Dimple Countersinking Tools 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.