Deep Hole Drilling Chip Handling and Swarf Management

Chip management is one of the most overlooked aspects of deep hole drilling. In BTA drilling, chips travel through a 10–40 mm annular gap at high velocity — any disruption in chip flow can cause tool jamming, workpiece scoring, or catastrophic tool failure. A properly designed chip handling system is not optional; it is integral to the process.

Chip Types in Deep Hole Drilling ​

The two primary deep hole drilling processes produce distinctly different chip forms, each requiring different handling strategies.

Gundrilling Chips ​

Gundrilling produces small, discontinuous chips that evacuate through a single V-shaped flute. The ideal chip shape is a tight conical curl — often described as "sixes and nines" — that flows freely through the flute without jamming.

Gundrilling chip control depends primarily on coolant pressure (20–80 bar) and the tool's lip geometry, which acts as a mechanical chip breaker. The velocity differential between the inner and outer cutting edges creates natural chip curl — larger diameters curl more easily than small ones.

BTA Drilling Chips ​

BTA drilling produces larger, heavier chips that evacuate through the annular gap between the tool head and the bore wall, then through the center of the drill tube. Chip shape control is critical because a chip that jams in the annular gap will score the finished bore surface.

BTA chips are typically larger and heavier than gundrill chips — a single BTA drilling operation can generate kilograms of swarf per minute in large diameters.

Chip Evacuation Principles ​

Chip evacuation in deep hole drilling relies on hydraulic transport — the coolant flow carries chips out of the hole. Understanding the fluid dynamics is essential for system design.

Gundrill Evacuation ​

Coolant enters through the tool's coolant hole at 20–80 bar, exits at the cutting edge, and flows back along the single V-shaped flute carrying chips with it. The flute acts as a chip channel, and the coolant velocity must be sufficient to overcome the chip settling velocity.

Key parameters:

• Coolant velocity at the flute exit: 5–15 m/s
• Minimum coolant velocity for chip transport: 3–5 m/s (depends on chip size and density)
• Pressure drop along the flute increases with depth — deeper holes require higher inlet pressure

BTA Evacuation ​

Coolant enters through the annular gap between the drill tube and the bore wall, flows to the cutting head, and returns through the center of the drill tube carrying chips. The return flow velocity is higher than the inlet velocity because the cross-sectional area of the return path is smaller.

Key parameters:

• Coolant flow rate: 200–2,000 L/min (diameter and depth dependent)
• Return velocity: 3–10 m/s
• Maximum chip size limited by the return passage diameter
• Chip shape control essential to prevent bridging in the return tube

Chip Conveyor Systems ​

Once chips exit the machine, they must be separated from the coolant stream and conveyed to a collection point. The choice of conveyor depends on chip type, volume, and material.

Conveyor Types ​

For deep hole drilling applications, hinged belt conveyors are the most common choice for BTA operations where chips are larger and may be stringy. Scraper conveyors are preferred for gundrilling operations where chips are small and well-broken.

System Integration ​

A typical deep hole drilling chip handling system follows this flow:

Machine tool → Chip conveyor → Magnetic separator → Filter → Coolant tank → Pump → Machine tool
                                                        ↓
                                              Chip wringer / centrifuge
                                                        ↓
                                              Briquetting press → Recycled swarf

Chip Conveyor Sizing ​

Conveyor capacity should be calculated based on the maximum material removal rate:

The chip volume factor accounts for the fact that chips occupy 3–8 times the volume of solid material, depending on chip shape and density. Conveyors should be sized with at least 20% safety margin above the calculated requirement.

Coolant Filtration Systems ​

Coolant cleanliness is critical in deep hole drilling. Unfiltered chips circulating through the coolant system will damage pump seals, block coolant passages in the tool, and recirculate through the cutting zone, causing built-up edge and poor surface finish.

Filtration Stages ​

Stage 1 — Chip Conveyor: Removes the bulk of large chips from the coolant return flow. A well-designed conveyor removes 85–95% of chip mass at this stage.

Stage 2 — Magnetic Separation: Removes fine ferrous particles. Magnetic separators use chains of magnetic rods or rotating magnetic drums to capture ferrous fines. For ferrous deep hole drilling, this stage is essential.

Stage 3 — Fine Filtration: Achieves the required coolant clarity for deep hole drilling (typically 10–30 µm). Several technologies are available:

Stage 4 — Coolant Conditioning: Temperature control and concentration monitoring.

Filtration Sizing Guidelines ​

Filter capacity is expressed as a multiple of pump flow because the return flow includes chip volume and aeration. A 3× factor means the filter can handle three times the pump's rated flow, which provides adequate margin for peak chip loads.

Chip Processing and Recycling ​

After separation from the coolant, wet chips require further processing before recycling or disposal.

