Two lines, one overlap zone, and a decision that is not about diameter alone.
Between Ø20 and Ø40 mm both processes can physically make your bore. What decides it is the ratio, the tolerance and whether the bore has to be trued afterwards. Here is how to place your part — and the two cases where the obvious answer is wrong.
Deep-hole work is not one process. It is two lines with different physics, and they overlap exactly where most enquiries land:
| Line | Diameter | Depth | What it is good at |
|---|---|---|---|
| Gun drilling | Ø3 – 40 mm | to 6,000 mm, drilled from both ends | Small precision bores and extreme length-to-diameter ratios — fuel-system bores, hydraulic galleries, sensor passages, cooling channels |
| BTA drilling & boring | Ø20 – 360 mm | Machine envelope 12,000 mm; 10,000 mm production-proven | Large bores, and bores that have to be trued after drilling — drill collars, hydraulic bodies, large shafts |
Between Ø20 and Ø40 mm, both lines can physically do the job. Diameter alone will not decide it — and a quote that names a process from diameter alone is a guess. In practice, most of the enquiries that land in this band are decided by four things, none of which is the diameter.
“Deep” is a ratio, not a depth. The same Ø25 mm bore is a routine job at 500 mm and a different problem at 5,000 mm. The ratio is the first number to put in front of a shop
BTA is a drilling-and-boring process: drilling opens the hole from solid, then boring follows with a guided head to make it straight, round and to size. If your callout is on roundness, on the bore being true along its length, or on holding a size across a long span, boring belongs in the route
Internal annular grooves, eccentric bores and multi-step seal bores are machined inside the deep hole — they belong to the boring envelope, not to drilling alone. If your part has them, say so on the drawing; they change the sequence
A bore drilled from both ends halves the unsupported length of the tool. On a long part this is often the single difference between a bore that stays straight and one that drifts — and it is a property of your part, not of the shop
Deep-hole capability is not automatically the right answer just because a hole is round and long. Two cases to watch:
Small and shallow. The usual rule of thumb puts deep-hole territory above roughly 10:1. Below that, a twist drill and a reamer are cheaper, faster and just as accurate. Sending a 3:1 hole to a gun-drilling line buys nothing but a longer lead time.
Large and very short. A big bore that is only one or two diameters deep is boring-mill work, not deep-hole work. The deep-hole line earns its place when the tool has to travel — at 1:1 or 2:1 there is nothing to travel.
The reverse is also worth knowing: when the ratio climbs and the tolerance tightens at the same time, a shop that only has one of the two lines will either decline the job or quote it on the wrong process. That is the moment the overlap zone stops being academic.
Seven things. With them, the process family follows almost by itself; without them, anyone who names a process is guessing:
A STEP file answers the geometry. These seven answer the process.
Both lines can run Ø25 mm. What decides it is the ratio, the tolerance, and whether the bore has to be trued afterwards — not the diameter. Send those three and the answer takes one look.
Yes, and it is common — a large bore drilled and bored, plus small cross-holes or galleries gun drilled, on the same part. The two lines sit under one roof here.
To 6,000 mm, drilled from both ends. Extreme length-to-diameter ratios — beyond 600:1 at small diameters — are routine work here.
Send it anyway. Parts that sit outside the catalogue are a normal starting point here, not an exception.
Send the drawing — the depth-to-diameter ratio and the tolerance callouts will tell us, and we will tell you. Within 24 hours an engineer replies with process feedback. NDA welcome.
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