A deep hole drilling machine is only as good as the coolant it receives. If the coolant mixing tank delivers the wrong concentration, or the day tank supplies bacteria-laden coolant, the drilling process suffers — tool life drops, surface finish degrades, and corrosion appears on parts. The mixing tank and day tank are the start of the coolant system. If they are not maintained, nothing downstream works correctly.
Tank Types
Mixing Tank Types
| Tank Type | Mixing Method | Capacity | Automation Level | Best For | Disadvantages |
|---|
| Batch mixing tank | Manual — operator adds concentrate and water — mixes with agitator | 200–2000 L | Manual | Small shops — single coolant type — low volume | Labor-intensive — inconsistent concentration |
| Automatic proportioning tank | Automatic water and concentrate metering — PLC controlled | 500–5000 L | Fully automatic | Medium to large shops — consistent concentration | Higher cost — requires calibration |
| Inline mixing system | Concentrate and water mixed in flow — no storage tank | Continuous (flow-through) | Fully automatic | Central systems — high volume — single coolant | No buffer volume — requires stable water supply |
| Eductor mixing system | Water flow through eductor draws concentrate from drum — mixes at point of use | Continuous | Semi-automatic | Small to medium — drum-to-machine direct | Limited to one machine — eductor ratio must be verified |
| Pre-mix concentrate tank | Pre-diluted concentrate delivered ready-to-use | Per delivery | None — ready to use | Facilities with bulk delivery service | Higher concentrate cost per liter — logistics dependency |
Day Tank Design
| Feature | Purpose | Specification |
|---|
| Capacity | Supply coolant to machine for minimum 1–2 shifts without refill | Sizing: 2–3× machine coolant tank volume minimum |
| Baffles | Prevent vortex formation at pump suction — separate return flow from suction | Minimum 3 baffles — first at inlet — last at pump suction — height 60–80% of tank depth |
| Inlet diffuser | Reduce return flow velocity — prevent aeration | Submerged inlet — discharge below liquid surface — velocity < 1 m/s |
| Bottom slope | Drainage for cleaning | Minimum 10 mm/m slope to drain point |
| Cover | Prevent contamination — reduce evaporation — prevent coolant splash | Full cover — hinged access for maintenance — sealed edges |
| Sight glass | Visual level indication | Minimum 300 mm visible length — marked with low and high levels |
| Temperature sensor | Monitor coolant temperature | RTD or thermocouple — display at tank |
| Low-level alarm | Alert when coolant is low — prevent pump cavitation | Float switch or ultrasonic sensor — alarm at 20% tank volume |
| Fill connection | For adding fresh coolant | Dedicated line from mixing tank or premix supply — automatic or manual valve |
Mixing Methods
Manual Batch Mixing
| Step | Action | Detail |
|---|
| 1 | Measure water volume | Use tank level markings or flow meter — record volume |
| 2 | Calculate concentrate required | Volume × target concentration ÷ concentrate factor |
| 3 | Start agitator | Establish water flow — prevent concentrate from settling |
| 4 | Add concentrate slowly | Pour into water stream or into turbulent zone — do not pour into stagnant water |
| 5 | Mix thoroughly | Continue agitation for 10–15 minutes after concentrate addition |
| 6 | Check concentration | Refractometer reading — adjust if needed — add water or concentrate |
| 7 | Record | Date — batch volume — concentration — operator |
| 8 | Transfer to day tank | Pump to day tank or open valve |
Automatic Proportioning
| Step | Action | Detail |
|---|
| 1 | Set target concentration | On PLC controller — typically 5–10% |
| 2 | Verify concentrate supply | Drum or tote has sufficient concentrate — suction line primed |
| 3 | Verify water supply | Pressure 3–6 bar — flow rate sufficient for system demand |
| 4 | Initiate batch cycle | Auto or manual start — PLC controls water valve and concentrate pump |
| 5 | Monitor mixing | PLC measures flow rates — adjusts concentrate/water ratio |
| 6 | Verify concentration | Inline refractometer or sample port — automatic shut-off at target |
| 7 | Transfer to day tank | Automatic or manual — pump or gravity |
| 8 | Log batch data | Date — volume — concentration — alarms — automatically logged |
Educator Mixing
| Step | Action | Detail |
|---|
| 1 | Connect concentrate drum suction line | To eductor suction port — check for airtight seal |
| 2 | Connect water supply | To eductor inlet — pressure 3–6 bar |
