Gun Drilling Without High Pressure Coolant
High-pressure coolant is why gun drilling reaches 100:1 and beyond — the oil pushes the chips back up the V-flute. Without it, you cannot simply drill deep holes and expect them to work. But for short holes and light-duty work, gun drilling with low-pressure coolant — or on a manual lathe with a flood pump or even an oil can — is genuinely practical. This guide explains what high pressure actually does, how low pressure changes the limits, and how to set up a shop-floor solution that does not break the drill.
What High Pressure Coolant Actually Does
A gun drill is a single-lip tool with a V-flute. Chips form at the cutting edge and must travel the full length of the groove back to the drill mouth. They do not leave on their own — the coolant jet carries them. Pressure overcomes the resistance of the narrow annulus between the drill and the hole wall; flow provides the volume that sweeps the chips out.
Without enough pressure, the first few diameters of a hole drill fine and then the flute packs solid, the cutting edge starves, and the drill snaps. This is why gun drilling machines run 100–300 bar. For the full pressure/flow logic, see coolant pressure optimization.
How Low Pressure Changes the Limits
The existing coolant guide lists a minimum and a recommended pressure for each diameter. The minimum keeps the process alive; the recommended gives clean production. Below the minimum, you are no longer “gun drilling” in the production sense — you are drilling shallow holes that you retract frequently to clear chips by hand.
| Hole diameter | Minimum pressure | Below-minimum practice |
|---|---|---|
| Ø3–6 mm | 24–35 bar | Only up to ~5:1 with frequent retraction |
| Ø6–12 mm | 17–24 bar | Up to ~8:1 with pecking |
| Ø12–25 mm | 10–17 bar | Up to ~10:1 with pecking |
The larger the hole, the more forgiving the coolant requirement — a 25 mm bore needs only 10 bar minimum. The catch is that a big hole also needs high flow, and a standard machine flood pump delivers pressure or flow, rarely both.
Peck Gun Drilling: The Retraction Method
The practical workaround for low coolant pressure is to not let the chips accumulate. Retract the drill at set intervals so chips fall out and coolant reaches the cutting edge fresh.
| Setting | Low-pressure peck value |
|---|---|
| Peck depth | 2–3 × diameter per peck |
| Return | Full retraction clears the flute best |
| Feed | 50–70% of normal feed |
| Speed | Normal surface speed |
Pecking roughly halves the practical depth penalty — it is the difference between “it will not work” and “it works slowly.” For the retract cycle on a CNC machine, see the deep hole G-code cycles.
Gun Drilling on a Manual Lathe
A manual lathe has no high-pressure coolant, but it has rigid workholding and a tailstock — enough for short holes when the geometry is controlled.
- Bore a true pilot into the part first, or fit a guide bushing in the tailstock, so the drill cannot walk at entry.
- Run a low surface speed — below the material’s normal range, because a hand-fed drill has no constant feed to keep chips broken.
- Feed by hand in small, steady increments, retracting every 2–3 diameters to clear chips. Do not let the drill dwell — a stopped feed in the cut work-hardens the hole wall and snaps the tip.
- Flood with cutting oil from a small pump, or apply oil by brush between retractions for very short holes.
- Watch the chips. If they stop coming out of the flute, retract now. Chip packing is the failure mode.
Realistically, a manual lathe setup with good technique handles about 3–5× diameter comfortably and up to ~10× diameter with effort and patience. Beyond that, whip and chip control defeat hand feeding.
What You Can Realistically Expect
| Setup | Practical depth | Notes |
|---|---|---|
| Manual lathe, hand feed | 3–5:1 (up to ~10:1) | Short holes, frequent retraction |
| CNC with low-pressure flood | 5–10:1 | Peck cycle; reduced feed |
| CNC with ~35–50 bar pump | 15–25:1 | Above minimum for most diameters |
| Dedicated high-pressure system | 50–150:1 | Full gun drilling capability |
If the hole is longer than about 10–20:1, the honest answer is that you need higher pressure. The coolant system guide covers retrofit pumps in the 50–300 bar range for machine tools.
When to Upgrade to High Pressure
Low-pressure gun drilling is a capability you use when you have to. Upgrade when:
- Depth exceeds about 20:1 for small diameters
- The material hardens — prehardened and stainless steel pack chips much faster
- Volume rises — pecking on a manual lathe does not scale
- Straightness and repeatability matter — high pressure feeds a steady, unjammed flute
A 100 bar retrofit pump is a fraction of the cost of a dedicated machine and covers most short-run and job-shop work. For machine selection, see the method comparison.
FAQ
Can you gun drill without high-pressure coolant? Yes, for short holes — roughly up to 5–10× diameter — with peck retraction and reduced feed. Beyond that depth, chip packing breaks the drill.
What is the minimum coolant pressure for gun drilling? Per the pressure guide, roughly 35 bar at Ø3 mm down to 10 bar at Ø25 mm. Below these, keep depth very short and retract frequently.
Can you gun drill on a manual lathe? Yes, up to about 3–5× diameter with a true pilot or guide bushing, low surface speed, hand feed in small increments, and regular retraction to clear chips.
Why does gun drilling need such high pressure? The coolant jet is the chip-evacuation system. Pressure pushes the oil and chips back up the V-flute; without it, the flute packs and the tool breaks.
Summary
High-pressure coolant is what unlocks gun drilling’s extreme depths, but its absence does not rule the method out — it just shrinks the depth envelope. With peck retraction, reduced feed, and honest expectations, low-pressure flood and even manual-lathe setups drill useful short holes in the 3–10× diameter range. The larger the hole, the lower the pressure it tolerates. When the job outgrows that envelope, a 50–100 bar retrofit pump is the next step up.
For the pressure and flow fundamentals, see coolant pressure optimization. For the depth-ratio framework, see maximum L/D by method. For coolant system design, see gun drilling coolant systems.