Dimple countersinking is accomplished by using male and female dies. The female die, shown in figure 13-14, contains a spring-loaded ram that flattens the bottom of the dimple as it is formed. This prevents cracks from forming around the dimple. The forming of a dimple is a combined bending and stretching operation. A circular bend is formed around the hole. As in any bending operation, the tension force at the upper side of the bend (break) creates the radius at the junction of the two surfaces-the top side of the sheet and the downward bent inner wall of the dimple depression. The stretch occurs around the hole as it is displaced from its original position and relocated at the bottom of the dimple. The female die must have a slightly larger cone diameter than the corresponding dimension of the male die. This allows for material thickness and relieves the bending load at the break in order to avoid circumferential cracks around the boundaries of the dimple. As a further safeguard, a slight radius is made on the female die at the junction of the top face with the dimple depression.

Dimpling dies are made to correspond to any size and degree of countersunk rivet head available. The dies are numbered, and the correct combination of dies to use is indicated in charts specified by the manufacturer. Both male and female dies are machined accurately and have highly polished surfaces. When you dimple a hole, place the material on the female die and insert the male die in the hole to be dimpled. The dies are generally brought together, forming the dimple by a mechanical or pneumatic force.

As newer aluminum alloys were developed to increase shear and tensile strength, they became more difficult to form, since these alloys are harder and more brittle. These aluminum alloys are subject to cracking when formed or dimpled cold. For this reason, it is necessary to use a hot dimpling process. The application of hot dimpling to the more brittle materials helps reduce cracking. The heat is applied to the material by the dies, which are maintained at a specific temperature by electrical heaters. The heat is transferred to the material to be dimpled only momentarily, and none of the heat-treat characteristics of the material are lost.

There are several models of dimpling machines used in the Navy, from the bulky floor models to portable equipment. One of the most popular portable types is shown in figure 13-15. Basically, it has three units: the dimpling control unit, the dimpling squeezer, and the thermo dimple gun.

The dimpling control unit is a small compact unit designed to regulate dimple die temperatures, prepressure, dwell time, and final forming pressure.

Figure 13-14.-Dimple countersinking.

Figure 13-15.-Hot dimpling kit.

This same unit is used with both the hot dimpling squeezer and the thermo dimple gun.

The hot dimpling squeezer is designed for use where stationary squeezer operation is impractical or impossible. It is capable of working all material gauges up to and including 0.091 of an inch. The squeezer is designed to dimple in areas that are inaccessible to other types of equipment. Male and female dies are independently warmed by electrical heaters. The heaters produce a short heat-up and recovery time. The male die is adjustable to provide the maximum squeeze on all gauges of material. The unit also has a cooling feature.

The thermo dimple gun is used to dimple in the center of panels and in those areas otherwise inaccessible to stationary dimpling equipment. When it is being used on the aircraft, the thermo dimple gun drives the dimple from the exterior while the female die and dolly bar are used on the inside. The thermo dimple gun is air cooled. This eliminates the need for cumbersome heat-resistant gloves. This tool is small, compact, well balanced, and easy to handle.

Before adjusting the control unit for dimpling, you should refer to the equipment manufacturers dwell time chart. When you set up any dimpling equipment, follow the step-by-step procedure outlined in the operating and maintenance manual supplied with the equipment. Since equipment types vary, it is impractical to specify a standard procedure; however, there are four general requirements of a dimple, and by examining each, it is possible to denote improper setting up of equipment.

1. Sharpness of definition. It is possible to get a dimple with a sharp break from the surface into the dimple. The sharpness of the break is controlled by two things: the amount of pressure and the material thickness.

2. Condition of dimple. The dimple must be checked for cracks or flaws that might be caused by damaged or dirty dies, or by improper heating.

3. Warpage of material. The amount of warpage may be held to a minimum if the correct pressure setting is held. When dimpling a strip with too much pressure, the strip tends to form a convex shape, as shown in figure 13-16. When insufficient pressure is used, it tends to form a concave shape. This can be checked by using a straight edge. 

Figure 13-16.-Checking dimple equipment air pressure.

4. General appearance. The dimple should bechecked with the fastener that is to be used, making sure it meets the flushness requirement. This is important because the wrong type or size of dies are sometimes used by mistake.