Counterbored holes

Define the recess and its function

A counterbore leaves a flat annular shoulder around a smaller coaxial hole. Specify the recess diameter, depth and reference face to control head seating and remaining material.

Original tolerance example: assume a finished plate thickness 10.00 ±0.10 mm and recess depth 6.00 ±0.05 mm, measured from the same top reference face. Remaining thickness beneath the recess shoulder is nominally 4.00 mm. Independent limits give 3.85–4.15 mm beneath the shoulder, from 9.90 − 6.05 and 10.10 − 5.95 mm.

For an assumed head height 5.90 ±0.05 mm with no washer, the same recess gives 0.00–0.20 mm head-top clearance below the reference face: minimum 5.95 − 5.95 and maximum 6.05 − 5.85. If a strictly positive recess is required, this hypothetical stack does not guarantee it. Real fastener dimensions and tolerances must replace these illustrative assumptions.

Gühring distinguishes the flat cutting geometry of counterbores from tapered countersinks. Its tooling guidance describes a pilot entering a predrilled hole to guide the counterbore, with fixed and interchangeable pilot arrangements. These are tooling options, not requirements that every counterbore be made using a pilot. No technical issue date was verified on the webpage. Gühring countersinking and deburring tools

Keep counterbores distinct from conical countersinks and from shallow seating preparations whose functional requirement is only a local bearing surface. Similar tool names do not establish identical drawing requirements.

Dimensions that belong together

A diameter and depth without an entry face can be ambiguous on a part accessible from both sides. Define depth from the intended finished reference surface, especially when machining, coating or facing will change that surface. A blind smaller hole also needs its own end condition; recess depth is not total drill depth.

Tolerances should express the functional need rather than an assumed capability of a named cutter. If coaxial alignment or seating orientation controls assembly, define that relationship explicitly in the drawing/model requirements.

A dated fastener-clearance example

Carr Lane's Trigonometry Tables & Handy References for Engineers, revision September 2021, page 9, tabulates counterbored holes for socket-head cap screws. Its 1/4-inch screw row gives:

The countersink field is separate from the cylindrical recess. Preserve the source's diagram and field definitions rather than treating every diameter as an alternative counterbore size. These are bounded reference-table values, not a universal requirement for every 1/4-inch fastener or a completed joint design. Carr Lane reference, page 9

Confirm the exact fastener head style, governing standard and edition, dimensional extremes, washers and installation-tool access before selecting final dimensions. Low-head, button-head and other screw forms cannot be assumed to use the same recess merely because their threads match.

ASME's official B18.3-2012 preview identifies an inch-series socket-screw standard and a printed issuance date of April 24, 2013. That historical preview is edition evidence, not confirmation that a supplier's current screw conforms to it or that it remains the latest edition. The order and drawing should identify the actual required fastener specification. ASME B18.3-2012 official preview

Seating and remaining material

Do not equate nominal recess depth with guaranteed flushness. The actual head height, any washer, depth tolerance, surface finish and reference-face variation determine whether the installed head is recessed or protruding. Verify underhead fillet clearance and seating contact as part of that stack.

This is dimensional bookkeeping only. It does not establish sufficient bearing strength, pull-through resistance or fatigue life. It also does not represent a solid floor at the hole center when the smaller hole is through.

Machining and access

Tool selection depends on recess size, depth, material, access and the existing hole. Gühring notes that its piloted tools use the predrilled hole for guidance; the pilot and hole must therefore be compatible in the actual process. Do not infer suitability from the nominal screw size alone. Gühring counterbore guidance

Harvey Tool's named 25511 flat-bottom long-reach counterbore illustrates separate tool dimensions: 0.1719-inch cutter diameter, 0.6250-inch flute length and 1.3750-inch overall reach. Reach and flute length are different tool attributes; neither alone is a guaranteed usable machining depth in an arbitrary setup. No technical revision date was verified on the product page. Harvey Tool 25511

Review chip evacuation, fixturing and interference with neighboring geometry. Specify acceptable shoulder finish and burr removal where they affect seating. A conventional drilled conical bottom does not satisfy a flat seating requirement merely because its diameter matches.

CAD and acceptance

SOLIDWORKS 2026 Help identifies counterbores as a Hole Wizard feature and describes separate hole-position and hole-profile sketches. The selected fastener can dynamically update parameters; the resulting values still require engineering review against the actual hardware and drawing. A library default is a modeling convenience, not evidence of fit or joint strength. SOLIDWORKS 2026 Hole Wizard overview

Before releasing the part:

1. Confirm the smaller hole, recess and entry side in both model and drawing.
2. Check head/washer/tool clearance with actual dimensional limits.
3. Evaluate remaining material and surrounding features for the intended loads.
4. Agree the machining route and inspect the functional diameters, depth and seating surface.
5. Accept the assembled condition separately where flushness or seating contact matters.

For agent-readable records, preserve units, source edition, reference face and whether a value is nominal, a tolerance limit or a catalogue recommendation. Keep recess geometry separate from thread designation, clearance class and fastener acceptance.