Designing Blind Holes for CNC Machining

Start With Functional Depth

A blind hole ends within the part rather than exiting an opposite face, so its depth is a functional design decision. The important question is not simply how deep a tool enters. It is which portion of the hole must accept a mating feature, guide a component, retain fluid, provide thread engagement, or establish clearance. That required portion should be identified as usable depth.

A drawing that labels only one blind-hole depth can leave room for conflicting interpretations. A mating dowel may need full-diameter engagement over a stated length, while a fastener may need a specified threaded engagement length. State the feature’s role, identify the reference surface from which depth is measured, and distinguish cylindrical working depth from the remaining bottom geometry.

Account for the Drill Point

A conventional drilled hole does not usually finish with a perfectly flat bottom. The drill tip creates a conical point, leaving less than the nominal diameter near the lowest part of the feature. If a component, pin, screw end, or sealing element must reach the bottom, this reduced-diameter region can prevent the intended fit even when the reported drilling depth appears adequate.

The designer should therefore decide whether the bottom zone is nonfunctional, available only for clearance, or required to be flat and fully usable. The applicable drawing, process plan, material behavior, tool geometry, and engineering agreement determine the allowance needed; a single numeric rule does not suit every diameter or operation. If bottom shape matters, call it out rather than assuming a generic drill result.

• For a locating feature, specify the minimum full-diameter engagement depth.
• For an inserted component, define any required bottom clearance.
• For a flat-bottom requirement, identify the permitted bottom form and corner condition.

Choose the Bottom Condition

The simplest blind hole usually accepts a drilled point because it limits added operations and leaves the designer flexibility in the unused lower region. A more controlled bottom may be useful when assembly stack-up, internal clearance, fluid volume, or a mating part makes the lower boundary important. That choice should follow function, not appearance in a CAD section.

A flat-bottom feature can require a different toolpath or finishing operation. It may also leave a corner transition that is not perfectly sharp. Deep, small-diameter, or difficult-to-access features can add further constraints. Instead of prescribing a method unnecessarily, define the functional geometry, surface requirement where relevant, and any prohibited interference. The manufacturer can then evaluate a process consistent with the drawing and agreed requirements.

Separate Drilling From Threading

A blind threaded hole contains several related but different dimensions: pilot-hole depth, thread depth, full thread engagement, chamfer or entry condition, and clearance below the thread. Calling out only a nominal thread designation and a depth can obscure whether the mating fastener must bottom out, stop on a clamped joint, or remain clear of the hole base.

Specify the thread standard, nominal size, pitch where applicable, class or fit if required, and minimum functional thread depth. Then define the mating fastener’s expected engagement and any allowable runout or clearance below it. Do not assume that every thread extends uniformly to the bottom of the drilled feature. The applicable standard and agreed manufacturing plan govern how the thread is produced and verified.

• Avoid using a blind-hole bottom as the intended screw stop unless that contact is explicitly designed and reviewed.
• Provide a relief or unused region below required engagement when the mating screw must not interfere.
• Define entry chamfer only when it serves assembly, protection, or inspection needs.

Control Tolerance by Function

Not every blind hole needs the same control. A clearance hole can often be governed by the mating fastener and assembly freedom, whereas a locating bore may require tighter control of size, position, straightness, surface, or depth. Applying a restrictive tolerance to every hole can obscure the truly critical features and make quotation review less informative.

Choose tolerances from the assembly function and inspection method. Diameter limits alone may not control where the hole begins, its orientation, or the effective depth available to the mating part. Use datums that reflect how the part is located in service. Where the feature interfaces with a purchased or mating component, cite the relevant drawing, standard, component data, material grade, and engineering agreement.

Make Inspection Intent Visible

Blind-hole inspection is more nuanced than checking an external length. A depth reading can be influenced by a drill point, burr, chamfer, surface condition, or the reference face selected by the inspector. Thread verification introduces its own question: is the requirement based on thread form, gauge acceptance, functional engagement, or another agreed criterion? The drawing should make the intended acceptance basis understandable.

For high-consequence features, identify the depth reference surface and clarify whether depth applies to the full-diameter bore, the theoretical drill-point apex, or the start and end of usable thread. If a special gauge, mating-part check, borescope review, or measurement report is needed, state it as a requirement before manufacture. Inspection planning should be proportionate to function and controlled by the approved documentation.

• Use the same datum logic for the feature callout and the inspection requirement.
• Identify functional depth rather than relying on an unlabeled CAD measurement.
• Flag inaccessible or visually concealed features early so measurement can be planned.

Review Before Requesting a Quote

A strong pre-quote package removes avoidable interpretation without dictating every machining step. Include the part drawing and model, material grade, revision status, hole schedule, thread information, general tolerances, critical datums, expected quantity, and any finish or cleanliness requirement that affects the feature. Make sure the modeled depth agrees with the dimensioned requirement.

Review blind holes in section view before release. Check for tool access from the proposed entry face, proximity to thin walls or intersecting features, required full-diameter depth, thread clearance, and interference from the mating component. If the part is governed by an assembly, share the applicable interface information. Questions raised during technical review are an opportunity to align the drawing, process plan, and inspection approach.

• Confirm whether each depth is total, usable, threaded, or a clearance requirement.
• Confirm the bottom shape required by the mating condition.
• Confirm the controlling drawing revision, standard, and material grade.
• Identify features whose acceptance depends on a mating part or special inspection.