Perforated Screen Plate: Clear- or Buhr-Punched Steel Plate (Richards, Ore Dressing, 1903)

The punched steel screens of ore mills: round holes or slots punched through steel plate, either 'clear punched' with a sharp square-edged tool that cuts out a wad, or 'buhr punched' with a thicker tool that tears the metal over a socket in the die. Clear punching suits holes of 0.03 inch and up; buhr punching suits fine holes of 0.02 inch and less, because a thicker plate can be used. Holes in rows at 60° give 1.154 times the open area of rows at 90°. Unknown: the plate thickness and hole spacing for a given screen. Source: R. H. Richards, Ore Dressing vol. I (1903), pp. 167-168. Picture: p. 167 with Figs. 117 and 118.