Counterbore vs countersink vs spotface for CNC bolted joints

Why bolted-joint callouts confuse CNC quotes

Counterbores, countersinks and spotfaces all clear fastener heads or create a flat seat for a washer or bolt head—but they are not interchangeable on a drawing. Buyers who write “C’sink for M6” without angle, diameter and depth force estimators to guess, and shops that guess wrong scrap the first article. This guide compares the three features for CNC bolted joints, shows typical drawing callouts and lists RFQ notes that keep aluminum and steel housings on schedule. Values below are common industry practice for metric fasteners; your OEM standard and fastener drawing override any rule of thumb.

For related fastener and hole practice, see designing threads and tapped holes, thread inserts vs tapped holes in aluminum, and GD&T basics for buyers. Surface finish on seats matters when sealing washers or soft gaskets ride on the joint—see surface roughness Ra for machined parts.

Counterbore, countersink and spotface defined

• Counterbore diameter (clearance for the head)
• Counterbore depth (from the top face to the flat seat)
• Through or pilot hole diameter for the shank
• Optional bottom fillet or corner break if the tool cannot leave a sharp internal corner

Counterbores are the default choice when you need a flat bearing surface under a hex socket head and a controlled depth so the head does not protrude into a mating cover.

• Included angle (commonly 82° or 90° for many inch and metric flat-head screws; some aerospace and specialty fasteners use other angles—match the fastener drawing)
• Major (outer) diameter of the cone at the face, or depth of the cone
• Pilot / through hole for the shank
• Whether the head must sit flush, proud or slightly underflush

Angle mismatch is the most common countersink scrap cause: an 82° screw in a 90° seat does not seat correctly and can crack thin heads under torque.

• Depth is minimal (often only a few tenths of a millimeter to a small fraction of the fastener head height)
• Diameter is sized for the washer or head bearing area
• The through hole is still required for the shank

Spotfaces are common on castings and flame-cut plate; on fully CNC-milled faces they are used when you want a local flat without sinking the head into a deep counterbore.

Side-by-side comparison for RFQ language

Use distinct words on the print. Do not write “C’bore / C’sink as required.”

Illustration 1 in this article summarizes those columns for buyers who prefer a visual matrix.

Drawing callouts that estimators can price

• “Ø11 counterbore × 6.5 deep; Ø6.6 through” for an M6 socket-head style joint (verify against your fastener and washer stack—these are example proportions only)
• Datum and position tolerance on the through hole if the bolt pattern is critical; do not assume the counterbore diameter automatically carries the same tolerance story
• State whether depth is measured to the flat seat and whether a small fillet at the counterbore bottom is allowed

Avoid “counterbore for SHCS” without sizes. Head catalogues differ by standard and manufacturer family.

• Call the included angle every time: “90° countersink to Ø12” or “82° × 6.5 deep”
• If flushness matters cosmetically, say “head flush to −0.1 / +0.0” or similar—shops need a target, not “flush”
• On thin walls, check remaining material under the cone; a deep countersink can break into a pocket or O-ring groove

• “Spotface Ø18 × 0.5 deep max” (depth as a maximum is common when the goal is only a flat seat)
• Relate the spotface to the bolt pattern with position if the washer must clear adjacent walls
• On castings, note whether the spotface must fully clean the as-cast surface or a partial land is acceptable

Geometry interactions shops watch for

CNC process implications

• Counterbores: end mills, dedicated counterbore tools or spot drills followed by milling; flat bottoms may need a second pass if a drill point remains
• Countersinks: single-flute or multi-flute countersink cutters at the correct angle; form tools matched to the fastener
• Spotfaces: short end mills or counterbore tools at shallow depth

Estimators care about quantity of holes, hardness of material and whether holes are on one face (single setup) or wrap a housing (multi-setup or five-axis). Five-axis aluminum housing tips cover when ports and bolt patterns on several faces drive setup count.

• Counterbore: depth micrometers, pin gauges for pilot, optical or CMM for position
• Countersink: angle gauges, major diameter at the face, flushness of a sample screw
• Spotface: depth as maximum, flatness of the seat when a sealing washer is used

First article inspection reports should list each recess type separately when angles or depths differ across the pattern.

Material notes for common CNC metals

RFQ checklist for bolted-joint recesses

Before you send the package:

1. Every fastener location states counterbore, countersink or spotface—not a generic “machined recess.”

2. Countersinks include included angle and major Ø or depth.

3. Counterbores include Ø and depth to the flat seat plus pilot Ø.

4. Spotfaces include seat Ø and max depth.

5. Coating sequence vs final size is stated when flushness matters after finish.

6. Edge distance and thin-wall risks are visible on the drawing or noted in the RFQ text.

7. Sample fastener standard or head style is named when “for SHCS” would otherwise be ambiguous.

8. Deburr / edge-break expectations on recess rims are written, not assumed.

Illustration 2 walks a simple decision path: flush flat-head → countersink; recessed socket head → counterbore; flat seat only → spotface.

Common specification pitfalls

• Mixing 82° and 90° language on the same drawing family without tying each hole to a fastener
• Calling a deep spotface that is actually a counterbore (inspectors then argue about depth tolerance)
• Applying one position tolerance balloon to a pattern where only the pilot holes are critical and the counterbore OD is clearance
• Leaving countersink depth “until flush” with no tolerance band on cosmetic covers
• Ignoring washer OD when specifying spotface diameter—too small a seat bruises soft washers

How 3DWK uses this on quotes

When your RFQ lists clear recess types, angles and depths, CNC estimators can select tools, count ops and plan deburr without a clarification loop. Ambiguous “C’sink / C’bore as needed” notes delay the quote. Attach the fastener standard or a head-dimension excerpt when the joint is non-standard. For production intent on machined housings and plates, request a quote with the drawing package and call out which holes are structural versus cosmetic.

Related reading and next steps

Pair this comparison with designing threads and tapped holes when the same part carries both recesses and threads, and with flatness / parallelism / perpendicularity when large covers bolt across a wide face. If your joint also seals, review O-ring groove design for CNC housings so bolt holes never cross the gland path.