Deep Hole Drilling Glossary: 150+ Terms and Definitions

The language of deep hole drilling has evolved over three centuries — from cannon boring in the 1700s to modern BTA and gun drilling systems. Understanding the terminology is essential for writing specifications, communicating with suppliers, and interpreting technical documents.

This glossary covers over 150 terms organized by category. Each term includes a concise definition relevant to deep hole drilling practice.

General Deep Hole Drilling Terms ​

Deep hole drilling A machining process for producing holes with a depth-to-diameter (L/D) ratio greater than 10:1. Requires specialized tools and coolant systems to manage chip evacuation, heat dissipation, and straightness control.

L/D ratio (length-to-diameter ratio) The ratio of hole depth to hole diameter. The defining characteristic of deep hole drilling. Ratios of 100:1, 200:1, and even 400:1 are achievable with specialized processes.

Self-guidance (self-piloting) The ability of a deep hole drill head to maintain its axis without external guidance once inside the hole. Achieved by guide pads that bear against the machined bore wall.

Chip evacuation The process of removing chips from the cutting zone during drilling. The primary challenge in deep hole drilling — inadequate chip evacuation causes tool failure.

Through-coolant A tool or system that delivers coolant through internal channels in the tool directly to the cutting edge. Essential for deep hole drilling.

High-pressure coolant (HPC) Coolant delivered at pressures exceeding 300 psi (20 bar). Deep hole drilling typically requires 300–3,000 psi depending on the application.

Penetration rate The axial feed speed of the drill into the workpiece, typically expressed in mm/min or inches per minute. In deep hole drilling, penetration rate is limited by chip evacuation capacity.

Guide pads (support pads, bearing pads) Carbide or high-speed steel pads mounted on the drill head that contact the machined bore wall. Provide guidance, vibration damping, and hole straightness.

Starter bushing (drill bushing) A precision guide bushing mounted at the workpiece entry point that aligns the tool before it enters the hole. Typically F7 or G6 grade tolerance.

Counter-rotation A machine configuration where the workpiece and tool rotate in opposite directions. Produces the best hole straightness by canceling drill drift.

Rotating tool A drilling configuration where the tool rotates while the workpiece remains stationary. Used for non-symmetric or off-center holes.

Rotating workpiece A drilling configuration where the workpiece rotates while the tool feeds axially. Produces better straightness for coaxial holes.

Drill wander (drill walk, drift) Deviation of the drill from its intended axis as it advances. Caused by asymmetric cutting forces, tool geometry errors, or inadequate guide pad support.

Chip packing Accumulation and compaction of chips in the bore or tool flute, blocking chip evacuation. Can cause tool jamming, breakage, or workpiece scrapping.

Brinelling Surface damage to bearing races or guide pads caused by static overload or impact. Creates indentations that cause vibration and premature failure.

Coolant bypass Leakage of coolant from the intended flow path, reducing pressure and flow at the cutting zone. Common at rotary union seals and pressure head seals.

Gun Drilling Terms ​

Gun drill (gundrill) A single-lip drilling tool with an internal coolant channel and a V-shaped chip groove along its exterior. Originally developed for drilling gun barrels.

Single-lip drill Another name for a gun drill, referring to its single cutting edge.

V-groove (chip groove, flute) The longitudinal groove on the outside of a gun drill shaft through which chips and coolant are evacuated.

Gun drilling machine A specialized horizontal machine designed for gun drilling, featuring high-pressure coolant delivery and precision spindle support.

Gun drill shank The steel body of a gun drill, typically hardened and ground. Transmits torque and houses the coolant channel.

Gun drill tip (gun drill head) The carbide cutting portion of a gun drill, brazed to the shank. Contains the cutting edge, coolant exit port, and guide pad surfaces.

Brazed tip A carbide cutting tip attached to a steel shank via brazing. Standard for smaller-diameter gun drills.

Coolant eyelet (coolant hole exit) The opening at the gun drill tip where high-pressure coolant exits to lubricate the cutting edge and flush chips.

Gun drill geometry The specific angular geometry of a gun drill tip, including point angle, rake angle, and relief angle. Differs from conventional drill geometry.

Inlet tube The tube inside a gun drilling machine that delivers coolant from the rotary union to the gun drill shank.

Whistle-notch A specific chip breaker geometry ground into the cutting edge of a gun drill to control chip formation.

