Countersunk holes

What the feature does

A countersink has a conical seat. Specify included angle and the dimensions that define the head’s seating condition. Protolabs sheet-metal guidelines

For the ideal 10 mm seat over a 6 mm pilot, a 90° cone needs 2.000 mm depth and an 82° cone needs 2.301 mm. In a 3.5 mm plate that leaves 1.500 mm versus 1.199 mm of cylindrical land. The calculation excludes tolerances, burrs and tool-tip details. It does not establish bearing strength.

For the sheet-metal tooling example with an 8 mm seat, see Countersink forming tools. It compares ideal recess depth with a dedicated forming tool’s depth guidance.

SendCutSend identifies 82° as common for imperial/SAE flat-head screws and 90° for metric screws. These are selection clues, not a substitute for the actual fastener drawing: Protolabs also lists 100° and 120° tooling options. Specify the included cone angle, not the half-angle used in a trigonometric calculation. SendCutSend countersinking, Protolabs guidelines

Information to carry into a drawing or agent task

Use the following as a proposed handoff checklist; the supplier's accepted drawing convention and the fastener specification govern the final callout.

Do not infer a recess-depth tolerance from the word “flush.” Agree on whether the functional acceptance is a measured diameter, an axial head position with the specified fastener, or both. A seat with the wrong angle can meet a major-diameter target without matching the head's conical surface; this is a geometric mismatch, not evidence of a particular joint load capacity.

Geometry: diameter, angle and depth are coupled

For an ideal cone intersecting a coaxial cylindrical pilot, let D be the major diameter, d the pilot diameter, θ the included angle and h the axial depth from the entrance surface to the cone/pilot intersection:

h = (D − d) / [2 tan(θ / 2)]

Calculated example, not a fastener standard: for D = 10 mm and d = 6 mm, a 90° seat requires h = 2.000 mm; an 82° seat requires approximately 2.301 mm. In an assumed 3.5 mm thick plate with a through pilot, the remaining cylindrical land is respectively 1.500 mm and about 1.199 mm. Holding depth at 2 mm while changing to 82° produces a major diameter of about 9.477 mm, rather than 10 mm.

These values exclude tolerances, tool tip details, burrs, coatings and formed-sheet geometry. They establish dimensions only. They do not establish minimum safe wall thickness, fastener bearing strength or allowable load. Check the widest cut against adjacent features and edges, and evaluate minimum remaining material using the actual dimensional limits.

Fabrication requirements are supplier-specific

SendCutSend's countersinking service recommends a depth no greater than 60% of material thickness and warns about interference with edges, nearby holes and bend lines. Treat this as that service's guidance for supported materials and tooling; do not apply it as a universal countersink design limit. Protolabs distinguishes formed countersink diameter tolerances from its other feature guidance, reinforcing the need to confirm the chosen manufacturing process. SendCutSend service guidance, Protolabs feature guidance

For SendCutSend's 2D upload workflow, draw the minor hole and configure the countersink in its ordering system; its instructions say not to draw the major diameter into that cut file. This upload convention does not remove the need to define major diameter and angle in the engineering requirements. Confirm the selected face in the supplier's preview. SendCutSend file setup and orientation

Inspection and finished condition

Starrett's countersink gauges seat against the angular sides and directly indicate the large diameter, whereas its chamfer gauges contact the top edge. Its countersink gauge offerings include 82°, 90° and 100° versions and use a master ring for calibration. Select a gauge matched to the specified angle and measurement method; a large-diameter indication alone does not verify every aspect of the cone or the assembled head position. Display increments are not an accuracy specification. Starrett countersink gauges

As a proposed acceptance sequence, identify the inspection face, check pilot and major diameter plus angle, inspect burrs or damaged seating surfaces, and check head position with the specified fastener when assembly flushness matters. Agree on the finishing stage: a requirement referenced to the finished surface needs an acceptance method consistent with that surface. SendCutSend notes that machining can leave oil residue and visible tool marks, so cosmetic expectations also need an explicit agreement. SendCutSend finish notes

Related reference

Part features

Source scope

The cited sources are live manufacturer/service pages reviewed on the date above, not a normative fastener or drawing standard. Service capabilities and ordering conventions require confirmation for the specific quotation. Equations, calculations and proposed handoff/acceptance steps are editorial engineering guidance under the assumptions stated here.