This is a video which shows in brief, the minting process for silver eagles at the West Point Mint. A few points of interest from the video include:
1.The planchets come from an outside vendor.
2.The planchet's edges are oiled (to help them slide through the feeder tubes.) This is one of the sources of grease which silver eagles are sometimes found "struck through."
3.The coin rolls are weighed by hand--every roll! This means that clips or underweight planchet errors are almost impossible to find for silver eagles, and are almost unknown for the series.
4.There is so much individual handling of the planchets and coins that any errors, beyond small strike-throughs, are unlikely to make it out of the mint. This is why even partial collar strikes are very rare for the silver eagle series. The most common errors found are small grease strike throughs or struck through small bits of plastic (which occasionally is found "retained" on the coin's surfaces.)