I wrote last month about the top-most part of a cable tool drill string: the socket. This column discusses jars, the next item on the drill string (which we may or may not want to use).
Jars for cable tool work look like two elongated lengths of a chain. Drilling jars have a gap of about 4 to 5 inches between the upper and lower parts. Fishing jars, which I will write about in a future column, have a much longer stroke or “free play.” For drilling, the jars come between the socket and the drill stem. They serve to bump loose, or “jar,” a stuck drill bit. How does a drill bit get stuck? When drilling in drift, a pebble or stone can wedge between the expanded or dressed-out part of the drill bit and the casing. In bedrock, the drill bit could get stuck in a crack or fissure.