Craftsman 137.248760 tablesaw blade comes loose
If the blade is spinning, but the nut remains secure add a washer between the blade and nut. If the nut is loosening look at it's threads for evidence of cross-threading; it may be stripped. If so apply a thread-locking fluid and snug it tight. Check that the blade opening is the correct size for the arbor (I'm sure it is but...) and that the blade doesn't have a knock-out bushing that has come detached.