Dewey Wu General Manager & senior mechanical engineer at EPOC CRAFTER, 15 years in design engineering, quality, and metallurgy. Hands-on across CNC machining, metalwork, sheet metal, and prototyping (subtractive + 3D printing).
A deep hole drilling RFQ needs a machining route, a chip-removal plan and inspection coverage that matches the bore’s function. Diameter and depth alone leave the quote incomplete. Define the useful cylindrical depth, material grade and condition, intersections and required geometry before comparing prices. Then agree on the regions that inspection must cover. The hydraulic valve-body case below shows how a 15:1 blind passage required separate allowances for guidance, coolant delivery, internal deburring and long-reach inspection.
1. When Hole Depth Changes the RFQ
In deep hole drilling, start with the length of bore that performs the function. Effective cylindrical depth, depth to the drill point and tool reach describe different dimensions. Resolve those references during the DFM design review, while the surrounding geometry can still change.
EPOC CRAFTER shop-floor data shows the distinction on a hydraulic valve body in Baosteel 42CrMo4, quenched and tempered to 260 to 300 HBW. Its Ø8 mm blind main passage had a 120 mm effective cylindrical depth, giving a hole depth-to-diameter ratio of 15:1. The 140° drill point extended the total depth to 121.46 mm. Two Ø4 mm transverse holes intersected the passage at 70 mm and 100 mm from the entrance.
The gun drill had a 180 mm effective working length. Its installed distance from the front of the guide device to the tip was 155 mm. A drill length-to-diameter ratio needs that length reference: tool working length, installed reach and hole depth describe different constraints.
The drilling depth-to-diameter ratio was 120 ÷ 8 = 15:1. That hole aspect ratio describes the passage, without establishing a universal deep-hole threshold. For high aspect ratio drilling, assess a supplier’s maximum drilling depth against diameter, material condition, guidance, coolant delivery and required bore quality.
Inspection coverage belongs in the initial review. The case checked diameter at 20 mm, 60 mm and 105 mm from entry. Entrance-axis perpendicularity applied over the first 30 mm; full-length axis straightness was process controlled and was not a full-length acceptance characteristic. Price the required coverage explicitly.
2. Confirm the Process and Supplier Capability
A machine travel specification cannot establish deep hole machining capability. Ask about guidance, coolant delivery, chip return, workholding and inspection on comparable material and geometry. A deep hole drilling machine and a machining center can require different preparation even when both reach the programmed depth. Review machining center selection alongside the complete operation sequence.
2.1 Gun Drilling and BTA Drilling
Gun drilling, also written gundrilling, uses a guided tool and a defined coolant and chip-return circuit. The gun drilling vs BTA drilling comparison starts with those circuits, the supported diameter range and the available machine configuration. UNISIG’s process guides distinguish external chip return in conventional single-lip gun drilling from internal return in BTA drilling.
| Process | Coolant and chip path | What the quote must cover |
| Single-lip gun drilling process | Coolant through the tool; chips along the external groove | Pilot or guide, support, supply circuit and chip-return clearance |
| BTA drilling process | Coolant around the drill tube; chips through its interior | Compatible pressure head, sealing, tube and internal chip exhaust |
| Ejector drilling | Coolant through the space between two tubes; chips through the inner tube | Compatible connector, two-tube assembly and required fluid delivery |
The hydraulic case used a horizontal gun drilling machine with a rotating tool and stationary workpiece. Deep hole boring may be added when the feature needs size or geometry correction. A quote for precision hole drilling should identify any such finishing operation, so you can compare the complete route.
2.2 Where Peck Drilling Fits
Peck drilling interrupts axial feed to manage chips, but the peck drilling cycle must suit the tool and coolant system. More withdrawals add non-cutting time and repeated entry. Sandvik Coromant specifies drilling without pecking for CoroDrill 861-GM under its application conditions. Use the toolmaker’s strategy before adding a generic peck cycle.
2.3 Deep Hole Drilling on Machining Centers
Deep hole drilling on machining centers requires more than fitting a long-series carbide drill. A solid-carbide deep-hole drill may need a dedicated pilot, a controlled entry sequence and sufficient coolant at the cutting edge. Small-diameter deep-hole drilling also leaves less space for chip transport and a narrower margin for tool damage.
Sandvik Coromant lists CoroDrill 861-GM in a 30D configuration for Ø3 to 8 mm holes, with a pilot and internal coolant. The range belongs to that product and its application conditions. Ask your CNC drilling services supplier which tool family and machine configuration the quote assumes.
3. Specify the Geometry the Part Needs
Blind hole drilling requires a defined bottom and usable depth. Angled hole drilling also needs entry geometry and a datum reference that the supplier can reproduce. Specify the features that serve the part’s function before selecting additional controls.
