Deep-hole drilling and gun drilling for CNC buyers

Deep holes on CNC parts: when buyers should plan beyond a stub drill

Many RFQs show a long hole through a housing, manifold or shaft and assume “just drill it.” Once depth-to-diameter grows, chip evacuation, straightness, heat and tool breakage dominate cost and risk. This guide helps buyers decide when a conventional twist or stub drill with peck cycles is enough, when to ask for gun drilling (or similar deep-hole methods), and what to put on drawings—without inventing shop-specific cycle times or guaranteed straightness numbers.

Related reading: wire EDM for precision parts when a hole must be started or finished differently; turned stainless shaft drawing guide when deep axial holes sit in shafts; surface roughness Ra when bore finish matters.

What “deep” usually means on an RFQ

Shops rarely share one universal depth-to-diameter cutoff. As a buyer-facing orientation only:

• Short holes (often under roughly 3× to 5× diameter) are routinely handled with standard drills on mills and lathes
• Moderate depths often need peck drilling, better coolant and attention to chip packing
• Deeper holes (commonly discussed around 10× diameter and beyond, depending on material and diameter) push shops toward gun drills, BTA-style systems or specialized deep-hole equipment

Material matters as much as the ratio. Gummy aluminum, work-hardening stainless and tough alloy steels behave differently at the same L/D. Do not paste a single ratio onto every drawing as a hard rule.

Stub drills, jobber drills and peck cycles

• Stub (screw-machine) drills: short flute length, stiffer, good for starting holes and shallow work
• Jobber-length drills: general-purpose length for many CNC holes
• Taper-length / aircraft-length drills: longer reach with more deflection risk

Peck drilling (periodic retract to clear chips) is common on CNC mills and lathes for deeper twist-drill holes. Coolant through the spindle or abundant flood coolant helps; blind holes without coolant path are harder.

• Moderate L/D in free-machining materials
• Tolerance and straightness are commercial
• Hole will be reamed, bored or honed afterward for size
• Diameter is large enough for robust twist drills and chip space

• Wandering and bell-mouth at the entry
• Chip packing and broken tools in deep blind holes
• Taper and size growth as the drill dulls
• Poor finish that will not meet a fine Ra callout without a secondary op

If the hole is a critical oil gallery, bearing seat precursor or long coaxial feature, ask the estimator whether twist-drill + peck is the real plan or only a hopeful default.

Gun drilling and related deep-hole methods

Gun drilling uses a single-flute (typical) tool with high-pressure coolant through the tool to flush chips out the flute. It is widely used for deep, straight holes in molds, manifolds, shafts and hydraulic components. Related processes (BTA / STS, ejector drilling) serve larger diameters and production volumes—your shop will propose what their equipment supports.

Buyer-relevant traits of gun drilling:

• Better suited to high L/D than twist drills in many materials
• Strong dependence on starting bushing or precise pilot, machine rigidity and coolant pressure
• Often paired with a drilled or milled start feature
• Diameter range and max depth depend on the machine—ask rather than assume

Gun drilling is not automatically “more accurate than everything.” It is a process family for deep holes; final size may still need ream, bore or hone. Do not claim a universal IT grade on a marketing page.

Straightness, runout and positional reality

Deep holes accumulate angular error. A hole that meets diameter at both ends can still miss a mid-span wall or a intersecting cross-hole. Buyers should:

• Define which end is the primary datum for position
• Avoid stacking tight true position at both ends of a very long hole without a process plan
• Call out intersecting features clearly (cross-drills, ports) so the shop sequences correctly
• Consider a relief or larger diameter zone if a mid-length feature only needs clearance

If wall thickness around a deep hole is thin, state minimum wall after machining. Wandering into a thin wall scrapes the part.

Coolant, chips and material notes

Drawing notes that keep quotes accurate

Vague notes such as “deep hole drill” without diameter, depth, blind/through and tolerance force guesswork. Prefer:

• Diameter and tolerance (and whether ream/bore/hone follows)
• Depth (blind) or through; which face is entry
• Maximum allowable runout, straightness or position if functionally required—only where needed
• Intersections with other holes or pockets

• “Gun drill acceptable” or “gun drill required” if your standard demands it
• Pilot / bushing requirements only if you mandate them
• Entry chamfer and exit burr expectations

• Fluid passage, clearance only, or precision bore precursor
• Cleanliness / deburr level for hydraulic use (without inventing particle counts)

CNC mill vs lathe vs dedicated deep-hole machine

• Mills: flexible for prismatic parts; deep holes compete with tool length and chip egress
• Lathes / Swiss: natural for axial holes in bar work; see Swiss vs CNC lathe for small parts
• Dedicated deep-hole machines: best for extreme L/D and production consistency when available

On RFQs, say whether the hole is in a prismatic housing or a turned blank. Routing changes the quote.

Secondary operations after the deep hole

Common follow-ons:

• Ream or bore for size
• Hone for finish and geometry
• Roller burnish in some ductile materials
• Cross-drilling and port spotting
• Pressure test of galleries (defined by you)

Call the finished requirement, not only the first drill diameter, when the function is a precision bore.

Inspection without over-specifying

Useful checks:

• Diameter at entry and reachable depth
• Through-hole pin or plug checks where accessible
• Optical or probing for position of entry
• Sectioned FAIR pieces for critical intersecting galleries (costly—use sparingly)

Long blind holes are hard to inspect fully. If mid-span diameter is critical, say how you will verify—or accept a process capability approach with your quality team. See first article inspection reports.

Common RFQ mistakes

• Showing a 20× diameter blind hole with no process note and a fine Ra on the full depth
• Tight true position from both ends of a long hole with no datum priority
• Forgetting exit burrs into intersecting pockets
• Specifying gun drill on a diameter the shop’s equipment cannot hold
• Omitting heat-treat sequence relative to deep holes in steel
• Asking for “aircraft drill” as a substitute for a real depth and tolerance callout

Cost and lead-time drivers (no fixed prices)

Expect quotes to move with L/D, material, blind vs through, tolerance/finish, intersecting features, scrap risk and whether dedicated deep-hole capacity is required. A clearance hole at moderate depth is cheap; a long hydraulic gallery with fine finish and intersections is not. Ask early—process choice after the PO often blows schedule.

Checklist before you release the drawing

• Diameter, depth, blind/through and entry face
• Functional tolerance and finish only where needed
• Straightness / position strategy that matches L/D reality
• Material and heat-treat sequence
• Intersections, deburr and cleanliness notes
• Process preference (twist+peck vs gun drill) when you have one

When the package is clear, estimators can route the job correctly. For CNC work with deep features, use the CNC machining service page and submit drawings through the quote form.