Chip Wringing / Drying ​

Centrifugal chip wringers (centrifuges) spin chips at high speed to remove residual coolant, reducing coolant content from 15–25% by weight down to 2–5%.

The recovered coolant is returned to the coolant system, reducing coolant consumption by 80–90%.

Briquetting ​

Briquetting presses compact dry chips into dense briquettes (70–90% of solid density depending on material). This reduces volume by 5–10× and increases scrap value.

Coolant Recovery Economics ​

A typical ROI calculation for a chip processing system:

Payback period for chip wringing and briquetting equipment is typically 12–24 months for high-volume operations.

System Design Guidelines ​

Single Machine System ​

For a single deep hole drilling machine:

1. Select a chip conveyor rated for the maximum chip volume
2. Size the coolant tank at 5–10× the pump flow rate per minute for thermal stability
3. Install a magnetic separator for ferrous operations
4. Install a paper band or cartridge filter for the required filtration level
5. Include a chiller sized for the total heat load (spindle power + hydraulic + coolant pump)
6. Provide a chip collection bin or conveyor to the central disposal point

Centralized Multi-Machine System ​

For multiple machines sharing a coolant and chip handling system:

1. Install a central chip conveyor (underground trench or overhead) collecting from all machines
2. Size the central filter for 4–6× the combined pump flow of all machines
3. Include a buffer tank to smooth peak flow variations
4. Install a central chiller system (more efficient than individual chillers)
5. Provide automated coolant concentration monitoring and make-up
6. Include a central chip wringer and briquetting press

Common Design Mistakes ​

• Undersized coolant tank: Small tanks allow temperature to rise during long cycles, causing thermal expansion and dimensional drift
• Inadequate filtration bypass: A bypass line around the filter prevents machine downtime during filter maintenance
• Ignoring chip volume factor: Conveyors and tanks sized for solid material volume are quickly overwhelmed by the actual chip volume
• Poor access for maintenance: Filters, magnetic separators, and tank clean-out ports need easy access
• Single-point failure in centralized systems: A single pump or filter failure stops all connected machines — include redundancy

Troubleshooting Chip Handling Problems ​

FAQ ​

Q: What is the ideal chip shape for BTA drilling? Short, broken arc segments approximately 5–15 mm long. They should flow freely through the return tube and not bridge or pack. If chips emerge as long ribbons, increase feed rate. If they emerge as powder, reduce speed or check tool condition.

Q: How often should coolant filters be changed? It depends on chip volume and filter type. Paper band filters advance automatically when differential pressure rises. Cartridge filters typically need replacement when the pressure gauge shows 1.5–2× the clean pressure drop. Monitor the indicator and change on condition, not on a fixed schedule.

Q: Can a single coolant system serve both gundrill and BTA machines? Yes, but the system must be designed for the most demanding requirements — BTA flow rate and gundrill filtration level. Use a two-stage filtration system with a magnetic separator and fine filter. Monitor coolant condition closely as the operating conditions differ significantly between processes.

Q: What coolant flow rate is needed for effective chip evacuation? For gundrilling, a minimum of 3–5 m/s return velocity in the flute is required. For BTA drilling, the return velocity in the drill tube should be 3–10 m/s. Calculate the required flow rate from the cross-sectional area of the return path and the minimum transport velocity.

Q: Is magnetic separation necessary for non-ferrous materials? No — magnetic separation only works for ferrous materials. For non-ferrous deep hole drilling (aluminum, brass, titanium), use paper band filters or hydrocyclones for primary chip separation.

Q: How do I handle stringy chips that jam the conveyor? First, adjust cutting parameters to produce shorter, more broken chips — increase feed rate or change insert geometry. If chip shape cannot be changed (material-dependent), switch from a scraper conveyor to a hinged belt conveyor that handles stringy chips better.

Q: What is the best way to recover coolant from wet chips? A centrifugal chip wringer is the most effective method, reducing coolant content from 15–25% down to 2–5%. For smaller operations, a simple drip tray or drainage bin can recover free coolant but leaves 10–15% residual oil in the chips.

Q: How does chip handling affect hole quality? Directly. Poor chip evacuation causes chips to recirculate through the cutting zone, leading to built-up edge, poor surface finish, and oversized holes. In BTA drilling, chips trapped in the annular gap score the bore surface. Chip handling is a quality system, not just a waste management system.

Q: What tank volume is recommended for a deep hole drilling coolant system? A minimum of 5× the pump flow rate per minute. For example, if the pump delivers 400 L/min, the tank should hold at least 2,000 L. Larger tanks (8–10×) provide better thermal stability for long-cycle drilling operations.

Q: What are the environmental regulations for swarf disposal? In most jurisdictions, machining swarf containing residual coolant is classified as hazardous waste. Chip wringing and briquetting reduces coolant content below the threshold for hazardous classification in many regions, significantly reducing disposal costs. Check local regulations for specific limits.