Gun drilling bushing A precision bushing used to guide the gun drill at the workpiece entry point. Must match the drill diameter within tight tolerance.

Oil groove The channel on the guide pad surface of a gun drill that distributes coolant for lubrication.

Chipping (gun drill tip) Fracture or breakage of the carbide cutting edge, typically caused by interrupted cuts, hard inclusions, or excessive feed.

Gun drill walking The tendency of a gun drill to drift off-axis at the entry point before the guide pads fully engage. Controlled by proper starter bushing alignment.

Keenan system A two-stage gun drilling system using a pilot drill and a follow-up gun drill for very deep holes.

BTA Drilling Terms ​

BTA drilling (Boring and Trepanning Association) A deep hole drilling method using a multi-edged cutting head attached to a hollow tube. Coolant is delivered externally (between the tube and hole wall), and chips are evacuated internally through the tube bore.

Single-tube system Another name for BTA drilling, referring to the single thick-walled tube used for torque transmission and chip evacuation.

BTA drill head The replaceable cutting head of a BTA system. Available in brazed carbide, indexable insert, or spade-type configurations.

BTA tube (drill tube) The thick-walled steel tube that connects the BTA head to the machine. Transmits torque, carries coolant externally, and evacuates chips internally.

Internal chip evacuation The defining characteristic of BTA drilling — chips are removed through the center bore of the drill tube, not along the hole wall.

Pressure head (coolant induction head) The assembly on a BTA machine that seals against the workpiece face and delivers high-pressure coolant into the annular gap between the drill tube and the hole wall.

Annular gap The space between the drill tube outer diameter and the machined bore wall through which coolant flows to the cutting zone.

BTA guide pads Carbide pads on the BTA drill head that provide self-guidance. Typically three pads arranged around the head circumference.

BTA cutting edges Multi-edged cutting configuration on a BTA head. Unlike gun drilling's single-lip design, BTA heads have multiple cutting edges for higher material removal rates.

BTA starter tube A short, thick-walled tube used at the start of a BTA operation to guide the drill head before full engagement.

Chip box (chip receiver) The enclosure at the machine headstock that collects chips and coolant as they exit the BTA drill tube.

BTA seal The sealing element at the pressure head that prevents coolant leakage at the workpiece face.

BTA guide bushing (drill bushing) A replaceable bushing in the pressure head that centers the drill tube and seals the coolant annulus.

Indexable BTA head A BTA drill head using replaceable carbide inserts rather than a brazed tip. Common for larger diameters.

BTA spade head A BTA drill head with a spade-type cutting blade, used for large-diameter applications.

BTA trepanning head A BTA head designed for trepanning — cutting an annular groove to remove a solid core.

Ejector, DF, and Trepanning Terms ​

Ejector drilling (double-tube system, DTS) A deep hole drilling method using concentric inner and outer tubes. Coolant flows between the tubes, and chips evacuate through the inner tube using a Venturi effect.

Venturi effect (ejector effect) The pressure drop created by fluid flowing through a constricted section. In ejector drilling, a portion of coolant is directed through crescent openings to create suction for chip removal.

Inner tube (ejector) The inner of two concentric tubes in an ejector drilling system. Carries chips and return coolant.

Outer tube (ejector) The outer of two concentric tubes in an ejector drilling system. Carries supply coolant to the cutting zone.

Crescent openings Shaped openings at the rear of the inner tube in an ejector drill that direct a portion of coolant to create the Venturi effect.

DF drilling (double feeder) A deep hole drilling method combining BTA and ejector principles. Uses a single tube with both push (pressure) and suction for chip evacuation.

Trepanning A method for producing large-diameter holes by cutting an annular groove, removing a solid cylindrical core (trepan) rather than reducing all material to chips.

Trepanning head A cutting head designed for trepanning operations. Cuts only the annular groove.

Core (trepan) The solid cylindrical piece removed during trepanning. Can be used as material for another part.

Skiving A finishing process where a cutting tool is pulled through an existing bore to improve surface finish and dimensional accuracy.

Burnishing (roller burnishing) A cold-working finishing process where rollers are pressed against the bore surface to improve finish (Ra 0.2–0.4 μm) and increase surface hardness.

