When drilling holes larger than 1/2 inch, drilling a smaller hole first is a good practice. This is called a pilot hole, and the pilot drill should have a diameter __________.
• Pilot hole purpose when using a large drill bit • How the chisel edge / web of a drill affects cutting vs. scraping • What happens if the pilot hole is too big or too small compared to the center of the larger drill
• Think about what part of a twist drill actually CUTS metal and what part only SCRAPES or forces material aside. • Consider how a pilot hole can reduce the thrust (pushing force) needed by the larger drill without destroying its ability to guide itself. • Ask yourself: if the pilot hole diameter is bigger than the solid center of the larger drill, what will the cutting edges be doing when they first contact the metal?
• Compare each choice to the size of the web / chisel edge (the solid center) of the larger drill. • Eliminate any option where the pilot hole would be so large that the larger drill’s chisel edge never contacts metal at all. • Verify which option best balances reduced drilling force with maintaining guidance and proper cutting action of the larger drill.
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