| 3 | Open water valve | Water flows through eductor — creates vacuum — draws concentrate |
| 4 | Adjust ratio | Flow control valve on concentrate line — check with refractometer |
| 5 | Verify concentration | At outlet — adjust if needed |
| 6 | Monitor drum level | Replace drum when empty — do not run air into system |
Tank Construction Materials
| Material | Coolant Compatibility | Durability | Cost | Cleaning Ease | Best For |
|---|
| Stainless steel 304 | Excellent — most common tank material | Excellent — 15–25 year life | High | Easy — smooth surface — no absorption | Standard coolant — all applications |
| Stainless steel 316L | Excellent — best corrosion resistance | Excellent — 20–30 year life | Very high | Easy — best surface finish | Aggressive coolant — high-chloride conditions — food-grade |
| Crosslinked polyethylene (XLPE) | Good — most coolants compatible | Good — 10–20 year life | Low–Moderate | Moderate — can absorb some chemicals | Low-cost tanks — small to medium size |
| Linear polyethylene (LLDPE) | Good — most coolants | Moderate — 5–15 year life | Low | Moderate — less chemical resistant than XLPE | Budget installations — small day tanks |
| Steel with epoxy liner | Moderate — liner dependent on condition | Moderate — 5–10 years before liner failure | Moderate | Difficult — liner damage risk | Retrofits — existing steel tanks — not recommended for new |
| Carbon steel (unlined) | Poor — rusts in coolant service | 1–3 years | Low | Difficult — rust contamination | Not recommended for coolant storage |
Cleaning Procedures
Tank Cleaning Frequency
| Tank Type | Recommended Cleaning Frequency | Signs That Cleaning Is Needed |
|---|
| Mixing tank — manual batch | Monthly or every 10 batches — whichever comes first | Visible residue on walls — concentration variation — sediment at bottom |
| Mixing tank — automatic proportioning | Quarterly | Sludge accumulation — bacterial growth — coating on level sensors |
| Day tank — clean coolant (< 50 µm filtration) | Quarterly | Sludge at bottom — wall coating — biofilm at liquid line |
| Day tank — standard coolant (50–100 µm filtration) | Monthly | Sludge at bottom — visible sediment — return line debris accumulation |
| Day tank — cast iron / graphite coolant | Weekly or after each 100 operating hours | Heavy sludge accumulation — settling of graphite or iron fines |
| Day tank — central system (multi-machine) | Monthly | Sediment — bacterial growth — coolant appearance change |
Manual Tank Cleaning Procedure
| Step | Action | Detail |
|---|
| 1 | Plan shutdown | Coordinate with production — schedule cleaning during downtime |
| 2 | Drain tank | Pump out coolant to waste or temporary storage if reusable |
| 3 | Remove residual sludge | Manual scraping — shovel — wet vacuum — dispose per hazardous waste regulations |
| 4 | Remove loose debris | Large chips — swarf — shop debris |
| 5 | Clean walls and floor | Nylon brush (not wire — steel scratches stainless) — tank cleaner or mild detergent |
| 6 | Rinse thoroughly | Clean water — drain completely — remove all cleaning solution residue |
| 7 | Inspect tank condition | Check for corrosion — cracks — liner condition — weld integrity — level sensor condition |
| 8 | Clean sensors and components | Level sensors — temperature sensor — sight glass — fill valve screen |
| 9 | Clean cover and seals | Wipe cover — check gasket condition — replace if damaged |
| 10 | Close drain | Verify drain valve closed — clean valve seat if needed |
| 11 | Refill with clean coolant | Pre-mixed to correct concentration — or batch mix in tank |
| 12 | Verify systems operational | Level sensors — temperature — fill valve — agitator — pump |
| 13 | Document cleaning | Date — person — observations — next scheduled cleaning |
Chemical Tank Cleaning (For Biofilm and Scale)
| Step | Action | Detail |
|---|
| 1 | Drain tank | Per standard procedure |
| 2 | Pre-rinse | Remove loose debris and sludge |
| 3 | Prepare cleaning solution | Per tank cleaner manufacturer — typically 1–5% concentration in water |
| 4 | Circulate cleaning solution | Use tank pump or temporary pump — circulate for 30–60 minutes at 40–50°C |
| 5 | Scrub stubborn deposits | Nylon brush — focus on liquid line and corners where biofilm accumulates |
| 6 | Drain cleaning solution | Dispose per hazardous waste regulations — solution now contains biocide and coolant residues |
| 7 | Rinse thoroughly | Clean water — drain completely — repeat until no foam or residue |
| 8 | Neutralize (if required) | Check pH of rinse water — should be 6–8 — repeat rinse if needed |
