Once a hole gets deeper than about ten times its diameter, an ordinary twist drill stops being a tool and starts being a liability. It wanders, it packs with chips, it burns, and it produces a bore that drifts off centre by millimetres over its length. Deep hole drilling services exist to solve that problem properly. Using self-guiding single-flute gundrills or BTA heads with high-pressure through-tool coolant, we routinely produce bores with depth-to-diameter ratios above 100:1 while holding straightness, roundness, and surface finish that a twist drill could never approach. For hydraulic manifolds, turbine shafts, injection nozzles, and firearm barrels, it is the only credible route.
What Makes Deep Hole Drilling a Separate Discipline
Three things change fundamentally when a hole gets deep: chip evacuation, tool guidance, and heat. A gundrill addresses all three at once. Coolant is pumped at 40-100 bar down an internal channel in the tool shank, exits at the tip, and flushes chips back along an external V-shaped flute. The tool has a single asymmetric cutting edge and hardened wear pads that burnish against the bore wall it has just created, so the tool guides itself rather than relying on the machine spindle for straightness.
That self-guiding action is the reason a gundrilled hole stays true. It also means the entry condition matters enormously: we start the hole with a pilot bushing or a pre-drilled pilot, because any wander in the first few millimetres is faithfully reproduced for the entire depth. Our machinists apply the same setup discipline you will find behind our standard CNC drilling and CNC boring services, but with dedicated deep hole equipment built on rigid box ways for vibration damping.
Gundrilling vs BTA vs Trepanning
Choosing between the three main deep hole methods comes down to diameter, depth, and how much material you are willing to turn into chips.
- Gundrilling — best for roughly 1 mm to 50 mm diameter. Coolant in through the tool, chips out along the external flute. Excellent straightness, superb finish, the standard choice for small and medium bores at extreme depth ratios.
- BTA (Boring and Trepanning Association) — best above about 20 mm. Coolant flows in around the outside of the drill tube and chips exit through the centre of the tube. Much higher metal removal rate than gundrilling, supports depth-to-diameter ratios up to 400:1, and handles large diameters efficiently.
- Trepanning — cuts an annular groove and leaves a solid core intact. Only the ring becomes chips, so it is dramatically faster and more energy-efficient on large diameters, and the recovered core can be reused or sold. Ideal for expensive alloys.
In practice we often combine them: BTA or trepanning to open the bore quickly, then a finishing pass or honing to reach final size and surface finish.
Achievable Tolerances, Straightness and Surface Finish
Deep hole drilling is far more accurate than most engineers expect. On typical steel components we hold:
- Diameter tolerance: IT8-IT9 as-drilled, IT6-IT7 after reaming or honing
- Straightness (hole drift): commonly 0.1-0.5 mm per metre, and better on shorter parts with careful setup
- Roundness: 0.005-0.02 mm
- Surface roughness: Ra 0.4-1.6 µm as-drilled, down to Ra 0.1-0.2 µm after honing
- Depth-to-diameter ratio: up to 100:1 with gundrilling in a single pass, higher with BTA
Where the print calls for a bore that is both dimensionally tight and geometrically perfect, we plan a two-stage route from the start. Deep drilling establishes position and straightness; honing or cylindrical grinding of the outside diameter then brings everything into concentric relationship. Our article on honing services for bores covers that finishing stage in detail, and the CNC drilling guide explains when a conventional hole is sufficient.
Materials, Depth Limits and Practical Design Rules
We deep-drill carbon and alloy steels (4140, 4340, tool steels), stainless grades including 17-4 PH and 300/400 series, Inconel and other nickel superalloys, titanium, aluminium, brass, and cast iron. Harder and gummier materials simply run slower and consume tooling faster; none of them are off limits.
Design guidance that keeps cost down:
- Drill from the largest practical diameter. Doubling diameter cuts the depth ratio in half and dramatically shortens cycle time.
- Avoid intersecting cross-holes mid-bore where possible. A gundrill exiting into an existing void loses guidance and can deflect or break. If cross-holes are unavoidable, tell us the sequence you want.
- Specify straightness realistically. Calling 0.05 mm/m when 0.3 mm/m would work can triple the cost.
- Allow access from both ends where the part is very long. Drilling from both ends halves the effective depth ratio.
- Keep the entry face square to the bore axis. An angled or curved entry needs a spot-face or bushing, which is cheap to add at design stage.
For long turned components we normally combine deep hole drilling with CNC turning in the same routing, or with Swiss turning for smaller diameters. Everything runs under one roof within our broader metal CNC machining capability, which removes the shipping delays and finger-pointing that come with outsourcing the deep hole step to a specialist subcontractor.
Typical Applications
The industries that buy deep hole drilling do so because a straight, smooth, deep bore is safety-critical:
- Hydraulics and pneumatics — cylinder bores, manifolds, high-pressure valve blocks where sealing depends on bore finish
- Power generation — turbine shaft lines, cooling channels, and lubrication galleries in Inconel and steel
- Oil and gas — drill collars, mud motor housings, downhole tool bodies
- Aerospace and defence — landing gear components, actuator bodies, gun barrels
- Mould and die — conformal cooling lines in large tool blocks
- Medical — cannulated bone screws, surgical instrument shafts and trocars
Long shaft-type components are a recurring theme, and the tolerancing considerations overlap heavily with those in our precision shaft manufacturing guide. If you are unsure how to dimension the bore relative to the outside diameter, our CNC machining tolerances guide shows how to write it so the print is both manufacturable and inspectable.
Frequently Asked Questions
What is the deepest hole you can drill?
With gundrilling we handle depth-to-diameter ratios up to roughly 100:1 in a single pass. With BTA on larger diameters, ratios of 200-400:1 are achievable. Very long parts can also be drilled from both ends, effectively doubling the reach.
What is the smallest diameter you can gundrill?
Around 1.0-1.5 mm, though tooling becomes fragile and feed rates drop sharply below 3 mm. Send us the depth requirement along with the diameter — the ratio matters far more than the diameter alone.
How straight will the hole be?
Typical drift is 0.1-0.5 mm per metre of depth. Straightness depends on the entry condition, material homogeneity, and whether cross-features interrupt the bore. We can quote a tighter straightness spec if it is genuinely required.
Can you deep drill hardened material?
Generally we drill before hardening, then finish the bore afterwards by honing or grinding. Drilling into fully hardened stock is possible in some cases but slow and expensive, so we plan the heat treat sequence into the routing.
Do I need honing after deep hole drilling?
Only if you need better than IT8 size, Ra below about 0.4 µm, or a specific crosshatch pattern for sealing. Many hydraulic and pneumatic bores are honed for exactly those reasons; many structural cooling channels are left as-drilled.
Can you drill deep holes at an angle or into a curved surface?
Yes, provided we can create a proper start. We machine a flat spot-face or use a guide bushing so the drill enters square. Tell us at RFQ stage so we can include the feature in the routing.
Send Us the Part, Not Just the Hole
The best deep hole quotes come from seeing the whole component. Send a STEP file plus a 2D print showing bore diameter, depth, straightness, finish, and any intersecting features, along with material grade and quantity. Our engineers will confirm whether gundrilling, BTA, or trepanning is the right method, flag any features that will fight the process, and return process, lead time, and pricing. Where the bore is one step in a larger part, we will quote the complete routing so you buy a finished component rather than a series of operations.
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