Tramp oil is the most destructive contaminant in deep hole drilling coolant. It floats on the surface, trapping chips and debris, starving the cutting zone of coolant, promoting bacteria growth, and creating a foul odor that fills the shop. A correctly selected and maintained coolant skimmer removes tramp oil continuously — keeping the coolant clean, the shop odor-free, and the drilling process stable.
Skimmer Types
Skimmer Comparison
| Skimmer Type | Operating Principle | Oil Removal Rate | Sensitivity to Coolant Flow | Best Application |
|---|
| Belt skimmer | Continuous loop belt passes through oil layer — oil wiped off at wiper blades | 2–20 L/hour — depends on belt width and speed | Low — works in moderate turbulence | General purpose — most deep hole drilling |
| Disk skimmer | Rotating disk passes through oil layer — oil scraped off | 1–10 L/hour — depends on disk diameter | Low — works in turbulence | Small tanks, limited space |
| Tube skimmer | Floating closed-loop tube attracts oil — passes through wiper | 1–5 L/hour — depends on tube material | Moderate — best in still areas | Light oil, small tanks |
| Coalescing separator | Oil droplets coalesce on media and rise — collected at surface | 5–50 L/hour — depends on flow rate | High — requires controlled flow | Central systems, high oil volume |
| Floating suction skimmer | Floating suction head skims surface layer | 10–100 L/hour — pump driven | High — turbulence affects skimming | Large central tanks |
| Weir skimmer | Adjustable weir plate skims surface into collection trough | 20–200 L/hour — high capacity | High — requires still surface | Very large systems, heavy oil input |
Oil Removal Efficiency
| Skimmer Type | Free Oil Removal | Dispersed Oil Removal | Oil-Free Coolant Discharge | Maintenance Requirement |
|---|
| Belt | Excellent | Poor | Clean — no coolant removed | Low |
| Disk | Excellent | Poor | Clean — no coolant removed | Low |
| Tube | Good | Poor | Clean — no coolant removed | Low |
| Coalescing | Good | Good | Clean — if designed correctly | Moderate |
| Floating suction | Excellent | Poor | Removes some coolant with oil | Moderate |
| Weir | Good | Poor | Removes significant coolant | High |
Belt Skimmer Details (Most Common Type)
| Parameter | Typical Range | Selection Guidance |
|---|
| Belt width | 50–300 mm | Wider belt = higher capacity |
| Belt material | Steel, stainless steel, polymer | Steel for high temp, polymer for chemical resistance |
| Belt speed | 1–10 m/min | Faster for high oil load |
| Motor power | 10–90 W | Higher for wider belts, colder environments |
| Oil removal per width | 0.1–0.3 L/hour per cm of belt width | Estimate capacity |
| Coolant temperature limit | 60–90°C (depends on belt material) | Verify for hot coolant |
| Minimum tank depth | 150 mm | Belt needs immersion depth |
| Installation | Mount on tank rim or bracket | Overhang into tank |
Selection Criteria
| Tank Volume | Typical Oil Input Rate | Recommended Skimmer Type | Minimum Capacity |
|---|
| < 200 L | < 0.5 L/hour | Tube skimmer or small disk | 1–2 L/hour |
| 200–1000 L | 0.5–2 L/hour | Belt skimmer (50–100 mm) or disk | 2–5 L/hour |
| 1000–5000 L | 2–10 L/hour | Belt skimmer (100–200 mm) | 5–20 L/hour |
| 5000–20000 L | 10–50 L/hour | Belt skimmer (200–300 mm) or coalescing | 20–50 L/hour |
| > 20000 L (central) | 50–200 L/hour | Coalescing or floating suction | 50+ L/hour |
Selection Factors
| Factor | Consideration | Impact |
|---|
| Oil type | Way lube, hydraulic oil, spindle oil | Viscosity affects skimmer choice — heavy oils need more aggressive skimming |
| Oil temperature | Hot coolant reduces oil viscosity | Hot oil is easier to skim — but may exceed belt material limits |
| Coolant turbulence | Return flow agitation | Turbulence breaks oil into droplets — needs still zone or larger skimmer |
| Tank access | Available space for skimmer installation | Belt and disk skimmers need rim access — coalescing needs floor space |
