A Dimple die set for sheet metal is typically used where flush fasteners are desired but the material is to thin to drill a countersink hole. Example 0.025 inch thick aluminum. In this case a spring back dimple die is used to form an indentation in the metal that is the proper angle and depth for the fastener being used. 100 degree is standard for flush mount rivets and some other fasteners. If unsure check the angle of your head before purchasing a set of dimple dies or a countersink. That information should be available from the company the fasteners were purchased from. In may cases it will be listed in the item description. Spring back dimple dies are available in 4 separate sizes and as a 4 piece kit. If buying only one size make sure it fits the diameter of your fastener. THIS LISTING IS FOR THE DIMPLE DIE ONLY! RIVET SQUEEZERS ARE SOLD SEPERATELY!