Use
Tool for measuring spindle tram (1-1/2 in x 1-1/2 in spindle)
With the Jackstick placed on the spindle, and the spindle and Jackstick are rotated 360° to check that a feather quill protruding thru the Jackstick is consistently the same distance from the bedstone, thus proving the spindle is perpendicular to the bedstone.
