Deep Hole Drilling Accuracy: What Tolerances Can You Actually Hold?
Every customer who walks through my shop door asks the same question: “What can you actually hold?” The textbook says one thing. My CMM reports say another. Here is what I have measured across thousands of parts over the last decade.
Diameter Tolerance by Process and Range
I run gun drilling, BTA, and reaming every day. Each process lands in a different IT-grade band, and the diameter range changes what is realistic.
| Diameter Range | Gun Drilling (IT) | BTA (IT) | Reaming (IT) |
|---|---|---|---|
| 3 – 10 mm | IT 7 – 9 | — | IT 6 – 7 |
| 10 – 30 mm | IT 7 – 8 | IT 8 – 10 | IT 6 – 7 |
| 30 – 80 mm | IT 8 – 9 | IT 7 – 9 | IT 6 – 8 |
| 80 – 150 mm | — | IT 7 – 8 | IT 7 – 8 |
I have held IT 6 on gun-drilled holes under 10 mm exactly twice in five years – both times on 4140 annealed, with a fresh tool and a machine that had just been scraped. Do not count on it. Quote IT 7 to IT 8 and you will have room to breathe.
For BTA in the 30 – 80 mm range I hold IT 7 consistently in 4140 and 4340. Aluminum wanders more – I budget IT 9 there unless the customer pays for a reaming pass.
Straightness
Straightness is where deep hole drilling separates from conventional machining. The L/D ratio is the dominant variable, not the material.
| L/D Ratio | Gun Drilling (mm/m) | BTA (mm/m) | Reaming (mm/m) |
|---|---|---|---|
| < 10:1 | 0.05 – 0.15 | 0.05 – 0.10 | 0.02 – 0.08 |
| 10:1 – 30:1 | 0.10 – 0.30 | 0.10 – 0.25 | 0.05 – 0.15 |
| 30:1 – 60:1 | 0.20 – 0.50 | 0.15 – 0.40 | — |
| 60:1 – 100:1 | 0.40 – 1.00 | 0.30 – 0.80 | — |
I have held 0.08 mm/m on a 12 mm gun-drilled hole at 25:1 L/D in pre-hardened 4140. The customer did not believe me until I air-gauged it in front of them. At 80:1 in the same material I measured 0.55 mm/m – still inside print, but not what the salesman promised.
BTA beats gun drilling on straightness above 30:1 because the multi-edge head self-centers better. Below 30:1 the difference is noise – your bushing fit and guide pad clearance matter more than the process choice.
Surface Finish
Ra depends on material, feed rate, and whether you are roughing or finishing. Here are the ranges I see from production runs, not test coupons.
| Material | Gun Drilling Ra (µm) | BTA Ra (µm) | Gun Reaming Ra (µm) |
|---|---|---|---|
| 4140 / 4340 (annealed) | 0.8 – 1.6 | 1.2 – 2.5 | 0.4 – 0.8 |
| 4140 / 4340 (pre-hard) | 0.6 – 1.2 | 0.8 – 1.8 | 0.3 – 0.6 |
| 303 / 304 Stainless | 0.8 – 2.0 | 1.0 – 2.8 | 0.4 – 1.0 |
| 17-4 PH H900 | 0.4 – 0.8 | 0.6 – 1.2 | 0.2 – 0.5 |
| Aluminum 6061 | 0.6 – 1.0 | — | 0.3 – 0.6 |
| Inconel 718 | 1.2 – 2.5 | 1.6 – 3.2 | 0.6 – 1.2 |
The low end of each range requires a sharp tool and a finish pass. On 17-4 PH I have hit 0.3 Ra with gun reaming – looks like a ground surface under a 50X scope. The high end is what you get from a worn tool you should have changed an hour ago.
I cover the relationship between feed, speed, and wiper geometry deeper in the surface finish guide.
