bur vs whetstone

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. 

whetstone

noun
  • A sharpening stone; a hard stone or piece of synthetically bonded hard minerals that has been formed with at least one flat surface, used to sharpen or hone an edged tool. 

  • A benchmark for evaluating the power and performance of a computer. 

  • A stimulant. 

verb
  • To sharpen with a whetstone. 

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