HELIX

HELIX is a simple program that will help you calculate the size of the annular pieces needed to yield a desired finished auger size.
One way to make an auger is to cut annular shaped (washer-shaped) pieces from flat stock, slit them along a radius, then stretch them out and weld them to a circular shaft. If the diameter of the finished auger is to be D, the od of the annulus must be larger than D because the act of stretching the piece to form one turn of the screw will cause the diameter of the annulus to shrink. The same argument applies to the id of the annulus. It must be larger than the diameter of the shaft. The allowance needed for these effects depends on the pitch of the auger.
Think of it this way. The outer circumference of the annulus must be equal to the distance measured along the edge of the flight as we move through one revolution. This distance is longer than the circumference of the auger and the amount of length depends on the pitch of the auger.