Process Comparison: Accuracy Table
This is the table I actually use when quoting. It collapses everything above into a single reference.
| Parameter | Gun Drilling | BTA | Reaming (after gun/BTA) |
|---|---|---|---|
| Diameter tolerance (typical) | IT 7 – 9 | IT 7 – 10 | IT 6 – 8 |
| Straightness (typical, < 30:1) | 0.10 – 0.30 mm/m | 0.10 – 0.25 mm/m | 0.05 – 0.15 mm/m |
| Surface finish Ra (typical) | 0.6 – 1.6 µm | 0.8 – 2.5 µm | 0.3 – 0.8 µm |
| Roundness (typical) | 0.02 – 0.08 mm | 0.01 – 0.05 mm | 0.005 – 0.02 mm |
| Min hole diameter | 1.5 mm | 8 mm | 3 mm |
| Max practical L/D | 200:1 | 100:1 | 20:1 |
I add reaming as a second operation on maybe 20 % of my jobs. It tightens everything, but it also adds a setup and a tool-change. The numbers in the reaming column assume a sound pilot hole – a banana-shaped gun-drilled hole will not straighten out in reaming. It just follows the banana.
Position Tolerance
Position is a function of your starting bushing, not the drill itself. If your bushing bore is worn 0.03 mm oversized, your hole position drifts by at least that much before the drill engages the full diameter.
I hold ± 0.1 mm positional on a CNC gun driller with a ground bushing plate. On a lathe with a homemade bushing fixture I have seen ± 0.3 mm and the customer accepted it. If you need better than ± 0.05 mm, you spot-face or pilot-drill first, then gun drill from the spot. I do this for hydraulic spool bores and it lands every time.
Roundness
Roundness is the unsung metric. A bore can be within diameter tolerance and still be tri-lobed from a dull corner or a clogged coolant hole.
- Gun drilling: 0.02 – 0.08 mm roundness in 4140. The single-lip cutting action naturally produces a slight triangle. Accept it.
- BTA: 0.01 – 0.05 mm. The multi-edge head averages out cutting forces and produces a truer circle. This is the main reason I switch to BTA above 30 mm diameter when roundness is critical.
- Reaming: 0.005 – 0.02 mm with a carbide expansion reamer. Hydraulic valve bores go here.
I check roundness with a three-point micrometer, not a bore gauge, because the tri-lobe pattern hides from a two-anvil gauge. Learned that one the hard way.
How Setup Quality Affects Every Tolerance
I can hand you a tolerance table, but it is worthless if your setup is soft. Here is what I have learned the hard way:
- Bushing clearance. A bushing worn 0.05 mm oversized doubles your start-up runout. I replace bushings at 0.02 mm over nominal. That means every six months for me.
- Coolant pressure. Dropping below 80 bar on gun drilling turns straightness from predictable into a coin flip. I watch the pressure gauge like a heart monitor.
- Guide pad wear. On BTA, worn guide pads produce chatter marks that look like a threading operation. I gauge pads every 20 parts.
- Workpiece clamping. A part that moves 0.01 mm during drilling creates a taper that grows with depth. I now clamp with hydraulic fixtures instead of strap clamps. Worth every penny.
If you are just getting into deep hole drilling, read the gun drilling primer first – it covers the basics of how the chip evacuation and coolant work.
Key Takeaways
- Quote IT 7 – 8 for gun drilling and IT 7 – 9 for BTA in common steels. Add a reaming pass to jump up two IT grades.
- Straightness is driven by L/D ratio, not material. Use the mm/m table above to set expectations before you cut chips.
- Surface finish below 0.6 µm Ra requires a finishing pass (gun reaming or skiving). Single-pass gun drilling will not reliably get there.
- Roundness is tighter with BTA than gun drilling because of the multi-edge head geometry. Do not try to measure tri-lobed bores with a two-point gauge.
- Setup quality – bushings, coolant, clamping – dominates every tolerance. A perfect tool in a sloppy fixture produces a sloppy hole.
- Real numbers beat textbook numbers every time. I keep a binder of CMM reports sorted by material and L/D. Start one.