counterbore vs countersink

counterbore

noun
  • A cylindrical recess, typically machined around a hole to admit a screw so that it sits flush with a surface. 

  • The tool with which a counterbore is machined. 

verb
  • To create such a cylindrical recess. 

countersink

noun
  • a cylindrical recess, typically machined around a hole to admit a screw so that it sits flush with a surface. 

verb
  • To create such a conical recess. 

  • To cause to sink even with or below the surface. 

How often have the words counterbore and countersink occurred in a corpus of books? (source: Google Ngram Viewer )