| Coolant type | Water-based or oil-based | Most skimmers work on water-based only |
| Oil removal target | Acceptable remaining oil level | < 1% tramp oil is target — verify skimmer can achieve |
| Budget | Initial cost vs ongoing maintenance | Belt skimmers offer best value for most installations |
Sizing Example
| Scenario | Tank Size | Oil Input | Recommended Skimmer |
|---|
| Single gun drilling machine | 600 L | ~1 L/hour way oil | 100 mm belt skimmer |
| BTA machine with hydraulics | 3000 L | ~5 L/hour hydraulic + way | 150 mm belt skimmer |
| Central coolant system (4 machines) | 15000 L | ~20 L/hour total | 200 mm belt or coalescing |
| Small gun drilling machine | 200 L | ~0.3 L/hour | Disk skimmer or tube skimmer |
Installation Guidelines
Belt Skimmer Installation
| Step | Action | Detail |
|---|
| 1 | Select skimmer location | Still zone — away from return flow, near tank corner |
| 2 | Verify tank depth | Belt must be immersed 100–150 mm minimum |
| 3 | Mount skimmer on tank rim | Secure bracket — level skimmer |
| 4 | Adjust belt tension | Per manufacturer — not too tight |
| 5 | Connect wiper blade assembly | Blade contacts belt at correct angle |
| 6 | Route oil discharge hose to collection container | Use open-top container — do not seal |
| 7 | Connect power | Verify voltage — typically 110–240 VAC |
| 8 | Test operation | Belt should rotate — oil should appear on belt and wipe off |
Skimmer Placement
| Placement Factor | Best Practice | Reason |
|---|
| Location in tank | Opposite end from coolant return | Stillest water — oil collects here |
| Distance from return | As far as possible | Turbulence breaks oil into droplets |
| Depth of belt immersion | 100–150 mm | Deep enough to pick up oil, not so deep that coolant is lifted |
| Skimmer height relative to tank rim | Belt hangs vertically into tank | Belt must not contact tank walls or baffles |
| Oil collection container | Below skimmer discharge | Gravity flow — do not elevate discharge |
| Clearance around skimmer | 100 mm minimum on each side | Access for maintenance |
Common Installation Mistakes
| Mistake | Result | Correction |
|---|
| Skimmer placed in turbulent area | Poor oil pickup — oil broken into droplets | Move to still zone or install baffle |
| Belt tension too high | Motor overload, belt wear | Reduce tension per manufacturer |
| Wiper blades worn or wrong angle | Oil not removed from belt — returns to tank | Replace wipers, adjust angle |
| Oil discharge hose too long or kinked | Oil backs up — overflows at skimmer | Shorten hose, eliminate kinks |
| Skimmer too small for oil load | Oil accumulates faster than removal | Upgrade to larger skimmer |
| No collection container or sealed container | Air lock prevents oil flow | Use open-top container |
Maintenance
Routine Maintenance Tasks
| Task | Frequency | Detail |
|---|
| Inspect belt condition | Weekly | Check for wear, stretching, damage |
| Clean wiper blades | Weekly | Remove oil residue — replace if worn |
| Check belt tension | Monthly | Adjust as needed |
| Lubricate motor bearings | Per manufacturer (typically annually) | Few drops of oil |
| Clean skimmer body | Monthly | Remove coolant residue, chips |
| Check oil collection container | Weekly | Empty before full |
| Inspect mounting bracket | Monthly | Tighten bolts if loose |
| Check belt tracking | Monthly | Belt should run centered on pulleys |
Wiper Blade Replacement
| Step | Action | Detail |
|---|
| 1 | Lock out skimmer | Disconnect power |
| 2 | Remove wiper blade assembly | Typically two screws |
| 3 | Remove old wiper blades | Note orientation |
| 4 | Install new wiper blades | Correct material for oil type |
| 5 | Adjust blade contact angle | Blade should contact belt at 15–30° |
| 6 | Verify blade pressure | Light contact — not pressing hard |
| 7 | Reinstall assembly | Tighten screws |
