This article is part of the DFM for CNC Machining: Design Rules and Tolerance Checklist on CNX Precision.
The counterbore vs countersink decision decides how your fastener head sits in the part. A counterbore creates a flat-bottom recess for a socket head cap screw. A countersink cuts a conical seat for a flat-head screw. In short, the fastener type determines the hole type. This guide explains both features, their dimensions, and when to use each. Therefore, you can call out the correct hole on every drawing.
What Is a Counterbore?
Understanding counterbore vs countersink starts with the fastener head. A counterbore is a flat-bottomed enlargement at the top of a hole. It provides a seat for a screw head, so the head sits flush or below the surface. Socket head cap screws, or SHCS, use counterbores. The feature has two key dimensions: diameter and depth. The diameter matches the screw head plus clearance. The depth matches the head height. For example, an M6 SHCS needs a 10 mm counterbore diameter and about 6 mm depth. The pilot hole through the part allows the screw shank. Meanwhile, a spotface is a shallow counterbore with minimal depth. Spotfaces provide a flat seat on castings and forgings. Consequently, they improve bolt seating on uneven surfaces.
Manufacturers machine counterbores with end mills. A CNC mill interpolates the bore to the exact diameter. This approach controls size and finish tightly. Alternatively, a pilot counterbore tool cuts both features in one operation. Therefore, counterbores work well in CNC machining. Designers should specify the standard counterbore size when possible. Standard sizes reduce tooling and inspection cost.
Counterbore tolerances typically run plus 0.1 to 0.2 mm on diameter. Depth tolerances are looser, around plus 0.3 mm. These values let the screw head seat without binding. Tighter tolerances raise cost without adding value. Therefore, use standard tolerances unless the design demands more.
Standard counterbore tables appear in fastener catalogs and machinery handbooks. Therefore, designers rarely need custom sizes. In addition, inch sizes follow ANSI standards, while metric sizes follow ISO. Consequently, the drawing should cite the standard when possible.
Counterbores also appear on tapped plates and flanges. A tapped hole with a counterbore lets a shoulder screw seat precisely. In addition, stacked plates stay aligned when counterbores register the screw heads. Therefore, counterbores support both fastening and assembly alignment.
Standard Counterbore Sizes
- M3 SHCS: 5.5 mm diameter, 3.1 mm depth
- M4 SHCS: 7.0 mm diameter, 4.1 mm depth
- M5 SHCS: 8.5 mm diameter, 5.1 mm depth
- M6 SHCS: 10.0 mm diameter, 6.1 mm depth
- M8 SHCS: 13.0 mm diameter, 8.1 mm depth
What Is a Countersink?
For a quick rule, remember that counterbore vs countersink maps to two screw families. A countersink is a conical recess at the top of a hole. It matches the taper of a flat-head screw. Therefore, the screw head sits flush with the part surface. Countersinks use standard angles. Inch screws use 82 degrees. Metric screws commonly use 90 degrees. In addition, 100 and 120 degree angles work for thin sheet metal. The countersink diameter depends on the head size and angle. For example, a 90 degree countersink with 8 mm top diameter produces a 4 mm deep cone. Machinists create countersinks with dedicated countersink tools or chamfer mills. CNC machines can also interpolate the cone with a single point tool. However, dedicated tools give smoother finish.
Countersink depth matters for screw seating. If the cone is too shallow, the head protrudes. If it is too deep, the head sinks below the surface. Inspection uses gauges or optical comparators. A simple go or no-go gauge checks the depth quickly. Therefore, most shops measure countersinks with dedicated gauges.
Inspection of countersink angles uses gauge balls or optical systems. For example, a ball gauge seats in the cone and measures depth by calculation. Therefore, verification stays fast and accurate.
CNC machines produce countersinks with precise angles and depths. A tool holder with depth control ensures repeatability. For large countersinks, interpolation avoids chatter. Therefore, CNC countersinking delivers consistent results across batches.
Thin sheet metal favors countersinks because they need less depth. A 90 degree countersink cuts into 1 mm sheet easily. In contrast, a counterbore of similar size may break through. Therefore, check the material thickness before choosing the feature.
Why Use a Countersink
- Flat-head screws sit flush for a clean surface.
- Flush surfaces prevent snagging and wear.
- Countersinks also deburr the hole edge.
- They center the screw during assembly.
Counterbore vs Countersink: Key Differences
The counterbore vs countersink difference comes down to shape and fastener type. Compare both features side by side.
| Feature | Counterbore | Countersink |
|---|---|---|
| Shape | Flat bottom cylinder | Cone |
| Fastener | Socket head cap screw | Flat-head screw |
| Head position | Flush or recessed | Flush |
| Key dimensions | Diameter and depth | Angle and diameter |
| Load capacity | Higher | Lower |
| Standard angles | None, flat bottom | 82, 90, 100, 120 degrees |
| Tooling | End mill or counterbore tool | Countersink tool |
Both features share one rule: the pilot hole matters. The pilot diameter controls thread engagement for tapping. In addition, pilots must be perpendicular to the surface. Otherwise, the screw head seats unevenly. Consequently, machinists machine pilots and counterbores in the same setup when possible.
When to Use a Counterbore
Use a counterbore for structural joints that carry high loads. Socket head cap screws provide greater strength than flat-head screws. The counterbore also protects the head from wear. The counterbore vs countersink choice depends on assembly needs and stress levels.
When to Use a Countersink
Use a countersink when the surface must stay flush and clean. Electronics panels, enclosures, and sheet metal parts benefit. In addition, countersinks suit thin material where counterbore depth is impossible.
Counterbore vs Countersink: FAQ
Which hole type is stronger?
A counterbore with a socket head cap screw carries more load. The flat head of a countersunk screw transfers less force. In the counterbore vs countersink comparison, strength favors the counterbore.
What angle should a countersink use?
Match the angle to the screw. Inch screws need 82 degrees. Metric screws usually need 90 degrees. Therefore, always check the fastener data sheet. In addition, 100 degree screws exist for thin panels. Therefore, confirm the exact head angle on the drawing.
Can a part use both features together?
Yes. Some designs combine a counterbore with a chamfer or a countersink above it. For example, a counterbore may include a 45 degree chamfer at the edge. However, such combinations increase machining time. In addition, countersinks often sit beneath a counterbore for shoulder screws.
Once you master counterbore vs countersink, hole callouts become simple. Choose the fastener first, then match the hole feature. At CNX Precision, our machinists cut both features daily on 3, 4, and 5-axis mills. In addition, our ISO 9001 system verifies every diameter and depth. Send your drawings for a free DFM review.
For related information, see our guide to cnc machining service and 5-axis cnc machining and cnc milling vs turning and cnc machining tolerances.