Method for determining propelling condition for shot medium, and method for manufacturing coil spring

A method for determining a propelling condition for a shot medium, the method comprising:

• a first step of forming a coating film on a surface of a coil spring;
• a second step of performing a shot peening treatment of propelling the shot medium onto the coil spring where the coating film is formed;
• and
• a third step of visually checking whether or not the coating film remains on the coil spring where the shot peening treatment is performed, wherein:
• in a case where it is visually confirmed that the coating film remains in the third step, at least one or more of conditions of the shot peening treatment including a propelling speed of the shot medium, a propelling time of the shot medium, a material of the shot medium, and an average particle diameter of the shot medium are changed and the second step and the third step are repeated until the coating film does not remain;
• and
• in a case where it is visually confirmed that the coating film does not to remain in the third step, the condition of the shot peening treatment in the second step in which the coil spring is obtained with no remaining coating film is determined as the propelling condition for the shot medium.