Skiving and burnishing (combined tool) A single-pass finishing operation combining skiving (cutting) and burnishing (cold working) for efficient bore finishing.

Pull boring A finishing operation where a tool is pulled through an existing bore to correct straightness and diameter.

Counterboring Enlarging a portion of an existing bore to a larger diameter, typically for a shoulder or seating surface.

Machine and Component Terms ​

Headstock The machine unit that houses the spindle, rotary coolant union, and workholding device. Provides rotation and coolant delivery.

Tailstock The movable machine unit on the opposite end from the headstock. Supports the workpiece or provides counter-rotation.

Guideway (machine bed) The precision-machined surfaces on the machine bed that guide carriage movement. Deep hole drilling machine guideways must maintain straightness within 0.02 mm/m.

Carriage (saddle) The movable machine component that carries the drill or workpiece along the guideway.

Spindle The rotating assembly that holds and drives the tool or workpiece. Deep hole drilling spindles must maintain runout below 0.005 mm.

Rotary coolant union (rotary union) A rotating seal assembly that transfers high-pressure coolant from the stationary supply line to the rotating spindle or tool.

Drawbar The mechanism inside the spindle that retains the tool holder or drill. May be hydraulic, pneumatic, or mechanical.

Steady rest A support device that contacts the workpiece or drill tube at intermediate points to prevent deflection.

Chip conveyor A mechanical system that removes chips from the machine coolant tank. Types include screw-type, belt-type, and hinge-belt conveyors.

Coolant tank (reservoir) The storage tank for drilling coolant. Deep hole drilling systems typically require tanks with 5–10 times the pump flow per minute capacity.

Pressure gauge An instrument that measures coolant pressure at a specific point in the system. Critical for monitoring tool condition and system health.

Flow meter An instrument that measures coolant flow rate. Used with pressure readings to assess system performance.

Accumulator A pressure storage device that dampens pressure pulsations from piston pumps in the coolant system.

Chip evacuation tube (coolant return tube) The tube through which chips and return coolant exit the drilling zone in BTA systems.

Machine base (bed casting) The main structural casting of the machine. Deep hole drilling machine beds are typically heavy cast iron or welded steel for rigidity.

Way covers (telescopic covers) Protective covers that prevent chips and coolant from contaminating the machine guideways.

Leveling wedges Adjustable supports under the machine base used for precision leveling during installation.

Anchor bolts Bolts that secure the machine base to the foundation. Cast-in-place J-bolts or anchor plate bolts are standard for deep hole drilling machines.

Foundation inertia block A massive reinforced concrete block that supports the machine and provides vibration isolation.

Tooling and Insert Terms ​

Carbide (cemented carbide) A hard, wear-resistant material used for cutting edges and guide pads in deep hole drilling tools. Tungsten carbide with cobalt binder.

Solid carbide drill A drill made entirely from carbide, used for smaller diameters and higher precision applications.

Indexable insert A replaceable carbide cutting insert with multiple cutting edges. Used in larger BTA drill heads and modern gun drills.

Insert grade The specific carbide grade of an indexable insert, selected based on workpiece material and cutting conditions.

Insert coating A thin layer (TiN, TiAlN, AlTiN, CVD diamond) applied to carbide inserts to increase wear resistance and tool life.

Chip breaker A geometric feature on the cutting edge that controls chip shape and breaks chips into manageable lengths for evacuation.

Brazed joint The connection between the carbide tip and steel shank on a gun drill. Must be free of voids for reliable performance.

Drill regrinding (tool reconditioning) The process of resharpening a worn gun drill or BTA head. Can extend tool life through multiple regrinds.

Tool life The total cutting time or material volume removed before a tool requires replacement or regrinding. Measured in linear meters of drilling or number of holes.

Tool wear Gradual material loss from the cutting edge during operation. Measured as flank wear, crater wear, or notch wear.

Flank wear Wear on the relief surface of the cutting edge. The most common wear mode and the standard measure of tool life.

Crater wear Wear on the rake surface of the cutting edge caused by chip friction at high temperature.

Notch wear Localized wear at the depth-of-cut line on the cutting edge. Common when machining work-hardening materials.

Built-up edge (BUE) Workpiece material that adheres to the cutting edge during machining. Causes poor surface finish and dimensional variation.

Edge chipping Small fractures along the cutting edge. Caused by mechanical or thermal shock.