| 8 | Test operation | Oil should wipe off cleanly |
Belt Replacement
| Step | Action | Detail |
|---|
| 1 | Release belt tension | Loosen tension adjuster |
| 2 | Remove old belt | Slide off pulleys |
| 3 | Clean pulleys | Remove oil residue |
| 4 | Install new belt | Correct length and material |
| 5 | Adjust tension | Per manufacturer — moderate tension |
| 6 | Check belt tracking | Run — belt should not walk off pulleys |
| 7 | Adjust tracking if needed | Tilt pulley or adjust position |
| Monitoring Method | What to Check | Acceptable | Action if Failed |
|---|
| Visual — tank surface | Oil layer thickness | No visible oil layer or < 1 mm sheen | Check skimmer operation |
| Visual — collected oil | Oil volume in collection container | Increasing at expected rate | Verify skimmer running |
| Tramp oil test (jar test) | Percentage oil in coolant sample | < 1% tramp oil | Increase skimming capacity |
| Oil sheen on surface | Rainbow effect | None or minimal | Clean wipers, check belt |
| Skimmer run time | Hours per day | Continuous preferred | Verify no nuisance trips |
| Belt condition | Wear, cracks, stretch | No visible damage | Replace belt |
FAQ
What is a coolant skimmer and why do I need one?
A coolant skimmer is a device that continuously removes tramp oil (hydraulic oil, way lube, spindle oil) from the surface of coolant in the tank. Tramp oil causes multiple problems: it reduces coolant cooling efficiency, traps chips and debris on the surface, promotes bacteria growth, produces foul odors (rotten egg smell), and degrades surface finish on machined parts. A skimmer removes tramp oil before these problems develop.
What type of coolant skimmer is best for deep hole drilling?
A belt skimmer is the best general-purpose choice for most deep hole drilling machines. Belt skimmers are simple, reliable, effective across a wide range of oil types and viscosities, and low maintenance. Select the belt width based on tank size and estimated oil input rate — 100 mm for single machines, 150–200 mm for larger tanks or central systems. Disk skimmers are a good alternative for small tanks with limited rim space.
How do I install a coolant skimmer?
Mount the skimmer at the stillest end of the tank — opposite the coolant return. The belt must be immersed 100–150 mm in the coolant. Adjust belt tension per manufacturer. Install wiper blades at 15–30° to the belt. Route the oil discharge hose to an open-top collection container below the skimmer. Connect power and verify the belt rotates and oil wipes off into the collection container.
How do I maintain a coolant skimmer?
Clean the wiper blades weekly — oil residue builds up and reduces wiping efficiency. Check belt condition and tension monthly. Empty the oil collection container before it overflows. Replace wiper blades every 6–12 months. Replace the belt when it shows wear, cracks, or stretching. Lubricate motor bearings per manufacturer schedule. A skimmer that stops working allows tramp oil to accumulate rapidly.
How much tramp oil should a skimmer remove?
A properly sized skimmer should keep tramp oil below 1% of coolant volume. The oil collection container should show steady accumulation during machine operation. If the tank surface develops a visible oil layer (sheen or thicker), the skimmer is undersized, not operating correctly, or needs maintenance. Measure tramp oil percentage monthly with a jar test — let a coolant sample settle and measure the oil layer thickness.
A coolant skimmer is the most effective tool for controlling tramp oil in deep hole drilling coolant. A correctly selected and maintained belt skimmer removes tramp oil continuously — keeping coolant clean, reducing bacteria growth, eliminating odor, and extending coolant life. Monitor skimmer performance by observing the tank surface and measuring tramp oil percentage regularly. This article reflects industry practice as of 2026.
