The coolant pressure gauge is the most informative instrument on a gun drilling machine. It tells you about chip evacuation, pump condition, filter status, and drill condition — all in real time. But only if you know what to look for.
I have spent years learning to read pressure patterns. Here is what I have found.
Where to Measure Pressure
The pressure at the pump gauge is not the pressure at the tool tip. I install gauges at two locations:
- Pump outlet. This tells me the pump is delivering pressure. A drop here means a pump or drive problem.
- At the drill holder or rotary inducer inlet. This is the pressure actually available to the drill. The difference between pump pressure and holder pressure is the system loss.
The pressure drop from pump to tool tip is typically 20-40% depending on hose length, fitting count, and rotary union condition. If the pump reads 1000 psi and the tool reads 600 psi, I have a 40% loss that needs investigation.
I use a glycerin-filled pressure gauge at the machine. The glycerin dampens needle oscillation from pump pulsation and gives a stable reading. Dry gauges on high-pressure systems vibrate so much that reading the exact pressure is impossible.
What Normal Pressure Looks Like
A healthy gun drilling operation shows a steady pressure reading that stays within the expected range for that drill diameter and material.
| Drill Diameter | Normal Pressure Range (Steel) | Normal Pressure Range (Aluminum) |
|---|---|---|
| 3-6mm | 1200-1800 psi | 800-1200 psi |
| 6-12mm | 800-1200 psi | 600-1000 psi |
| 12-20mm | 500-800 psi | 400-700 psi |
| 20-30mm | 300-600 psi | 200-500 psi |
The needle should be steady, not oscillating more than 50 psi in either direction. A steady reading at the expected pressure means chip evacuation is working and the drill is cutting normally.
I cover the relationship between pressure and flow in Coolant Flow Rate vs Pressure.
Low Pressure: Causes and Diagnosis
When the pressure reads below the expected range, the problem is usually one of three things:
| Reading | Most Likely Cause | Check |
|---|---|---|
| 20-40% below normal | Pump wear or incorrect speed | Pump condition, VFD speed |
| 40-60% below normal | Filter clogged or bypassing | Filter pressure drop, change filter |
| 60-80% below normal | Major leak in system | Hoses, fittings, rotary union seals |
| Near zero | Pump not running or suction blocked | Pump motor, suction strainer |
I check pressure first thing every morning. If the baseline has dropped more than 10% from last week, I investigate before starting production.
A gradual pressure drop over days or weeks usually means pump wear or filter loading. A sudden drop means a leak or a pump issue.
Fluctuating Pressure: The Diagnostic Signal
A pressure gauge that oscillates more than 50 psi tells me something is wrong inside the hole.
| Pattern | What It Means | Action |
|---|---|---|
| Slow oscillation (1-2 seconds per cycle) | Chips are packing and clearing intermittently | Increase pressure if possible, check chip shape |
| Fast oscillation (multiple times per second) | Pump cavitation or drill vibration | Check suction strainer, check drill for runout |
| Random spikes followed by drops | Tool is chipping or breaking | Stop the machine and retract immediately |
| Steady drop during the cut | Filter loading under flow | Change filter, check coolant cleanliness |
The most dangerous pattern is random pressure spikes. I have seen this pattern about 2-3 seconds before a drill breaks. If the pressure spikes suddenly and the needle jumps 200+ psi, I stop the feed immediately.
I cover pump cavitation in more detail in Coolant Pump Cavitation with Small Drills.
Pressure at Startup vs During Cutting
The pressure reading changes when the drill enters the cut. At startup, with the drill spinning but not cutting, the pressure should be at the pump’s idle pressure. When the drill engages, the pressure rises as coolant is forced through the drill’s coolant holes and the chip flute.
The pressure increase from idle to cutting should be consistent from part to part. If the pressure is higher than normal at the start of a new part, the drill’s coolant hole may be partially blocked. If it is lower, the drill may have a worn or broken tip that is letting coolant escape.
What to Do With This Information
I train every operator on my machines to check the pressure gauge at three points during each cycle:
- At startup — confirm baseline pressure is normal
- 10 seconds into the cut — confirm pressure rises correctly
- Mid-hole — confirm pressure is steady
Any deviation from the normal pattern triggers a stop-look-listen response. If the pressure is off, I do not let the machine keep running and hope it fixes itself. It will not.
Key Takeaways
- Install a pressure gauge at the tool holder, not just at the pump. The pressure at the tool is what matters.
- Use a glycerin-filled gauge on high-pressure systems. Dry gauges vibrate too much to read accurately.
- Normal pressure is steady within 50 psi. Fluctuation means something is wrong.
- Sudden pressure spikes are a warning of imminent drill breakage. Stop immediately.
- Track baseline pressure daily. A gradual drop of 10%+ over time indicates pump wear or filter loading.
- Train operators to read the gauge at startup, at engagement, and mid-hole. Three checks per cycle catch problems before they cause scrap.