Deep Hole Drilling: definition, working principle and applications
Deep Hole Drilling is drilling holes deeper than about ten times the diameter, with guided tools and high pressure coolant. It is used for cooling channels, hydraulic bores and long oil ways that cannot be reached with a twist drill. This entry explains what it is, what the numbers on a data sheet mean, and what to put on a drawing.
It is written from the point of view of the buyer and the drawing office: the values that are realistic in production, the ones that cost money, and the way the result is verified. Where an industry sets the requirement, rail is used as the example: documented welds, coatings and fire smoke toxicity compliance.
Definition
Deep Hole Drilling is drilling holes deeper than about ten times the diameter, with guided tools and high pressure coolant.
Working principle
The principle is straightforward once the jargon is set aside: material is affected in a controlled way, the result is measured, and the process is adjusted from that measurement. For Deep Hole Drilling the control points are depth to diameter ratio and hole diameter, and everything else follows from them.
What makes it repeatable is not one setting but the order of operations and the way the part is held. Bore gauge and a straightness check, and the surface condition afterwards is Ra 3.2 µm as drilled.
Typical parameters
| Specification | Typical value |
|---|---|
| Depth to diameter ratio | 10:1 to 100:1 |
| Hole diameter | 3 to 40 mm |
| Straightness | 0.5 mm per metre achievable |
| Surface finish | Ra 1.6 to 3.2 µm |
| Coolant pressure | 20 to 100 bar |
| Tool type | gun drill or BTA system |
Applications
Deep Hole Drilling is used for cooling channels, hydraulic bores and long oil ways that cannot be reached with a twist drill, and in practice it appears most often in mould cooling channels, hydraulic cylinder bores, spindle oil ways, heat exchanger plates.
How it fits into a machining route
Deep Hole Drilling is one operation in a sequence: roughing to leave even stock, any stress relieving the geometry needs, semi finishing, then the finishing operation that holds the tolerance. The order is what keeps the result stable as the machine warms up and the batch runs.The route is fixed before the first part is cut, and it is the route rather than the individual setting that holds the result: set up and coolant filtration, not the cutting time. Where a drawing calls for something the route cannot hold economically, that is raised at quotation rather than after the parts are made.
Standards, documentation and what is issued
The documentation requirement is in-house approval with dimensional reports. Material is supplied against a certificate where the application calls for it, inspection results are recorded with the job, and the records are kept so that a repeat order can be compared with the first instead of being measured from scratch.
Bore gauge and a straightness check is the verification step. Where a customer format or a standard report is required, it is agreed before the batch starts, because a report written after the event is a report nobody can use.
Frequently asked questions
What tolerance should be specified for Deep Hole Drilling?
±0.05 mm on diameter, straightness by process is realistic in production. Tightening it beyond the functional requirement increases cost without improving the part.
Which material is usually used?
4140 alloy steel is the usual choice, supplied against in-house approval with dimensional reports.
How is the result verified?
Bore gauge and a straightness check, with the results recorded so a repeat order can be compared with the first.
What is the most common mistake?
The entry face not square, which walks the drill off position.
How long does it take and what does it cost?
A realistic cycle is 10 to 60 minutes per hole, and the cost driver is set up and coolant filtration, not the cutting time.
Related capability
If you are specifying Deep Hole Drilling on a drawing and want a second opinion on the route, send the model and the quantity. The first response is a manufacturability review with the likely risk points and a quotation, so the decision can be made on facts rather than on a price alone.
Machining, inspection and documentation are handled in one place, which is why the dimensional report and the material certificate arrive with the parts rather than after them.
Related entries
These entries sit next to Deep Hole Drilling in the encyclopedia: High Speed Machining, Thread Milling, 2024-T3 Aluminium. Reading them together gives a fuller picture of how the process, the material and the measurement affect each other.
