breakage from chip packing, which costs 30–120 minutes of downtime, a $200–800 tool, and potentially a scrapped component. The net economic effect of reducing feed is typically negative when the chip packing risk is factored in. The correct approach is to select the feed that provides reliable chip breaking first, then adjust cutting speed for tool life. Tool life is more sensitive to cutting speed than to feed (the Taylor tool life equation exponent for speed is typically n = 0.15–0.25, while for feed it is n = 0.30–0.50, meaning speed has a stronger effect on tool life), so reducing speed to extend tool life is more effective than reducing feed. A 15% reduction in cutting speed typically increases tool life by 40–80%, while a 15% reduction in feed increases tool life by only 15–25% but significantly reduces chip-breaking reliability. The recommendation is to set the feed at or above the minimum for reliable chip breaking (based on the manufacturer's chip breaker recommendation for the specific material), then adjust cutting speed to achieve the target tool life. If the resulting material removal rate is too low for productivity requirements, the solution is to use a more aggressive chip breaking method (MAM instead of mechanical chip breaker, CNC oscillation instead of peck drilling) that can maintain reliable chip breaking at higher speeds and feeds, rather than reducing feed to extend tool life at the expense of process reliability.

Can chip breaking be reliably achieved in all materials, or are some materials fundamentally more difficult than others? ​

Chip breaking reliability varies fundamentally across material classes, determined primarily by the material's ductility (elongation at break), work hardening rate, thermal conductivity, and shear localisation behaviour. Materials can be classified into three categories of chip breaking difficulty. Easy-to-break materials (grey cast iron, ductile cast iron, hardened steels above 45 HRC, sintered metals, powder metallurgy materials, and carbon steels with > 0.35% C in the normalized condition) produce short, naturally broken chips without any chip breaking assistance. In these materials, the chip breaks at the cutting edge or within 2–5 mm of it due to the material's inherent brittleness, graphite flakes (cast iron), or carbide content. Deep hole drilling of these materials requires no chip breaking strategy beyond standard tool geometry — the chip control challenge is chip evacuation (preventing fine particles from packing) rather than chip breaking. Moderate-to-break materials (low-carbon steels, alloy steels up to 35 HRC, aluminium alloys, and most brasses) produce chip forms that can be reliably controlled with standard mechanical chip breakers or low-frequency CNC oscillation. These materials have sufficient ductility to form a continuous chip (like the easy-to-break category) but respond predictably to chip breaking geometry — the chip curls and fractures at the chip breaker groove, producing a chip length of 3–15 mm. The primary requirement for these materials is that the chip breaker geometry is correctly matched to the feed range and that the feed is maintained above the minimum for reliable chip breaking.

Difficult-to-break materials (austenitic stainless steels 304/316/316L, copper and copper alloys, nickel-based superalloys, titanium alloys, aluminium bronzes, and beryllium copper) present the most significant chip breaking challenges. These materials share three characteristics: high ductility (elongation 25–55%), high work hardening rate (the material becomes stronger as it deforms, making the chip stronger and harder to break the farther it travels from the cutting edge), and low thermal conductivity (7–20 W/m·K for stainless and Inconel versus 50–60 W/m·K for carbon steel), which means the chip stays hot and ductile as it flows across the rake face, resisting fracture. In these materials, even with an optimally designed mechanical chip breaker, chip breaking may be unreliable at the low feed rates typical of deep hole drilling (0.01–0.05 mm/rev). The chip forms a continuous ribbon that does not break at the chip breaker groove because the thin chip cannot generate enough bending stress to overcome the work-hardened material's fracture strength. For difficult-to-break materials, MAM or CNC oscillating feed is strongly recommended as the primary chip breaking method, with the mechanical chip breaker serving as a secondary measure that improves the consistency of the chip segments produced by the oscillation. The minimum feed for reliable chip breaking in these materials is higher than for moderate materials (0.02–0.03 mm/rev minimum for stainless, 0.03–0.05 mm/rev for copper), and the feed must be maintained above this threshold regardless of tool life considerations. Of all the difficult materials, copper is the most challenging for chip breaking — its extremely high ductility (up to 50% elongation) combined with its high thermal conductivity (400 W/m·K, which rapidly carries heat away from the chip and prevents thermal softening) makes it the most resistant to chip fracture. Copper deep hole drilling typically requires the most aggressive chip breaking parameters (MAM at > 15 Hz with A_osc / f_nom > 1.2) and even then may produce occasional long chip segments that require careful coolant-flow monitoring.

The information provided in this article is for general informational purposes only and does not constitute professional engineering advice. Always consult qualified manufacturing engineers, equipment manufacturers, and tooling suppliers for specific chip breaking applications. Data and parameter recommendations are based on published research and industry experience as of 2026.