3.1 Diameter Limits and Fit
A hole diameter tolerance defines permissible size. Gun drilling tolerances quoted without diameter, depth, material and inspection conditions give buyers little basis for comparison. Deep hole drilling tolerances must also identify whether they concern size, axis geometry, form or finish.
ISO 286-1:2010 establishes the limits and fits system; ISO 286-2:2010 supplies the tabulated deviations. Table 6 gives 20 H7 as 20.000 to 20.021 mm in the size interval above 18 mm through 30 mm, and 90 H7 as 90.000 to 90.035 mm above 80 mm through 120 mm. The same grade has different widths in those intervals. A review of ISO 286 hole tolerances belongs with the mating feature’s limits, not with an assumed straightness guarantee.
The hydraulic case specified Ø8.000 to 8.050 mm directly. A drilled hole tolerance should reflect the mating or flow function. Tightening drilling tolerances without a functional reason can add machining and gauging work without resolving hole alignment.
3.2 Straightness, Position and Bore Form
Hole straightness, hole cylindricity and hole position tolerance control different characteristics. ISO 1101:2017, Clause 17.2, distinguishes surface-line straightness from derived median line straightness. With the diameter symbol, the latter uses a cylindrical zone. Clause 17.5 defines cylindricity through two coaxial cylinders; Clause 17.13 describes position relative to theoretically exact dimensions and the indicated datum system. ISO 1101 defines the geometric requirement; it does not select the tolerance value for your part.
Drill wander can change the bore path while local diameters remain within limits. Drill deflection, guidance and setup alignment need review when investigating drill wandering. Ask what a supplier means by deep hole straightness or drilling straightness: which feature, reference and evaluation length? Define hole alignment against its functional datum. Allocate functional hole tolerances feature by feature, and check whether combined geometric zones already constrain the variation you need to control.
4. Review Chip Evacuation and Process Stability
Chip packing can obstruct coolant delivery, score the wall and contribute to drill bit breakage. When reviewing deep hole drilling problems, connect the symptom to chip return, tool condition, setup and cutting data. Specify the parameter and measurement region for bore surface roughness; a poor hole surface finish alone cannot identify the cause.
4.1 Chip Geometry and Process Stability
Chip control starts with cutting geometry and feed suited to the material. Chip breaking produces a manageable shape; chip evacuation removes it. The return path still needs to clear broken chips; chip packing can contribute to drill bit breakage. Drill chatter, drilling vibration and deteriorating drilling surface finish are symptoms to investigate alongside flow and tool condition.
ISCAR’s 2025 article “Achieving the Hole Solution with Deep Hole Drilling” describes chip splitting on TRIDEEP replaceable inserts. Assess an indexable gun drill by supported diameter, chip breaker geometry and material suitability. An indexable deep hole drill also needs a repeatable insert-change procedure: confirm the specified insert and check its seating before restarting. The RFQ question is how the proposed geometry will support chip evacuation during drilling on your part.
4.2 Coolant Flow, Tool Reach and Support
A through-coolant drill needs a supply circuit that delivers fluid under cutting load. Specify where pressure and drilling coolant flow are checked. A pump nameplate does not establish either value at the tool. For high-pressure coolant drilling, review the tool circuit, fluid condition and filtration with the supplier.
In the hydraulic case, deep hole drilling coolant was dedicated drilling oil. The circuit used magnetic separation and 10 µm fine filtration. Tool-inlet delivery under cutting load was 50 bar, equivalent to 5 MPa, at 15 L/min. The Ø8 mm tool ran at 1,800 rpm and 0.035 mm/rev, giving 63 mm/min axial feed. These settings belong to this material condition and setup; they are not a recipe for another alloy or bore.
Assess the gun drill, internal coolant drill or other deep hole drill bit with its guide and supply circuit. Coolant optimization starts with delivery and return-path checks. Tool overhang, also described as drill overhang, needs a defined support reference before it helps assess drill rigidity. The case used a Ø8.02 to 8.04 mm pilot, 15 mm deep, and an 8.03 mm guide-bush inside diameter; the tool entered at low speed before high-pressure supply.
For through-coolant drilling, ask which deep hole drilling tools and fluid circuit are included, and how delivery will be checked after a tool or filter change.
4.3 Process Monitoring and Adaptive Control
Drilling process monitoring can reveal changes before a finished bore is checked. The hydraulic job monitored pressure, flow, thrust, spindle load and chip return. Transient chip accumulation and thrust variation occurred around 95 to 105 mm during main-hole drilling, before the transverse holes were made. Cross-hole burrs were a separate later operation.
The team cleaned the filter and checked the chip groove and guide alignment, keeping tool diameter, speed, feed and pressure/flow settings unchanged. The response to falling flow or a thrust surge was to stop feed, maintain coolant and withdraw for tool and circuit checks.
