Deep Hole Drilling Cost: Quoting Gundrilling and BTA Work

A hole that goes ten, twenty, or fifty times deeper than it is wide does not behave like a hole you drill on a machining centre. The tool runs slow, the chips have to travel a long way out, the coolant has to fight its way in, and the bore has to stay straight the whole distance. That is why deep hole drilling cost is one of the line items estimators most often guess at — and most often get wrong. Quote it like an ordinary drilled feature and you will lose the job on the easy parts and lose money on the hard ones.

This article builds a deep-hole price the same disciplined way you would build any machined feature — material, cycle, setup, tooling, secondary ops, overhead, margin — but with the specifics of gundrilling, BTA, and peck drilling spelled out so the number holds up against a bore that actually has to be straight, round, and on-size at the bottom.

When a hole becomes a "deep" hole

The rule of thumb most shops use is the depth-to-diameter ratio (often written L/D). Below roughly 5:1 a hole is ordinary — twist drill, peck a little, done. Past that, swarf evacuation and bore straightness start to dominate, and the process — not the geometry — sets the cost.

L/D ratio	Typical method	Cost behaviour
Under 5:1	Twist drill on a mill/lathe	Priced as a normal drilled feature
5:1–10:1	Peck drilling, parabolic drill	Slower cycle, frequent retract
10:1–40:1	Gundrilling (single-flute)	Dedicated machine, high-pressure coolant
20:1–100:1+	BTA / STS drilling	Large bores, specialist machine + tooling

The jump that catches estimators out is the one from "peck it on the mill" to "this needs a gundrilling or BTA machine." That is not a longer cycle on the same machine — it is a different machine, a different hourly rate, dedicated tooling, and often an outside-service buy if you do not own the equipment. Treat that boundary as a hard cost step, not a slope.

The three processes you are actually quoting

Peck drilling is deep drilling on equipment you already own. A standard or parabolic drill bores a short distance, retracts to clear chips, plunges again. It is cheap on capital but slow per hole, because every retract is non-cutting time, and it loses straightness past 10:1 or so.

Gundrilling uses a single-flute tool with high-pressure coolant fed down the centre and chips flushed out along a V-groove. It holds tight straightness and good finish at high L/D, runs continuously without pecking, but needs a dedicated gundrilling machine and slow, careful penetration rates.

BTA (boring and trepanning association), also called STS, feeds coolant around the outside of the tool and pulls chips back through the centre. It is the method for large-diameter, very deep bores — think hydraulic cylinders and gun barrels — and carries the highest machine and tooling cost of the three.

The estimating mistake is pricing all three as "drilling." They share a feature on the drawing and almost nothing else on the cost sheet. Pin down the process first; the rest of the numbers follow from it. This is the same logic that underpins any CNC machined part estimate — the operation, not the outline, drives the cost.

Cycle time: penetration rate is the whole story

Deep-hole cycle time is dominated by one number: the penetration rate (feed) the process allows, which on a gundrill or BTA machine is deliberately slow to keep the bore straight and the tool alive. The cycle is simply:

Drilling time = hole depth ÷ penetration rate + (peck retract time, if pecking) + entry/exit handling

A gundrill might penetrate at 20–60 mm/min in steel depending on diameter and grade — so a 400 mm deep bore is a 7-to-20-minute cut before you add anything. Peck drilling looks faster per millimetre but pays it all back in retracts: every peck is a full withdrawal and re-approach, and a deep hole can need dozens.

Two details swing the number hard. First, material: free-cutting steel and aluminium let you push the feed; stainless, titanium, and hardened steel force it down, so the same 400 mm bore can double or triple in time on the same machine. Second, the start. Deep-hole tools need a pilot hole or a bushing to start straight, and the entry is run slow to avoid deflection — that lead-in is real cycle time, not rounding. Always cost the cycle against the fully burdened rate of the specific machine, built the way the machine shop hourly rate calculation lays out — a dedicated deep-hole machine carries a higher rate than the mill it replaces.

