welding however to protect the metal element from corrosion and to ensure a smooth trough wall which is easy to clean it is preferred to arrange the metal element such that is fully enclosed within the trough wall the position at which the metal element 30 is arranged in the trough wall 21 22 23 24 25 depends on the design of the trough 20 however it is preferred to arrange the metal element 30 in a position in the trough wall where the concentration of metal particles in the grinding liquid is high during grinding preferably the metal element 30 it therefore arranged in the circumferential wall 21 22 23 24 of the trough 20 and close to the bottom 25 of the trough 20 by u201c close to the bottom 25 u201d is meant that the metal element 30 may be arranged in a position adjacent to the bottom 20 or in a position on half of the height of the circumferential wall or anywhere between these positions for example the metal element 30 may be arranged in a position on a quarter of the height of the circumferential wall the metal element 30 may also be arranged in the bottom 25 of the trough 20 it is preferred that the metal element 30 is arranged on one of the front or rear end walls 21 22 of the trough 20 when arranging the metal element 30 in the trough wall consideration is preferably given to the direction of rotation of the grindstone since the rotating grindstone creates a flow of.