UNISIG’s 2025 automation guidance describes real-time monitoring, parameter adjustment and interruption at defined limits. Ask what tool condition monitoring observes and what triggers a stop. A deep hole drilling monitoring plan should identify the signals, limits and recovery actions, including drill breakage detection where offered. For adaptive drilling, the supplier should explain which parameters the control can change. Adaptive control in machining does not replace dimensional acceptance.
4.4 Flow Optimization Supported by Experiments
A Production Engineering study by Rupasinghe and colleagues, published on 23 February 2026, combined drilling simulation, manufactured ejector drill heads and experiments. Changes to the chip mouth reduced vortex formation or changed local flow, lowering the fluid flow needed for stable drilling under the tested conditions.
Ask whether a proposed head or coolant change has been tested with comparable chips, fluid and tooling. The study supports geometry-specific evaluation; it does not establish a transferable pressure reduction or productivity gain.
5. Resolve Cross Holes, Burrs and Bottom Geometry
5.1 Internal Intersections and Exit Edges
Cross drilling changes the cutting sequence and cleaning scope. Drilling through an existing transverse opening interrupts the cut. Performing cross-hole drilling afterward can leave drilling burrs inside the main passage. State which hole is made first and how the intersection will be reached.
The hydraulic case drilled the main passage first, then the transverse holes. The intersection edges were required to have no detachable burrs. Internal deburring used a flexible brush through accessible main and transverse openings, followed by pressure flushing, reverse flushing, filtered collection and borescope viewing. No obvious hanging swarf was visible at the inspected main opening and intersections. The observation covers those visible regions.
A drill burr at the entrance, an exit burr and an internal breakthrough burr need different access. An entry burr may be reachable from outside; deep-hole deburring and cross-hole deburring need tools that fit the passage and intersection. Back deburring is an option only where the tool can reach and cut without damaging a functional edge. Define the required hole deburring result. Specify hole edge chamfers only on edges that need a defined angled face.
For intersecting holes in a hydraulic circuit, agree on edge condition and any particle-cleanliness requirement separately. Specify a separate particle-cleanliness test when the circuit requires quantified residual-particle limits.
5.2 Blind Hole Depth and Bottom Shape
Blind hole drilling leaves a bottom shape determined by the tool and any following operation. A flat-bottom hole requires a suitable flat-bottom drilling route; a conventional pointed drill does not produce a fully flat floor at its tip depth.
The case’s drill point angle was 140°. Effective hole depth was 120.0 mm, while drill point depth was 121.46 mm. Blind hole depth must state which reference applies. A change in drill depth can change the remaining wall or intersect another feature, even while the useful cylindrical length remains unchanged.
6. Agree on Inspection Before Quoting
Inspection accessibility determines whether the proposed method can evaluate the required feature. Your deep hole inspection plan should name the characteristic, region, instrument and reporting scope. Include cleaning and handling in the quote. Review the supplier’s manufacturing quality controls, then agree on the part-specific measurement plan.
6.1 Diameter and Straightness Verification
Bore measurement at selected sections establishes local size. A bore diameter measurement cannot establish the path between those sections. For hole straightness measurement, agree on the controlled feature and evaluation length; a bore straightness measurement method must reach and evaluate that scope.
The hydraulic job used a long-stem Ø8 mm air bore gauge with 110 mm reach, calibrated with a Ø8 mm ring. Parts were cleaned, dried and stabilized at 20 ± 2°C. Diameter measurement uncertainty was U = 0.006 mm at k = 2. Agree on the acceptance decision rule when uncertainty matters near a limit.
| Check on one hole | Location from entry | Reading | RFQ and acceptance meaning |
| Diameter | 20 mm | 8.018 mm | Local size within 8.000 to 8.050 mm; no conclusion about the axis |
| Diameter | 60 mm | 8.024 mm | Local size within the same limits; retain this depth in the inspection scope |
| Diameter | 105 mm | 8.031 mm | Local size within the same limits; 110 mm gauge reach covers this section |
| Surface roughness | 50 mm | Ra 2.4 µm | Local result below Ra 3.2 µm; axial evaluation length 4.0 mm |
EPOC CRAFTER shop-floor data above describes one hole. The 30-part production run used the same inspection arrangement. The diameter readings are local results within the specified limits; they do not establish geometry between sections or batch capability. The roughness requirement covered a 20 to 90 mm inspection region, while the listed trace was taken at 50 mm.
The entrance-axis requirement was perpendicularity Ø0.15 mm to datum A over 0 to 30 mm, evaluated with a coordinate measuring machine. Full-length axis deviation was process controlled and was not a full-length acceptance characteristic. Keep local orientation and full-length bore inspection as separate scopes.
6.2 What a Borescope Can Verify
Borescope inspection reveals visible burrs, chips and wall damage within the accessible field of view. The case used a Ø3.9 mm scope with 120 mm reach, with intersection checks at 70 mm and 100 mm. Blind hole measurement needs a method matched to the dimension; seeing a bottom does not establish its depth or the bore axis.
