I’ve run deep hole drilling machines from most of the major builders — TBT, UNISIG, Mollart, Chinese machines, and some custom-built ones. They all drill holes, but each type has its strengths depending on what you’re making.
Here’s how I think about the different machine categories.
Single-Spool CNC Gun Drills
This is the most common type you’ll see in a job shop. A single-spindle machine with a CNC control, capable of drilling diameters from maybe 1mm to 50mm depending on the build.
What they’re good at:
- Small batches and frequent changeovers
- Parts with multiple hole sizes (change tools between cycles)
- High-precision work where each hole needs attention
What they’re not good at:
- High-volume production of identical parts
- Very large diameters (over 50mm, you’re better off with BTA)
- Parts with many holes (each hole takes its own cycle time)
I’ve seen shops buy a single-spindle machine for prototype work and then try to run production on it. The machine can do it, but the per-part cost is too high. At around 500 parts per month, you should be looking at a dual-spindle or dedicated solution.
Dual-Spindle and Multi-Spindle Machines
Two spindles drilling at once doubles the output without doubling the floor space. I’ve run four-spindle machines on production lines, and the throughput difference is dramatic.
Multi-spindle machines are purpose-built. You can’t change the spindle spacing easily, so they’re for parts that have fixed hole patterns. The most common application I’ve seen is transmission valve body drilling — a valve body might have 12 holes of different diameters, and a 4-spindle machine drills them in 3 cycles instead of 12.
The trade-off: setup changes take longer. Changing tool diameters on a multi-spindle machine means setting up all the spindles, not just one. For high-volume production of the same part, it’s worth it. For low-volume job shop work, it’s not.
BTA Machines
BTA machines are built differently from gun drilling machines. They’re heavier, with more torque and higher coolant flow. The spindle might run at lower RPM but deliver 10 times the torque of a gun drilling spindle.
The workpiece on a BTA machine is usually stationary. The tool rotates and feeds into the part, with coolant delivered through the gap between the drill tube and the bore wall. The chips come back through the center of the tool.
BTA machines handle diameters from about 20mm to 300mm or more. I’ve run them on hydraulic cylinder bores, heat exchanger tube sheets, and large valve bodies. The material removal rate is 3 to 5 times what a gun drill can do at the same diameter.
Combination Drill + Mill Machines
Some machines combine deep hole drilling with milling capability. CHETO is the best example I’ve seen — their machines can gun drill, mill, tap, and bore in a single setup.
The advantage is obvious: a part that needs drilling and milling goes in once and comes out finished. No second setup, no alignment issues between operations.
The disadvantage is that these machines cost more and are more complex. For parts that only need drilling, a dedicated gun drill is simpler and cheaper. But for complex mold work with angled cooling channels and milled features, the combination machine saves time.
What I Tell People Looking to Buy
If you’re buying your first deep hole drilling machine, here’s my advice:
| Your Work | Machine Type | Why |
|---|---|---|
| Job shop, various parts | Single-spindle CNC gun drill | Most flexible, fastest changeover |
| Production line, one part | Multi-spindle or dedicated | Lowest per-part cost |
| Large-diameter bores (>50mm) | BTA machine | Gun drilling is too slow at this size |
| Complex parts with multiple features | Drill + mill combo | Saves second operations |
| Budget under $100k | Used single-spindle gun drill | You can get a good used machine at this price |
Machine Duty Cycle Considerations
One factor that does not get enough attention is duty cycle. Deep hole drilling machines run at high coolant pressures for extended periods, and not all machines are built for continuous operation.
Light-duty machines are built for job shop work with runs of 1-4 hours. They have smaller coolant tanks, lighter construction, and less robust filtration. These machines are fine for prototype work and small batches, but they overheat if run continuously for 8+ hours.
Heavy-duty machines are built for production work with continuous 24-hour operation. They have larger coolant tanks (500+ liters), redundant filtration, and heavier castings that dissipate heat better. These machines cost 30-50% more but last longer under production conditions.
I’ve seen shops burn out light-duty machines by running them 20 hours a day on production work. The coolant overheated, the spindle bearings failed early, and the machine was down for repairs every few months. The machine was not designed for that duty cycle.
Maintenance Differences Between Types
Each machine type has different maintenance requirements:
| Machine Type | Key Maintenance Items | Interval |
|---|---|---|
| Single-spindle gun drill | Spindle bearings, coolant union seals, guide bushings | 2000 hours |
| Multi-spindle | Same as single-spindle but multiplied by spindle count | 1500 hours |
| BTA machine | Coolant pump seals, drill tube support bushings, way wipers | 1000 hours |
| Combination drill-mill | All of the above plus tool changer and milling spindle maintenance | 1000 hours |
Multi-spindle machines require more maintenance because there are more components to service. A four-spindle machine has four sets of bearings, four coolant unions, and four sets of seals to maintain. The maintenance cost per operating hour is higher than a single-spindle machine.
BTA machines have heavier maintenance requirements because of the higher forces and coolant flow. The coolant pump seals wear faster at higher pressures, and the drill tube support bushings need regular replacement.
Combination machines are the most complex to maintain. They have two spindles (drilling and milling), a tool changer, and a more complex control system. If you buy a combination machine, budget for higher maintenance costs.
Key Takeaways After Years of Running Different Types
The single-spindle gun drill is the most forgiving machine type. It is simple, reliable, and easy to maintain. If I were starting a deep hole drilling shop today, I would buy a single-spindle gun drill first and add specialized machines as the work grew.
The worst mistake I’ve seen is buying a machine that’s too specialized for the work available. A multi-spindle BTA machine is great if you have steady production of large parts. But if your work mix changes month to month, the flexibility of a single-spindle gun drill will serve you better.
Key Takeaways
- Single-spindle gun drills are the most flexible and easiest to maintain — a good first machine for most shops
- Multi-spindle machines cut cycle time by dividing holes between spindles but require more setup and maintenance
- BTA machines handle diameters over 20mm at 3-5 times the material removal rate of gun drilling
- Combination drill-mill machines save second operations but cost more and are more complex to maintain
- Duty cycle matters — light-duty machines overheat under continuous production runs
- Buy a machine that matches your work mix, not the one that looks most impressive on paper