Wipe off the fresh cuttings to locate the bare spot. It should have scrapes from the blade hitting the deck. If you are sure there isn't a crack, then I would check the spindle. If it's vibrating after hitting the deck, you will need to tighten or replace the spindle. (it will need tightening if the blade is moving around) You may also want to check that the bearings are in tact.
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