Runout (tool runout) The radial deviation of the tool from true rotation. Excessive runout causes oversize holes, poor surface finish, and accelerated tool wear.

Tool holder The device that connects the drill or tool to the machine spindle. Must provide accurate centering and secure retention.

Collet A spring-loaded clamping device that holds the drill shank in the tool holder.

Hydraulic chuck A tool holder that uses hydraulic pressure to clamp the tool shank. Provides high clamping force and excellent concentricity.

Coolant and Filtration Terms ​

Coolant (cutting fluid) The fluid used for lubrication, cooling, and chip evacuation during drilling. May be oil-based or water-miscible emulsion.

Oil-based coolant Straight cutting oil used as coolant. Preferred for deep hole drilling because of superior lubricity and chip evacuation.

Emulsion (water-miscible coolant) A mixture of oil concentrate and water. Used for general deep hole drilling but provides less lubricity than oil.

Coolant concentration The ratio of oil concentrate to water in an emulsion. Measured with a refractometer. Typically 5–15% for deep hole drilling.

Coolant pressure The fluid pressure at the system outlet or tool entry point. Measured in psi, bar, or MPa.

Coolant flow rate The volume of coolant delivered per unit time. Measured in GPM or L/min.

Filter rating (micron rating) The particle size at which a filter captures a specified percentage of contaminants. Deep hole drilling typically requires 20–50 micron filtration.

Drum filter (paper filter) A self-cleaning filtration system using continuous paper filter media. Common in deep hole drilling coolant systems.

Magnetic separator A device that removes ferrous particles from coolant using magnetic fields. Used as a pre-filter stage.

Cyclonic filter (centrifugal filter) A filter that separates particles using centrifugal force. No disposable media required.

Bag filter A disposable filter bag used for final filtration. Less common in production deep hole drilling because of high maintenance.

Differential pressure (ΔP) The pressure drop across a filter. Used to monitor filter loading and indicate when media replacement is needed.

Coolant chiller A refrigeration system that controls coolant temperature. Recommended for systems above 50 HP pump power.

Coolant temperature stability The consistency of coolant temperature at the machine entry point. Target ±2°C for precision drilling.

Tramp oil Unwanted oil (hydraulic oil, way oil) that contaminates the coolant. Causes bacterial growth in emulsions and reduces lubricity.

Coolant bacteria (biological contamination) Microbial growth in water-mixed coolant caused by tramp oil contamination and inadequate maintenance.

Coolant pH A measure of coolant acidity or alkalinity. Emulsion coolants typically require pH 8.5–9.5 for corrosion protection and bacterial control.

Refractometer An optical instrument that measures coolant concentration by measuring the refractive index of the fluid.

Sump (coolant tank sump) The lowest point in the coolant tank where solids settle. Must be regularly cleaned to prevent bacterial growth.

Metrology and Quality Terms ​

Runout (spindle runout) Radial deviation of the spindle from true rotation. Measured at the spindle taper with a dial indicator.

Bore diameter tolerance The allowable size variation for a drilled bore. Expressed as IT grade (e.g., IT7, IT8). Deep hole drilling typically achieves IT7–IT10.

Straightness The deviation of the bore axis from a true straight line. Measured in mm per meter of bore length.

Roundness (circularity) The deviation of the bore cross-section from a true circle. Measured with a roundness tester or CMM.

Surface finish (surface roughness) The texture of the machined bore surface. Measured as Ra (μm) using a profilometer.

Ra (roughness average) The arithmetic average of surface profile deviations from the mean line. Most common surface finish parameter.

CMM (coordinate measuring machine) A precision measurement device that measures dimensional characteristics of machined parts using a touch probe or optical sensor.

Air gauge (air plug gauge) A precision measurement tool that uses airflow to measure bore diameter. Capable of 0.001 mm resolution.

Profilometer An instrument that measures surface roughness by tracing a stylus across the surface.

Roundness tester A precision instrument that measures bore roundness by rotating a probe around the bore circumference.

Borescope An optical inspection tool used to visually examine the interior surface of deep bores.

Cpk (process capability index) A statistical measure of how well a process produces parts within specification limits. Deep hole drilling processes typically target Cpk ≥ 1.33.

First-pass yield The percentage of parts passing inspection without requiring rework. A measure of process reliability.