Some commercial scopes support calibrated local dimensional measurements, subject to probe geometry and viewing conditions. The case used visual hole inspection. Specify the characteristic and region for each deep hole measurement result so a visual check is not substituted for sizing or full-length axis evaluation.
7. Build an RFQ That Suppliers Can Evaluate
Use a deep hole drilling RFQ checklist to align supplier scope before comparing prices. The fields below identify work that can otherwise disappear inside a single drilling line item.
| RFQ field | Information to include | Why the supplier needs it |
| Function and material | Service conditions, material grade and material condition, stock form and relevant hardness | Select tooling and review the process for the supplied condition |
| Bore geometry | Diameter limits, cylindrical and tip depths, entry and bottom shape | Distinguish useful bore length from tool travel and remaining wall |
| Intersections | Diameters, locations, sequence and accessible edges | Price interrupted cutting, internal deburring and cleaning |
| Geometric controls | Required position, orientation, straightness or form; datums and evaluation lengths | Separate functional requirements and avoid redundant controls |
| Process scope | Guidance, tool reach, coolant delivery, filtration, monitoring actions and finishing operations | Compare complete routes and abnormal-condition responses |
| Inspection scope | Characteristics, covered regions, methods, reporting and acceptance rules | Establish what the results will demonstrate |
| Order scope | Quantity, repeat orders, setup, tooling and delivery requirements | Separate fixed preparation from recurring work |
Application changes the review. In manifold drilling, list every intersection and its cleaning access. Shaft drilling needs a functional reference for the bore axis. For hydraulic cylinder drilling, distinguish the initial hole from subsequent bore finishing. Cooling hole drilling and mold cooling holes need the intended connections and remaining wall, so a misplaced passage cannot be accepted solely on diameter.
For automotive component manufacturing, add the dimensional-report or submission requirements stated by the customer. Those documents should reflect the agreed bore characteristics and measured regions.
Deep hole drilling in aluminum such as 6061-T6 needs a review for that grade and state; do not transfer the 42CrMo4 case settings. Apply the same discipline to deep hole drilling in stainless steel such as 316L and deep hole drilling in titanium such as Ti-6Al-4V. For gun drilling in aluminum or gun drilling in stainless steel, ask which tool geometry and cutting-data range the supplier proposes for the named grade.
Compare gun drilling services by the complete operation sequence. A quote for deep hole drilling services or broader deep hole machining services should identify pilot preparation, drilling, deburring, cleaning and inspection. Gun drilling cost is only part of the total when other operations remain outside that line item. Deep hole drilling cost also includes setup and tooling, whose allocation matters in low volume production.
The hydraulic case shows why the RFQ should separately cover the 15 mm pilot, gun drilling, 50 bar oil supply, 10 µm filtration, cross-hole deburring, long-reach gauging and borescope checks. Use the same scope in competing quotations. A price that omits internal deburring or inspection is not a price for the same work.
8. Deep Hole Drilling Questions
8.1 What makes a hole a deep hole?
Deep hole drilling involves a depth-to-diameter ratio that calls for a dedicated review of tooling, chip transport or bore quality. No single ratio in ISO 286 or ISO 1101 determines the process. State the cylindrical depth and diameter, then ask which tool and setup support that geometry.
8.2 What is gun drilling?
Gun drilling uses a guided, slender tool for deep bores. A conventional single-lip arrangement supplies coolant through the tool and returns chips along an external groove. The gun drilling process review should identify the guide and coolant circuit included in your quote.
8.3 How can a deep hole stay straight?
Start a review of how to drill a straight hole by defining what must be straight and over which length. An entrance location or acceptable local diameter cannot establish the complete bore path. To decide how to prevent drill wandering, review guidance, alignment, support and cutting conditions against that requirement.
8.4 How should a supplier plan deep hole drilling?
A supplier planning how to drill deep holes should quote a route it can machine and inspect. Ask whether guidance, coolant delivery and long-reach measurement are included, and whether another operation follows drilling. A machine name alone cannot answer those scope questions.
8.5 How can chip evacuation be improved?
Investigate chip shape and the return circuit together when deciding how to improve chip evacuation. Check cutting geometry, feed and actual fluid delivery before changing the cycle. A short chip can still block the return path, so chip breaking alone does not establish evacuation capacity.
8.6 How can a deep hole be inspected?
Match the method to the characteristic and accessible region. Local diameter gauging, a roughness trace and visual inspection answer different questions. Before accepting a straightness report, check that its evaluation covers the specified feature and length.
Send your deep hole drilling RFQ to EPOC CRAFTER with the diameter, effective depth, functional tolerances, material condition, quantity and inspection requirements. Machining and verification can then be priced together.
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