The "spoil" board is exactly as described. When you cut out a part, you cut through the part into the spoil board a small amount to make sure the cut is completely through and to give the bottom of the part a better surface. Eventually, the spoil board is full of cuts and gets uneven, at which time it is surfaced with a large flat end mill to make it smooth again. When the spoil board burns down to a thin layer, it's replaced with a new one.
The spoil board is typically made from 3/4 MDF and for a small machine, probably $10-20 (in these times of high priced wood).
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