Tooling and consumables: the cost that isn't the machine

Deep-hole drilling carries a tooling and consumables bill that ordinary drilling does not, and it has to sit in cost, not get absorbed into the hourly rate by accident:

1. The drill itself — gundrills and BTA heads are specialist, sometimes made-to-order for an odd diameter, with a real per-part amortisation if the run is short.
2. Regrinds and tool life — deep-hole tools are reground on a schedule; spread that cost across the holes the tool produces.
3. High-pressure coolant / cutting oil — deep drilling needs high flow and pressure, and BTA in particular often runs on neat cutting oil, a consumable with its own cost and disposal.
4. Pilot drilling and bushings — the lead-in operation and any guide bushing are part of the job.

On a one-off or short run, a custom-diameter gundrill can dominate the per-part price all by itself. On a long run it disappears into amortisation. Quoting both the same way is how short runs lose money. Where the deep hole is one step among several, write it into a work plan for multi-step operations so the tooling and the pilot op are both costed, not assumed.

Tolerance and straightness: what actually drives the price up

A deep hole is rarely "just a hole." The drawing usually demands bore diameter to a tolerance, straightness over the length, and a surface finish — and each tightens the process. A gundrilled hole holds straightness on the order of a fraction of a millimetre per metre, but if the print calls tighter than the drill delivers, you are now adding a secondary operation: reaming, skiving, roller-burnishing, or honing the bore to size and finish. That secondary pass can cost as much as the drilling itself.

Reading those callouts correctly is the whole game, and it is exactly where deep-hole quotes go wrong: an estimator prices the drilling and misses that the bore finish callout forces a hone. The GD&T-for-estimators guide covers how diameter, straightness, and finish callouts translate into added operations and cost rather than decoration on the drawing.

A worked example: a 12 mm bore, 360 mm deep, batch of 25

Take an alloy-steel spacer needing a 12 mm bore 360 mm deep — an L/D of 30:1, squarely gundrilling territory. Quantity 25.

1. Material and pilot — bar plus a pilot drill and face op ≈ €2.10 per part.
2. Gundrilling cycle — 360 mm at ~40 mm/min ≈ 9 min, plus 1.5 min handling = 10.5 min at a €95/h burdened gundrill rate = €16.63.
3. Tooling amortisation — share of the gundrill + regrinds across the run ≈ €3.20 per part.
4. Setup — €220 ÷ 25 = €8.80 per part.
5. Secondary — deburr both ends + bore inspection ≈ €1.50.
6. Cost per part = 2.10 + 16.63 + 3.20 + 8.80 + 1.50 = €32.23.
7. Price at 35% margin — using price = cost ÷ (1 − margin), 32.23 ÷ 0.65 = €49.58.

Add a honed-bore callout and step 5 might carry another €6–8 of secondary work, pushing the price past €60 — which is exactly why the tolerance line has to be read before the number is sent. The margin step uses the same arithmetic as any margin-versus-markup pricing; confuse the two and a 35% intended margin quietly becomes 26%.

From a drawing to a deep-hole price

The slow, error-prone step in deep hole drilling cost estimation is not the arithmetic — it is reading the drawing completely under deadline and recognising, before you quote, that a hole has crossed from "peck it on the mill" into a process that needs a dedicated machine, specialist tooling, and possibly a honing pass to hit the bore tolerance.

QuoteBuddy reads the technical drawing and surfaces those features — hole diameter and depth, the depth-to-diameter ratio, material, and the tolerance and finish callouts that decide whether a secondary operation is needed — so the estimator builds the price from a complete picture instead of treating a 30:1 bore like an ordinary drilled hole. The cost engine then assembles material, cycle, tooling, setup, secondary operations, and your target margin the same way every time.

Start a 30-day trial and run a few real deep-hole drawings through it — from upload to a complete, itemized quote PDF — and see whether the unit price it builds matches what the job actually costs, with the margin landing exactly where you set it.