Guides / Process
Drilling a Long, Straight Bore Through a Large Part
September 29, 2026
A hole that is a few diameters deep is a routine drilling operation on almost any machine. A hole that is fifteen or twenty diameters deep, straight through a long housing or a hydraulic cylinder body, is a different job entirely, and it narrows the list of shops that can quote it long before table size does.
Why a standard drill wanders
A conventional twist drill is guided by nothing but the hole it is cutting. At shallow depths that is enough, because the drill has not had room to drift off centre. Past a certain depth-to-diameter ratio, several things start working against it at once.
Flex. A long, slender drill bends slightly under cutting load, and once it bends it follows the path of least resistance rather than the intended centreline.
Chip evacuation. Chips have to travel the full length of the hole to get out. If they pack, they push the drill sideways or cause it to re-cut its own chips, both of which degrade straightness and finish.
Regrind and web condition. A standard drill’s point geometry is not designed to self-centre over long distances the way a dedicated deep hole tool is.
The result is a bore that may be within tolerance at the entry and noticeably off centre by the exit, sometimes without any obvious sign at either end until it is checked along its length.
Where gun drilling and BTA drilling come in
Once the ratio gets past what a standard drill can hold, shops move to processes built specifically for depth.
Gun drilling. A single-flute tool with an internal coolant passage that flushes chips out through a V-shaped flute while high pressure coolant is pumped through the centre. It is slow but produces a straight, round, well-finished bore, and it is the usual choice for smaller diameter deep holes such as those in long shafts or cylinder bodies.
BTA and ejector drilling. Used for larger diameters than gun drilling suits well. Coolant is delivered around the outside of the tool and chips are carried back through the centre of the boring bar, which handles higher metal removal rates on bigger bores.
Both methods depend on the tool being supported and guided as it advances, which is what standard drilling lacks at depth. That support is also why the machine and tooling for this work is specialised and not every shop that can bore a large diameter can also drill a long, small one.
What straightness and runout to expect
Numbers here are typical ranges, not promises, and the achievable result depends heavily on diameter, length, material and the specific machine.
| Method | Typical depth-to-diameter range | What it is good at |
|---|---|---|
| Standard drilling | Up to roughly 5:1 | Shallow holes, low cost |
| Gun drilling | Well beyond 20:1 | Smaller diameter, fine finish, tight straightness |
| BTA / ejector drilling | Similarly deep | Larger diameter, higher stock removal |
Straightness over a long bore is usually expressed as total deviation over the length rather than a single tolerance value, and it is worth asking the shop what they can hold on a part of your specific length and diameter rather than assuming a catalogue number applies.
Why this narrows the shortlist further than table size
A shop can have a large enough table and spindle travel to hold your housing and still not own a gun drilling or BTA setup, because deep hole drilling is its own dedicated capability with its own tooling, coolant system and often its own machine. This is one of the reasons the machine class that fits your envelope is not automatically the machine class that can produce your bore.
A part with a long, deep, precision bore should be flagged on the RFQ as a deep hole feature specifically, not folded into a general note about tight tolerances. That is what lets a matching request go only to shops that actually run this process, rather than to every shop that happens to have a big enough table.
What to put on the RFQ
- Bore diameter, depth, and the ratio between them
- Straightness or total runout requirement over the full length
- Surface finish inside the bore, since it differs from finish elsewhere on the part
- Whether the bore is drilled from one end or two, and any requirement on where they meet
- Material and hardness, since both affect tool selection and cycle time
What to do next
If your part has a bore deeper than roughly ten diameters, call it out as a deep hole drilling requirement in the RFQ rather than a general tolerance note. It is the single detail most likely to determine which shops can actually take the job.
Related reading: large bore machining and coaxiality, horizontal boring mill vs gantry mill, what to put in a large part RFQ.
Frequently asked questions
At what depth-to-diameter ratio does a hole become a deep hole problem?
There is no fixed cutoff and it depends on the diameter, the material and the machine, but ratios above roughly 5:1 to 10:1 are where standard drilling starts to wander and shops begin planning for gun drilling or a boring bar operation instead.
Is gun drilling more expensive than a standard drilled bore?
Usually, yes, because it is a slower, more specialised process with its own tooling and coolant setup. It buys straightness and finish that a standard drill cannot hold at depth, so the extra cost is what it takes to get a bore that stays straight rather than a bore that looks fine on paper.
Can an existing bore be corrected if it drifted during drilling?
Sometimes, by boring it out afterward on a machine that can reach the full length and correct the centreline, but that removes more material and needs a larger finished diameter than originally planned. It is better caught before machining starts than after.
Need a quote for a part like this?
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