Scrap rate The percentage of parts rejected as unusable. Deep hole drilling scrap is costly because the workpiece is often near-complete when drilled.

Test bore A sample bore drilled during machine commissioning or process qualification to verify machine capability.

Dial indicator (test indicator) A mechanical measurement tool used for runout measurement, alignment checking, and positional accuracy verification.

Test bar (alignment test bar) A precision-ground bar used in the spindle to check spindle-to-guideway parallelism and alignment.

Troubleshooting and Defect Terms ​

Chatter (vibration marks) Regularly spaced marks on the bore surface caused by vibration between the tool and workpiece. Indicates insufficient rigidity or incorrect cutting parameters.

Spiral marks (helical marks) Continuous spiral patterns on the bore surface caused by guide pad issues or chip packing.

Bell-mouth (entry bell) Enlargement of the bore at the entry end. Caused by misalignment between the starter bushing and spindle axis.

Taper (diameter variation) Gradual change in bore diameter along the bore length. Caused by tool wear, deflection, or temperature changes.

Bore oversize A drilled bore that exceeds the upper diameter tolerance. Caused by excessive runout, incorrect tool size, or vibration.

Bore undersize A drilled bore that falls below the lower diameter tolerance. Caused by tool wear, incorrect tool size, or built-up edge.

Chip jamming (chip blockage) Accumulation of chips in the coolant path that blocks flow and stops chip evacuation. Can cause immediate tool failure.

Tool breakage (drill breakage) Fracture of the drill inside the bore. Often caused by chip packing, excessive feed, or hitting a hard inclusion.

Guide pad galling Material transfer from the workpiece to the guide pads. Causes surface finish degradation and potential tool seizure.

Pressure fluctuation Variation in coolant pressure during drilling. Can indicate chip packing, pump cavitation, or seal failure.

Coolant leakage Loss of coolant from seals, connections, or the pressure head. Reduces pressure at the cutting zone.

Bore straightness deviation Departure of the bore axis from a straight line. Caused by guideway misalignment, uneven cutting forces, or drill drift.

Surface burn (heat discoloration) Discoloration of the bore surface caused by excessive heat. Indicates inadequate coolant flow or incorrect cutting parameters.

Recutting chips Chips that are not evacuated and are re-cut by the tool. Causes increased tool wear and surface damage.

Washboard surface A regular wavy pattern on the bore surface. Caused by low-frequency vibration in the machine system.

Out-of-round bore A bore cross-section that deviates from circular. Caused by uneven cutting forces, bearing wear, or workpiece deflection.

Cross-hole burr A burr formed at the intersection of a cross-hole with the main bore. Must be removed for proper part function.

Tool push-off Deflection of the tool away from the intended cutting path. Caused by asymmetric forces or inadequate guide pad support.

Material and Process Terms ​

Work-hardening An increase in material hardness at the machined surface caused by plastic deformation during cutting. Common in stainless steels and nickel alloys.

Built-up edge formation Accumulation of workpiece material on the cutting edge. Causes poor surface finish and dimensional variation.

Chip morphology The shape and form of chips produced during cutting. Deep hole drilling requires short, broken chips for reliable evacuation.

Segmental chip Chips formed by a chip breaker geometry that breaks the chip into segments. Ideal for deep hole drilling.

Stringy chip (continuous chip) A long, continuous chip that does not break. Dangerous in deep hole drilling because it can pack and block chip evacuation.

L/D ratio limits The maximum practical depth-to-diameter ratio for a given drilling method and tool diameter. Limits are determined by tool rigidity, coolant pressure, and chip evacuation.

Peck drilling (intermittent feed) A drilling cycle where the tool periodically retracts to clear chips. Not recommended for deep hole drilling because the tool must re-enter the bore.

Starting drill A short, rigid drill used to create a starting bore before the deep hole tool engages. Ensures accurate hole location and straight entry.

Pilot hole (pre-drilled bore) An existing bore that is subsequently enlarged or finished by a deep hole drilling operation.

Through-bore A bore that extends completely through the workpiece from one end to the other.

Blind bore (blind hole) A bore that does not extend completely through the workpiece. Requires special consideration for chip evacuation at the bore bottom.

Step bore (stepped bore) A bore with two or more different diameters along its length. Drilled in multiple operations.

Annular groove A recess cut into the bore circumference, typically for seal or retaining ring applications.

