A countersink hole is one in which the outer edge of the hole has been opened up to a conical entry. For ANSI/Imperial fasteners, it is commonly at 82°, 90° for metric fasteners, 100° for BA threaded fasteners, and 120° for sheet metal rivets.
The countersink is generally sized according to the head dimensions of the fastener being fitted, such that a flat-faced fastener will sit flush with the countersunk surface. It is common to further recess the head to sit below the countersunk surface by increasing the diameter of the countersink. This process allows for variations in the head and potential distortion of the drive recess in the fastener, caused by driver bruising.
A countersink is identical to a deburring, except that most deburring tools have a 90° included angle. The purposes are different, in that a deburring intends to remove a burr, whereas countersinking is generally deeper and is used to make fasteners flush or below flush with the part surface. This article will further discuss: “What are countersink holes in engineering?”, how to drill one, their applications and uses, as well as the countersink hole size charts.
How To Drill a Countersink Hole
Countersinking is usually performed as a secondary operation after the hole is formed/drilled. The countersink tool does not cut at its center of rotation and can, in some cases, have a smooth post that locates the tool into the hole to ensure a smoother start and precise centering.
Countersinking tools are specialist drill bits and cutters that can have multiple cutting faces. In the case of larger countersinks, and particularly those for machine use, they commonly have only one flute/cutter. This feature aids in centering on the hole and producing a more even and regular cut.
Countersinking operations can be manual or machine performed, but involve plunge cutting a specialist cutter of the correct included tip angle into a pre-made hole, to a defined and controlled depth, to produce a standardized and repeatable conical recess.
Choosing the Right Tap for a Countersink Hole
Where the countersunk hole is formed/drilled through to the other side of the part, the tap required for threading such a hole is a taper or middle type. These two forms of tap have the initial threads ground away, allowing the tap to enter the hole and correctly center from the start of tapping/thread cutting.
For a through hole, the selection of taper or middle taps depends on the depth of the hole to be tapped. There must be enough total length of unaltered tap and shank combined to allow the tap to penetrate deeply enough to complete the thread through the part.
Where the hole is blind, tapping requires a two-stage operation. The thread is initiated with a taper or middle tap and then completed with a bottoming tap to the required depth. The choice between taper and middle taps is driven by the blind hole depth. Where its use is possible, the taper tap gives a softer and more certain start to the thread cutting. But if the taper prevents effective cutting as it is longer than the blind hole is deep, it is necessary to use a middle tap.
Applications and Uses of Countersink Holes
Countersunk holes are most commonly used for the aesthetic flush fitting of fasteners, to render a product surface more smooth. Where the countersink is further drilled parallel, this is referred to as a “counterdrilled” hole. This allows plugs or caps to be fitted, which renders the fasteners invisible, taking the improved appearance to its logical conclusion. This generally applies to fasteners that are not at risk of requiring later removal.
Additional practical reasons for countersinking holes to bury the heads of fasteners are: in moving equipment, to remove obstacles and reduce the need for clearances; in walkways, and on stairs and gantries, to remove obstructions and trip hazards; and to bury fastener heads where additional components must overlay the fastener, removing the need for precise relief holes/recesses in the overlying part.
Countersink Hole Size Charts
Tables 1, 2, and 3 below list typical countersink dimensions for socket flat head screws to sit flush with the surface they are sunk into:
