A manual lathe can be converted for gun drilling with the right equipment. The conversion costs a fraction of a dedicated machine but comes with real limitations. I’ve done this conversion on a 14-inch Colchester lathe and gotten usable results.
The main requirements are a through-coolant toolholder, a high-pressure coolant system, and a guide bushing support. The toolholder mounts on the lathe carriage and holds the gun drill. The coolant system delivers fluid through the tool at the required pressure. The guide bushing supports the drill at the workpiece entry.
What You Need for the Conversion
For the coolant system, I use a positive displacement pump rated for at least 1000 psi. A hydraulic gear pump or a pneumatic booster pump both work. I prefer the pneumatic booster pump because it is easier to control the pressure and it stops automatically when the feed stops.
The pump feeds through a high-pressure hose to the toolholder. The coolant tank needs a filter system to remove chips. I use a 40-micron paper band filter that handles the fine chips from gun drilling. Without proper filtration, chips recirculate and scoring the drill shank.
The toolholder mounts on the lathe carriage using a custom bracket. I had a local machine shop make an adapter plate that bolts to the carriage T-slots. The toolholder needs to be aligned with the spindle axis within 0.02mm.
The guide bushing support mounts on the lathe bed and holds the bushing at the workpiece entry. I use an adjustable support with a V-block that can be aligned to the spindle axis. The support needs to be rigid enough to handle the cutting forces.
Here is the equipment list I recommend:
| Component | Specification | Estimated Cost |
|---|---|---|
| Coolant pump | Pneumatic booster, 1000-2000 psi | $800-1200 |
| Coolant tank | 20-gallon with 40-micron filter | $300-500 |
| Toolholder | Through-coolant, for gun drill shank | $200-400 |
| Guide bushing | Hardened steel, drill diameter + 0.01mm | $80-150 |
| Adapter brackets | Custom machined for your lathe | $200-400 |
| High-pressure hose | 3000 psi rated, 6ft length | $100-150 |
Total conversion cost runs between $2000 and $3000, depending on what you can fabricate yourself.
Setup and Alignment Process
The setup takes about 2 hours for the first job. Here is my step-by-step process:
First, I mount the guide bushing support on the lathe bed and align it to the spindle axis. I use a test bar in the spindle and a dial indicator on the support. I adjust until the runout is under 0.01mm.
Second, I mount the toolholder on the carriage and align it to the spindle axis. The toolholder must be parallel to the bed in both planes. I check this with a test bar running through both the toolholder and the guide bushing.
Third, I connect the coolant hoses and test the system at full pressure. I check for leaks at every connection before starting the cut.
The first few cuts are done at conservative parameters. I start at 50% of the recommended speed and feed, then increase based on chip formation and surface finish.
Once set up, changing between gun drilling and normal turning takes about 30 minutes. I remove the toolholder bracket and guide bushing support, then remount the tailstock.
Limitations and What You Can Expect
The conversion has real limitations. The tailstock limits the maximum drilling depth. On most manual lathes, you can drill about 200-300mm before the tailstock body interferes with the guide bushing support.
The lathe spindle speed limits the minimum drill diameter. A typical manual lathe has a maximum speed of 2000-2500 RPM. For a 6mm drill at 20 m/min cutting speed, you need about 1060 RPM. For a 3mm drill at the same speed, you need 2120 RPM. Most manual lathes cannot reach that speed.
I have used my conversion for:
- Drilling 8mm holes through 200mm mild steel with 0.05mm straightness
- Drilling 10mm holes through 250mm 4140 steel with 0.08mm straightness
- Drilling 12mm holes through 180mm aluminum with excellent surface finish
The straightness is not as good as a dedicated gun drilling machine. I get about 0.05-0.10mm straightness on the conversion compared to 0.01-0.03mm on a dedicated machine.
For occasional gun drilling work, the conversion is a practical solution. For production work or holes deeper than 300mm, a dedicated gun drilling machine is a better investment.
Key Takeaways
- Total conversion cost is $2000-3000, much less than a dedicated machine
- Alignment to within 0.01mm is essential for straight holes
- Maximum depth is limited to about 300mm by the tailstock
- Minimum diameter is limited by the lathe’s maximum spindle speed
- Expect 0.05-0.10mm straightness, not the 0.01-0.03mm from a dedicated machine
- Changeover between gun drilling and turning takes about 30 minutes
For more on getting started, see my guide on planning a deep hole drilling job.