Ejector drilling parameters are different from gun drilling or BTA drilling. The speeds, feeds, and coolant settings all shift because ejector drilling uses a double-tube system with a venturi effect for chip evacuation. Here are the ranges I use after running ejector drilling on dozens of production jobs.
Cutting Speed by Material
Ejector drilling operates at cutting speeds similar to BTA drilling but on the lower end of the range. The double-tube design restricts coolant flow slightly, so I run 10-15% slower than BTA speeds in the same material.
| Material | Cutting Speed (m/min) | My Starting Point | Notes |
|---|---|---|---|
| Mild steel (1018, A36) | 80 - 120 | 90 m/min | Increase to 110 if chips break well |
| Alloy steel (4140, 4340) | 60 - 90 | 70 m/min | Reduce to 60 for hardened over 35 HRC |
| Stainless steel (304, 316) | 50 - 70 | 55 m/min | Watch for work hardening at low feed |
| Stainless 15-5 PH | 45 - 60 | 50 m/min | Use coated inserts for tool life |
| Cast iron (gray, ductile) | 60 - 100 | 75 m/min | Cast iron is forgiving on speed |
| Aluminum (6061, 7075) | 100 - 200 | 150 m/min | Chips can be long — ensure coolant flow |
| Titanium (6Al-4V) | 25 - 40 | 30 m/min | Low speed is critical for tool life |
| Inconel 718 | 15 - 25 | 18 m/min | Use high-pressure coolant mode |
I start every new material at 60% of the listed range and work up. Chip color tells me when I am in the right zone. Steel chips should come out straw-colored. Blue chips mean I am burning the cutting edge.
Feed Rate by Diameter
Feed rate depends on diameter more than material for ejector drilling. The double-tube system is less rigid than a BTA single-tube system, so I keep feeds modest.
| Diameter Range | Feed (mm/rev) | Feed (IPR) | My Default in Steel |
|---|---|---|---|
| 18 - 25 mm | 0.05 - 0.10 | 0.002 - 0.004 | 0.07 mm/rev |
| 25 - 40 mm | 0.08 - 0.15 | 0.003 - 0.006 | 0.10 mm/rev |
| 40 - 60 mm | 0.10 - 0.18 | 0.004 - 0.007 | 0.12 mm/rev |
| 60 - 80 mm | 0.12 - 0.20 | 0.005 - 0.008 | 0.14 mm/rev |
| 80 - 100 mm | 0.15 - 0.22 | 0.006 - 0.009 | 0.16 mm/rev |
I watch chip shape closely on the first holes. If I get long stringy chips, I increase feed by 0.02 mm/rev until the chips break. If I get powder chips, the feed is too high and the insert is chipping.
Coolant Pressure and Flow by Diameter
Ejector drilling needs lower coolant pressure than gun drilling but higher flow rates than BTA. The venturi effect at the ejector nozzle creates the suction that pulls chips through the inner tube.
| Diameter | Pressure Range | Flow Rate | Chip Evacuation |
|---|---|---|---|
| 18 - 30 mm | 200 - 300 psi | 30 - 50 L/min | Good — small chips clear easily |
| 30 - 60 mm | 200 - 400 psi | 50 - 100 L/min | Moderate — larger chips need more flow |
| 60 - 100 mm | 300 - 500 psi | 100 - 200 L/min | Critical — large annulus needs high flow |
The pressure reading at the machine gauge is not the pressure at the cutting edge. I have measured 400 psi at the pump and 250 psi at the ejector head due to pressure drop through the outer tube. I install a pressure tap at the ejector head for critical jobs.
Low coolant pressure is the most common cause of chip packing in ejector drilling. When the venturi effect weakens, chips accumulate in the inner tube and the drill seizes. I set the pressure 15% above minimum to account for filter loading over the shift.
Setup Procedure
I follow this setup sequence for every ejector drilling job:
- Mount the ejector head to the machine spindle. Align within 0.01 mm runout measured at the head OD.
- Install the inner tube through the ejector head. The inner tube must extend 5-10 mm past the ejector nozzle.
- Connect coolant lines to the outer tube. Use swivel fittings that do not restrict flow.
- Set coolant pressure to the minimum for the diameter. Run the pump for 30 seconds to purge air.
- Check chip evacuation by feeding a test hole at 50% feed. Observe chips exiting the inner tube.
- Verify pressure at the head with a secondary gauge at the ejector head, not the pump.
- Increase feed to the target rate after confirming chip flow.
- Run the first production hole at 75% of target parameters. Check diameter and finish.
Skipping the air purge step is a common mistake. Air in the system causes erratic chip evacuation and the first hole often packs chips.
Common Parameter Mistakes
I have made every mistake on this list. Here are the ones that cost me the most time and tooling.
Running feed too low. Low feed produces thin, stringy chips that tangle in the inner tube. The chips pack and the drill seizes. I run feed at 0.08 mm/rev minimum in steel regardless of diameter.
Setting pressure by the machine gauge only. The pressure drop between pump and ejector head can be 100-150 psi. I install a gauge at the head for setup and check it monthly.
Ignoring return flow. The coolant returning through the inner tube should be steady and continuous. Pulsing return flow means the venturi is starving. I check return flow by watching the coolant exit at the chip basket.
Using the wrong nozzle size. The ejector nozzle diameter controls the venturi effect. A nozzle that is 0.5 mm too small cuts flow by 30%. I keep a nozzle selection chart on the machine for reference.
| Nozzle Diameter | For Tube OD | Flow Capacity |
|---|---|---|
| 8 mm | 20 - 30 mm | 40 L/min |
| 12 mm | 30 - 60 mm | 80 L/min |
| 16 mm | 60 - 100 mm | 140 L/min |
Not pecking on deep holes. Ejector drilling works best in continuous feed, but on holes over 300 mm deep I add a 0.5 mm retract every 50 mm of depth. This pulse clears any accumulated chips from the inner tube without stopping the spindle.
Key Takeaways
- Start cutting speed at 60% of the range and increase based on chip color — straw is good, blue is too hot.
- Feed rate should be 0.08 mm/rev minimum in steel to prevent stringy chips.
- Coolant pressure at the ejector head is 50-100 psi lower than the pump gauge — measure at the head.
- Air purge before drilling prevents chip packing on the first hole.
- Match nozzle diameter to tube OD using the table above to maintain proper venturi effect.
- For deep holes over 300 mm, add a 0.5 mm peck every 50 mm to clear accumulated chips.