Method for cleaning nozzle and laser machining apparatus

A method for cleaning a nozzle of a laser machining apparatus to process a work piece to be processed by a laser beam introduced into a jet liquid column, the method comprising the steps of:

• forming the jet liquid column by jetting a liquid from the nozzle;
• and
• irradiating a spot of the laser beam onto a vicinal section of an inlet opening of the nozzle in such a state that a peak power per unit area of the spot of the laser beam irradiated on a surface provided with the inlet opening of the nozzle does not go beyond a threshold value at which the nozzle is damaged and removing foreign matter attached on the surface,
• wherein the vicinal section is within a distance range from a center of the inlet opening which is five times or more, but 15 times or less as large as a diameter of the inlet opening,
• the threshold value is 1.0 MW/mm2,
• the step of removing the foreign matter comprises making the spot of the laser beam scan the vicinal section on the surface,
• the laser machining apparatus comprises:
• the nozzle;
• a liquid reception chamber to receive the liquid for the inlet opening;
• and
• a window disposed on an upstream side with regard to the liquid reception chamber in an optical axis direction of the laser beam, and
• the step of removing the foreign matter comprises:
• making the laser beam converged within the liquid reception chamber;
• and
• having the spot of the laser beam formed on the surface with the spot of the laser beam spread from a position where the laser beam is converged.