| 9 | Inspect | Verify tank is clean — no residue — no chemical smell |
| 10 | Refill | With fresh coolant at correct concentration |
Component Maintenance
| Component | Function | Maintenance Task | Frequency |
|---|
| Agitator / mixer | Keep coolant mixed — prevent settling | Check motor bearing temperature — listen for unusual noise — check seal for leaks | Monthly |
| Level sensor (float switch) | Detect coolant level | Clean float stem — check free movement — verify electrical continuity | Quarterly |
| Level sensor (ultrasonic) | Detect coolant level | Clean sensor face — check calibration — verify reading against sight glass | Quarterly |
| Temperature sensor | Monitor coolant temperature | Compare to calibrated thermometer — verify controller reading | Quarterly |
| Fill valve (automatic) | Control water addition to day tank | Clean valve seat — check solenoid operation — verify no leaks | Monthly |
| Sight glass | Visual level indication | Clean glass — replace if clouded or cracked — verify markings legible | Monthly |
| Tank cover | Prevent contamination | Check seal/gasket condition — verify closed — hinges operating | Quarterly |
| Drain valve | Tank draining | Operate fully — check for leaks — clean seat if needed — lubricate stem | Quarterly |
| Pump suction strainer | Protect pump | Clean strainer — inspect condition | Weekly (heavy chip load) — monthly (clean) |
| Concentrate pump (automatic systems) | Transfer concentrate to mixing tank | Check priming — check for leaks — verify flow rate — listen for cavitation | Monthly |
| Inline refractometer (if equipped) | Monitor coolant concentration | Clean sensor window — verify with handheld refractometer — recalibrate if needed | Weekly |
| Water flow meter | Measure water volume for batch mixing | Compare to tank level measurement — clean if magnetic — verify calibration | Quarterly |
Troubleshooting
| Problem | Likely Cause | Corrective Action |
|---|
| Inconsistent coolant concentration | Mixing time too short — agitator ineffective — proportioner calibration drifted — water quality variation | Increase mixing time — repair agitator — recalibrate proportioner — test water hardness |
| Sludge accumulation in day tank | Inadequate filtration upstream — coolant degradation — bacterial growth — tramp oil accumulation | Improve filtration — shock treat with biocide — increase cleaning frequency — install skimmer |
| Bacterial growth in day tank | Warm coolant — stagnant zones — infrequent cleaning — insufficient biocide | Clean tank — shock treat — increase biocide — eliminate dead zones — install circulation pump |
| Foam in day tank | Return flow aerating coolant — incorrect coolant concentration — defoamer depleted | Check return line termination (should be below surface) — adjust concentration — add defoamer |
| Level sensor false readings | Sensor coated with coolant residue — float stuck — electrical fault | Clean sensor — free float mechanism — check wiring and controller |
| Tank leaking | Corrosion — crack — seal failure — liner damage | Identify leak source — patch if minor — replace if structural — repair liner |
| Automatic fill valve not closing | Debris in valve seat — solenoid failure — controller fault | Clean valve — replace solenoid — check controller output |
| Concentrate pump not priming | Suction line leak — pump running dry — filter clogged — drum empty | Check suction connections — prime pump — clean filter — replace drum |
FAQ
What is the difference between a mixing tank and a day tank?
A mixing tank (also called a make-up tank or batch tank) is where coolant concentrate is blended with water to create the correct concentration. It typically has an agitator for mixing, water and concentrate supply connections, and a means to measure the batch volume. The mixed coolant is then transferred to the day tank. A day tank is the supply reservoir that feeds coolant directly to the deep hole drilling machine(s). It receives coolant from the mixing tank (or from inline mixing systems), provides a buffer volume to ensure consistent supply, and includes level controls, temperature monitoring, and return flow management. In small systems, a single tank may serve both functions — the coolant is mixed directly in the day tank. In larger or more sophisticated systems, separate mixing and day tanks provide better control over concentration accuracy and allow the mixing tank to be cleaned without interrupting coolant supply to the machine.
How often should coolant mixing and day tanks be cleaned?
