bur vs knurl

bur

noun
  • A rotary cutting implement having a selection of variously shaped heads. 

  • Any of several plants having such husks. 

  • A rough, prickly husk around the seeds or fruit of some plants. 

knurl

noun
  • A lined or crossgrained pattern of ridges or indentations rolled or pressed into a part for grip. 

  • A crossgrained protuberance; a nodule; a boss or projection. 

  • A contorted knot in wood. 

verb
  • To roll or press a pattern of ridges or indentations into a part for grip. 

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