Figure 4-94.  Table riveter. Figure 4-95.  Dimpling techniques. Figure 4-96.  Predimple and countersink method. Hot Dimpling Radius Dimpling rivet hole needing to be drilled to correct rivet size before the  dimpling operation is accomplished. Since the metal stretches  during the dimpling operation, the hole becomes enlarged and  the rivet must be swelled slightly before driving to produce  a close fit. Because the rivet head causes slight distortions in  the recess, and these are characteristic only to that particular  rivet head, it is wise to drive the same rivet that was used as  the male die during the dimpling process. Do not substitute  another rivet, either of the same size or a size larger. Radius dimpling uses special die sets that have a radius  and are often used with stationary or portable squeezers.  Dimpling removes no metal and, due to the nestling effect,  gives a stronger joint than the non-flush type. A dimpled  joint reduces the shear loading on the rivet and places more  load on the riveted sheets. NOTE: Dimpling is also done for flush bolts and other  flush fasteners. Dimpling is required for sheets that are thinner than the  minimum specified thickness for countersinking. However,  dimpling is not limited to thin materials. Heavier parts may  be dimpled without cracking by specialized hot dimpling  equipment. The temper of the material, rivet size, and  available equipment are all factors to be considered in  dimpling.  [Figure 4-95] Hot dimpling is the process that uses heated dimpling dies to  ensure the metal flows better during the dimpling process. Hot  dimpling is often performed with large stationary equipment  available in a sheet metal shop. The metal being used is  an important factor because each metal presents different  dimpling problems. For example, 2024-T3 aluminum alloy  can be satisfactorily dimpled either hot or cold, but may crack  in the vicinity of the dimple after cold dimpling because of  hard spots in the metal. Hot dimpling prevents such cracking.  7075-T6 aluminum alloys are always hot dimpled.  Magnesium alloys also must be hot dimpled because, like  7075-T6, they have low formability qualities. Titanium is  another metal that must be hot dimpled because it is tough  and resists forming. The same temperature and dwell time  used to hot dimple 7075-T6 is used for titanium.  Metals of different thicknesses are sometimes joined  by a combination of dimpling and countersinking.  [Figure 4-96]  A countersink well made to receive a dimple  is called a subcountersink. These are most often seen where  1 2 3 Gun draw tool Flat gun die Bucking bar Male die Female die Hole Dimpled hole This top sheet is dimpled Thick bottom material is countersunk 100° Combination Predimple and Countersink Method 4-43