Recommended cleaning intervals: mixing tank — monthly or every 10 batches (manual batch mixing) or quarterly (automatic proportioning systems — the closed system reduces contamination). Day tank — monthly for standard coolant service — weekly for heavy chip load materials like cast iron or graphite (fine settling solids accumulate quickly) — quarterly for clean coolant systems with good filtration and central systems. Signs that cleaning is needed regardless of schedule: visible sludge accumulation at the bottom of the tank (more than 5 mm), biofilm or slime on tank walls at the coolant liquid line, bacterial or fungal growth visible in the tank, coolant concentration variation that cannot be explained by other causes, or a foul odor from the tank (indicating bacterial or fungal activity). The most skipped cleaning is the day tank in cast iron service — operators assume the black sludge is normal — but it accumulates rapidly, harbors bacteria, and reduces coolant effectiveness.
What is the best way to mix coolant concentrate with water?
The best mixing method depends on volume and precision requirements: for small volumes (< 200 L), manual batch mixing is acceptable — fill the tank with water first — start the agitator — add concentrate slowly to the turbulent water — continue mixing for 10–15 minutes — verify concentration with a refractometer. For medium to large volumes (200–5000 L), automatic proportioning is recommended — PLC-controlled water and concentrate metering provides consistent concentration (± 0.2–0.5%) with minimal operator involvement — verify with inline or handheld refractometer. For central systems or high-volume operations, inline mixing systems blend concentrate and water in the flow stream — ideal for continuous supply but requires stable water pressure and regular ratio verification. Regardless of method, the critical rules are: always add concentrate to water — never water to concentrate (concentrate can gel or invert if water is added to it). Mix thoroughly before using — inadequate mixing causes concentration gradients in the tank. Verify concentration with a calibrated refractometer before transferring to the day tank.
What tank material is best for coolant storage?
Stainless steel 304 is the standard and best overall material for coolant mixing and day tanks. It provides: excellent corrosion resistance (does not rust in coolant service — unlike carbon steel), smooth surface finish (easy to clean — does not harbor bacteria as readily as rough surfaces), long service life (15–25 years in coolant service with proper maintenance), and excellent compatibility with all common coolant chemistries. For aggressive coolant formulations or high-chloride water conditions, use stainless steel 316L — it provides additional corrosion resistance at a moderate cost premium (20–30% over 304). Crosslinked polyethylene (XLPE) is an acceptable lower-cost alternative for small to medium tanks — it will not corrode and is compatible with most coolants — but it is more difficult to clean (scratches harbor bacteria) and has a shorter service life (10–20 years). Avoid carbon steel tanks for coolant service — they rust, contaminate the coolant with red iron oxide, and require replacement within 1–3 years. Avoid galvanized steel — the zinc coating reacts with coolant chemistry.
How do I prevent bacterial growth in the day tank?
Prevent bacterial growth in the day tank by: maintaining proper coolant concentration (coolant concentrate contains biocides — maintaining the correct concentration ensures adequate biocide levels — running coolant too dilute reduces biocide effectiveness). Maintaining coolant pH (bacteria thrive at pH < 8.0 — keep pH at 8.5–9.5 for semi-synthetic coolants). Keeping the tank covered (day tanks should have a sealed cover — open tanks allow airborne bacteria and fungi to enter). Cleaning the tank on schedule (biofilm that accumulates on tank walls harbors bacteria — a clean tank has minimal biofilm — a dirty tank has abundant biofilm). Eliminating tramp oil (tramp oil floating on the coolant surface creates an environment where bacteria thrive — use a skimmer to remove tramp oil). Maintaining proper coolant temperature (bacteria multiply faster in warm coolant — above 35°C, bacterial growth rate increases significantly — keep coolant at 20–30°C). Adding biocide as needed (test bacterial counts weekly — add biocide shock treatment if counts exceed 1000 CFU/mL). The most effective prevention is a combination of concentration maintenance, tank cleaning, and tramp oil removal — biocide alone cannot compensate for a dirty tank.
The coolant mixing tank and day tank are the foundation of the coolant system. Mix coolant correctly — add concentrate to water, mix thoroughly, verify concentration. Keep tanks clean — monthly for mixing tanks and standard day tanks, weekly for heavy chip load materials. Use stainless steel tanks for best compatibility and longest life. Monitor and maintain coolant concentration, pH, and bacterial counts at the tank — if the coolant is wrong at the tank, it will be wrong at the drill tip. A well-maintained mixing and day tank system delivers consistent, clean coolant to the drilling process — and consistent coolant means consistent drilling results. This article reflects industry practice as of 2026.
