On motion and for good cause shown, and upon such terms as the court prescribes, but not ex parte, the court may set aside a default or the judgment thereon, for mistake, inadvertence, surprise, excusable neglect or unavoidable casualty. Such motion must be filed promptly after the discovery of the grounds thereof, but not more than 60 days after entry of the judgment. Its filing shall not affect the finality of the judgment or impair its operation.