Countersunk Hole

A countersunk hole is a cone-shaped recess, and it is cut at the top of a drilled hole. It is primarily used to hold a screw and bolt with a tapered head. It allows the screw to sit flush with the surface and slightly below it, and creates a uniform finish.

Besides this, this hole feature is helpful in tight spaces, where a raised screw head might cause interference with nearby components. It makes the products functional and visually clean.

Most countersunk holes are made by custom CNC machining at standard angles of 82° or 90°. The angle matches the taper of the screw head, while the depth is controlled by a countersink tool. This ensures a precise fit for the fastener in every application.

This article explains everything you need to know about countersunk holes. It will cover what they are, their standard sizes, and how they are used in design and engineering.

You will learn about:

What a countersunk hole is

How countersinking helps in design

Standard angles and sizes to use

Difference between Countersink vs Counterbore holes

What Are Countersunk Holes?

Countersunk Hole Profile

A countersunk is a mounting hole that has a cone-shaped profile. It is designed to screw the head sinks into the hinge surface and stay level with it. This creates a flat, smooth finish instead of leaving the screw raised above the surface.

With a straight hole, the fastener head always sticks out. That can catch on objects, look untidy, or affect the hinge's movement. A countersunk hole solves this problem. The screw head fits neatly inside the recess. This keeps the hinge surface clear and gives it a clean, professional look.

The dimensions of the hole are essential as both the angle and cone depth are cut to match the screw being used. For example, common countersink angles are 82° or 90°. If these don’t match, the screw will not sit properly, and it may sit too high or sink too deep.

Dimensions of a Countersunk Hole

Standard Dimensions for Countersunk Hole

A countersunk hole dimension has three major aspects:

1. The countersink diameter
2. The pilot hole diameter
3. Angle of the conical section

The correct values are selected in accordance with the fastener used, such that the screw or bolt head is flush with a surface. In engineering drawings, these dimensions are usually indicated in one callout.

The larger top portion of the hole is created by countersinking. It is conical in form and must be broad enough to receive the head of the screw in it. The diameter of the pilot hole is the reduced part below through which the screw shank passes. These two values must be carefully correspond with the fastener size.

The slope of the conical part depends on the included angle. Metric fasteners tend to be 90 degrees, but imperial fasteners tend to be 82 degrees. Other angles, e.g., 60, 100, 110, or even 120 degrees, are also used on special items like rivets or hardware. The appropriate angle allows the screw head and countersink surface to come into contact to match.

These dimensions are shown together when they appear in a technical drawing. A standard countersunk hole callout consists of a hole size (usually the pilot hole diameter), the diameter of the countersink, and the angle, sometimes with a tolerance, such as saying, e.g., 5/27. The pilot hole may read “Ø 5/27. V 11/32 ±0.005 x 82°”, whereas the countersink hole is 11/32, the tolerance of the hole is ±0.005, and the angle of the hole is 82.

Screw Heads and Why Countersinking Can Be a Problem

Countersunk Screw

Countersinking makes screws lie level and does not always go well. When there is an angle, depth, or alignment that is off, the screw will not fit as intended. This tends to weaken the joint and affects the finish.

Fit and Angle

Tapered screw heads are of varying sizes, depending on the screw standard. The drill bits cut at 118 or 135 degrees and cannot cut the appropriate shape. To have a good fit, a countersink tool with the corresponding screw head angle is needed.

Choosing the Right Tool

Various countersink tools can obtain varying results. Multi-edge cutters are fast, but uneasy without lubricant. Three-flute cutters plug less and are more difficult to steer. The smoothest finish and most reliable countersinks are conical and typically more expensive. Regardless of the tool, it must be lubricated.

Getting Accurate Results

Countersinks are declared to be self-centred, but are inclined to wander. When so, the head of the screw is not in equilibrium. A special countersink, preferably one with a centring guide, used exclusively on the screw with which it is to be used, is preferable.

Keeping Depth Consistent

A set of holes at the same depth is far more difficult. When depths do not align, there will be a disparity in the screw height. A drill press with a depth stop can assist, but minor alterations are still required to keep things uniform.

Cap head screws are lenient if the holes are not perfect. The screw, when not placed well, is left in a bad position and leads to insecure fastening.

Countersunk Hole Size Guide for 90° Flat Head Screws (Metric)

Table: Countersunk Hole Sizes in ISO Metric (90 Degree)

Note: All dimensions are in millimetres.

Countersunk Hole Size Chart (82° Flat Head, ANSI Inch Standard)

Table # 2: Countersunk Hole Sizes in ANSI Standard.

Note: All dimensions are in inches, and the Countersunk angle is 82°.

Countersink vs Counterbore: What Are the Differences

Counterbore Vs. Countersink Holes

Both countersink and counterbore holes are used to allow screws and bolts to sit neatly. But, they are not alike and keep the different aspects. A countersink is drilled at a slope, while a counterbore is a deep, flat hole.

Here’s a simple breakdown:

Conclusion

This guide has explained a countersunk hole, its use, and how its size and angles are determined. You also learned about the tools and care needed to make them accurate and neat.

We also looked at the difference between a countersink and a counterbore. A countersink is sloped for flat-head screws, while a counterbore is flat and deep for bolts. So, it's essential to understand which one to use to help make parts stronger and cleaner.