Pa. t. and pple. countersunk. [COUNTER-, app. in a sense akin to 8 b, the hole being the counterpart of that which is to be sunk in it.]
1. trans. To enlarge the upper part of (a hole or cavity), usually into a conical form, to receive the head of a screw, bolt, etc.; to bevel the edge of a hole.
1831. J. Holland, Manuf. Metal, I. 170. The practice of countersinking the holes, to receive the nail heads.
1861. Wynter, Soc. Bees, 189. Counter-sinking the eyes in order that they may not cut the cotton.
1869. Sir E. J. Reed, Shipbuild., xxi. 482. The holes in the plates to receive the bolt-heads are countersunk one-half through the plates.
2. To sink the head of (a screw, bolt, etc.) in a depression made to receive it, so that it lies flush with the surface.
1816. J. Smith, Panorama Sci. & Art, I. 70. The heads of the bolts must not project above the surface of the plate; therefore the grooves must be … wider at the top than the bottom, and the heads may then be countersunk.
1868. Regul. & Ord. Army, § 573 b. The horse’s shoe is not to be grooved but simply punched, and the nails countersunk.
Hence Countersinker, a tool for countersinking; = COUNTERSINK sb. 1; Countersinking vbl. sb.
1881. Mechanic, § 319. The rose or roundheaded countersinker.
1888. Daily News, 25 April, 6/1. Countersinking machines.
