Means associated with a yarn package building mechanism such as a winder automatically to assure that the package is built to a predetermined diameter, thus assuring that such packages are efficiently used in subsequent processes. The invention provides a simple, economical attachment which may be placed on existing winders and other package builders without major change therein. The mechanism incorporates a knife which is engaged by the oscillating thread as it is level wound onto the package, the knife being raised as the package is built, so that finally the thread engages the knife and is cut, thus stopping the building of the package.