Cross-hole (intersecting bore) A hole drilled perpendicular to or at an angle intersecting the main bore. Creates a difficult drilling condition.

Gun drilling oil A high-viscosity cutting oil specifically formulated for gun drilling. Contains extreme-pressure (EP) additives for lubrication.

Sulfurized oil Cutting oil containing sulfur additives for extreme-pressure lubrication. Common in deep hole drilling of steel.

Chlorinated oil Cutting oil containing chlorine additives for extreme-pressure lubrication. Being phased out due to environmental regulations.

EP additive (extreme-pressure additive) A chemical additive that reacts with the metal surface at high temperature to prevent welding and reduce friction.

Penetration rate optimization The process of maximizing feed rate while maintaining reliable chip evacuation and acceptable tool life. The key economic parameter in deep hole drilling.

Specific cutting force (kₓ) The force required to cut a unit area of material. Used in power and torque calculations for tool selection.

Material removal rate (MRR) The volume of material removed per unit time. Higher for BTA drilling than gun drilling at equivalent diameters.

Tool life equation (Taylor's equation) The empirical relationship between cutting speed and tool life: VTⁿ = C. Used to optimize cutting parameters.

Chip volume ratio The ratio of chip volume to solid material volume. Affects chip evacuation capacity in the available chip space.

Summary Table ​

FAQ ​

What does L/D ratio mean in deep hole drilling? ​

L/D ratio is the length-to-diameter ratio of a hole. A hole with an L/D ratio greater than 10:1 is generally considered a deep hole requiring specialized techniques. The ratio determines the drilling method, tool geometry, coolant pressure requirements, and expected process capability. Gun drilling can achieve L/D ratios up to 400:1, while BTA drilling typically operates below 100:1. The higher the L/D ratio, the more challenging chip evacuation and straightness control become.

What is the difference between gun drilling and BTA drilling? ​

The fundamental difference is the chip evacuation path. Gun drilling evacuates chips externally through a V-shaped groove along the tool's exterior (coolant goes through the tool and exits at the tip, pushing chips back along the groove). BTA drilling evacuates chips internally through the center bore of the hollow drill tube (coolant flows between the tube and hole wall, and chips are pushed through the tube center). Gun drilling is used for smaller diameters (0.5–50 mm) while BTA handles larger diameters (20–500+ mm) at higher material removal rates.

What does "self-piloting" mean in deep hole drilling? ​

Self-piloting (or self-guidance) refers to the ability of a deep hole drill to maintain its direction without external guidance once the tool is fully engaged in the bore. This is achieved by guide pads — carbide pads on the drill head that contact the machined bore wall. The guide pads create a three-point contact that centers the tool and resists deflection. Self-piloting is what distinguishes deep hole drilling from conventional drilling, where the tool must be guided by the machine structure alone.

What is a BTA pressure head? ​

A BTA pressure head is the assembly on a BTA drilling machine that seals against the workpiece face and delivers high-pressure coolant into the annular gap between the drill tube and the hole wall. It contains a guide bushing that centers the drill tube, seals that prevent coolant leakage, and a connection for the high-pressure coolant supply. The pressure head must maintain a tight seal against the workpiece surface — any leakage reduces coolant pressure at the cutting zone and can cause chip evacuation failure.

What coolant pressure is required for deep hole drilling? ​

Coolant pressure requirements vary by application. Gun drilling small diameters (1–4 mm) requires 1,500–3,000 psi (100–207 bar). Standard gun drilling up to 25 mm typically runs at 300–1,500 psi (20–100 bar). BTA drilling operates at 500–750 psi (35–52 bar) for most applications. The pressure must be sufficient to overcome friction losses in the coolant path and maintain adequate velocity at the cutting zone for chip evacuation. Pressure at the pump outlet will always be higher than pressure at the tool — the difference is the system pressure drop.

What is chip packing and why is it dangerous? ​

Chip packing occurs when drilling chips accumulate and compact in the bore or tool flute, blocking the chip evacuation path. In deep hole drilling, chip packing can happen in seconds and causes immediate tool jamming and breakage. The tool may seize in the bore or fracture, often scrapping the workpiece. Prevention requires adequate coolant pressure and flow, proper chip breaker geometry on the tool, and cutting parameters matched to the chip evacuation capacity of the system.