Abstract: Milled screen units can be tack welded to existing pockets after the conventional screen has been removed or can be recessed into the top of modified pockets and tack welded. In both cases the construction of the screen unit permits minimal tack welding thus obviating the change in metallurgical structure normally present thereby reducing chemical fretting when used with active solutions and/or abrasive particles. The screen is formed from a cylinder in which the slits are milled. The sections are then cut from the cylinder and flattened so that the slits take up a cross sectional configuration having a narrow width on one side and gradually widening towards the other side thus preventing clogging during use.